Abstract
Accelerating age at first calving (AFC) is a strategy for sustainable dairy farming, whereas the impact of a reduction in AFC on long-term performance remains unclear. In this study, longevity and milk productivity until the end of the third lactation period were investigated retrospectively according to AFC. A total of 169 cows were categorized according to AFC as young, moderate, old, and very old (< 22.5, 22.5 –< 24.0, 24.0 –< 25.5, and > 25.5 months). The young AFC group had approximately 70 kg lower body weight before first calving (620 vs. 695 kg, P < 0.05) and experienced their first calving approximately 4.2 months earlier than the very old AFC group (21.9 vs. 26.1 months, P < 0.05). The survival rate at the third calving stage was 61% in the young AFC group, which was higher than those in the moderate (42%), old (35%), and very old (33%) AFC groups. In the young AFC group, no cows were culled because of low productivity and hoof disease, compared to 5.0–8.1% of older AFC cows. The young AFC group had a higher overall lifetime milk yield (cumulative milk yield/days from birth to the end of final lactation) than the old AFC group (14.3 vs. 8.7 kg/d, P = 0.11). The cows that survived the third calving had better reproductive performance than non-surviving cows; however, no statistical difference was detected among the AFC groups. In conclusion, AFC as early as 22.5 months could be associated with better survivability and higher overall lifetime milk yield than older AFC without impairing reproductive performance. Our results suggest that accelerating AFC may lead to higher profitability.
Keywords: Fertility, First calving age, Holstein heifer, Milk yield
Owing to the progress in genetics and nutrition to increase milk yield, modern dairy replacement heifers can achieve higher body growth rates and greater body weight (BW) at their first calving. Consequently, the age at first calving (AFC) of modern Holstein heifers has decreased [1]. Over the past 20 years, infertility has become an important issue in high-production dairy cows. Poor fertility results in longer calving intervals and a lower average annual milk yield per cow, which means that more cows are required in the herd to meet milk production requirements, resulting in increased herd replacements that contribute towards total greenhouse gas emissions [2]. Additionally, the return on investment in rearing replacement heifers from birth to first lactation does not fully recover until at least the end of the first lactation [3]. Accelerating AFC is a strategy for sustainable dairy farming [4].
To maximize the advantages of reducing AFC, adequate BW control for heifers until first calving is essential. Unsuccessful weight management can frequently cause dystocia during first calving, which contributes to an increase in postpartum uterine diseases such as postpartum metritis and endometritis [5]. The most common cause of calving difficulties is fetal–maternal disproportion, that is, a calf that is too large and/or a pelvis that is too small [5]. Therefore, replacement heifers must rapidly acquire a mature frame and sufficient BW before their first calving.
Modern Holstein heifers are recommended to be bred from 15 months of age to calves at approximately 24 months of age, weighing 550–625 kg [1]. However, considerable variations in AFC have been reported for dairy cows; for example, the average AFC is 26.0 months in the USA, 26.4 months in the United Kingdom (UK), 28.8 months in Australia, 29.3 months in China, and 31.0 months in Kenya [6]. In Japan, approximately 38% of Holstein heifers have their first calves at 24–26 months; however, in the past decade, 24% had their first calves at over 27 months [7]. Significant variations in AFC may be related to differences in age at first breeding, farm management, growth rate, or heifer fertility.
There is considerable interest in reducing AFC. However, most previous studies focused on heifer performance during the first lactation period. A negative association between reduced AFC and subsequent fertility has never been reported and the impact of AFC on milk production remains controversial. Hoffman et al. (1996) compared the performance of heifers with a higher average daily gain (calving earlier) with those fed the control (0.93 vs. 0.78 kg/day, 21.7 vs. 24.6 months AFC) and indicated that the earlier-calving heifers had significantly lower milk fat and protein yields than the later-calving heifers [8]. Similar negative effects of early AFC on milk production have been reported [9, 10]. Previous studies have shown that rapid prepubertal weight gain has a deleterious effect on mammogenesis when accompanied by excess body fat deposition. However, recent studies have indicated that an AFC of less than 23 months in dairy heifers does not reduce or may even increase the first lactation yield, provided that the animals are sufficiently well-grown [11], and do not show an increased frequency of problems at calving [3].
