Skip to main content
NCBI home page
Journal of Animal Science logoLink to Journal of Animal Science
. 2021 Nov 20;99(12):skab348. doi: 10.1093/jas/skab348

Effects of physical or fenceline boar exposure and exogenous gonadotropins on puberty induction and subsequent fertility in gilts

Robert V Knox 1,✉, Lidia S Arend 1, Ashley L Buerkley 1, Jennifer L Patterson 2, George R Foxcroft 2
PMCID: PMC8717829  PMID: 34967902

Abstract

The present study was part of a larger experiment that evaluated litter of origin effects on gilt production. The objectives of this study were to determine the effect of physical or fenceline boar exposure and exogenous gonadotropins on puberty induction and subsequent fertility in a commercial farm environment. The experiment was performed in three replicates. Prepubertal gilts were assigned by pen (13/pen) to receive 15 min of daily Fenceline (FBE, n = 153) or Physical (PBE, n = 154) Boar Exposure (BE) for 3 weeks starting at 184 d of age in a purpose-designed Boar Exposure Area (BEAR). At the start of week 3, prepubertal gilts were randomly assigned to receive PG600 or none (Control). From weeks 4 to 6, estrus was checked using only FBE. During weeks 1 to 3, measures of reproductive status were obtained weekly or until expression of estrus. Upon detection of first estrus, gilts were relocated into stalls and inseminated at second estrus. PBE reduced age (P = 0.001) and days to puberty (P = 0.002), increased the proportion of gilts in estrus (P = 0.04) in week 1 (38.3 vs. 27.5%), and tended (P = 0.08) to improve estrus in week 2 (37.6 vs. 26.1%) compared to FBE, respectively. In week 3, more prepubertal gilts receiving PBE-PG600 exhibited estrus (P = 0.04; 81.8%) compared to PBE-Control (40.3%), FBE-PG600 (56.4%), and FBE-Control (47.8%). Overall, expression of estrus through week 6 tended (P = 0.08) to be greater for PBE than FBE (91.5 vs. 85.0%). PBE increased (P ≤ 0.05) or tended to increase (P > 0.05 and ≤0.10) service and farrowing rates in parities 1 through 4, but within parity, there were no effects (P > 0.10) on pig production or wean to service interval. Analyses also indicated that weeks from start of boar exposure to puberty, litter of origin traits, and follicle measures at puberty were related to the subsequent fertility. The results of this study confirm the advantages of using increased intensity of boar exposure, combined with PG600 treatment, for effective induction of pubertal estrus in a commercial setting.

Keywords: boar exposure, estrus, fertility, gilt, longevity, puberty

Introduction

Effective management of puberty induction in gilts is important for achieving high production efficiency and longevity in the sow herd. Gilts are needed to replace sows that are older, less productive, ill, unsound, or infertile. Annually, > 45% of breeding females are culled for reasons mostly related to poor reproductive performance and structural failures (Engblom et al., 2008; Tani et al., 2018; PigChamp, 2019). This high replacement rate can increase crowding of gilts and result in lower productivity when compared with herds with lower replacement rates. The economics of early culling are also not sustainable because 15% to 20% of gilts produce only a single litter before removal (Lucia Jr et al., 2000; Engblom et al., 2008), and an economic return on gilt investment costs occurs only after a third litter is produced (Stalder et al., 2003; Rodriguez-Zas et al., 2006). The conditions that limit lifetime performance and lead to early culling have been attributed to improper age, weight, and reproductive maturity at the time of first service (Hughes et al., 2010). In support of this association, high performing breeding herds have been reported to have lower culling rates after the first litter compared to those of less productive farms (Sasaki and Koketsu, 2012).

The major problems associated with replacement gilt entry into the breeding herd include delayed puberty (Eliasson, 1991) and failure to express pubertal estrus (Heinonen et al., 1998). Numerous reports indicate that > 20% of gilts may fail to display puberty within 60 to 80 d of starting boar exposure (Stancic et al., 2011). For gilts expressing delayed puberty, less than optimal body condition or maturity at first service can result in early removal from the herd due to reproductive or structural failures (Patterson and Foxcroft, 2019). The available data suggest that lifetime performance and longevity can be associated with gilts expressing pubertal estrus soon after the start of boar exposure (Knauer et al., 2010), adequate growth rate (Amaral Filha et al., 2009; Kummer et al., 2009; Tummaruk et al., 2009), and the timing and intensity of boar stimulation (Karlbom, 1982; Hughes et al., 1997; Patterson et al., 2002). Genetic selection and improved management have increased litter size in swine, but with higher frequencies of decreased birth weights (Patterson and Foxcroft, 2019) and increased litter birthweight variation (Quesnel et al., 2008). This could have important implications for traits of selection for replacement gilts (Patterson et al., 2020). Gilts with lower birthweight often display reduced growth rate (Beaulieu et al., 2010) and contribute to increased variation in the pubertal response to boar exposure.

The present study was part of a larger investigation designed to determine whether litter of origin traits would impact the efficiency of gilt replacement programs and lifetime productivity in a commercial system (Patterson et al., 2020). Previous studies demonstrated that direct boar stimuli improved the proportion of gilts showing pubertal estrus compared to fenceline stimulation in stalls (Patterson et al., 2002), and use of exogenous gonadotropins in prepubertal gilts after an initial period of boar stimulation helped meet industry benchmarks for gilt and sow productivity (Patterson et al., 2016). Therefore, the objectives of the present experiment were to confirm that the intensity of boar stimulation critically affects the synchrony of boar-induced pubertal estrus and to assess whether this method would impact the response of prepubertal gilts to exogenous gonadotropins (PG600).

Materials and Methods

The use of animals in this experiment was approved by the Institutional Animal Care and Use Committee of the University of Illinois at Urbana-Champaign (#15194).

Experimental design

The present experiment was performed in three replicates during the winter months of November 2015 to January 2016 on a commercial breed to wean 2,800 sow farm in northern Iowa. Camborough R gilts (PIC) with litter of origin records (n = 264) for birthweight (0.59 to 2.50 kg) and litter size (3 to 20 piglets), and their dam’s litter birth weight phenotype, and a smaller number of gilts without litter of origin records (n = 43), were used in this study. The gilts were transferred from an off-site nursery into an internal isolation/acclimation room at the farm in batches of 210 females at ~70 d of age. After 4 wk, the females were relocated into an adjacent gilt development unit (GDU) that was environmentally controlled with a fully slatted concrete floor. The GDU had four rows of 10 pens that were each 3 × 6 m. The GDU also had two boar exposure areas (BEAR) in the center of the barn, with each housing five Meishan x Large White F1 cross boars that were 6 to 18 mo of age. The gilts remained in their group pen until ~170 d of age when final pre-selection for conformation and minimum target weight (106.5 kg) was performed. Gilts that did not meet the pre-selection criteria were not mixed with selected gilts or assigned to treatment. Groups of selected gilts (n = 13) were moved into treatment pens to allow ~1.4 m2/female. While in the GDU, gilts housed in pens were provided ad libitum access to water and a corn-soybean meal diet.

Starting at 184 ±0.3 d of age (range: 173 to 200 d), pens of prepubertal gilts were randomly assigned to receive physical (PBE, n = 154) or fenceline (FBE, n = 153) boar exposure (BE) for 15 min once daily in the morning for 3 wk. At the start of week 3, half of the prepubertal gilts from each treatment were randomly selected to receive PG600 (400 IU PMSG and 200 IU of hCG, Merck Animal Health, Madison, NJ) or none (Control), with the assigned BE treatment continuing for the third week. Pens of gilts were alternated by day between the two BEAR pens. The FBE provided nose-to-mouth contact between the group of gilts and all five of the boars in the BEAR stalls. In addition to this level of stimulation, PBE involved the release of one boar out of the stall for direct physical contact with the pen of gilts. The boar that was released was rotated daily. A gilt was identified in pubertal estrus when displaying the immobility reflex either in response to the presence of boar stimuli or the back-pressure test applied by a stockperson. Once a gilt was confirmed in standing estrus, her body weight was measured using a digital scale and the gilt was moved into a row of stalls within the GDU where reproductive measures could occur. Additionally, at the end of each week, prepubertal gilts from each treatment pen were weighed and moved into open stalls to record reproductive measures before returning them to their group pen. Gilts assigned to treatment and that displayed pubertal estrus within 1 to 6 wk were moved out of the GDU and into the gestation barn, and bred on their second estrus. At day 30 after breeding, pregnancy was confirmed using transabdominal ultrasound. Pregnant gilts remained within the gestation barn until 1 wk before moving into the farrowing barn. Confirmed non-pregnant gilts were either culled or returned to the breeding herd.

