Data from the F2 generation of a Large White (LW) ✕ Meishan (MS) crossbred population were analysed to detect quantitative trait loci (QTL) for leg and gait scores, osteochondrosis and physis scores. Legs, feet and gait score were assessed in 308 F2 animals at 85 ( + 5) kg and osteochondrosis and physis scores were recorded for the right foreleg after slaughter. A genome scan was performed using 111 genetic markers chosen to span the genome that were genotyped on the F2 animals and their F1 parents and purebred grandparents. A QTL on chromosome 1 affecting gait score was significant at the genome-wide significance level. Additional QTL significant at the chromosome-wide 5% threshold level (approx. equivalent to the genome-wide suggestive level) were detected on chromosome 1 for front feet and back legs scores, on chromosome 13 for front legs and front feet scores, on chromosome 14 for front legs, front feet and back legs scores and on chromosome 15 for back feet score. None of the QTL for osteochondrosis score exceeded the chromosome-wide suggestive level, but one chromosome-wide QTL for physis score was found on chromosome 7. On chromosome 1, gait and front feet scores mapped to the middle of the chromosome and showed additive effects in favour of the LW alleles and no dominance effects. The QTL for back legs score mapped to the distal end of the chromosome and showed a dominant effect and no additive effect. On chromosomes 14 and 15, the LW allele was again superior to the MS allele. On chromosome 13, there were both additive and dominance effects in favour of the MS allele. The MS alleles on chromosome 13 may have potential for introgression into a commercial LW population. The other putative QTLs identified may have value in marker-assisted selection in LW or MS-synthetic populations.

Lactational performance was measured in Large White gilts from lines that had been divergently selected over seven generations for daily food intake (DFI), lean food conversion ratio (LFC), lean growth rate with ad-libitum (LGA) or restricted (LGS) feeding during performance test. Control line gilts were included in the study. During the lactation period of 21 days, gilts were given to appetite five isoenergetic diets differing in ileal digestible lysine: energy (0·40, 0·58, 0·76, 0·94 and 1·12 g/MJ digestible energy). The study consisted of 223 gilts with a similar number of animals in each selection line.Live-weight loss was greater in the low LFC and LGA lines than in the high lines and food intake was significantly lower in the low LGA line than in the high line. Litter-weight gain of the low LFC and high LGA lines were greater than in the complementary selection lines. Prediction equations for nutrient utilization were used to express the effect of diet and selection line in terms of energy and protein inputs and outputs. Selection on DFI or LGS resulted in gilts that did not mobilize lipid during lactation as sufficient energy for milk and maintenance was provided by dietary intake. In contrast, there was insufficient dietary energy with selection on LFC or on low LGA so lipid mobilization was required to achieve energy balance. The energy required to excrete excess protein and energy from lipid mobilization increased as the dietary lysine energy ratio increased, but there were no other dietary effects on energy and protein utilization. Genotype with nutrition interactions were detected for energy intake and lipid mobilization, which were due to the lines selected for low DFI and LGA. The general absence of genotype with nutrition interactions for lactational performance in gilts selected for components of efficient lean growth and the lack of significant dietary effects on energy utilization indicated that the consequences of changing nutritional inputs will be broadly similar for genotypes within the set of genotypes studied. Selection strategies which result in reduced food intake during lactation should be avoided if lipid mobilization is then required to attain energy balance and there are resultant negative effects on subsequent reproductive performance.

