Animals and design – gilts

Gilts (Large White×Landrace) were selected as replacement breeding stock at birth on a commercial breeding company’s (Hermitage AI, Sion Road, Kilkenny, Ireland) multiplier farm and were exposed to the same housing and feeding regime up to 22 weeks of age. At this time backfat depth of gilts was measured at the last rib and 65 mm from the back bone using an ultrasound scanner (Lean-meater, Renco Corporation, Minneapolis, Minnesota, USA) on both the left and right side and the mean value was recorded.

One hundred and forty gilts were selected based on their backfat depth at 22 weeks of age and Thin (n=68; P2 backfat ~8 mm) and Fat (n=72; P2 backfat ~12 mm) groups were formed. The difference in backfat depth between groups was accentuated by feeding different diets up to service at 32 weeks. The Thin gilts were restricted to 1.8kg/day of gestation diet (Table 1) and the Fat gilts were provided with ad libitum access to a gilt developer diet (Table 1) up to 2 weeks before service. All gilts were provided with ad libitum access to a lactation diet (Table 1) for flushing for 2 weeks before insemination (30 to 32 weeks). To allow each gilt to exhibit at least one standing oestrus before insemination, they were provided with constant boar contact from an adjoining pen for 4 weeks before planned mating. Nine mililitres of Regumate Equine (altrenogest, 2.2 μg/ml; Intervet Productions S.A., France), was added to the diet of each gilt on each day for 6 days before planned insemination to synchronise oestrus. Gilts were artificially inseminated at onset of standing oestrus and again 24 h later using semen pooled from eight closely related Hylean Large White boars (Hermitage AI, Sion Road, Kilkenny). At Insemination, backfat depth was 12±0.6 mm and 19±0.6 mm for the Thin and Fat gilts, respectively.

Table 1

The composition and chemical analysis of experimental diets (on air dry basis g/kg as fed)

Gilt developer, gestation diet and lactation diet provided (mg/kg completed diet): Cu, 38; Fe, 70; Mn, 62; Zn, 80; I, 0.6; Se, 0.2; vitamin A, 10 000 IU; vitamin D₃, 1000 IU; vitamin E, 100 IU; vitamin K, 2 mg; vitamin B₁₂, 15 µg; riboflavin, 5 mg; nicotinic acid, 12 mg; pantothenic acid, 10 mg; choline chloride, 500 mg; Vitamin B1, 2 mg, Biotin, 200 µg; Folic acid, 5 mg; thiamin, 2 mg and pyridoxine, 3 mg.

Weaner diet provided (mg/kg completed diet): Cu, 155; Fe, 90; Mn, 47; Zn, 120; I, 0.6; Se, 0.3; vitamin A, 6,000 IU; vitamin D3, 1000 IU; vitamin E, 100 IU; vitamin K, 4 mg; vitamin B₁₂, 15 µg; riboflavin, 2 mg; nicotinic acid, 12 mg; pantothenic acid, 10 mg; choline chloride, 250 mg; vitamin B₁, 2 mg; vitamin B₆, 3 mg; and endox, 60 mg.

Finisher diet provided (mg/kg completed diet): Cu, 40; Fe, 40; Mn, 31; Zn, 80; iodine, 0.3; Se, 0.2; vitamin A, 2,000 IU; vitamin D3, 500 IU; vitamin E, 40 IU; vitamin K, 4 mg; vitamin B₁₂, 15 µg; riboflavin, 2 mg; nicotinic acid, 12 mg; pantothenic acid, 10 mg; vitamin B₁, 2 mg; vitamin B₆, 3 mg.

^* Sow diets contained 500 FTU phytase per kg finished feed from Natuphos 5000 (BASF, Ludwigshafen, Germany).

Immediately after insemination, pregnant gilts were moved to individual gestation pens (2.4 m × 0.6 m; O’Donovan Engineering, Coachford, Ireland) and fed once per day 1.8 kg/day (23.4 MJ DE/day) of a gestation diet (Table 1) until day 25 of gestation. Gilts that repeated to first service were removed from the experiment. On day 25 of gestation, pregnant gilts from each body condition group (Thin or Fat) were blocked according to weight and expected farrowing date and allocated at random to one of three feeding allowances of the gestation diet: (a) Restricted (1.8 kg/day), (b) Control (2.5 kg/day) or (c) High feed level (3.5 kg/day), until day 90 of gestation. The experiment was a 2×3 factorial design with 2 factors for gilt backfat and 3 factors for gestation feed allowance giving a total of six treatment groups; Thin Restricted, Thin Control, Thin High feed level, Fat Restricted, Fat Control and Fat High feed level. From days 90 to 110 of gestation, all gilts were fed 2.5 kg/day of the gestation diet.