Vulva measures

Three-dimensional measures were obtained on a sub-population of the gilts at pubertal estrus and in prepubertal gilts at the end of weeks 1, 2, and 3. The base-width, mid-length, and base-depth were measured by ruler and used to create a single measure based on a three-sided pyramid (Figure 1). The area for the base (B) of the vulva was calculated using ½ (b*h), where b is the base width and h is the depth. The volume of the vulva was determined using the formula 1/3 (B*H), where B is the area of base and H is the height of the vulva. Vulva temperature (Figure 2) was also assessed in a sub-population (n = 91) of gilts at the same intervals using a thermal imaging camera (Fluke Ti-55, Everett, WA).

Figure 1.

Figure 1.

Open in a new tab

Image of the vulva of a gilt (left) and the measurements obtained for width, depth, and height used to calculate the area for triangular pyramid based on the approximate shape of the vulva.

Figure 2.

Figure 2.

Open in a new tab

Vulva temperature was assessed in a sub-population of gilts (n = 91) using a thermal imaging camera (Fluke Ti-55, Everett, WA) at pubertal estrus and in prepubertal gilts at the end of boar exposure weeks 1, 2, and 3.

Assessment of ovarian follicles

Ovarian follicle development was assessed in a sub-population of gilts (n = 176) by transrectal ultrasound using an Aloka 500 V console and a 7.5 MHz linear array transducer (Hitachi Aloka Medical, Ltd., Wallingford, CT) attached to a PVC stabilizing rod. Scanning sessions were digitally recorded using a computer for playback and obtaining measures at a later date. Data measures included the size, and number of follicles on both ovaries from the day of pubertal estrus and at the end of weeks 1, 2, and 3 in prepubertal gilts.

Gilt measures of weight and backfat

Animal weight was measured using an electronic scale and backfat was assessed using an Aloka 500 V ultrasound with a 7.5 MHz linear array transducer at the 10th rib at the P2 site with digital recording. Measures were obtained while gilts were in stalls at pubertal estrus and in prepubertal gilts at the end of weeks 1, 2, and 3. Backfat depth was assessed during digital playback.

Reproductive records

Gilt litter of origin records included the dam’s parity, total born litter size, gilt birthweight and birthdate, and the dam’s litter birth weight phenotype. Using the birth records, age at boar exposure, growth rate (g/d) to puberty, age at puberty, first service, farrowing, and removal were determined. Weight at puberty and days to second estrus were assessed for all gilts. Subsequent performance data for gilts included service and farrowing events, litter measures (total born, born alive, stillborn, and mummified piglets), lactation length, and wean-to-service intervals for parity 1 to parity 4 (P1 to P4).

Statistical analysis

Data were analyzed using ANOVA procedures in SAS (SAS Institute Inc., Cary, NC). Continuous response measures were analyzed using PROC MIXED and binary responses tested using PROC GLIMMIX using a binary distribution and a logit-link function. Replicate was included in all models as a fixed effect. Significance of the main effects was determined using the F-test and differences between least squares means were identified using the t-test. The effects of treatment on measures related to fertility included all gilts and responses related to litter of origin included only tagged animals with those records. Pubertal estrous responses were analyzed by week from the start of boar exposure. The effects of treatment and PG600 and their interaction were included in the models for week 3 and for all subsequent fertility responses. Treatment and other class variables were evaluated for their effects on service and farrowing rates for all assigned gilts to analyze retention through fourth parity. Assessment of lifetime pigs included the total born from females that produced a first litter with ≥1 pig. Data with repeated measures over weeks or multiple parities were analyzed using PROC MIXED with the REPEATED statement and using a variance component (VC) structure. Litter of origin traits were also included in the treatment models for assessing the specific effects of birth weight class (< 1.30 kg [n = 54], 1.30 to 1.69 kg [n = 146], and ≥1.7 kg [n = 64]) and total born litter size class (< 12 [n = 35], 12 to 16 [n = 147], or ≥17 [n = 82]) on response variables. The effect of week of pubertal estrus after the start of boar exposure (1, 2, 3, or 4 to 6) was also tested in a one-way ANOVA using the MIXED procedure for effects on fertility measures. The assumptions for ANOVA were confirmed by visual assessment of data plots for normality using PROC UNIVARIATE, whereas homogeneity of variance was confirmed using Levene’s test. Several response variables from the data set (follicle size, days to puberty, estrous interval, stillborns, and mummies) could not meet the assumptions for ANOVA and could not be transformed for normality, and were subsequently analyzed using nonparametric analysis (NPAR1WAY) with the Wilcoxon test option. Relationships between selected variables were also assessed using PROC CORR and significant correlations (> r = 0.20) were reported. Significant differences were identified at P ≤ 0.05 and non-significance at P > 0.10.

Results

Effects of intensity of boar stimulation with or without PG600

The responses to BE treatment on measures associated with pubertal estrus are shown in Table 1. For gilts with litter of origin measures, the dam’s parity, total born litter size, and gilt birthweight, were similar (P > 0.10) between treatments. From the start of BE treatment at 184 d of age, days to pubertal estrus (P = 0.002) and age at puberty (P = 0.001) were reduced with PBE compared to FBE. Growth rate (778 g/d), body weight (134 kg), and backfat (18.7 mm) at pubertal estrus did not differ between treatments (P > 0.10). Pubertal estrous induction rates were greater (P = 0.04) for PBE in week 1 and tended (P = 0.08) to be greater in week 2. In week 3, there was an interaction of BE treatment with PG600 (P = 0.04), and estrous induction was increased in PBE-PG600 compared to the other treatments (P < 0.05). There was no effect of BE treatment on pubertal estrus in weeks 4 to 6, but by the end of the 6-wk period, overall pubertal estrous induction rates tended (P = 0.08) to be ~7% greater for PBE than FBE. There were no effects of treatment (P > 0.10) on the number of large follicles (12.8) or follicle size at pubertal estrus, but average size of follicles was greater at pubertal estrus (7.3 mm) compared to measures for prepubertal gilts (6.0 to 6.5 mm) in weeks 1 to 3. Vulva temperature at pubertal estrus did not differ in response to BE treatment (34.4 °C) and varied across weeks and treatment (31.4 to 35.5 °C). Although there were no effects of BE treatment on vulva size at pubertal estrus or in prepubertal gilts in weeks 1 through 3, vulva size was greater (P < 0.0001) at pubertal estrus (11.1 cm3) compared to the size of prepubertal gilts in weeks 1 (7.3 cm3), 2 (7.5 cm3), and 3 (6.6 cm3).

Table 1.

