Effect of a dual enteric and respiratory pathogen challenge on swine growth, efficiency, carcass composition, and pork quality

Abstract

The objective of this study was to determine the influence of a dual respiratory and enteric pathogen challenge on growth performance, carcass composition, and pork quality of high and low feed efficient pigs. Pigs divergently selected for low and high residual feed intake (RFI, ~68 kg) from the 11th generation of Iowa State University RFI project were used to represent high and low feed efficiency. To elicit a dual pathogen challenge, half of the pigs (n = 12/line) were inoculated with Mycoplasma hyopneumoniae (Mh) and Lawsonia intracellularis (MhLI) on days post-inoculation (dpi) 0. Pigs in a separate room of the barn were not inoculated and used as controls (n = 12/RFI line). Pigs were weighed and feed intake was recorded to calculate ADG, ADFI, and G:F for the acclimation period (period 1: dpi −21 to 0), during peak infection (period 2: dpi 0 to 42), and during the remaining growth period to reach market weight (period 3: dpi 42 to harvest). At ~125 kg, pigs were harvested using standard commercial procedures. Carcasses were evaluated for composition (weight, fat free lean, loin eye area, 10th rib fat depth) and meat quality (pH decline, temperature decline, Hunter L, a, and b, subjective color and marbling, star probe, drip loss, cook loss, proximate composition, and desmin degradation). Challenged pigs had lesser ADFI than controls during period 2 (P < 0.05), but had greater ADG and G:F during period 3 (P < 0.05). Selection for feed efficiency did not result in a differential response to MhLI (P > 0.05). Loin chops from the less feed efficient, high RFI pigs, had greater drip loss, greater cook loss, lesser moisture content, greater Hunter L values, and greater Hunter b values (P < 0.05) than loin chops from low RFI pigs. Infection status did not significantly affect carcass composition or pork quality traits (P > 0.05). These results indicate that a MhLI challenge early in growth did not significantly affect ultimate carcass composition or meat quality traits. Selection for greater feed efficiency in pigs did not affect their response to pathogenic challenge.

Introduction

The hypothesis of energy resource allocation proposes that energy-demanding processes (behavioral, physiological, or immunological) may be compromised in animals selected for high production efficiencies (Rauw et al., 1998; Rauw, 2012). The rationale for this hypothesis is that animals that are pre-dispositioned for efficient accretion of muscle and adipose tissue are less likely to divert nutrients to address other physiological stressors. Pigs that differ in residual feed intake (RFI) can be used as a model to test the resource allocation hypothesis. Residual feed intake is defined as the difference between observed and expected feed intake based on ADG and back fat (Koch et al., 1963; Kennedy et al., 1993). Low RFI (LRFI) animals are more feed efficient than their high RFI (HRFI) counterparts. Genetic variation of RFI is attributed to feeding behavior, maintenance requirements, nutrient digestion, energy homeostasis, and energy partitioning (Luiting, 1998). To date, the energy resource allocation hypothesis has not been supported by inflammatory (Merlot et al., 2016), viral (Dunkelberger et al., 2015), or bacterial (Helm et al., 2018) challenges of pigs differing in feed efficiency as a result of divergent selection for RFI.

Mycoplasma hyopneumoniae and Lawsonia intracellularis are 2 common pathogens encountered in commercial swine production (United States Department of Agriculture, 2016). In fact, coinfections of these 2 pathogens are also common. Helm et al. (2018) documented that a dual challenge of M. hyopneumoniae and L. intracellularis in growing pigs (~68 kg) immediately reduced ADG, ADFI, G:F, fat accretion, and protein accretion. The extent to which a challenge of growing pigs influences performance during the finishing phase, carcass composition, and pork quality is not known. Because carcass composition and quality affect pork value, it is important to define how production factors such as feed efficiency and pathogen challenges influence them. It is suggested that pigs that demonstrate greater feed efficiency—such as those selected for improved RFI (Young and Dekkers, 2012)—have fewer available resources to respond to dual pathogenic challenge. Therefore, it is hypothesized that those pigs would demonstrate poorer growth performance, carcass composition, and pork quality in response to that challenge. The objective of the study was to determine the influence a dual respiratory and enteric pathogenic infection had on the growth, carcass composition, and pork quality of pigs differing in RFI.

Materials and Methods

Procedures with animals were approved by the Iowa State University Institutional Animal Care and Use Committee (IACUC # 6-16-8298-S) and adhered to the ethical and humane use of animals for research. Twenty-four HRFI and 24 LRFI barrows (49.34 ± 7.52 kg BW; 116 ± 5 d of age), from the 11th generation of Iowa State University RFI selection project (Cai et al., 2008), were split evenly across 2 rooms in 1 barn and penned individually. One room (n = 12 pigs/line) was designated as a control room; a separate room with identical conditions (n = 12 pigs/line) was designated as the challenge room. These pigs were a subset of a larger group of pigs (n = 100) used in Helm et al. (2018) in a study to determine the impact of a dual pathogen challenge on the growth performance of pigs for 42 d post-inoculation (dpi). Helm et al. (2018) determined performance and tissue gain during the key period after challenge. The 48 pigs that were maintained for this experiment were used to address the question of how challenge status influenced performance during the finishing phase and carcass composition and pork quality. These pigs were selected to ensure uniform live weights at harvest and equal representation of lines and challenge groups. The results presented herein are specifically from those pigs grown to market weights. Pigs had free access to water and were ad libitum fed a standard corn and soybean diet that met or exceeded their nutrient and energy requirements (Table 1).

Table 1.

Diet composition, as fed

Ingredient	%
Corn	66.67
Soybean meal	8.40
Corn DDGS1	22.50
Fat	0.51
Lysine	0.25
Salt	1.03
Vitamin–mineral premix2	0.13
Ronozyme phytase	0.01
Calculated composition
ME, kcal/kg	3,400
CP, %	15.50
SID lysine, %	0.87
STTD P, %	0.28

¹DDGS = dried distiller’s grains with solubles.