At day 110 of gestation, gilts were moved to individual farrowing pens and were liquid-fed (Big Dutchman, Vechta, Germany) 2.03 kg/day of a lactation diet (28.8 MJ DE/day) in three daily meals (morning, noon and evening). The backfat depth at farrowing was 14.4±0.34 mm for Thin gilts and 19.0±0.34 mm for Fat gilts (Amdi et al., Reference Amdi, Giblin, Hennessy, Ryan, Stanton, Stickland and Lawlor2013). Gilts were housed individually in national pig development company type farrowing crates (O’Donovan Engineering) with hinged bottom bars in farrowing rooms with 10 gilts per room. After farrowing, gilts were fed the lactation diet according to a feeding curve, which provided 28.8 MJ DE/day (2.03 kg/day) at day 0 of lactation increasing to 112.7 MJ DE/day (7.94 kg/day) on day 28 of lactation. Access to water was available on an ad libitum basis throughout the experiment and temperature was maintained at 20^oC, except for 48 h after farrowing when temperature was increased to 24^oC.

Litter information

At farrowing, litter weights, total born, born alive, stillbirths, mummified pigs and individual birth weights of all piglets were recorded. Piglets were tagged at birth for identification purposes. Litter size was standardised to 12±2 pigs per litter by cross-fostering within treatment groups within 24 h of birth. Creep feed (16.5 MJ/DE, lysine 1.6%, Startrite 88; Nutec, Monread Road, Naas, Co Kildare, Ireland) was fed to all litters from day 12 postpartum until weaning at day 28 postpartum. Pre-weaning mortality and the number of pigs weaned per litter were recorded.

Piglets were categorised on birth weight as low (LBW; <1.2 kg), medium (MBW; ~1.5 kg) and heavy (HBW; >1.7 kg). All pigs were weighed at weaning on day 28 of lactation. One hundred and fifty-five pigs were sacrificed at weaning and their organs weighed. The m. semitendinosus was fully removed from the right side of the piglet and weighed, as were the liver, the kidneys and the heart. All organs were trimmed of excess fat, cut open, rinsed and blotted dry before weighing.

Three same gender pigs (LBW, MBW and HBW) were selected within litter at weaning, with litter representation balanced for the six gestation treatments so that in total 270 pigs were allocated to individual pens to be followed through to slaughter at day 130 post-weaning. From weaning, these pigs were individually fed 2 kg of commercial starter diet (16.25 MJ DE/kg; lysine 1.6%; Easistart, Devensh, NI), followed by 5 kg of a commercial link diet (15.4 MJ DE/ kg; lysine 1.5%; Vital, Devenish, NI) after which a weaner diet (Table 1) was fed to day 49 post-weaning. A finisher diet was fed from day 49 post-weaning to slaughter (Table 1). At all times feed was available on an ad libitum basis. Pig weights were recorded at birth, weaning, days 14, 28, 49, 91 post-weaning and at slaughter. Feed disappearance was measured between days 0 (weaning) and 14, 14 and 28, 28 and 49, 49 and 91 and between 91 and 130 post-weaning. Average daily gain (ADG) and average daily feed intake (ADFI) were calculated for each of the growth stages above and feed conversion efficiency (FCE) was calculated as ADFI/ADG. Weighing took place between 1000 and 1300 h and feed was available right up to weighing except in the case of day 130 post-weaning when pigs were fasted for 12 h before weighing.

Diets and feed analyses

The ingredient content and chemical composition of the gilt developer, gestation, lactation, weaner and finisher diets are shown in Table 1. Representative samples of each diet type were collected and stored at −21°C until analysis. Dry matter, crude ash, CP, crude fat and crude fibre were analysed by the methods described previously (Lawlor et al., Reference Lawlor, Lynch, Caffrey and OD2002).

Housing

During the weaner stage (days 0 to 49 post-weaning), pigs were housed individually in fully slatted pens (1.2 m × 0.9 m). Slats were moulded plastic (Faroex, Manitoba, Canada), and pen divisions were of plastic panelling. Each pen had a single stainless steel feeder 30 cm wide and water was available ad libitum from bowls (BALP, Charleville-Mezieres, Cedex, France). Temperatures were maintained at 28°C to 30°C in the 1st week and reduced by 2°C per week to 22°C in the 4th week post-weaning.

During the finisher stage (days 49 to 130 post-weaning), pigs were individually housed in fully slatted pens (concrete slats, 75 mm solid width 20 mm slots), 1.81 m × 1.18 m with plastic partitions. Air temperature was maintained at 20°C to 22°C. Feeders were stainless steel dry feed hoppers 30 cm in width (O’Donovan Engineering, Coachford, Co. Cork, Ireland). Water was available ad libitum from one drinking bowl (BALP, Charleville-Mezieres, Cedex, France) per pen.