Least square means for reproductive measures of prepubertal gilts assigned to receive once daily fenceline (FBE) or physical (PBE) boar exposure for 15 min from weeks 1 to 3 and in week 3

Treatment TRT
Item n2 FBE PBE SE P-value
n1 153 154
Litter of origin
 Dam’s parity at farrowing 264 4.8 4.8 0.1 0.60
 Gilts’s birth litter total born 264 15.0 14.7 0.3 0.44
 Gilt birthweight, kg 264 1.5 1.6 0.02 0.12
 Gilt age at start of boar exposure, d 269 184.6 184.4 0.3 0.59
 Days to pubertal estrus 235 13.7x 10.4y 0.7 0.002
 Age at pubertal estrus, d 235 198.3x 194.7y 0.8 0.001
 Growth rate from birth to pubertal estrus, g/d 231 775.7 779.3 5.1 0.60
Weight
 Weight at pubertal estrus, kg 272 137.9 136.8 0.9 0.35
 Prepubertal gilt weight in week 1, kg 191 133.4 135.1 1.0 0.25
 Prepubertal gilt weight in week 2, kg 134 139.0 140.1 1.1 0.52
 Prepubertal gilt weight in week 3, kg 39 145.8 145.4 2.6 0.91
Backfat
 Backfat at pubertal estrus, mm 213 19.1 18.2 0.4 0.12
 Prepubertal gilt backfat in week 1, mm 176 17.4 17.3 0.5 0.88
 Prepubertal gilt backfat in week 2, mm 105 18.6 17.6 0.5 0.24
 Prepubertal gilt backfat in week 3, mm 40 19.1 17.0 1.3 0.27
Estrus
 Weeks to pubertal estrus 272 2.3x 2.0y 0.1 < 0.03
 Pubertal estrus in week 1, % 307 27.5x 38.3y 3.8 0.04
 Pubertal estrus in week 2, % 206 26.1 37.6 4.6 0.08
 Pubertal estrus in week 3, % Control3 61 47.8x 40.3x 5.6 0.50
PG600 74 56.4x 81.8y 7.8 0.04
 Pubertal estrus in weeks 4 to 6 67 47.2 48.2 8.4 0.93
 Cumulative pubertal estrus, % 307 85.0 91.5 2.6 0.08
Follicle measures
 Number of follicles at pubertal estrus 190 12.4 13.1 0.6 0.42
 Pubertal estrous follicle size, mm 207 7.2 7.3 0.1 0.61
 Prepubertal follicle size in week 1, mm 194 6.1 6.2 0.1 0.44
 Prepubertal follicle size in week 2, mm 115 6.3 6.1 0.2 0.30
 Prepubertal follicle size in week 3, mm 32 6.0 6.5 0.3 0.20
Vulva measures
 Pubertal estrus vulva temperature, °C 91 36.0 35.4 0.4 0.29
 Prepubertal vulva temperature week 1, °C 75 35.4x 32.4y 0.5 < 0.0001
 Prepubertal vulva temperature week 2 °C 29 33.0 33.5 0.9 0.73
 Prepubertal vulva temperature week 3, °C 21 33.7 36.5 1.1 0.09
 Vulva size at pubertal estrus, cm3 240 11.5 10.7 0.4 0.18
 Prepubertal gilt vulva size in week 1, cm3 190 7.3 7.3 0.3 0.93
 Prepubertal gilt vulva size in week 2, cm3 130 7.5 7.4 0.4 0.88
 Prepubertal gilt vulva size in week 3, cm3 45 6.9 6.3 0.7 0.57
Open in a new tab

1Total number of prepubertal gilts assigned to each treatment.

2Number of gilts with data for the measure listed and included in the analysis.

3Prepubertal gilts at start of week 3 randomly assigned by treatment to receive hormone (PG600) or none (Control).

xyMeans for estrus in week 3 with different superscripts in a row or column differ (P ≤ 0.05). Boar exposure x hormone treatment effect (P = 0.04).

For gilts that expressed pubertal estrus within 6 wk, treatment did not affect interval between first and second estrus, but age at first service was slightly reduced with PBE (P = 0.009, Table 2). For evaluation of retention rate, all gilts assigned to treatment were assessed. Physical BE increased (P < 0.05) or tended (P < 0.10) to increase the proportions of gilts and P1 sows served (inseminated) and farrowed in the first through fourth parities (Figure 3). However, within each parity, for gilts and sows that expressed estrus and were inseminated, there were no effects of BE treatment on measures related farrowing rate, or litter traits in parities one to three, except in the fourth parity with more sows in PBE farrowing and having more stillborns (Table 2). There were no effects of PG600 treatment on measures related to fertility except for an increase (P < 0.05) in total born (16.4 ±0.7 vs. 14.0 ±0.6) and numbers of liveborn pigs in the fourth parity. Lifetime litters produced did not differ (P > 0.10) between FBE (3.19 ± 0.10) and PBE (3.38 ± 0.09) nor did lifetime pigs produced for gilts that farrowed a first litter (Table 2). For females that were culled, there was no difference (P > 0.10) between FBE (1.47 ± 0.18) and PBE (1.15 ± 0.22) in the number of litters produced.

Table 2.

Least square means for reproductive measures in parity 1 (P1) to 4 (P4) sows for prepubertal gilts that received daily fenceline (FBE) or physical (PBE) boar exposure from weeks 1 to 31

Treatment
Item n FBE PBE SE P-Value
Interval from first to second estrus, d 258 22.5 22.5 0.3 0.97
Age at first service, d 228 222.1x 218.3y 1.0 0.01
Age at first farrowing, d 228 333.4 334.2 2.5 0.83
Served (%)2
 Gilts 263 94.8 95.3 1.9 0.87
 P1 230 95.6 96.1 1.9 0.82
 P2 201 95.7 95.5 2.0 0.95
 P3 169 91.1 92 2.9 0.81
Wean to estrus, d
 P1 230 6.3 6.1 0.5 0.70
 P2 199 4.9 4.5 0.3 0.31
 P3 168 4.8 4.8 0.3 0.92
 P4 141 4.5 4.5 0.2 0.78
Farrowed (%)2
 P1 263 90.4 92.7 2.4 0.49
 P2 230 90.0 91.9 1.8 0.62
 P3 201 90.0 94.4 2.6 0.26
 P4 169 86.9 94.6 3.1 0.10
Total born
 P1 241 14.1 13.9 0.3 0.72
 P2 207 14.7 14.0 0.4 0.24
 P3 186 15.5 15.6 0.4 0.90
 P4 153 15.9 16.0 0.4 0.93
Liveborn
 P1 241 13.0 12.7 0.3 0.42
 P2 207 13.6 12.9 0.4 0.17
 P3 186 14.3 14.1 0.3 0.56
 P4 153 14.7 14.2 0.4 0.38
Stillborn
 P1 241 0.7 0.9 0.1 0.05
 P2 198 0.8 0.9 0.1 0.69
 P3 176 0.8 1.1 0.2 0.24
 P4 144 1.0x 1.5y 0.2 0.05
Mummies
 P1 241 0.4 0.3 0.1 0.58
 P2 189 0.3 0.3 0.1 0.89
 P3 163 0.4 0.4 0.1 0.51
 P4 138 0.3 0.3 0.1 0.93
Gestation length, d3
 P1 245 116.1 116 0.1 0.89
 P2 212 115.7 115.7 0.1 0.92
 P3 165 116.1 116.4 0.2 0.19
 P4 133 116.4 116.3 0.1 0.92
Age at removal, d 152 462 481.2 24.1 0.58
Lifetime pigs4 241 47.2 49.7 1.7 0.31
Open in a new tab

1At the start of week 3, prepubertal gilts in each treatment were randomly assigned to receive PG600 or none (Control). There were no effects of PG600 or interaction with treatment (P > 0.10) for any measure and therefore only the main effects of treatment are shown.

2Measure based on gilts served at second estrus and for weaned sows served.

3Gestation lengths <110 d with abortion and those > 121 d due to repeat services excluded.

4Must have had a first litter.

xyMeans with different superscripts in a row differ (P ≤ 0.05).

Figure 3.

Figure 3.

Open in a new tab

Proportions of prepubertal gilts assigned to receive once daily physical (PBE) or fenceline (FBE) boar exposure and were inseminated (served) and farrowed (farrow) a litter in parities 1 to 4 (P1 to P4). *P ≤ 0.05 or †P ≤ 0.10.

Analysis for measures of litter of origin indicated that the total born litter size of the gilt was inversely related to its birthweight (r = -0.26, P < 0.0001). Inclusion of the gilt’s litter of origin birth weight as a class variable in the models indicated significant (P < 0.05) effects, with the heaviest gilts at birth (≥ 1.7 kg) more likely to be heavier at puberty and have more stillborns in their first litter, compared to lighter gilts. There were also class effects of gilt total born litter size of origin (P < 0.05): Gilts from small litters (≤ 11) were more likely to farrow their first litter at an earlier age, but have lower farrowing rates in the first and third litters. Gilts from small litters were also more likely to have lower total and liveborn in the third litter, and more stillborns in the first, third, and fourth litters compared to gilts from larger litters. However, gilts from the smallest litter sizes were more likely to have the largest vulva at pubertal estrus.