²Vitamin–mineral premix supplied (per kg of diet): 66,000,000 IU vitamin A; 12,120,000 IU vitamin D3; 35,200 IU vitamin E; 0.022 g vitamin B; 3.3 g riboflavin; 13.2 g d-pantothenic acid; 16.61 g niacin; 1.496 g ethoxyquin; 0.6 g I as ethylenediamine dihydroiodide; 0.133 g Se as sodium selenite; 1.6 g Cu as copper chloride; 4 g Mn as manganous oxide; 88 g Zn as zinc oxide; and 88 g Fe as ferrous carbonate and ferrous sulfate.

After a 3-wk acclimation period (dpi 0), pigs (approximately 68 kg) in the challenge room were snare restrained and inoculated intratracheally with 10 mL of crude lung inoculum (strain 232, containing 10⁵ genomic units/mL) of live Mycoplasma hyopneumoniae (Mh). Pigs were then inoculated via intragastric gavage with 40 mL Lawsonia intracellularis (LI) inoculum (2-mL crude gut homogenate, containing 2 × 10⁷ organisms). This dual pathogen challenge of Mh and LI will be denoted as MhLI. Both inoculums were prepared at the Iowa State University Veterinary Diagnostic Laboratory (ISUVDL, Ames, IA). The pigs (approximately 68 kg) in the control room were snare restrained and inoculated with a sham on dpi 0.

Jugular venipuncture was used on all pigs to collect blood samples (10 mL), on dpi 0, 7, 14, 28, and 42 in BD Vacutainer serum tubes (Becton, Dickinson and Company, Franklin Lakes, NJ), allowed to clot, centrifuged (2,000 × g for 10 min at 4 °C), and stored at −80 °C. Serum samples were submitted to the ISUVDL to quantify the Mh (IDEXX Laboratories, Inc., Westbrook, ME) and LI (SVANOIR Ileitis ELISA, Boehringer Ingelheim Svanova, Uppsala, Sweden) antibody responses using routine ELISA (Helm et al., 2018). Antibody response to LI was reported as percent inhibition of LI antibodies (20% to 30% inhibition = suspect; >30% inhibition = positive) and antibody response to Mh was reported as a ratio of sample to positive (S:P; 0.30 to 0.40 = suspect; >0.40 = positive; Helm et al., 2018). One pig was removed from the study, for reasons that were not related to the experimental conditions. The 47 remaining pigs represented all treatment groups (LRFI control, n = 11; LRFI MhLI, n = 12; HRFI control, n = 12; HRFI MhLI, n = 12).

On dpi −21, 0, 7, 14, 21, 28, 35, 42, and the day before harvest [3 groups; dpi 83 (n = 18; LRFI control n = 2, LRF MhLi n = 2, HRFI control n = 7, HRFI MhLI n = 7), dpi 118 (n = 16; LRFI control n = 5, LRF MhLi n = 5, HRFI control n = 3, HRFI MhLI n = 3), and dpi 127 (n = 16; LRFI control n = 5, LRF MhLi n = 5, HRFI control n = 3, HRFI MhLI n = 3)], BW and feed disappearance were recorded to calculate ADG, ADFI, and G:F for each pig. Once pigs weighed approximately 125 kg BW, they were transported 8 km to the Iowa State University Meat Laboratory (Ames, IA). Pigs had free access to water during lairage (17 to 20 h). Pigs were weighed, electrically stunned, and immediately exsanguinated. All harvesting procedures are standard methods used in the United States and were done under USDA inspection. Once passing inspection, dressed carcasses were weighed and then placed in the cooler (−2 °C).

The pH and temperature of the LM were measured at 45 min, 3 h, 6 h, and 1 d postmortem. The pH meter was calibrated before use, with pH buffers 4 and 7 at 20 °C for measurements taken at 45 min and 3 h, and 4 °C for measurements taken at 6 h and 1 d postmortem. Before each pH measurement, calibration of pH meter was confirmed or recalibrated. The pH probe was placed in the LM at the 13th rib with a portable HANNA HI9025 pH meter (HANNA Instruments, Woonsocket, RI). The temperature was measured in the LM approximately 5 cm cranial to the location pH was measured. Each carcass was ribbed at the 10th rib, on the left side, to measure fat depth and loin eye area (LEA) 1 d postmortem (NPPC, 1991). These measurements were used to calculate fat free lean (FFL) % (Burson and Berg, 2001).

Chop Allocation

The loin from the right side of each carcass was removed and chops were allocated (Fig. 1) 1 d postmortem. A 10-cm section was removed from the sirloin end of each loin before cutting chops. Chops 1 to 5 were cut 1.27 cm thick and chops 6 to 11 were cut 2.54 cm thick. Samples for biochemical evaluation were vacuum packaged and aged for 1, 7, or 14 d postmortem (chops 3 to 5). Once reaching the designated aging time, each sample was cubed, frozen with liquid nitrogen, and pulverized in a Waring Blender (Waring Commercial, New Hartford, CT) to homogenize. Powdered samples remained frozen at −80 °C prior to gel sample preparation.

Figure 1.

Pork chop allocation guide. Starting at the sirloin end, chops 1 to 5 were cut 1.27 cm thick and 6 to 11 were cut 2.54 cm thick.

Hunter L, a, and b values were measured on chops 8 and 9, 1 d postmortem using a Minolta Chroma Meter (CR-410; Konica Minolta Sensing Americas Inc., Ramsey, NJ), with a D65 light source, 50-mm aperture, and 2° observer (AMSA, 2012). Chops 8 and 9 were then weighed and placed in a sealed Ziploc bag and stored at 4 °C. Chops were reweighed 3 d postmortem to calculate drip loss [(chop wt. d 1 − chop wt. d 3)/chop wt. d 1) × 100]. The drip loss for each chop was averaged for each loin. Chops 10 and 11 were trimmed just to include the LM, frozen with liquid nitrogen, and homogenized using a Waring Blender (Waring Commercial, New Hartford, CT). Intramuscular moisture, fat, and protein were determined on the homogenized sample. Protein content was determined using the combustion method (AOAC, 1993). Moisture content was determined by using the oven-drying method, and hexane extraction was used to determine crude fat content (AOAC, 1990).