Recent global trends in dairy science place greater emphasis on longevity traits. In general, the lifetime productivity of dairy cows increases as the number of calves increases, and approximately 35% of Holstein cows in Japan are culled before their third calving [7]. Few previous studies have examined the association between AFC and longevity; therefore, the impact of a reduction in AFC on long-term performance remains unclear. A prospective study by Gardner et al. (1988) compared milk production and reproductive performance between heifers with an AFC of 22.2 months and 24.6 months (accelerated growth at 0.89 kg/day and normal growth at 0.78 kg/day during the rearing period, respectively) [12]. The results indicated that calving as early as 22.2 months of age did not affect milk yield and did not increase the incidence of reproductive problems from the first to the seventh lactation. Recent retrospective studies on Holstein-type herds have suggested that Holstein cows calving at relatively young ages have better survival and higher productivity [13,14,15,16], whereas AFC has been reported to have a negative effect on lifetime efficiency [17].
In Japan, a retrospective analysis of the calving records of 267 Holstein heifers at the Hokkaido Agricultural Research Center, National Agriculture and Food Research Organization (NARO), from 1979 to 1997 reported an association between AFC and BW at first calving [18]. BW at first calving and milk yield have increased substantially over 20 years; thus, early initiation of first breeding had no detrimental impact on first lactation and led to better reproductive performance. A second prospective study using the same experimental herd showed that a reduction in AFC from 25 to 22 months was not associated with an increase in dystocia when the BW of the heifers reached 600 kg at calving. It was concluded that an AFC of 22 months had no adverse effects on the overall milk yield up to the third lactation period, without impairing reproductive performance [19]. However, the sample size was too small to draw definitive conclusions.
Therefore, this study aimed to investigate the association among AFC, longevity, and lifetime productivity until the end of the third lactation period. Our recent study, using the calving records of 203 Holstein heifers from NARO, indicated that heifers at < 22.5 months AFC had the highest milk yield during the first lactation compared to heifers that calved later [20]. Therefore, we followed the subsequent performance of the 203 cows and compared longevity traits, the reasons for culling, and lifetime productivity until the end of the third lactation between cows that calved at < 22.5, 22.5 –< 24.0, 24.0 –< 25.5, and > 25.5 months. Secondly, we focused on whether the cows survived until the third calving period. The prevalence of stillbirths, incidence of dystocia, resumption of reproductive activity, and fertility were compared among the four AFC groups in survivors and non-survivors.
Materials and Methods
Animals and feed management
This study was conducted retrospectively using calving records of heifers from 1999 to 2012 at the Hokkaido Agricultural Research Center, a NARO facility in Hokkaido, Japan. Heifers were managed as previously described [19]. The animals were reared using a feeding regimen to meet their maintenance, growth, and lactation requirements following the Japanese Feeding Standards for Dairy Cattle (Agriculture, Forestry, and Fisheries Research Council Secretariat, 1999; see https://jlia.lin.gr.jp/info/archives1627/, accessed January 6, 2023). Whole milk (for the first 12 weeks) and a calf starter and grower ration (for the first 15 weeks) were fed to calves at a maximum of 5.0 and 1.2 kg/day, respectively. The calves were housed individually in stalls and, after weaning, they were moved to a tie-stall barn and fed concentrate with grass silage until 12 months of age. Weaned heifer calves were pastured for 6 h/day during summer (May–October) and fed hay ad libitum in the paddock for 6 h/day during winter (November–April). From the age of 12 months to 1 month before parturition, the heifers were raised on pastures supplemented with alfalfa silage and grass hay (packaged in bales during summer) or fed corn silage and grass hay in a tie-stall barn during winter. Heifers confirmed to be pregnant in summer were pastured during the day and night, whereas in winter they were fed baled alfalfa silage and grass hay ad libitum in an open paddock. From 1 month before calving to the end of each lactation period, the cows were fed concentrate and grass silage with grass hay ad libitum. Lactating cows were housed in a free-stall barn and milked twice daily (0900 and 1900 h). Milk yield was measured for each milking session. During summer, the cows were pastured for 3–4 h/day and the amount of feed was reduced to meet their nutritional requirements. Pregnant dry cows were pastured daily for 6 h during summer.