Other factors associated with fertility responses

Week of pubertal estrus was associated with several fertility measures (Table 3). Gilts that expressed pubertal estrus in week 1 were lighter at puberty than those in later weeks (P < 0.0001). Although growth rate was greater in gilts expressing puberty in week 2 (P = 0.002), there was no clear pattern for the remaining weeks. Gilts expressing pubertal estrus in the earlier weeks had reduced age at first service (P < 0.0001) and first farrowing (P < 0.001). There were effects of week of pubertal estrus on service rates but not farrowing rates, with fertility highest in the gilts induced into pubertal estrus in the earliest week. There were effects of week on litter traits and wean-to-estrous interval, but no clear pattern was evident. Pubertal estrus in the earlier weeks tended to be associated with greater age before removal (P = 0.08) and associated with more lifetime pigs produced (P = 0.003). Within the parameters of the present study, for the gilts that failed to express puberty within the six-week induction period, the number of non-productive days averaged ~50 d before removal from the farm. When examining differences between prepubertal and pubertal gilts during the 6-wk induction period, no single birth or growth measure could significantly account (P > 0.10) for variation in the pubertal response.

Table 3.

Least square means (± SE) for the main effects of week of pubertal estrus after start of boar exposure on associations with fertility measures

N Week of pubertal estrus
1 2 3 4 to 6 P-value
101 65 74 32
Age at pubertal estrus, d 189.3 ± 0.5w 195.1 ± 0.7x 201.8 ± 0.6y 209.4 ± 1.0z <0.0001
Days to pubertal estrus, d 4.2 ± 0.4w 10.7 ± 0.5x 17.9 ± 0.5y 25.9 ± 0.7z <0.0001
Weight at pubertal estrus, kg 131.5 ± 0.9x 139.7 ± 1.2y 140.3 ± 1.1y 144.7 ± 1.7z <0.0001
Growth (birth to puberty), g/d 775.4 ± 5.4x 801.5 ± 7.3y 768.6 ± 6.6x 761.6 ± 10.2x 0.002
Follicle size at pubertal estrus, mm 7.1 ± 0.1x 7.4 ± 0.1xy 7.5 ± 0.1y 6.9 ± 0.4xy 0.09
Age at first service, d 213.6 ± 0.9w 218.5 ± 1.2x 224.3 ± 1.2y 235.6 ± 1.7z <0.0001
Age at first farrowing, d 328.9 ± 2.7 x 334.2 ± 3.6 x 333.5 ± 3.4x 352.7 ± 5.1y <0.001
Served (%)1
 Gilts 99.1 ± 2.2x 97.5 ± 2.7xy 89.7 ± 2.5y 89.5 ± 3.9y 0.05
 P1 92.1 ± 3.5x 85.2 ± 4.5xy 78.2 ± 4.2y 78.7 ± 6.3y 0.06
 P2 84.1 ± 4.3x 77.6 ± 5.5xy 66.3 ± 5.1y 66.0 ± 7.8y 0.04
 P3 73.4 ± 4.8x 57.6 ± 6.0y 51.9 ± 5.6y 61.1 ± 8.6xy 0.03
Wean to estrus, d
 P1 6.0 ± 0.6x 5.6 ± 0.7xy 7.8 ± 0.7y 5.0 ± 1.1x 0.08
 P2 4.4 ± 0.3x 4.8 ± 0.4xy 5.7 ± 0.4y 4.0 ± 0.6x 0.05
 P3 4.9 ± 0.3 4.9 ± 0.5 4.5 ± 0.5 4.9 ± 0.7 0.90
 P4 4.5 ± 0.2x 4.5 ± 0.3x 4.1 ± 0.3y 5.3 ± 0.3z 0.04
Farrow (%)
 P1 93.1 ± 2.7 92.1 ± 3.5 87.6 ± 3.4 95.8 ± 5.2 0.50
 P2 94.6 ± 3.0 90.6 ± 4.0 89.5 ± 4.0 81.3 ± 5.8 0.29
 P3 97.7 ± 2.9 89.4 ± 3.9 85.0 ± 3.9 94.3 ± 0.6 0.11
 P4 93.0 ± 3.4 88.8 ± 4.9 86.8 ± 4.8 92.5 ± 6.8 0.72
Total born
 P1 14.3 ± 0.3 13.3 ± 0.4 14.3 ± 0.4 13.5 ± 0.6 0.12
 P2 14.8 ± 0.4x 13.3 ± 0.5y 14.7 ± 0.5xy 13.5 ± 0.8xy 0.09
 P3 15.8 ± 0.4 14.9 ± 0.5 15.5 ± 0.5 16.3 ± 0.8 0.41
 P4 16.4 ± 0.4 16.2 ± 0.6 14.8 ± 0.6 15.7 ± 0.8 0.14
Liveborn
 P1 13.1 ± 0.3 12.1 ± 0.4 13.1 ± 0.4 12.8 ± 0.6 0.19
 P2 13.8 ± 0.4 12.4 ± 0.5 13.3 ± 0.5 12.5 ± 0.8 0.11
 P3 14.5 ± 0.5 13.4 ± 0.5 14.3 ± 0.5 14.5 ± 0.7 0.25
 P4 13.1 ± 0.3 12.1 ± 0.4 13.1 ± 0.4 12.8 ± 0.6 0.19
Mummies
 P1 0.40 ± 0.06x 0.39 ± 0.08x 0.34 ± 0.08x 0.01 ± 0.12y 0.03
 P2 0.32 ± 0.06 0.21 ± 0.08 0.20 ± 0.08 0.31 ± 0.13 0.57
 P3 0.48 ± 0.07 0.34 ± 0.10 0.31 ± 0.10 0.36 ± 0.15 0.51
 P4 0.39 ± 0.09 0.27 ± 0.12 0.33 ± 0.12 0.12 ± 0.17 0.54
Age at removal, d 562.9 ± 30.0x 551.5 ± 33.9xy 472.0 ± 31.4y 448.6 ± 52.4y 0.08
Lifetime pigs2 54.1 ± 1.9x 44.1 ± 2.5y 45.8 ± 2.5y 45.1 ± 3.6y 0.003
Open in a new tab

1Measure based on gilts served at second estrus and weaned sows served.

2Must have had a first litter.

w–zMeans with different superscripts differ, P ≤ 0.05.

The number and size of follicles counted and measured on the first day of pubertal estrus were correlated (r = 0.26, P = 002) and represented the following distribution for follicle numbers counted in gilts: ≤ 5 follicles (10.0%); 6 to10 (28.8%); 11 to 15 (26.9%); 16 to20 (25.0%); and 21 to 30 (9.4%). Most gilts (85.3%) had large, ovulatory sized follicles on the first day of estrus (≥ 6.49 mm), but a smaller proportion of gilts had follicles 5.0 to 6.49 mm (11.9%), and even < 5 mm (2.9%) in diameter. There was a positive correlation of follicle size at pubertal estrus with numbers of follicles and increased age at puberty (r ≥ 0.25, P < 0.005), but with no relationship to vulva temperature or size at estrus or increased total born.

Discussion

Efficient management of replacement gilts is important to maintain consistent production flows when replacing older genotypes, less fertile sows, and animals with health or structural problems (Engblom et al., 2007). Most breeding farms have to deal with problems and inefficiencies associated with replacement gilts that fail to express estrus within 6 wk of starting boar exposure, cycle irregularities, and gilt weights that are too light or heavy for breeding (Cronin et al., 1983; Lucia Jr et al., 2000; Díaz et al., 2017). The cumulative impact of these problems can lead to greater costs from more non-productive days, reduced lifetime performance, and early removal (Heinonen et al., 1998; Stancic et al., 2011). Consistently attaining the targeted number of eligible replacement gilts to breed is challenging, especially when attempting to breed within the optimal ranges for age, weight, and at second estrus (Hughes et al., 2010). Methods for improving the selection and development of replacement gilts are a priority, and the results of this study indicate that more intense boar exposure followed by PG600 can help improve the delivery of high-quality breeding eligible gilts to the sow herd as a result of decreased days to puberty, and increased service and farrowing rates to the fourth parity.