Chops for star probe analysis (chops 6 to 7, Fig. 1) were vacuum packaged and aged 14 d at 1 °C in the dark. On day 14, chops were removed from refrigeration and allowed to bloom for 15 min at room temperature (approximately 22 °C). Subjective color and marbling scores were assigned to chops using standard pictures established by the National Pork Board (2000; color: 6-point scale, 1 = pale pinkish gray to white; 6 = dark purplish red; marbling: 10-point scale, 1 = 1.0% intramuscular fat; 10 = 10.0% intramuscular fat). Hunter L, a, and b values were measured as previously described. The pH was measured, on the same chop following color evaluation, with the same pH meter as previously described.

Each chop was trimmed to remove adipose tissue and epimysium. Chops were cooked to an internal temperature of 68 °C on clamshell grills (Cuisinart, Conair Group; Santos et al., 2018). Cook loss was measured using the following equation: [(raw wt. − cook wt.)/raw wt.] × 100. Cooked chops were cooled to room temperature (approximately 20 °C). An Instron (Instron Products, Grove City, PA) was fitted with a 5-point star probe attachment to measure instrumental tenderness (Schulte et al., 2019).

Whole Muscle Protein Sample Preparation

Proteins were solubilized using whole muscle extraction buffer (2% sodium dodecyl sulfate, 10 mM sodium phosphate, pH 7.0), as described by Huff-Lonergan et al. (1996b). Concentrations of solubilized proteins were determined using a modified Lowry’s assay with premixed reagents (DC protein assay; Bio-Rad Laboratories, Hercules, CA). Gel samples were adjusted to 4 mg/ml protein in Wang’s tracking dye [3 mM EDTA, 3% (wt/vol) SDS, 30% (vol/vol) glycerol, 0.01% (wt/vol) pyronin-Y, 30 mM Tris–HCl, pH 8.0], vortexed, heated (approximately 50 °C for 15 min), and then stored at −80 °C.

Running and Transferring Conditions

Sodium dodecyl sulfate–PAGE was performed to quantify the rate and extent of desmin degradation in samples aged 1, 7, and 14 d. Forty microgram of protein per sample was loaded onto 15% polyacrylamide separating gels and 5% polyacrylamide stacking gels as described by Carlson et al. (2017). One well on each gel included 40-µg protein of the same reference sample (4 mg/mL protein), which was from an LM chop, aged 14 d, from a pig outside of this study. A broad-range molecular weight standard was loaded into the first lane of each gel to use as a visual reference for approximate molecular weight. All samples were run in duplicate. SE 260 Hoefer Mighty Small II electrophoresis units (Hoefer, Inc., Holliston, MA) were used to run the 15% polyacrylamide gels at constant 80 V for 360 V-h. Running buffer was composed of 25 mM Tris, 192 mM Glycine, 2 mM EDTA, and 0.1% [wt/vol] SDS. Following electrophoresis, proteins were transferred onto polyvinylidene difluoride membranes, as described by Carlson et al. (2017).

Western Blotting

Each membrane was incubated with blocking buffer, as described by Carlson et al. (2017). Immunoblots were then incubated in primary antibody solution for 16 to 18 h at 4 °C. Rabbit-anti-desmin primary antibody, produced at Iowa State University (Huff-Lonergan et al., 1996a; Carlson et al., 2017), was diluted in 15-mL PBS-Tween solution using a 1:40,000 dilution. Blots were incubated in secondary antibody solution for 1 h at room temperature. A goat-anti-rabbit-HRP (conjugated to horseradish peroxidase) secondary antibody (# 31460, Thermo Fisher Scientific, Waltham, MA) was diluted in 15-mL PBS-Tween (1:20,000). Both primary and secondary antibody incubations were followed by 3, 10-min rinses with PBS-Tween solution.

Immunoblots were incubated using ECL Prime Western Blotting Detection Reagent (GE HealthCare, Amersham, Piscataway, NJ) and then imaged using a ChemiImager 5500 (Alpha Innotech, San Leandro, CA). Densitometry was used to quantify the abundance of intact desmin (55 kDa) and desmin degradation products (38 to 54 kDa), which were calculated as the ratio of the immunoreactive desmin intact band or degraded bands of each sample to the intensity of the internal reference sample’s desmin immunoreactive intact or degradation band(s), respectively (Fig. 2). Alpha Ease FC software (v. 3.03 Alpha Innotech, San Leandro, CA) was used to conduct densitometry analysis. All samples were run in duplicate.

Figure 2.

Representative Western blots of intact and degraded desmin in pork Longissimus dorsi whole muscle samples from low residual feed intake (LRFI) or high residual feed intake (HRFI) pigs in control or Mycoplasma hyopneumoniae + Lawsonia intracellularis (MhLI) challenge groups. The desmin intact band (55 kDa) and degradation products (38 to 54 kDa) of each sample were compared with corresponding intact and degradation products of a 14-d aged reference sample (Ref). Each treatment group is represented: LRFI control, LRFI MhLI, HRFI control, and HRFI MhLI.

Muscle Fiber-Type Analysis

Myosin heavy chain (MHC) isomers were determined as described by Carlson et al. (2017). A 6% separating acrylamide gel and a 4% stacking acrylamide gel were used for electrophoresis. Gels were then stained with Colloidal Coomassie Blue stain for 20 to 24 h. Gels were then destained, and proteins were detected using a ChemiImager 5500 (Alpha Innotech) and Alpha Ease FC software (version 3.03; Alpha Innotech). Densitometry was used to determine the abundance of MHC type IIa+x and type IIb bands, reported as a percentage of the total type II fibers detected (Table 7). All gels were run in duplicate.

Table 7.