Observations of postpartum ovaries and uterus
The ovaries of all cows were monitored using a real-time linear array ultrasound scanner (SSD114 620 with a 5 MHz probe or SSD-900 with a 7.5 MHz probe; ALOKA, Tokyo, Japan). The presence or absence of corpora lutea (CL; diameter > 10 mm) was confirmed for each cow at 26 days postpartum (D26: mean ± standard deviation (SD):26.0 ± 0.7 d and 25.9 ± 0.7 d after the first and second calving, respectively). In our previous study, the mean interval from calving to the first ovulation was 30.9 days [21], with a median of 26 days in Holstein cows. Early ovulation was defined as the presence of CL on day 26. The uteri of all cows were ultrasonographically monitored concurrently with ovarian monitoring. When uterine fluid was confirmed or purulent vaginal discharge from the vulva was observed, the cow was defined as having uterine dysfunction [22].
Reproductive management
Artificial insemination (AI) was initiated at the first estrus for each heifer weighing ≥ 350 kg. Hormonal treatments were not administered to the heifers. All inseminations were performed using freeze-thawed semen from Holstein bulls with normal fertility. Conception was confirmed by the detection of the fetal heartbeat using ultrasonography 35–40 days after each insemination. After the first calving, postpartum cows were observed twice daily for at least 30 min before milking, and those exhibiting standing estrus or mounting activity accompanied by other signs such as mucous, vaginal discharge, and swelling of the vulva were considered to be in estrus. Cows in estrus were inseminated after a minimum 45-day voluntary waiting period post-calving. If ovulation did not occur 24 h after the end of the estrous activity, it was diagnosed as delayed ovulation. When necessary, the use of PGF2α and GnRH was performed to recover some reproductive dysfunctions such as delayed ovulation, persistent corpus luteum, developing cystic follicles, and uterine endometritis.
Data collection
The original dataset consisted of 203 cows that first calved between 1999 and 2012, as described in our previous study [20]. After the first calving, subsequent performance continued until the end of the third calving. All data were evaluated to ensure accuracy and 34 animals whose culling was due to experimental culling were excluded from the study. In total, 169 cows were included in the analysis.
BW was measured before and within one week of parturition and each delivered calf was weighed within 24 h of birth. The average daily live weight gain (ADG) of the heifers from birth to first calving was calculated by subtracting the calf weight from the BW before calving and dividing it by AFC.
The reproductive and productive outcomes of the 169 cows were tracked until the end of the third lactation period. At each calving, the prevalence of stillbirths and incidence of dystocia were recorded. Calving was not induced. When there was no visible progress for 2 h after the appearance of the membranes, the cow was examined to determine the cause of the delay and the type of assistance required. The dystocia score was based on the amount of assistance provided during parturition as indicated in a previous report [23]. A dystocia score of one was assigned to births that required no assistance. A score of two was assigned to a calving event that required intervention by a single person without mechanical assistance (mild dystocia). A score of three was assigned to any calving event requiring assistance from two or more people. A dystocia score of 4 was assigned when mechanical extraction was performed. Finally, a score of 5 was assigned when surgical procedures were required. After evaluating the data, dystocia scores of 4 and 5 were combined with a score of 3 (severe dystocia).