In this study, we applied a high level of stimuli using a purpose-designed boar exposure area (Beltranena et al., 2005; Levis, 2008) with or without physical boar contact. Previous reports have indicated that direct physical contact is more effective for inducing pubertal estrus than even well-managed fenceline contact (Patterson et al., 2002). In the current study, physical boar contact increased estrous induction by ~10% in each of the first 2 wk. Furthermore, at the start of week 3, prepubertal gilts treated with PG600 and receiving PBE showed a 25% increase in estrus within the same week compared to gilts only “primed” with fenceline boar stimulation. In the present study, prepubertal gilts were treated with PG600 at the start of week 3 to improve the synchrony of estrus, compared to hormone treatment after week 3 as performed by Patterson et al. (2016), who re-grouped their prepubertal gilts after 14 d. In the present study, we could not identify any detrimental effects of PG600-induced puberty on measures of fertility or retention in the herd. Overall, PBE was most effective at inducing pubertal estrus and reducing the stimulation-to-estrous interval. A cumulative expression of pubertal estrus in 91% and 85% of gilts by the end of the 6-wk period using PBE and FBE, respectively, in combination with PG600 in some of these gilts, demonstrates the effectiveness of using purpose-designed BEAR facilities for puberty induction in replacement gilt programs. The weight of pubertal gilts averaged 137 kg, and 85% were served at ~220 d of age, to help meet the proposed industry benchmarks (Patterson and Foxcroft, 2019). Most importantly, as a consequence of a more effective puberty induction program using PBE, more gilts responded to PG600 treatment, and more PBE gilts were served, farrowed, and retained through four parities compared to gilts on the FBE treatment. In support of this response, a previous study evaluating the effects of gilt age at PG600 treatment, reported that boar exposure occurring for days or weeks before PG600, each improved the pubertal response by ~18% (Breen et al., 2005).

Previous data also indicated that increasing the duration (Caton et al., 1986), frequency, or proximity of boar exposure can decrease the interval to estrus and increase the proportion of pubertal gilts (Karlbom, 1982; Hughes et al., 1997; Zimmerman et al., 1998; Patterson et al., 2002). The enhanced estrus response to the added physical stimuli of the boar is presumed to result from greater olfactory stimulation from boar pheromones (Kirkwood et al., 1981), additional auditory (Hughes et al., 1985), and more intense tactile stimulation (Signoret, 1970). The use of PG600 in combination with boar exposure has resulted in the majority of treated gilts displaying pubertal estrus, and when these gilts are inseminated at second estrus, litter sizes and farrowing rates are similar to those gilts mated at a natural second estrus (Bartlett et al., 2009). As in the study of Patterson et al. (2016), we can add to the observation that we could not detect any detrimental effects of using PG600 to induce pubertal estrus after an initial period of boar stimulation and when breeding at second estrus observed no effects on measures for P1 farrowing rate, litter size, lifetime pig production, or retention in the herd. The hormone response has also been used as a gilt selection tool, and those gilts responding to PG600 and then later used for breeding showed no detrimental effects on farrowing and litter traits (Hidalgo et al., 2014).

Analysis of fertility effects by week of pubertal estrus after the start of boar exposure showed the greatest overall effects. In fact, fertility advantages for week 1 responders were clear compared to the later weeks, for service rates, lifetime pigs produced, and age at removal, with some advantages evident for week 2 responders. The results suggest that selection for early responders can result in fertility advantages that persist through the first four parities. Although pubertal response week is likely related to a complex array of traits, this outcome appears to be a key indicator for fertility. Previous studies have also reported that selection for early age at puberty (See and Knauer, 2019) could associate with improvements in farrowing rate (Knauer et al., 2011), longevity (Knauer et al., 2010), and wean to service intervals in sows (Sterning et al., 1998). The consequences of delayed puberty and age at first service have been linked to early removal and fewer lifetime pigs produced (Koketsu et al., 2020). Interestingly, younger gilts selected for an early response to PBE showed no effects on fertility or retention to third parity (Patterson et al., 2010). When gilts were older at the start of boar exposure, age at puberty had no effect on total lifetime pig production (Li et al., 2018). The age of the gilts in the present study averaged ~185 d (range 173 to 200 d) at the start of boar exposure and could be considered intermediate to slightly older. However, the age classification used for analysis in this study correlated with days to pubertal estrus, as well as age and weight at puberty. In modern commercial swine operations, age at start of boar exposure can vary considerably and can affect the interval to estrus, with older gilts expressing estrus sooner than younger gilts (Amaral Filha et al., 2009).

Other traits of interest were also notable: Although lowly correlated, larger follicle size at pubertal estrus was associated with increased numbers of follicles, but also with increased age at puberty. Yet, increased number and size of follicles did not correlate with larger vulva size or vulva temperature at estrus or increased total born in any subsequent parity. The lack of a relationship is somewhat unexpected, because larger follicle size during the follicle phase in weaned sows has been associated with reduced wean to estrous interval and increased estrous expression and fertility (Knox, 2019). Nevertheless, although heterogeneity in follicle size at estrus has been reported (Hunter et al., 1989), no differences in the size of the three largest follicles have been reported in weaned sows treated with gonadotropins (Knox et al., 2001) or in relation to different wean to estrous intervals (Knox and Rodriguez-Zas, 2001). In the present data set, the follicle relationship with more days required for a gilt to reach puberty could partially explain why fertility may be similar at pubertal estrus in later maturing gilts in comparison to early puberty gilts at second estrus (Walker et al., 1989; Patterson and Foxcroft, 2019) and parity one sows (Werlang et al., 2011). Some studies have reported that an increase in selected follicles leads to a greater ovulation rate (Kirkwood and Aherne, 1985; Anderson, 1993) and improved litter size (Holtz et al., 1999; Kirkwood et al., 2000), but the outcome for both has not been observed in all cases. One explanation for the inconsistency is that the increase in ovulation rate and litter size may only occur in those gilts that display earlier puberty at a reduced body maturity (Kirkwood and Aherne, 1985; Kirkwood et al., 2000). In the present study, although unsubstantiated, the early puberty gilts having smaller sized and fewer follicles than their older counterparts could be related to selection for higher prolificacy and greater sensitivity to estrogen feedback from smaller-sized ovulatory follicles. It is also important to note that because we included measures for all visible follicles ≥ 3 mm, unlike measures for just the three largest follicles, technical limitations for counting and measuring all surface follicles may skew the results. Data from our lab (unpublished) examining ultrasonic assessment of ovarian follicles in a water bath showed that rotating the ovary and the target large-sized follicle to the opposite side of the ovary and changing its orientation relative to the scanning plane of the transducer could reduce the measure for the largest diameter of that follicle by 2 mm. This phenomenon for visualization and measure of all surface follicles on both ovaries in the pig would occur in vivo and would also likely be affected by the distance from the transducer.

There were other measures in the present study that did not associate with the pubertal response or fertility in subsequent parities. Reported problems with replacement gilts most often involve delayed puberty or cases of silent estrus (Stancic et al., 2011) and Rydhmer et al. (1994) indicated that intensity of vulva symptoms can be associated with problems in estrous expression. Vulva temperature assessed using thermography has been reported to change during estrus and with ovulation when repeated measures and corrections for gluteal temperature can be performed (Scolari et al., 2011; Simões et al., 2014). Our single thermography measure was likely a limiting factor for identifying differences between pubertal and prepubertal gilts, especially without any correction for environmental or gluteal temperatures. Visual assessment or measures for changes in vulva size and color has long been used as an indicator for impending estrus in gilts. This change in the vulva would reflect changes in estrogen that impacts blood flow and fluid retention in the reproductive tissues. However, although gilt vulva size at pubertal estrus is moderately heritable, increases in vulva size could not be associated with pubertal estrus or ovulation in PG600-treated gilts (Lewchalermwong et al., 2020). In the present study, an increase in vulva size was clearly evident when comparing pubertal estrus to prepubertal gilts. However, the changes did not associate with any other indicator for fertility in the present study and were similar to that reported by Knauer et al. (2011). Nevertheless, recent studies assessed gilts at 15 wk of age and were able to associate prepubertal vulva size at this early stage with age at puberty (Graves et al., 2020), age at first farrowing, and litter size measures in subsequent parities (Romoser et al., 2020). This would suggest that selection for vulva size at a much earlier age is more sensitive and a better indicator for puberty and the subsequent fertility than the later assessment at 24–26 wk of age.