Main effects of residual feed intake (RFI) line, infection status (IS), and the interaction of line × IS on technical meat quality and proximate composition of fresh pork Longissimus muscle chops

Trait	Treatment					P-value
	LRFI1	HRFI2	Control	MhLI3	SEM	Line	Infection status	Line × IS
n	23	24	23	24
Subjective color4	3.4	3.0	3.2	3.2	0.159	0.032	0.578	0.179
Subjective marbling5	1.7	1.8	1.8	1.7	0.184	0.747	0.740	0.740
Drip loss, %6	2.62	3.66	3.17	3.11	0.410	0.023	0.878	0.633
Cook loss, %7	19.74	22.26	21.16	20.85	0.806	0.006	0.692	0.921
Star probe, kg8	5.89	6.12	5.96	6.04	0.241	0.381	0.737	0.253
% Moisture9	73.86	73.41	73.66	73.61	0.174	0.019	0.767	0.869
% Protein10	24.94	24.64	24.84	24.74	0.184	0.130	0.568	0.565
% Fat9	1.67	1.74	1.67	1.74	0.185	0.716	0.724	0.667
Myosin heavy-chain type IIb, %11	79.9	79.4	79.0	80.1	0.517	0.683	0.323	0.973
Myosin heavy-chain type IIa+IIx, %11	20.2	20.5	21.0	19.8	0.545	0.801	0.314	0.898

Subjective color and marbling, cook loss, and star probe were evaluated on chops aged 14 d.

¹LRFI, low residual feed intake.

²HRFI, high residual feed intake.

³Pigs inoculated with Mycoplasma hyopneumoniae and Lawsonia intracellularis.

⁴National Pork Board standards, 6-point scale (1 = pale pinkish gray to white; 6 = dark purplish red).

⁵National Pork Board standards, 10-point scale (1 = 1.0% i.m. fat; 10 = 10.0% i.m. fat).

⁶% Drip loss = [(initial weight (day 1) − final weight (day 3))/initial weight] × 100.

⁷Cooked to an internal temperature of 68 °C; % cook loss = [(raw weight − cooked weight)/raw weight] × 100.

⁸Force utilized to compress sample to 20% of its original height.

⁹As determined by proximate composition (AOAC, 1990).

¹⁰As determined by proximate composition (AOAC, 1993).

¹¹Proportion of myosin heavy chain detected as type IIa + IIx or IIb based on SDS–PAGE migration. Protein samples prepared from LM muscle removed from carcass at 45 min postmortem (Carlson et al., 2017).

Statistical Analysis

A 2 × 2 factorial design was used to evaluate the fixed effects of line (LRFI and HRFI), infection status (control and MhLI), and their interaction. The MIXED procedure of SAS (version 9.4; SAS Inst. Inc., Cary, NC) was used to analyze these data. Harvest group was included as a fixed effect for carcass composition and pork quality traits. Repeated measures were used for pH, temperature, and Hunter L, a, and b data. The Akaike information criterion fit statistic was used to select the most appropriate covariate parameter for each evaluated trait. Body weight at dpi 0 was used as a covariate for ADG. Age at dpi 0 was used as a covariate for ADFI and feed efficiency. The quadratic relationship of each covariate was used if the linear effect of the covariate was significant. Gel was included as a random effect for the analysis of desmin. Differences were considered significant when P ≤ 0.05 and tendencies when 0.05 < P ≤ 0.10.

Results

Response to Infection

Pooled serum samples and fecal swabs from a subset of pigs confirmed the absence of antibodies against both Mh and LI prior to inoculation (data not shown). Pigs inoculated with MhLI were confirmed positive, using serology antibody titers, for both Mh and LI by 14 dpi (Table 2). During this time (dpi 0 to 42), a subset of control pigs tested negative for Mh and LI antibodies and LI fecal shedding (Helm et al., 2018).

Table 2.

Serology of barrows (n = 24) dual inoculated with Mycoplasma hyopneumoniae and Lawsonia intracellularis (MhLI). (A) Serum antibody titers to Mycoplasma hyopneumoniae (Mh) in which a S:P ratio between 0.30 and 0.40 is suspect a S:P ratio > 0.40 is positive for Mh. (B) Serum antibody titers to Lawsonia intracellularis (LI) in which % inhibition between 20% and 30% is suspect and % inhibition > 30 is positive for LI. Pigs were inoculated on days post inoculation (dpi) 0 and assessed weekly until dpi 42

(A)
dpi	0	14	21	28	42
Mean	0.035	0.548	0.845	1.182	1.915
SE	0.012	0.058	0.069	0.075	0.470
Maximum	0.187	1.139	1.507	1.687	2.247
Minimum	−0.037	0.128	0.168	0.397	1.482
(B)
dpi	0	14	21	28	42
Mean	11.4	72.5	81.8	72.9	56.7
SE	2.5	3.8	2.5	2.7	0.8
Maximum	38.5	91.2	91.9	89.8	57.2
Minimum	−8.4	32.6	50.4	46.3	56.0

Growth Performance

There were no line × infection status interactions overgrowth periods 1, 2, or 3 (dpi −21 to 0, 0 to 42, and 42 to harvest, respectively) for ADG (P > 0.05, Table 3). Average daily gain for LRFI and HRFI pigs did not differ during periods 1 and 2 (P > 0.05), but HRFI pigs had a greater ADG compared with LRFI pigs during period 3 (P = 0.009). The ADG of MhLI and control pigs was not different in periods 1 and 2, but MhLI pigs had a greater ADG during period 3 (P < 0.01) than control pigs. This evidence suggests that MhLI challenged pigs experienced compensatory growth during period 3. The greater ADG in MhLI pigs during period 3 was most evident in harvest group 1 (from 42 to 83 dpi; Figure 3) and in LRFI control pigs in harvest group 3 (from 42 to 127 dpi; Figure 3).

Table 3.