Ages at first behavioural oestrus and at elevated plasma progesterone concentration were measured in three selection groups, after seven generations of divergent selection for lean growth rate (LGA), lean food conversion (LFC) and daily food intake (DFI) in a population of Large White pigs. First physiological oestrus was defined to have occurred when a blood plasma progesterone concentration of at least 1 μg/l was detected from weekly sampling of gilts. The study consisted of 146 gilts, which were given 0·75 , 0·81 , 0·88 , 0·94 or 1·0 g/g of daily ad-libitum food intake during performance test and then 1.9, 2.05, 2.2, 2.35 or 2.5 kg/day, respectively, until conception, to determine if there were differences between selection lines in their sensitivity to changes in nutritional inputs.Responses in oestrus and performance test traits were dependent on selection group. First physiological oestrus was later with selection for high LFC than for low LFC (234 v. 215, s.e.d. 9.1 days) but there was no significant response within each of the LG A (224 v. 226 days) and DFI (218 v. 206 days) selection groups. The probability of exhibiting oestrous behaviour signs at first physiological oestrus was significantly lower in the high LG A line (0·62 v. 0·93 or 0·5 v. 2.5, s.e.d. 0·75 on the logit scale) than in the low line but there were no responses in the LFC and DFI groups. For animals exhibiting oestrous behaviour signs at first physiological oestrus, there were no significant responses in oestrous behaviour score for the three selection groups. Live weight at first physiological oestrus in the LFC and LG A selection groups was greater in the high lines than in the low lines (120 v. 109 and 123 v. 112, s.e.d. 4.3 kg) but not in the DFI selection group (116 v. 111 kg). Responses in ultrasonic backfat (-7.3, -8.2 and 5.0, s.e.d. 1.5 mm) and muscle depth (4.9, 6.1 and -3.5, s.e.d. 1.4 mm) at first physiological oestrus were of similar magnitude in the LGA, LFC and DFI selection groups.Increasing the ration (amount of food offered) did not have a linear effect on performance test traits and reproductive development, such that ration had to be included in the model as a fixed effect, rather than a covariate. There was no significant effect of ration or of selection line with ration interaction for traits associated with first oestrus.Selection for lean growth rate had no adverse effect on reproductive development, unlike selection for lean food conversion. Detection of first oestrus with oestrous behaviour signs combined with physiological assessment may be required in genotypes selected exclusively for lean growth rate, rather than relying only on observed behavioural signs of oestrus.Ages at first behavioural oestrus and at elevated plasma progesterone concentration were measured in three selection groups, after seven generations of divergent selection for lean growth rate (LGA), lean food conversion (LFC) and daily food intake (DFI) in a population of Large White pigs. First physiological oestrus was defined to have occurred when a blood plasma progesterone concentration of at least 1 μg/l was detected from weekly sampling of gilts. The study consisted of 146 gilts, which were given 0·75 , 0·81 , 0·88 , 0·94 or 1·0 g/g of daily ad-libitum food intake during performance test and then 1.9, 2.05, 2.2, 2.35 or 2.5 kg/day, respectively, until conception, to determine if there were differences between selection lines in their sensitivity to changes in nutritional inputs.Responses in oestrus and performance test traits were dependent on selection group. First physiological oestrus was later with selection for high LFC than for low LFC (234 v. 215, s.e.d. 9.1 days) but there was no significant response within each of the LG A (224 v. 226 days) and DFI (218 v. 206 days) selection groups. The probability of exhibiting oestrous behaviour signs at first physiological oestrus was significantly lower in the high LG A line (0·62 v. 0·93 or 0·5 v. 2.5, s.e.d. 0·75 on the logit scale) than in the low line but there were no responses in the LFC and DFI groups. For animals exhibiting oestrous behaviour signs at first physiological oestrus, there were no significant responses in oestrous behaviour score for the three selection groups. Live weight at first physiological oestrus in the LFC and LG A selection groups was greater in the high lines than in the low lines (120 v. 109 and 123 v. 112, s.e.d. 4.3 kg) but not in the DFI selection group (116 v. 111 kg). Responses in ultrasonic backfat (-7.3, -8.2 and 5.0, s.e.d. 1.5 mm) and muscle depth (4.9, 6.1 and -3.5, s.e.d. 1.4 mm) at first physiological oestrus were of similar magnitude in the LGA, LFC and DFI selection groups. Increasing the ration (amount of food offered) did not have a linear effect on performance test traits and reproductive development, such that ration had to be included in the model as a fixed effect, rather than a covariate. There was no significant effect of ration or of selection line with ration interaction for traits associated with first oestrus. Selection for lean growth rate had no adverse effect on reproductive development, unlike selection for lean food conversion. Detection of first oestrus with oestrous behaviour signs combined with physiological assessment may be required in genotypes selected exclusively for lean growth rate, rather than relying only on observed behavioural signs of oestrus.

The sensitivity to dietary lysine: energy content of pigs from lines divergently selected over seven generations for components of efficient lean growth rate was quantified by the within-selection line regression of performance test traits on diet class. Large White pigs were performance tested over three 14-day test-periods starting at 30, 50 and 75 kg and offered, to appetite, isoenergetic diets differing in dietary total lysine: energy (0•59, 0•69, 0•81, 0•91, 1•01, 1•12 and 1•23 g/MJ digestible energy (DE)). Within each litter, full-sibs were performance tested on different diets. Pigs were also performance tested on a diet-choice procedure using diets with total lysine: energy of 0•69 and 1•12 g/MJ DE to determine the correlation between performance on phase-feeding and diet-choice.The study consisted of 578 animals with 459 pigs tested with phase-feeding and 119 tested on diet-choice procedures. The study detected significant selection line and dietary effects on performance test traits, but no significant between-selection line differences in sensitivity to dietary total lysine: energy. When combinations of performance test traits were transformed into predicted lysine and energy utilization traits there were significant selection line effects on predicted nutrient allocation, but not in responses to increasing dietary total lysine: energy. The lack of between-selection line differences in sensitivity to dietary total lysine: energy indicated that the consequences of changing dietary total lysine: energy will be broadly similar across a range of genotypes, as spanned by the selection lines of the study.Genetic correlations between performance traits of phase-fed and diet-choice pigs suggested that predictions of genetic merit will be similar with animals tested on either procedure between 30 and 75 kg but post 75 kg predicted genetic merit for growth rate and lysine intake should be estimated separately for performance on diet-choice or for performance on a given diet. In contrast, predicted genetic merit for fat deposition with performance testing on diet-choice will be highly correlated with predicted genetic merit with testing on a single diet.

Nutrition models of energy and protein utilisation have been developed for lactating pigs but there is little or no evidence in the scientific literature of the validation of such models. If there are systematic discrepancies in such models, then inappropriate supply of nutrients will either be an inefficient use of resources with oversupply or have an impact on piglet growth and subsequent reproductive performance of the sow with undersupply. In the current study, lactating gilts were fed isoenergetic diets differing in ileal digestible lysine: energy. Protein and energy utilisation were predicted using existing nutrition equations. The validity of the energy and protein utilisation equations was tested by determining if the predicted energy balance differed significantly from zero or if the predicted protein balance was not significantly less than zero.