The gilts used in the present experiment were part of a larger study that evaluated the effects of litter of origin traits on puberty and lifetime fertility (Patterson et al., 2020). Similar to the larger study, in this subset of gilts, we also observed an inverse relationship of birth litter size and gilt birthweight and that heavier gilt birthweights associated with more stillborns in the first litter. Additionally, when examining the effects of litter size of origin, gilts selected from small litters with ≤ 11 pigs were less likely to farrow, more likely to have fewer pigs in the third litter, and more stillborns in the subsequent litters. The reduced performance in these gilts born in smaller litters is likely attributed to their heavier birth weight and heavier weight at first service. This in turn has repercussions on their feed intake and weight loss during lactation as sows and may manifest itself in the later parities (Patterson et al., 2020). Differences in which measures of fertility are affected and in which parity, might be explained by the differences in how litter size and birthweight are classified for analysis. Although there were differences in observed litter of origin traits in comparison to the larger study for lifetime fertility performance, the overall relationship of smaller litter size and heavier birthweight persisted. The fertility problems after puberty are probably due to excessive weight and body condition at breeding and the problems caused during the farrowing and lactation periods (Patterson et al., 2020). It is also possible that if management intervened to correct over-conditioning, then feed restriction at less than optimal stages in gilt development could associate with fertility failures. In the present experiment, the average growth rate of the gilts from birth to pubertal estrus was 779 g/d, and well above the slow growth rate (< 550 g/d) associated with delayed puberty and prepubertal gilts (Beltranena et al., 1991; Kummer et al., 2009; Tummaruk et al., 2009). Calderón Díaz et al. (2015) noted no effects on puberty when gilts had growth rates between 650 and 850 g/d. Data from the present study support the hypothesis proposed by Patterson et al. (2020) that heavier gilts at birth grow faster and are heavier at puberty. Because the low litter birthweight phenotype repeats in subsequent parities (Patterson and Foxcroft, 2019), it would appear that selection of gilts from larger litters with intermediate birthweights would have the greatest potential for avoiding excessive weight, while improving fertility and retention in the herd. This approach for litter size selection would also be supported by Tummaruk et al. (2001), who reported that gilts born and selected from larger litters were more likely to produce larger litters of their own.

Summary and Conclusions

The results of this study illustrate significant advantages for using direct physical contact with boars, even when compared with very effective fenceline exposure to boar-derived stimuli, for inducing an earlier and more synchronous expression of pubertal estrus. The advantages of using PBE were evident as improved puberty induction within the first 2 wk and then in response to PG600 administered to anestrous gilts at the start of week 3. Furthermore, an early response to boar stimulation improved the proportion of gilts served and farrowing through four parities. The induction of pubertal estrus at less than 200 d of age is increasingly important for controlling the excessive breeding weights of gilts seen in many less efficient gilt replacement programs. The study also points to the importance for selection of gilts based on their birth litter size and birth weight to help optimize targets for age and weight at puberty to help improve their retention in the herd. We conclude that the use of physical boar exposure to intensify boar-derived stimuli can make an important contribution to overall breeding herd performance.

Acknowledgments

We extend our sincere thanks to Holden Farms Incorporated for their assistance with this experiment and greatly appreciate funding from the National Pork Board to support this project. This project was supported by the National Pork Board (grant RES0021623).

Glossary

Abbreviations

FBE

fenceline boar exposure

PBE

physical boar exposure

BE

boar exposure

BEAR

boar exposure area

Conflict of interest statement

The authors declare no real or perceived conflicts of interest.