Growth performance parameters of low residual feed intake (LRFI) or high residual feed intake (HRFI) pigs in control and Mycoplasma hyopneumoniae + Lawsonia intracellularis (MhLI) challenge groups during 3 periods of time

Trait	Treatment				SEM	P-value
	LRFI	HRFI	Control	MhLI		Line	Infection status	Line * infection status
n	23	24	23	24
ADG, kg P11	0.88	0.92	0.89	0.90	0.041	0.322	0.780	0.976
ADG, kg P22	0.69	0.73	0.73	0.68	0.036	0.276	0.130	0.665
ADG, kg P33	0.60	0.74	0.60	0.74	0.049	0.009	0.009	0.333
ADG, kg P2 and P3	0.64	0.72	0.67	0.69	0.037	0.027	0.693	0.765
ADFI P1	2.27	2.48	2.36	2.39	0.069	0.008	0.662	0.155
ADFI P2	2.49	2.78	2.78	2.49	0.076	0.001	0.0003	0.318
ADFI P3	2.92	3.51	3.15	3.29	0.144	0.0004	0.336	0.719
ADFI P2 and P3	2.66	3.04	2.90	2.80	0.085	0.0002	0.235	0.408
G:F P1	0.40	0.36	0.38	0.38	0.018	0.026	0.933	0.103
G:F P2	0.28	0.25	0.27	0.27	0.011	0.009	0.430	0.542
G:F P3	0.21	0.20	0.19	0.22	0.009	0.222	0.004	0.454
G:F P2 and P3	0.25	0.23	0.23	0.234	0.008	0.106	0.193	0.660

¹Period 1, 21-d acclimation period before infection.

²Period 2, days post inoculation 0 to 42.

³Period 3, days between period 2 and harvest.

Figure 3.

Effect of dual Mycoplasma hyopneumoniae + Lawsonia intracellularis (MhLI) challenge on ADG of pigs divergently selected for low or high residual feed intake (LRFI and HRFI, respectively). Pigs were harvested in 3 separate groups. The ADG data from groups 1 (dpi 42 to 83, n = 18; LRFI Control n = 2, LRF MhLi n = 2, HRFI Control n = 7, HRFI MhLI n = 7), 2 (dpi 42 to 118, n = 16; LRFI control n = 5, LRF MhLi n = 5, HRFI control n = 3, HRFI MhLI n = 3), and 3 (dpi 42 to 127, n = 13; LRFI control n = 4, LRF MhLi n = 5, HRFI control n = 2, HRFI MhLI n = 2) are represented as kg/d. Within group, means with a different letter are significantly different.

There were no significant line × infection status interactions regarding ADFI during the 3 evaluated periods of growth (P > 0.05, Table 3). Pigs from the LRFI line consumed 8.8% less feed during period 1 (P < 0.01), 11.0% less feed during period 2 (P = 0.001), and 18.4% less feed during period 3 (P < 0.001) compared with HRFI pigs. Infection status had no effect on ADFI during period 1 or 3 (P > 0.05), but MhLI pigs consumed 11.01% less feed than controls during period 2 (P < 0.001). Average daily feed intake during periods 2 and 3 combined was not affected by infection status (P > 0.05).

Significant line × infection status interactions were not detected for feed efficiency during the 3 growth periods (P > 0.05, Table 3). As expected, LRFI pigs had greater G:F than HRFI pigs during periods 1 (P < 0.05) and 2 (P < 0.01) by 10.5% and 11.3%, respectively. There was no difference of G:F between RFI line during period 3 (P > 0.05). Differences in G:F of MhLI and control pigs were not detected during periods 1 and 2, but MhLI pigs had a 14.6% greater feed efficiency during period 3 compared with controls (P < 0.01).

Carcass Composition and Pork Quality

Line, infection status, and their interaction did not significantly affect BW loss of pigs during trucking and overnight lairage (shrink; P > 0.05, Table 4). Carcass weight was not different between lines, infection status, or line × infection status (P > 0.05). Line did not affect dressing percentage (P > 0.05), but there was a tendency for control pigs to have a greater dressing percentage than the infected pigs, by 0.65% (P = 0.064). An interaction between line and infection status was not detected (P > 0.05).

Table 4.

Main effects of residual feed intake (RFI) line, infection status, and interaction of line × infection status on shrink, hot carcass weight (HCW), dressing percentage (DP), fat-free lean (FFL), loin eye area (LEA), and 10th rib fat depth

	Treatment				SEM	P-value
Trait	LRFI1	HRFI2	Control	MhLI3		Line	Infection status	RFI line × infection status
n	23	24	23	24
Shrink4, %	4.35	4.05	4.09	4.31	0.305	0.371	0.459	0.849
HCW, kg	92.53	93.82	92.72	93.63	1.687	0.478	0.581	0.118
DP, %	77.75	77.49	77.95	77.30	0.353	0.498	0.064	0.853
FFL5, %	50.23	50.11	49.65	50.68	0.962	0.909	0.275	0.641
LEA, cm2	41.49	43.05	40.95	43.58	1.827	0.431	0.147	0.649
10th rib fat depth, mm	23.75	24.61	24.75	23.61	1.698	0.639	0.493	0.454

¹LRFI, low residual feed intake.

²HRFI, high residual feed intake.

³Pigs inoculated with Mycoplasma hyopneumoniae and Lawsonia intracellularis.

⁴BW lost during transportation and lairage.

⁵Calculated using HCW, LEA, and 10th rib fat depth (Burson and Berg, 2001).

Line, infection status, and their interaction were not significant factors affecting FFL, LEA, or 10th rib back fat (P > 0.05, Table 4). Line, infection status, line × time, infection status × time, or line × infection status also did not significantly affect pH decline (P > 0.05, Table 5). As expected, time postmortem significantly affected pH (P < 0.0001). A tendency for a line × time × infection status interaction (P = 0.086) for pH was noted. This tendency is due to a more rapid pH decline of the HRFI MhLI group, compared with the LRFI control pigs, in particular at 6 h postmortem.

Table 5.