The following reproductive outcomes were evaluated: interval from calving to first service, first service conception rate, final conception rate, and number of days open. Reasons for culling, such as temperament, mortality, metabolic diseases, low productivity, hoof diseases, udder disorders, mastitis, and reproductive disorders, were also recorded. The number of days of milk production and milk yield were recorded until the end of the third lactation period.
Statistical analysis
All statistical analyses were performed using the JMP statistical software (JMP Pro Statistics and Graphics Guide, ver.17.0, SAS Inst., Inc., Cary, NC). Continuous variables were presented as median values within the range. A statistical probability of P < 0.05 was considered significant, with probabilities between P > 0.05 and P < 0.10 indicating that the difference was approaching significance (trend).
AFC ranged from 20.8 to 28.7 months. A total of 169 cows were categorized according to AFC as young (< 22.5 months, n = 23), moderate (22.5 to < 24.0 months, n = 67), old (24.0 to < 25.5 months, n = 52), and very old (> 25.5 months, n = 27). These categories were based on a previous study on the same herd [20] which proposed that, for a heifer with an AFC of less than 22.5 months, there were no adverse effects on fertility and milk production during the first lactation.
Age in months, BW, and average daily gain
The age in months when BW reached 350 kg, BW at 12 months of age, AFC, BW prior to first calving, and ADG from birth to first calving were not normally distributed and were therefore transformed into ranks. Comparisons of the mean values among the four AFC groups were performed using the Kruskal–Wallis test. If a significant difference was detected, multiple comparisons were performed using the Steel–Dwass test to analyze the differences between groups.
Longevity traits and lifetime productivity
We evaluated the longevity traits and lifetime productive performance in the AFC groups until the third lactation period. The parameters of longevity, proportion of cows with three or more calvings, and number of parities were used. The maximum number of parities was set to three, even when the cows had three or more calvings. Cumulative actual milk yield and overall lifetime milk yield up to the third lactation period were used as parameters for lifetime productive performance. Cumulative actual milk yield was calculated as the sum of milk yields up to the end of the third lactation period. The total amount of milk produced until the final lactation period was used for cows culled before the end of the third lactation period. The overall lifetime milk yield was calculated by dividing the cumulative actual milk yield by age at the end of lactation. The productive period was the sum of days spent in milk until the end of the final lactation period and the non-productive period was the age at the end of the final lactation minus the productive period. Additionally, the total calf production per cow and BW of live calves per calving were evaluated.
A likelihood ratio test was performed to analyze whether the proportion of qualitative variables differed among the four AFC groups. Quantitative variables were not normally distributed; therefore, they were ranked. Comparisons of the mean values among the four AFC groups were performed using the Kruskal–Wallis test. If a significant difference or trend towards significance was detected, multiple comparisons were performed using the Steel–Dwass test to analyze the differences between groups.
Comparison between the surviving and non-surviving cows
Sixty-nine of the 169 cows survived until the third calving and the other cows were culled before. The former was defined as the surviving group and the latter as the non-surviving group.
The prevalence of stillbirths, incidence of severe dystocia, incidence of early ovulation, and proportion of cows with abnormal uteri during the first and second lactations were compared between the surviving and non-surviving groups using Pearson’s chi-squared test. Furthermore, for each group, the performances of the four AFC cows were compared. Reproductive performance (days to first service, first-service conception rate, final conception rate, and days open during the first and second lactation) was compared. Comparisons of the mean values and proportions between the two groups were performed using the Wilcoxon signed-rank test and Pearson’s chi-square test. We then examined whether there was a difference in the distribution between the AFC groups using Pearson’s chi-square and Kruskal–Wallis tests.
Results
AFC, BW before first calving, and average daily gain
Table 1 shows the age at which BW reached 350 kg and the BW at 12 months of age. The heifers in the young AFC group reached 350 kg BW faster and had greater BW at 12 months of age than those in the very old AFC group (P < 0.05). AFC and BW before first calving for the four AFC groups are also presented in Table 1. The cows in the young AFC group experienced their first calving approximately 4.2 months earlier than those in the very old AFC group (P < 0.05). Before the first calving, the BW of the young AFC group was lower than that of the moderate, old, and very old groups (P < 0.05). The ADG from birth to first calving was higher in the young and moderate AFC groups than in the old and very old groups (P < 0.05).