Literature Cited

  1. Amaral Filha, W. S., Bernardi M. L., Wentz I., and Bortolozzo F. P.. . 2009. Growth rate and age at boar exposure as factors influencing gilt puberty. Livest. Sci. 120:51–57. doi: 10.1016/j.livsci.2008.04.009 [DOI] [Google Scholar]
  2. Anderson, L. L. 1993. Pigs. In: Hafez E. S. E., editor. Reproduction in farm animals. 6th ed. Malvern (PA):Lea and Febiger. p. 343–360. [Google Scholar]
  3. Bartlett, A., Pain S. J., Hughes P. E., Stott P., and van Wettere W. H.. . 2009. The effects of PG600 and boar exposure on oestrus detection and potential litter size following mating at either the induced (pubertal) or second oestrus. Anim. Reprod. Sci. 114:219–227. doi: 10.1016/j.anireprosci.2008.08.020 [DOI] [PubMed] [Google Scholar]
  4. Beaulieu, A. D., Aalhus J. L., Williams N. H., and Patience J. F.. . 2010. Impact of piglet birth weight, birth order, and litter size on subsequent growth performance, carcass quality, muscle composition, and eating quality of pork1. J. Anim. Sci. 88:2767–2778. doi: 10.2527/jas.2009-2222 [DOI] [PubMed] [Google Scholar]
  5. Beltranena, E., Aherne F. X., Foxcroft G. R., and Kirkwood R. N.. . 1991. Effects of pre- and postpubertal feeding on production traits at first and second estrus in gilts. J. Anim. Sci. 69: 886–893. doi: 10.2527/1991.693886x [DOI] [PubMed] [Google Scholar]
  6. Beltranena, E., Patterson J. L., and Foxcroft G. R.. . 2005. Designing effective boar stimulation systems as a critical feature of the Gilt Development Unit. Proceedings of Leman Pre-Conference Reproduction Workshop on “Effective Management of Replacement Gilts,”Univ. Minnesota Swine Extension Program, MN, USA, p. 42–46. [Google Scholar]
  7. Breen, S. M., Farris K. L., Rodriguez-Zas S. L., and Knox R. V.. . 2005. Effect of age and physical or fence-line boar exposure on estrus and ovulation response in prepubertal gilts administered PG600. J. Anim. Sci. 83:460–465. doi: 10.2527/2005.832460x [DOI] [PubMed] [Google Scholar]
  8. Calderón Díaz, J. A., Vallet J. L., Lents C. A., Nonneman D. J., Miles J. R., Wright E. C., Rempel L. A., Cushman R. A., Freking B. A., Rohrer G. A., . et al. 2015. Age at puberty, ovulation rate, and uterine length of developing gilts fed two lysine and three metabolizable energy concentrations from 100 to 260 d of age. J. Anim. Sci. 93:3521–3527. doi: 10.2527/jas.2014-8522 [DOI] [PubMed] [Google Scholar]
  9. Caton, J. S., Jesse G. W., Day B. N., and Ellersieck M. R.. . 1986. The effect of duration of boar exposure on the frequency of gilts reaching first estrus. J. Anim. Sci. 62:1210–1214. doi: 10.2527/jas1986.6251210x [DOI] [PubMed] [Google Scholar]
  10. Cronin, G. M., Hemsworth P. H., Winfield C. G., Muller B., and Chamley W. A.. . 1983. The incidence of, and factors associated with, failure to mate by 245 days of age in the gilt. Anim. Reprod. Sci.5: 199–205. doi: 10.1016/0378-4320(83)90027-1 [DOI] [Google Scholar]
  11. Díaz, J. A. C., Vallet J. L., Boyd R. D., Lents C. A., Prince T. J., DeDecker A. E., Phillips C. E., Foxcroft G., and Stalder K. J.. . 2017. Effect of feeding three lysine to energy diets on growth, body composition and age at puberty in replacement gilts. Anim. Reprod. Sci. 184:1–10. doi: 10.1016/j.anireprosci.2017.06.007 [DOI] [PubMed] [Google Scholar]
  12. Eliasson, L. 1991. Relationships between puberty and production traits in the gilt. 2. Oestrous symptoms at puberty. Anim. Reprod. Sci. 25: 255–264. Doi: 10.1016/0378-4320(91)90020-Z [DOI] [Google Scholar]
  13. Engblom, L., Lundeheim N., Dalin A.-M., and Andersson K.. . 2007. Sow removal in Swedish commercial herds. Livest. Sci. 106: 76–86. Doi: 10.1016/j.livsci.2006.07.002 [DOI] [Google Scholar]
  14. Engblom, L., Lundeheim N., Strandberg E., Schneider M. d. e. l. P., Dalin A. M., and Andersson K.. . 2008. Factors affecting length of productive life in Swedish commercial sows. J. Anim. Sci. 86:432–441. doi: 10.2527/jas.2007-0310 [DOI] [PubMed] [Google Scholar]
  15. Graves, K. L., Mordhorst B. R., Wright E. C., Hale B. J., Stalder K. J., Keating A. F., and Ross J. W.. . 2020. Identification of measures predictive of age of puberty onset in gilts. Transl. Anim. Sci. 4:285–292. doi: 10.1093/tas/txz173 [DOI] [PMC free article] [PubMed] [Google Scholar]
  16. Heinonen, M., Leppävuori A., and Pyörälä S.. . 1998. Evaluation of reproductive failure of female pigs based on slaughterhouse material and herd record survey. Anim. Reprod. Sci. 52:235–244. doi: 10.1016/s0378-4320(98)00105-5 [DOI] [PubMed] [Google Scholar]
  17. Hidalgo, D. M., Friendship R. M., Greiner L., Manjarín R., Amezcua M. R., Domínguez J. C., and Kirkwood R. N.. . 2014. Influence of gonadotrophin-induced first oestrus on gilt fertility. Reprod. Domest. Anim. 49:899–902. doi: 10.1111/rda.12390 [DOI] [PubMed] [Google Scholar]
  18. Holtz, W., Schmidt-Baulain R., Welp C., and Wallenhorst C. K.. . 1999. Effect of insemination of estrus-induced prepuberal gilts on ensuing reproductive performance and body weight. Anim. Reprod. Sci. 57:177–183. doi: 10.1016/s0378-4320(99)00054-8 [DOI] [PubMed] [Google Scholar]
  19. Hughes, P. E., Hemsworth P. H., and Hansen C.. . 1985. The effects of supplementary olfactory and auditory stimuli on the stimulus value and mating success of the young boar. Appl. Anim. Behav. Sci. 14: 245–252. Doi: 10.1016/0168-1591(85)90005-X [DOI] [Google Scholar]
  20. Hughes, P. E., Philip G., and Siswadi R.. . 1997. The effects of contact frequency and transport on the efficacy of the boar effect. Anim. Reprod. Sci. 46:159–165. doi: 10.1016/s0378-4320(96)01594-1 [DOI] [PubMed] [Google Scholar]
  21. Hughes, P. E., Smits R. J., Xie Y., and Kirkwood R. N.. . 2010. Relationships among gilt and sow live weight, P2 backfat depth, and culling rates. J. Swine Health Prod. 18(6):301–305. https://www.aasv.org/shap/issues/v18n6/v18n6p301.html [Google Scholar]
  22. Hunter, M. G., Grant S. A., and Foxcroft G. R.. . 1989. Histological evidence for heterogeneity in the development of preovulatory pig follicles. J. Reprod. Fertil. 86:165–170. doi: 10.1530/jrf.0.0860165 [DOI] [PubMed] [Google Scholar]
  23. Karlbom, I. 1982. Attainment of puberty in female pigs: influence of boar stimulation. Anim. Reprod. Sci. 4:313–319. doi: 10.1016/0378-4320(82)90045-8 [DOI] [Google Scholar]
  24. Kirkwood, R. N., and Aherne F. X.. . 1985. Energy intake, body composition and reproductive performance of the gilt. J. Anim. Sci. 60:1518–1529. doi: 10.2527/jas1985.6061518x [DOI] [PubMed] [Google Scholar]
  25. Kirkwood, R. N., Aherne F. X., Monaghan P. G., and Misutka S. C.. . 2000. Breeding gilts at natural or a hormone-induced estrus: effects on performance over four parities. J. Swine Health Prod. 8: 177–179. https://www.aasv.org/library/swineinfo/Content/SHAP/2000/v8n4p177.pdf [Google Scholar]
  26. Kirkwood, R. N., Forbes J. M., and Hughes P. E.. . 1981. Influence of boar contact on attainment of puberty in gilts after removal of the olfactory bulbs. J. Reprod. Fertil. 61:193–196. doi: 10.1530/jrf.0.0610193 [DOI] [PubMed] [Google Scholar]
  27. Knauer, M. T., Cassady J. P., Newcom D. W., and See M. T.. . 2011. Phenotypic and genetic correlations between gilt estrus, puberty, growth, composition, and structural conformation traits with first-litter reproductive measures. J. Anim. Sci. 89:935–942. doi: 10.2527/jas.2009-2673 [DOI] [PubMed] [Google Scholar]
  28. Knauer, M., Stalder K. J., Serenius T., Baas T. J., Berger P. J., Karriker L., Goodwin R. N., Johnson R. K., Mabry J. W., Miller R. K., . et al. 2010. Factors associated with sow stayability in 6 genotypes. J. Anim. Sci. 88:3486–3492. doi: 10.2527/jas.2009-2319 [DOI] [PubMed] [Google Scholar]
  29. Knox, R. V. 2019. Physiology and endocrinology symposium: factors influencing follicle development in gilts and sows and management strategies used to regulate growth for control of estrus and ovulation1. J. Anim. Sci. 97:1433–1445. doi: 10.1093/jas/skz036 [DOI] [PMC free article] [PubMed] [Google Scholar]
  30. Knox, R. V., Rodriguez-Zas S. L., Miller G. M., Willenburg K. L., and Robb J. A.. . 2001. Administration of p.g. 600 to sows at weaning and the time of ovulation as determined by transrectal ultrasound. J. Anim. Sci. 79:796–802. doi: 10.2527/2001.794796x [DOI] [PubMed] [Google Scholar]
  31. Knox, R. V., and Zas S. L.. . 2001. Factors influencing estrus and ovulation in weaned sows as determined by transrectal ultrasound. J. Anim. Sci. 79:2957–2963. doi: 10.2527/2001.79122957x [DOI] [PubMed] [Google Scholar]