Main effects of residual feed intake (RFI) line, infection status (IS), and interactions of line, time, and IS on pH and temperature (°C) decline of the Longissimus muscle

Trait	Treatment				SEM	P-value
	LRFI1 control	HRFI2 control	LRFI MhLI3	HRFI MhLI		Line	Time	Infection status (IS)	Line × time	Line × IS	Time × IS	Line × time × IS
n	11	12	12	12
pH 45 min	6.42	6.36	6.24	6.32	0.07
pH 3 h	5.92	5.95	5.90	5.76	0.08
pH 6 h	5.78	5.73	5.73	5.60	0.06
pH ~24 h	5.60	5.59	5.61	5.57	0.03
pH 14 d	5.52	5.48	5.52	5.49	0.02	0.354	<0.001	0.144	0.428	0.713	0.299	0.086
Temp. 45 min	39.9a,w	38.9b,w	40.2a,w	40.1a,w	0.33
Temp. 3 h	23.9x	23.5x	22.7x	24.5x	0.73
Temp. 6 h	11.7a,b,y	11.7a,b,y	10.7b,y	12.6a,y	0.51
Temp. 24 h	0.0b,z	0.1a,z	0.0a,b,z	0.1a,b,z	0.03	0.153	<0.001	0.631	0.040	0.041	0.087	0.157

¹LRFI, low residual feed intake.

²HRFI, high residual feed intake.

³Pigs inoculated with Mycoplasma hyopneumoniae and Lawsonia intracellularis.

^a,bWithin a row, means without a common superscript differ (P ≤ 0.05).

^w–zWithin a column and for a specific trait, means without a common superscript differ (P ≤ 0.05).

Line, infection status, time × infection status, line × time × infection status did not significantly affect temperature decline (P > 0.05, Table 5). As expected, temperature was affected by time postmortem (P < 0.0001). There was a significant interaction of line × time for temperature decline (P < 0.05), with the temperature of LM from HRFI control carcasses at 45 min (38.9 °C) being cooler than LRFI control carcasses (39.9 °C), LRFI MhLI carcasses (40.2 °C), and HRFI MhLI carcasses (40.1 °C). At 3 h postmortem, LM temperatures averaged 23.6 °C and there were no differences in temperature between treatment groups. At 6 h postmortem, the temperature of LRFI MhLI (10.7 °C) loins were cooler than loins from HRFI MhLI pigs (12.6 °C, P = 0.01).

Hunter L values from adjacent chops, documented that LM chops were lighter in color at day 14 postmortem than at day 1 (P < 0.001, Table 6). Hunter L values were lesser for LM chops from LRFI carcasses (47.18 ± 0.50) compared with chops from HRFI carcasses (49.11 ± 0.51; P = 0.013). Hunter a values increased over the 14-d aging period (P = 0.001). The line × days aged interaction (P = 0.040) and the infection status × days aged interaction (P < 0.05) were significant for Hunter a values, with Hunter a values of chops from the LRFI MhLi pigs significantly increasing from 14.58 at day 1 to 15.43 at day 14, whereas there were no changes in Hunter a values for chops from the LRFI control, HRFI control, or HRFI MhLI treatment groups. Hunter b values were greater for chops from HRFI pigs (6.88 ± 0.14) than for chops from LRFI pigs (6.27 ± 0.14, P = 0.006), and Hunter b values were greater at day 14 than at day 1 (P < 0.0001). Chops from MhLI pigs tended to have greater Hunter b values than chops from control pigs (P = 0.077) and chops from MhLI pigs had a greater increase in Hunter b values over the aging period than chops from control pigs (P < 0.01).

Table 6.

Main effects of residual feed intake (RFI) line, infection status (IS), and interactions of RFI line, days aged, and IS on Hunter¹L, a, b from Longissimus muscle chops aged 1 and 14 d

Trait	Treatment				SEM	P-value
	LRFI2 control	HRFI3 control	LRFI MhLI4	HRFI MhLI		Line	Days aged	Infection status (IS)	Line × days aged	Line × IS	Days aged × IS	Line × days aged × IS
n	11	12	12	12
L, day 1	46.27	48.73	47.36	48.27
L, day 14	46.87	49.78	48.23	49.68	0.74	0.013	0.0004	0.491	0.347	0.270	0.536	0.932
a, day 1	14.82	15.19	14.58z	15.01
a, day 14	15.22	15.09	15.43y	15.44	0.24	0.417	0.001	0.882	0.040	0.811	0.029	0.855
b, day 1	5.58b,z	6.35a,z	5.87a,b,z	6.18a,b,z
b, day 14	6.46b,y	7.21a,y	7.17a,y	7.77a,y	0.22	0.006	<0.0001	0.077	0.497	0.427	0.007	0.451

¹Hunter L, a, and b, D65 light source, 50-mm aperture, 0° observer.

²LRFI, low residual feed intake.

³HRFI, high residual feed intake.

⁴Pigs inoculated with Mycoplasma hyopneumoniae and Lawsonia intracellularis.

^a,bWithin a row, means without a common superscript differ (P ≤ 0.05).

^y,zWithin a column and for a specific trait, means without a common superscript differ (P ≤ 0.05).

Quality traits of fresh, never frozen, LM chops [subjective color and marbling, cook loss, drip loss, star probe, or proximate composition components (moisture, protein, and fat)] were not significantly affected by infection status or the interaction between line and infection status (P > 0.05; Table 7). Line did not significantly affect subjective marbling, star probe, protein %, or fat % (P > 0.05). Subjective color scores aligned with results for Hunter L value, with chops from LRFI pigs being darker (3.4 ± 0.1) than chops from HRFI pigs (3.0 ± 0.1, P = 0.032). Chops from LRFI pigs had less drip loss (2.62 ± 0.29) and cook loss (19.74 ± 0.58) than chops from HRFI pigs (3.66 ± 0.30 drip loss and cook loss 22.26 ± 0.59 cook loss; P < 0.05 and < 0.01, respectively). Moisture composition was greater in chops from LRFI pigs (73.86 ± 0.12) than chops from HRFI pigs (73.41 ± 0.13, P < 0.05). Neither line or infection status had a significant effect on muscle fiber type of the LM muscle.

Desmin degradation was significantly different between chops from high and low RFI pigs, with chops from HRFI pigs having greater abundance of intact desmin (P < 0.001, Table 8). Abundance of desmin degradation products did not differ between lines (P > 0.05). Abundance of intact desmin or desmin degradation products were not different between chops from MhLI and control pigs (P > 0.05). The effect of aging time was significant for both intact (P < 0.0001) and degraded (P < 0.0001) desmin, as abundance of intact desmin decreased during aging (1, 7, and 14 d), and abundance of desmin degradation products became increased over 1, 7, and 14 d of aging.