Table 1. Age at first calving, body weight prior to the first calving, and average daily gain from birth to first calving.
| AFC group * |
|||||
|---|---|---|---|---|---|
| Young (n = 23) |
Moderate (n = 67) |
Old (n = 52) |
Very old (n = 27) |
P-value | |
| Age when body weight reached 350 kg; months | 11.0 a (10 to 13) |
12.0 ab (10 to 14) |
12.0 ab (10 to 15) |
12.0 b (11 to 14) |
0.025 |
| Body weight at 12 months of age; kg | 375.0 a (342 to 436) |
368.0 ab (331 to 421) |
366.5 ab (307 to 446) |
355.0 b (321 to 409) |
0.040 |
| Age at first calving; months | 21.9 a (20.8 to 22.4) |
23.3 b (22.5 to 23.9) |
24.6 c (24.0 to 25.4) |
26.1 d (25.6 to 28.7) |
< 0.001 |
| Age at first calving; days | 666.0 a (634 to 684) |
709.0 b (685 to 728) |
747.5 c (730 to 775) |
793.0 d (779 to 872) |
< 0.001 |
| Body weight before the first calving; kg | 620.0 a (577 to 662) |
648.0 b (536 to 729) |
664.0 b, c (561 to 769) |
695.0 c (587 to 803) |
< 0.001 |
| Average daily gain from birth to first calving; kg per day § | 0.87 a (0.81 to 0.93) |
0.86 a (0.72 to 0.97) |
0.84 b, c (0.67 to 0.96) |
0.81 c (0.70 to 0.93) |
< 0.001 |
* Young, < 22.5 months; moderate, 22.5 to < 24.0 months; old, 24.0 to < 25.5 months; very old, ≥ 25.5 months of AFC. § Value = {(bodyweight of heifer before first calving) – (bodyweight of newborn calf)} ÷ AFC. a, b, c, d Values with different letters are significantly different (P < 0.05).
Longevity traits
Table 2 presents the longevity traits of the four AFC groups. Sixty-nine of the 169 cows survived until the third calving. The proportion of cows with three or more calvings was higher in the young AFC group than in the other AFC groups. The number of parities tended to differ among the AFC groups; cows categorized into the young AFC group tended to have more parities than those categorized into the old AFC group (P = 0.09). Table 3 shows the reasons for culling the AFC groups according to parity until the end of the third lactation period. Except for cows with unknown culling reasons, until the end of the second and third lactations, the primary and second culling reasons were udder disorders, mastitis, and reproductive disorders, respectively. There was no difference in distribution among the four AFC groups. In the young AFC group, no cows were culled because of low productivity and hoof disease until the end of the third lactation.
Table 2. Longevity traits and lifetime productivity until the end of the third lactation for the four AFC groups *.
| AFC group * |
|||||
|---|---|---|---|---|---|
| Young (n = 23) |
Moderate (n = 67) |
Old (n = 52) |
Very old (n = 27) |
P-value | |
| Proportion of cows with three or more calving | 60.9% (14) | 41.8% (28) | 34.6% (18) | 33.3% (9) | 0.154 |
| Number of parities § | 3.0 e (1 to 3) |
2.0 ef (1 to 3) |
1.5 f (1 to 3) |
2.0 ef (1 to 3) |
0.090 |
| Productive period; days | 693.0 (50 to 977) |
601.0 (0 to 1,038) |
315.5 (0 to 1,041) |
481.0 (0 to 878) |
0.123 |
| Non-productive period; days | 844.0 ab (661 to 1,047) |
785.0 a (691 to 1,062) |
770.5 a (730 to 1,206) |
907.0 b (779 to 1,277) |
<0.001 |
| Cumulative actual milk yield of life (kg) | 24,682 (1,282 to 28,633) |
17,401 (784 to 32,127) |
10,074 (395 to 34,896) |
15,020 (2,443 to 28,391) |
0.256 |
| Overall lifetime milk yield (per day, kg) | 14.3 (1.8 to 16.4) |
11.9 (0.0 to 17.2) |
8.7 (0.0 to 18.4) |
9.9 (0.0 to 16.6) |
0.090 |
* Young, < 22.5 months; moderate, 22.5 to < 24.0 months; old, 24.0 to < 25.5 months; very old, ≥ 25.5 months of AFC. a, b Values with different letters are significantly different (P < 0.05). e, f Values with different letters tended to be different (P < 0.1). § The maximum value was set to 3.