Blood sampling

For all blood sampling, pigs were restrained appropriately and blood was collected by jugular venipuncture into BD Vacutainer^® serum (BD, Franklin Lakes, NJ, USA). At day 80 of gestation, gilts were blood sampled before feeding for IGF-1 measurements. At weaning, piglets were also blood sampled for IGF-1 measurements and pigs that were followed through to slaughter at 158 days of age were blood sampled (before feeding) at day 49 post-weaning for cortisol measurements and at slaughter (during exsanguination after CO₂ stunning) for IGF-1 measurements. Blood was allowed to clot for 1 h at room temperature and serum was separated by centrifugation at 2500 × g for 20 min at 15°C. All samples were then stored at −20°C until analysed. Serum IGF-1 concentrations were measured using a human IGF-I ELISA kit (BioVendor R&D, Brno, Czech Republic) which was IGFBP blocked. Samples were performed in duplicate and intra-assay variation was 2.5%. Inter-assay variation was 9.4% and was calculated using standards, and high and low controls from each plate. Serum cortisol concentrations were outsourced and quantified, in duplicate, by Core Biochemical Assay Laboratory at the Clinical Biochemistry Department at Cambridge Biomedical Campus after validation of method (linearity of dilution and spike-recovery). The average intra-assay coefficient of variability (CV) for the cortisol was 5.6% and inter-assay CV was 6.9%.

Carcass composition

Pigs were fasted for 12 to 16 h before slaughter. On the day of slaughter, pigs were mixed, transported 90 km and slaughtered by exsanguination after CO₂ stunning. Following evisceration, muscle depth and backfat thickness were measured using a Hennessy grading Probe (Hennessy and Chong, Auckland, New Zealand) 6 cm from the edge of the split back at the level of the third and fourth last rib. Lean content was estimated according to the following formula (Department of Agriculture and Food (Ireland), 2001): Estimated lean meat content (%)=60.30–0.847x+0.147y, where x=backfat depth (mm); y=muscle depth (mm). Carcass weight (cold) was estimated as the weight of the hot eviscerated carcass (minus tongue, bristles, genital organs, kidneys, flare fat and diaphragm) 45 mins after harvest×0.98. Dressing percentage was calculated as (carcass weight/live weight prior to slaughter)×100.

Statistical analyses

Data were analysed using the mixed models procedures of SAS (SAS Inc., Cary, NC, USA). The model used to examine the effect of body condition and feed allowance of gilts during gestation was as follows:

$$\eqalign{Y_{{ijklm}} \,=\,\mu \,{\plus}\,\alpha _{i} {\plus}\beta _{j} {\plus}\left( {\alpha \beta } \right)_{{ij}} {\plus}\gamma_{k} {\plus}\delta _{{\rm l}} {\plus}\zeta _{m} {\plus}\eta _{n} {\plus}\theta _{{isj}} \cr \quad{\plus}\lambda _{{psj}} {\plus}{\varepsilon}_{{ijklm}} $$

where Y _ijklm is the dependent variable measured on the pig (weight, ADG, ADFI, FCE, carcass composition, organs, IGF-1, cortisol), μ denotes the overall mean, α _i denotes the effect of body condition (i=Thin, Fat), β _j is the effect of feed level (j=Restricted, Control, High feed allowance), γ _k is the effect of gender of piglet (k=male, female), δ _l is the effect of birth weight (l=LBW, MBW, HBW), ζ _m is the effect of block (m=1,2, ..., 17), covariate η _n is the effect of the litter size (n=6,7, ..., 18), (αβ) _ij is the interaction effect between body condition and feed level, θ _isj is the random effect of sow and λ _psj is the random effect of piglet and ε _ijklm describes the error term. If the interaction was non-significant it was removed from the model. The number of piglets born alive and block were included in the model as covariates for the analysis of production data, but not for analyses of IGF-1 concentrations or cortisol concentrations in piglets. Random effects included gilt nested within treatment (feed level). For the analysis of the organ weights at weaning, weaning weights were included in the model as a covariate. Where variables of interest were recorded at multiple time points, day of sampling was regarded as the repeated variable in the analysis.

To analyse maternal IGF-1 concentrations at day 80, the following model was used:

$$Y_{{ij}} =\mu {\plus}\alpha _{i} {\plus}\beta _{j} {\plus}(\alpha \beta )_{{ij}} {\plus}{\varepsilon}_{{ij}} $$

where Y _ij is the dependent variable measured on the gilt (IGF-1), μ denotes the overall mean, α _i denotes the effect of body condition (i=Thin, Fat), β _j is the effect of feed level (j=Restricted, Control, High feed allowance), (αβ) _ij is the interaction effect between body condition and feed allowance, and ε _ij describes the random error.

The Bonferroni correction was used to adjust for pairwise comparison of means. For all data, results were considered statistically significant when P<0.05 and as trends when 0.05<P⩽0.10. Values above P>0.10 were reported as non-significant. The results were reported as the least square means (LSMeans) with standard errors of the means (s.e.m.).