  32. Koketsu, Y., Iida R., and Piñeiro C.. . 2020. Increased age at first-mating interacting with herd size or herd productivity decreases longevity and lifetime reproductive efficiency of sows in breeding herds. Porcine Health Manag. 6:2. doi: 10.1186/s40813-019-0142-9 [DOI] [PMC free article] [PubMed] [Google Scholar]
  33. Kummer, R., Bernardi M. L., Schenkel A. C., Amaral Filha W. S., Wentz I., and Bortolozzo F. P.. . 2009. Reproductive performance of gilts with similar age but with different growth rate at the onset of puberty stimulation. Reprod. Domest. Anim. 44: 255–259. doi: 10.1111/j.1439-0531.2007.01050.x [DOI] [PubMed] [Google Scholar]
  34. Levis, D. G. 2008. Gilt management in the BEAR system. Pork Information Gateway factsheet. http://porkgateway.org/resource/gilt-management-in-the-bear-system/—[accessed September 6, 2016].
  35. Lewchalermwong, D., Tummaruk P., and Knox R. V.. . 2020. Investigation into the variation in follicular and endocrine responses of prepubertal gilts treated with exogenous gonadotropins. Anim. Reprod. Sci. 223:106622. doi: 10.1016/j.anireprosci.2020.106622 [DOI] [PubMed] [Google Scholar]
  36. Li, Q., Yuan X., Chen Z., Zhang A., Zhang Z., Zhang H., and Li J.. . 2018. Heritability estimates and effect on lifetime reproductive performance of age at puberty in sows. Anim. Reprod. Sci. 195:207–215. doi: 10.1016/j.anireprosci.2018.05.025 [DOI] [PubMed] [Google Scholar]
  37. Lucia, T.Jr, Dial G. D., and Marsh W. E.. . 2000. Lifetime reproductive performance in female pigs having distinct reasons for removal. Livest. Prod. Sci. 63: 213–222. doi: 10.1016/s0301-6226(99)00142-6 [DOI] [Google Scholar]
  38. Patterson, J. L., Beltranena E., and Foxcroft G. R.. . 2010. The effect of gilt age at first estrus and breeding on third estrus on sow body weight changes and long-term reproductive performance. J. Anim. Sci. 88:2500–2513. doi: 10.2527/jas.2008-1756 [DOI] [PubMed] [Google Scholar]
  39. Patterson, J., Bernardi M. L., Allerson M., Hanson A., Holden N., Bruner L., Pinilla J. C., and Foxcroft G.. . 2020. Associations among individual gilt birth weight, litter birth weight phenotype, and the efficiency of replacement gilt production. J. Anim. Sci. 98. doi: 10.1093/jas/skaa331 [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Patterson, J., and Foxcroft G.. . 2019. Gilt management for fertility and longevity. Animals 9: 434. doi: 10.3390/ani9070434 [DOI] [PMC free article] [PubMed] [Google Scholar]
  41. Patterson, J., Triemert E., Gustafson B., Werner T., Holden N., Pinilla J. C., and Foxcroft G.. . 2016. Validation of the use of exogenous gonadotropins (PG600) to increase the efficiency of gilt development programs without affecting lifetime productivity in the breeding herd1. J. Anim. Sci. 94: 805–815. doi: 10.2527/jas.2015-9705 [DOI] [PubMed] [Google Scholar]
  42. Patterson, J. L., Willis H. J., Kirkwood R. N., and Foxcroft G. R.. . 2002. Impact of boar exposure on puberty attainment and breeding outcomes in gilts. Theriogenology 57:2015–2025. doi: 10.1016/s0093-691x(02)00674-x. [DOI] [PubMed] [Google Scholar]
  43. PigCHAMP . 2019. Benchmarking USA Summary. https://www.pigchamp.com/benchmarkingPigChamp. Accessed August 23, 2021.
  44. Quesnel, H., Brossard L., Valancogne A., and Quiniou N.. . 2008. Influence of some sow characteristics on within-litter variation of piglet birth weight. Animal 2:1842–1849. doi: 10.1017/S175173110800308X [DOI] [PubMed] [Google Scholar]
  45. Rodriguez-Zas, S. L., Davis C. B., Ellinger P. N., Schnitkey G. D., Romine N. M., Connor J. F., Knox R. V., and Southey B. R.. . 2006. Impact of biological and economic variables on optimal parity for replacement in swine breed-to-wean herds1. J. Anim. Sci. 84: 2555–2565. doi: 10.2527/jas.2005-635 [DOI] [PubMed] [Google Scholar]
  46. Romoser, M. R., Gall T., Baumgard L. H., Keating A. F., Stalder K. J., and Ross J. W.. . 2020. Evaluating the efficacy of visual assessment of gilt vulva size prior to puberty on subsequent reproductive performance*. Applied Anim. Sci. 36:694–700. Doi: 10.15232/aas.2020-02034 [DOI] [Google Scholar]
  47. Rydhmer, L., Eliasson-Selling L., Johansson K., Stern S., and Andersson K.. . 1994. A genetic study of estrus symptoms at puberty and their relationship to growth and leanness in gilts. J. Anim. Sci. 72:1964–1970. doi: 10.2527/1994.7281964x [DOI] [PubMed] [Google Scholar]
  48. Sasaki, Y., and Koketsu Y.. . 2012. A herd management survey on culling guidelines and actual culling practices in three herd groups based on reproductive productivity in Japanese commercial swine herds1. J. Anim. Sci. 90. doi: 10.2527/jas.2011-4313 [DOI] [PubMed] [Google Scholar]
  49. Scolari, S. C., Clark S. G., and Knox R. V.. . 2011. Vulvar skin temperature changes significantly during estrus in swine as determined by digital infrared thermography. J. Swine Health Prod. 19: 151–155. https://www.aasv.org/shap/issues/v19n3/v19n3p151.html [Google Scholar]
  50. See, G. M., and Knauer M. T.. . 2019. Associations with four generations of divergent selection for age at puberty in swine. J. Anim. Sci. 97:2320–2328. doi: 10.1093/jas/skz129 [DOI] [PMC free article] [PubMed] [Google Scholar]
  51. Signoret, J. P. 1970. Reproductive behaviour of pigs. J. Reprod. Fertil. Suppl. 11:Suppl 11:105+. [PubMed] [Google Scholar]
  52. Simões, V. G., Lyazrhi F., Picard-Hagen N., Gayrard V., Martineau G. P., and Waret-Szkuta A.. . 2014. Variations in the vulvar temperature of sows during proestrus and estrus as determined by infrared thermography and its relation to ovulation. Theriogenology 82:1080–1085. doi: 10.1016/j.theriogenology.2014.07.017 [DOI] [PubMed] [Google Scholar]
  53. Stalder, K. J., Lacy C., and Cross T. L.. . 2003. Financial impact of average parity of culled females in a breed-to-wean swine operation using replacement gilt net present value analysis. J. Swine Health Prod. 11: 69–74. https://www.aasv.org/shap/issues/v11n2/v11n2p69.pdf [Google Scholar]
  54. Stancic, I., Stancic B., Bozic A., Anderson R., Harvey R., and Gvozdic D.. . 2011. Ovarian activity and uterus organometry in delayed puberty gilts. Theriogenology 76:1022–1026. doi: 10.1016/j.theriogenology.2011.05.004 [DOI] [PubMed] [Google Scholar]
  55. Sterning, M., Rydhmer L., and Eliasson-Selling L.. . 1998. Relationships between age at puberty and interval from weaning to estrus and between estrus signs at puberty and after the first weaning in pigs. J. Anim. Sci. 76:353–359. doi: 10.2527/1998.762353x [DOI] [PubMed] [Google Scholar]
  56. Tani, S., Piñeiro C., and Koketsu Y.. . 2018. Culling in served females and farrowed sows at consecutive parities in Spanish pig herds. Porcine Health Manag. 4:3. doi: 10.1186/s40813-018-0080-y [DOI] [PMC free article] [PubMed] [Google Scholar]
  57. Tummaruk, P., Lundeheim N., Einarsson S., and Dalin A. M.. . 2001. Effect of birth litter size, birth parity number, growth rate, backfat thickness and age at first mating of gilts on their reproductive performance as sows. Anim. Reprod. Sci. 66: 225–237. doi: 10.1016/s0378-4320(01)00095-1 [DOI] [PubMed] [Google Scholar]
  58. Tummaruk, P., Tantasuparuk W., Techakumphu M., and Kunavongkrit A.. . 2009. The association between growth rate, body weight, backfat thickness and age at first observed oestrus in crossbred Landrace × Yorkshire gilts. Anim. Reprod. Sci. 110: 108–122. Doi: 10.1016/j.anireprosci.2008.01.004 [DOI] [PubMed] [Google Scholar]
  59. Walker, N., Kilpatrick D. J., and Courtney D. J.. . 1989. The effect of conception in gilts at puberty or second oestrus on reproductive performance over two parities. Irish J. Agric. Res. 28: 115–121. http://www.jstor.org/stable/25556239 [Google Scholar]
  60. Werlang, R. F., Argenti L. E., Fries H. C., Bernardi M. L., Wentz I., and Bortolozzo F. P.. . 2011. Effects of breeding at the second oestrus or after post-weaning hormonal treatment with altrenogest on subsequent reproductive performance of primiparous sows. Reprod. Domest. Anim. 46: 818–823. doi: 10.1111/j.1439-0531.2010.01747.x [DOI] [PubMed] [Google Scholar]
  61. Zimmerman, D. R., McGargill T., and Rohda N.. . 1998. Efficacy of once (1x) vs twice (2x) daily physical or fence-line contact with boars for stimulating earlier puberty in gilts. Nebraska Swine Reports. 159. https://digitalcommons.unl.edu/coopext_swine/159. Accessed August 23, 2021. [Google Scholar]

Articles from Journal of Animal Science are provided here courtesy of Oxford University Press

RESOURCES