Table 3. Reasons for culling for the AFC groups * by parity until the end of the third lactation.
| Timing for culling | AFC group * | Tempera-ment | Mortality | Metabolic disease | Low productivity | Hoof disease | Udder disorder & Mastitis | Reproductive disorders | Unknown | Total |
|---|---|---|---|---|---|---|---|---|---|---|
| First lactation | Young | 20.0 (1) | 0.0 (0) | 0.0 (0) | 0.0 (0) | 0.0 (0) | 60.0 (3) | 0.0 (0) | 20.0 (1) | 5 |
| Moderate | 4.3 (1) | 8.7 (2) | 4.3 (1) | 13.0 (3) | 4.3 (1) | 47.8 (11) | 4.3 (1) | 13.0 (3) | 23 | |
| Old | 3.8 (1) | 0.0 (0) | 0.0 (0) | 7.7 (2) | 3.8 (1) | 42.3 (11) | 23.1 (6) | 19.2 (5) | 26 | |
| Very old | 0.0 (0) | 9.0 (1) | 0.0 (0) | 9.0 (1) | 9.0 (1) | 36.4 (4) | 9.0 (1) | 27.3 (3) | 11 | |
| Total | 4.6% (3) | 4.6% (3) | 1.5% (1) | 9.2% (6) | 4.6% (3) | 44.6% (29) | 12.3% (8) | 18.5% (12) | 65 | |
| Second lactation | Young | 0.0 (0) | 25.0 (1) | 0.0 (0) | 0.0 (0) | 0.0 (0) | 50.0 (2) | 25.0 (1) | 0.0 (0) | 4 |
| Moderate | 0.0 (0) | 0.0 (0) | 0.0 (0) | 18.8 (3) | 12.5 (2) | 25.0 (4) | 12.5 (2) | 31.3 (5) | 16 | |
| Old | 0.0 (0) | 0.0 (0) | 0.0 (0) | 0.0 (0) | 0.0 (0) | 25.0 (2) | 12.5 (1) | 62.5 (5) | 8 | |
| Very old | 0.0 (0) | 14.3 (1) | 0.0 (0) | 0.0 (0) | 0.0 (0) | 57.1 (4) | 14.3 (1) | 14.3 (1) | 7 | |
| Total | 0.0% (0) | 5.7% (2) | 0.0% (0) | 8.6% (3) | 5.7% (2) | 34.3% (12) | 14.3% (5) | 31.4% (11) | 35 | |
| Third lactation | Young | 16.7 (1) | 0.0 (0) | 16.7 (1) | 0.0 (0) | 0.0 (0) | 16.7 (1) | 16.7 (1) | 33.3 (2) | 6 |
| Moderate | 0.0 (0) | 0.0 (0) | 0.0 (0) | 0.0 (0) | 25.0 (3) | 58.3 (7) | 0.0 (0) | 16.7 (2) | 12 | |
| Old | 0.0 (0) | 18.2 (2) | 0.0 (0) | 0.0 (0) | 18.2 (2) | 9.1 (1) | 9.1 (1) | 45.5 (5) | 11 | |
| Very old | 0.0 (0) | 0.0 (0) | 0.0 (0) | 0.0 (0) | 16.7 (1) | 16.7 (1) | 16.7 (1) | 50.0 (3) | 6 | |
| Total | 2.9% (1) | 5.7% (2) | 2.9% (1) | 0.0% (0) | 17.1% (6) | 28.6% (10) | 8.6% (3) | 34.3% (12) | 35 | |
* Young, < 22.5 months; moderate, 22.5 to < 24.0 months; old, 24.0 to < 25.5 months; very old, ≥ 25.5 months of AFC.