Table 8.

Main effects of residual feed intake (RFI) line, infection status (IS), days aged, and their appropriate interactions on intact desmin and desmin degradation (Deg) products of pork Longissimus muscle aged 1, 7, or 14 d

Trait	Treatment				SEM	P-value
	LRFI1 control	HRFI2 control	LRFI MhLI3	HRFI MhLI		Line	IS	Days aged	IS × days aged	Line × days aged	Line × IS	Line × IS × days aged
n	11	12	12	12
Intact4 day 1	3.17b,x	4.82a,x	3.46b,x	3.72b,x	0.273
Intact day 7	1.96a,b,y	2.53a,y	1.85b,y	2.32a,b,y	0.276
Intact day 14	0.90b,z	1.82a,z	1.93a,y	1.47a,b,z	0.277	0.0003	0.588	<0.0001	0.088	0.100	0.0008	0.100
Deg5 day 1	0.36x	0.40x	0.39x	0.40x	0.047
Deg day 7	0.92y	0.90y	0.82y	0.79y	0.046
Deg day 14	1.02y	1.04z	1.00z	1.09z	0.045	0.425	0.327	<0.0001	0.056	0.432	0.796	0.660

¹LRFI, lw residual feed intake.

²HRFI, high residual feed intake.

³Pigs inoculated with Mycoplasma hyopneumoniae and Lawsonia intracellularis.

⁴55 kDa intact band was measured.

⁵38 to 54 kDa degradation products were measured.

^a,bWithin a row, means without a common superscript differ (P ≤ 0.05).

^x–zWithin a column and for a given trait, means without a common superscript differ (P ≤ 0.05).

The infection status × days aged interaction tended to affect the abundance of intact desmin (P = 0.088), as a result of less continued proteolysis of intact desmin in LM from LRFI MhLi pigs. The IS × days aged interaction tended to affect the abundance of degraded desmin products (P = 0.056), probably because of no significant increase in abundance of desmin degradation products in LM from LRFI controls pigs as a result of aging from days 7 to 14.

The abundance of intact desmin was not significantly different between LRFI MhLI and HRFI MhLI groups at 1, 7, or 14 d of aging. Chops from LRFI control pigs had significantly less intact desmin than HRFI control and LRFI MhLI pigs did not significantly differ from HRFI MhLI pigs at 14 d postmortem. Because of these differences, the line × infection status interaction was significant (P < 0.001). This interaction was not significant for desmin degradation products (P > 0.05). There was a tendency for a significant interaction of line × infection status × days aged for intact desmin (P = 0.10), but not for desmin degradation products (P > 0.05, Table 8).

Discussion

Grow-finish pigs in commercial production systems probably encounter singular or dual pathogen challenges at some point in their life (United States Department of Agriculture, 2016). The consequences of these encounters for growth performance, carcass composition, and pork quality are not defined because dual pathogenic infections are complex and intensities (subclinical to clinical) vary.

Residual feed intake is defined as the difference between observed and predicted feed intake based on average requirements for growth and maintenance (Koch et al., 1963; Kennedy et al., 1993). RFI in growing pigs is moderately heritable (Gilbert et al., 2007; Cai et al., 2008; Young and Dekkers, 2012) and therefore is a valuable tool for sourcing pigs that differ in feed efficiency. Selection of LRFI has resulted in pigs with 8% to 10% greater feed efficiency for a similar rate of growth (Boddicker et al. 2011). Young and Dekkers (2012) proposed that the differences in efficiency between the RFI lines were due to feeding behavior and a reduced maintenance requirement. In addition, there is evidence of decreased muscle protein turnover in pigs selected for low RFI (Cruzen et al., 2013). Because LRFI pigs have greater feed efficiency, they may have fewer resources available to respond to a dual pathogenic challenge. Thus, it was hypothesized that LRFI pigs would demonstrate poorer growth performance, carcass composition, and pork quality in response to that dual challenge than HRFI pigs.

The pigs included in this study are a subset of those included in an investigation of the response of pigs to an MhLI challenge for 42 dpi (Helm et al., 2018). The feed efficiency advantage of LRFI pigs was evident during periods 1 and 2 and is consistent with Helm et al. (2018). This investigation included performance beyond peak infection (period 3). The feed efficiency advantage of the LRFI pigs was not detected during period 3 (dpi 42 to harvest). The feed efficiency of control and MhLI-challenged pigs was not different during periods 1 and 2, but the feed efficiency of MhLI pigs was greater than that of control pigs following peak infection (period 3). A significant MhLI effect (P < 0.05) was detected with greater ADG in the MhLI pigs compared with controls during period 3. This was most evident in the first harvest group (dpi 42 to 118) and within the LRFI pigs in the third harvest group (dpi 42 to 127). It could be proposed that the MhLI-challenged pigs experienced compensatory growth following the MhLi challenge. The LRFI and HRFI pigs responded similarly to the MhLI challenge in terms of growth, indicating that the greater propensity of LRFI pigs to allocate resources toward growth did not hinder their ability to respond to a dual MhLI challenge early in growth. Furthermore, regardless of RFI, pathogen challenges are typically accompanied by reductions in ADG and ADFI, whereas feed efficiency reductions may or may not be observed, depending on the pathogen and severity of the disease (Ciprián et al., 2012; Paradis et al., 2012; Helm et al., 2019).

Therkildsen et al. (2004) reported that greater protein synthesis was observed in pigs that experienced compensatory growth when compared with pigs that did not demonstrate compensatory growth. In contrast, protein degradation was not different in pigs that showed a difference in compensatory growth. Altered protein turnover in vivo, because of compensatory growth, caused a greater extent of postmortem tenderization, due to greater protein degradation (Kristensen et al., 2002), and may (Kristensen et al., 2002) or may not (Therkildsen et al., 2002) ultimately improve pork tenderness compared with pigs that did not exhibit compensatory growth after a period of feed restriction. In this study, chops from the HRFI MhLI pigs had less intact desmin on day 1 postmortem than chops from HRFI control pigs, but the extent of degradation did not significantly differ between the 2 lines. Star probe was not significantly different between pigs that experienced compensatory growth (MhLI) and controls. These results indicate that a pathogenic infection, early in growth, does not ultimately affect meat tenderness or postmortem protein degradation of desmin.