Lifetime productivity
Table 2 shows the lifetime productivity of the four AFC groups. The non-productive periods differed between the AFC groups; the cows categorized into the moderate and old AFC groups had shorter non-productive periods than those categorized into the very old AFC group (P < 0.05). The median value of the BW for live calves per calving was 43.0 kg; cows categorized as moderate AFC tended to have smaller calves than those categorized as very old AFC (41.5 vs. 43.5 kg, P = 0.07). The total calf production per cow was 2.0, which was not significantly different among the AFC groups. The overall lifetime milk yield, but not the cumulative 305-day milk yield, tended towards significance between the AFC groups, with young AFC cows having a higher overall lifetime milk yield than old AFC cows (14.3 vs. 8.7 kg/d, P = 0.11).
Comparison between the surviving and non-surviving cows
At first calving, the non-surviving group had a higher prevalence of stillbirths and severe dystocia than the surviving group (14.0% vs. 4.4%, P < 0.05; and 16.0% vs. 8.7%, P = 0.16, respectively; Table 4); however, there was no statistical difference in the distribution among the four AFC groups. The incidence of early ovulation and the proportion of cows with abnormal uteri on D26 were similar between the surviving and non-surviving groups and among the four AFC groups.
Table 4. Prevalence of stillbirth and incidence of severe dystocia at the first (A) and second (B) calving between the surviving and non-surviving groups.
Table 5 presents reproductive performance between the surviving and non-surviving groups during the first and second lactations. A difference was detected in the conception rate and days open during the second lactation between the surviving and non-surviving groups; the non-surviving groups had a lower first service conception rate and final conception rates and further longer days open than the surviving group (28.5% vs. 62.3%, 62.9% vs. 100% and 127.7 vs. 103.0 days, P < 0.05, respectively). No significant difference was detected in the reproductive performance among the four AFC groups; however, in the non-surviving group, young AFC cows had numerically lower first service conception rates and longer days open compared with the older AFC cow.
Table 5. Reproductive performance during the first (A) and second (B) lactation between the surviving and non-surviving groups.
Discussion
According to a summary of lifetime performance in typical UK Holstein–Friesian dairy heifers, the 21- to 23-month AFC group had higher survival rates of up to 5 years than the 24- to 25-month AFC group (80% vs. 60%) [15]. In this study, cows calving at 21–23 months had higher total milk production over the 5 years from birth than those calving at 26–30 months (24,000 vs. 17,000 l) [15]. A similar trend was reported by Cooke et al. (2013) [16]. Another study indicated that Holstein heifers calving at relatively young ages have better survival rates [13]. Consistent with this, in the present study, a greater number of young AFC cows survived until the third calving than older AFC cows and a higher overall lifetime milk yield until the end of the third lactation. Although previous studies focusing on the first lactation were concerned about low milk productivity caused by reduced AFC [8,9,10], in the present study, younger AFC cows maintained better milk productivity in the long term and were not culled by low milk production. These findings suggest that Holstein heifers calve at younger ages have the potential to increase herd productivity.
Adequate BW control in heifers before the first calving is essential to avoid dystocia, which can inhibit stable milk production and improve reproductive performance. No significant association was detected between AFC and the occurrence of difficult calving. A previous study reported that heifers with early AFC and low BW (< 23.0 months and < 578 kg) had a higher incidence of dystocia and a greater prevalence of stillbirths compared with the older heifers (23.0–24.0 months and 579–613 kg); thus, the former had poorer fertility than the latter [24]. Our previous results also indicated that heifers weighing < 625 kg experienced more difficulty during first calving than those weighing > 626 kg [20]. Although it is recommended that modern Holstein heifers first calve when weighing between 550 and 625 kg [1], it is desirable to reach a BW of at least 600 kg prior to the first calving.
In addition to being underweight, overconditioning could be an important inhibitor of long-term production. Schöpke et al. (2013) indicated that heavier BW had a significant impact on susceptibility to hoof disorder; namely, lighter cows were less susceptible to laminitis and white-line disease than heaver cows [25]. Furthermore, Becker et al. (2012) suggested that large-line cows had significantly greater total health costs than small-line cows during the first lactation and tended to have greater total health costs for the first three lactations [26]. In the present study, young AFC cows had no culling reason for hoof disease, while 5.0–8.1% of older AFC cows were culled due to hoof disease. A significant difference was detected in the mean BW values at first calving between the young and very old AFC groups, which implied that older AFC cows weighing heavier could experience foot wasting, whereas young AFC cows with lower BW could maintain their foot health.
Contradictory results have been reported regarding the effects of AFC on reproductive performance. For spring-calving Irish cows, heifers calving at 25–26 months tended to have a shorter subsequent calving interval than younger and older age groups at first calving [27]. Similarly, lower conception rates in the first lactation were found for both early (< 23.3 months) and late (> 25 months) calving heifers compared to those calving between 23 and 24 months [28]. In contrast, a previous prospective study indicated that 21.6-month AFC cows had similar reproductive performance during the first and second lactations compared to 24.9-month AFC cows [24]. A retrospective study using UK Holstein cows also indicated that there were no differences in the days open and first service conception rates during the first lactation between cows calving at < 23 months and 23–25 months [16]. In the present study, no significant difference was detected in reproductive performance between the surviving and non-surviving AFC groups. In addition, the proportion of culling reasons for reproductive disorders did not differ among the AFC groups. These results suggested that a reduction in AFC may not be associated with poor reproductive performance.
A younger AFC (as early as 22.5 months) could contribute to increased herd profitability in this experimental herd. According to an estimate of the cost of a heifer produced in-house, using data from Japanese commercial dairy herds, the average rearing cost was 386 yen per day per herd [29]. In the present study, the young AFC group conceived approximately 4.2 months earlier than the very old AFC group; thus, it is expected that the rearing cost of the young AFC group in this herd could be reduced by approximately 48,000 yen compared to the very old AFC group.
In the present study, the reduction in AFC did not have a negative effect on milk yield or reproductive performance over the long term. However, the effect may not have been completely examined because genetic, nutritional, climatic, and other factors may have synergistic effects on yearly milk production and fertility traits. In particular, the effects of AFC and BW are difficult to separate because they are highly correlated. Our analysis could not evaluate the factors that had a large impact on milk production and fertility because of the small sample size. Moreover, the present study did not consider the costs before first calving. The occurrence of diseases and differences in food intake may be strongly associated with changes in BW during the rearing period, which may affect the timing of the first calving and performance after calving [30]. Further studies are required to clarify the long-term effects of AFC on dairy profitability, by simultaneously considering multiple factors and using larger samples.
In conclusion, an AFC as early as 22.5 months could be associated with better survival than an older AFC until the end of the third lactation period. There were no significant differences in the prevalence of stillbirths, incidence of severe dystocia, or reproductive performance between the AFC groups. Consequently, young AFC cows maintained a higher overall lifetime milk yield than older AFC cows. Our results suggest that accelerating AFC may lead to higher profitability. To obtain stable profitability improvements by accelerating AFC, future work will need to investigate the association between AFC and other factors that impact lifetime productivity and fertility.
Conflict of interests
The authors declare that they have no conflict of interest.
Acknowledgments
We thank the anonymous reviewers and editors for their constructive input which helped improve the manuscript. This study did not receive any specific grants from public, commercial, or nonprofit funding agencies.
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