Evidence suggests that selection for improved feed efficiency, through selection for LRFI, does not hinder the ability of pigs to respond to an inflammatory (Merlot et al., 2016) PRRS virus (Dunkelberger et al., 2015), or MhLI challenge (Helm et al. 2018). The greater feed efficiency of LRFI compared with HRFI pigs in this study before and during peak infection is slightly less than previous reports of healthy, never challenged pigs from these 2 lines, wherein, LRFI pigs had 13.6% (Grubbs et al., 2013) to 35% (Harris et al., 2012) greater feed efficiency than HRFI pigs.

Infection with MhLI early in the growth period did not significantly affect the % FFL, LEA, 10th rib back fat, or dressing percentage. In previous experiments, LRFI carcasses had greater loin depth than HRFI carcasses (Smith et al. 2011; Arkfeld et al. 2015). In a commercial feeding system, Arkfeld et al. (2015) demonstrated no difference in fat depth between RFI lines. It is interesting to note the tendency for an effect of infection to decrease dressing percentage, but more data are required to appropriately draw conclusions about this tendency. Dressing percentage was not significantly different between the 2 RFI lines, which is in agreement with dressing percentage comparisons between LRFI and HRFI pigs from the INRA RFI selection experiment (Faure et al., 2013). Helm et al. (2018) did show less lean weight, and less protein at 42 dpi in MhLI pigs compared with control pigs. Infection status did not significantly affect carcass composition (LEA or 10th rib fat depth) of market pigs, which may be because pigs were infected early in the growth period and had time to replenish fat stores that may have been depleted during peak infection.

Both the rate and extent of pH decline in the LM can influence fresh pork loin quality. A lower ultimate pH was correlated with lighter color, greater drip loss, greater cook loss, greater toughness (Kim et al., 2016), and with less juiciness, less flavor, and greater off-flavor (Huff-Lonergan et al., 2002). LM muscle of pigs selected for lean gain efficiency had lower pH values at 15 and 45 min postmortem than those from randomly selected controls, but ultimate pH was not different (Lonergan et al., 2001). Similarly, LM from the more efficient pigs had lower pH values at 15 and 45 min postmortem, but ultimate pH was not different between efficient and control pigs (Lonergan et al., 2001). Like the present study, ultimate pH of the LM was not different between LRFI and HRFI pigs from ISU RFI generations 5, 8, or 9 (Smith et al., 2011; Arkfeld et al., 2015). The ultimate pH for chops from INRA RFI pigs shows that the chops from LRFI pigs consistently had a lower ultimate pH than chops from HRFI pigs (Lefaucheur et al., 2011; Faure et al., 2013). In the present study, early postmortem pH was not significantly different due to RFI line, infections status, or their interaction. Temperature decline was not significantly affected by line or infection status. Therefore, rate of pH or temperature decline was probably not the primary contributors to RFI line differences in color, drip loss, or cook loss reported in this study.

Ultimate pH is not the only quality parameter for which differences between pork from LRFI and HRFI pigs are inconsistent when comparing the INRA and ISU RFI selection lines. Drip loss and cook loss from LM chops in ISU studies were either not different between the ISU RFI lines (Smith et al., 2011) or the LRFI pigs had less drip loss and a tendency for less cook loss in chops (Arkfeld et al., 2015). Results from the prsent study are in alignment with results from previous generations of the ISU RFI lines, as drip loss and cook loss were significantly less in chops from LRFI pigs compared with chops from HRFI pigs. In contrast, for the INRA RFI lines, Lefaucheur et al. (2011) demonstrated greater drip loss of chops from LRFI pigs compared with HRFI pigs, which is consistent with their observations of lower ultimate pH in the pork from the LRFI pigs.

The effect of RFI line on Hunter L, a, and b values in this study showed Hunter L values were lesser, Hunter a values were not different from, and Hunter b values were lesser for chops from LRFI pigs than HRFI pigs. Lightness values of LM chops were not found to be significantly different between RFI line generations 5, 8, or 9 of the ISU RFI lines (Smith et al., 2011; Arkfeld et al., 2015), but INRA studies showed LM chops from generations 4 and 6 of the INRA LRFI line to be lighter than chops from the INRA HRFI line (Lefaucheur et al., 2011; Faure et al., 2013). Hunter a values were not significantly different between the ISU RFI lines (Smith et al., 2011) or were greater in loin chops from the HRFI line (Arkfeld et al., 2015), whereas Hunter a values of loin chops between the INRA lines were either not significantly different (Lefaucheur et al., 2011) or were greater for the LRFI line (Faure et al., 2013).

In the INRA lines, fiber-type composition was evaluated in LRFI and HRFI after 4 generations of selection. Results showed that divergent selection for improved feed efficiency based on RFI caused the fiber-type composition of the LM to have a greater percentage of type IIb, glycolytic fibers for the LRFI than the HRFI line (Lefaucheur et al., 2011). This alteration of muscle fiber-type composition (based on myosin heavy chain isoform) was not detected in generation 5 of the ISU RFI lines (Smith et al., 2011) or in the present study. Therefore, differences in pork quality trait between the ISU and INRA LRFI and HRFI lines could potentially be attributed to differences in fiber-type composition.

Conclusions

Pigs selected for LRFI had greater feed efficiency and the same ADG as the less feed efficient, HRFI pigs, both before and during peak MhLI infection. After peak infection (period 3), the feed efficiency advantage of LRFI pigs was not detected. The MhLI challenged pigs had less ADFI during peak infection (period 2) than the control pigs and experienced compensatory gains during period 3 based on greater ADG and G:F during period 3 compared with controls. Carcass composition and meat quality traits measured in this study were not affected by MhLI infection early in growth, which may be because the timing of the challenge allowed for recovery during the finishing phase.