C.P. McPhee

The effects of selection for lean growth and the halothane allele on carcass and meat quality of pigs transported long and short distances to slaughter

Carcass and lean quality traits were studied in two genetic lines of pigs in which the halothane allele n was segregating with the normal allele N. One line had been selected for high growth rate and low backfat depth and the other line maintained as an unselected control. Pigs of the three segregants NN, Nn and nn from both lines were grown at two locations, one near the abattoir (1 h by road) and the other far (10 h) from the same abattoir. Pigs from both locations were fasted for 20 h before slaughter. Relative to control line pigs, those from the selected line had lower backfat depths, greater ham weights, eye muscle areas and carcass lengths but lower dressing percentages. The n allele had little effect on carcass traits but for lean quality traits, the n allele acted additively to increase acidity, paleness, water lost through drip, centrifugation and cooking and to reduce cured yield. Independently of the n allele, selection for lean growth increased the incidence of dark, firm and dry (DFD) pork. This was highest in carcasses of NN pigs from the selected line grown far from the abattoir. nn pigs from the control line grown near the abattoir produced the most pale, soft, exudative (PSE) pork.

Genetic parameters and responses of performance and body composition traits in pigs selected for high and low growth rate on a fixed ration over a set time

Two lines of Large White pigs of common genetic origin were divergently selected over four years for high and low growth rate during a 6 week post-weaning test period in which all pigs were fed the same total amount of food (80% of estimated ad libitum intake). Genetic parameters and direct and correlated responses in performance and carcass traits were estimated on 2884 pigs with pedigrees comprising a total of 5324 animals, with restricted maximum likelihood and best linear unbiased prediction methods applied to a multi-trait animal model. Estimates of heritability (± SE) were 0.19 ± 0.04 for lifetime daily gain (LDG), 0.16 ± 0.03 for test daily gain (TDG), 0.25 ± 0.04 for ultrasound P2 backfat (UBF) and 0.16 ± 0.03 for food conversion ratio during test (TFC), and 0.15 ± 0.04 for daily carcass weight gain (CDG), 0.43 ± 0.06 for carcass backfat (CFT) and 0.40 ± 0.06 for carcass lean percentage (LEAN). Common litter effects for TDG, UBF and TFC were less than 5% and for LDG, 17% of total phenotypic variance. Genetic correlations between performance and carcass traits were moderately to highly favourable. After four years of divergent selection for growth rate, the selection responses in estimated breeding value (EBV) for TDG were 40.14 and -41.11 g (SED 2.95) for the high and low growth lines, respectively. The regressions of EBV on year of birth, indicate that the annual genetic trend for TDG, was 8.73 g/yr in the high and -8.48 g/yr in the low lines (P < 0.001). Correlated genetic responses in the high and low lines respectively were 5.28 g and -12.40 g (SED 2.09) in LDG, -0.35 mm and 0.56 mm (SED 0.009) in UBF, -0.145 units and 0.185 units (SED 0.012) in TFC, 3.17 g and -10.97 g (SED 1.53) in CDG, -1.13 mm and 1.01 mm (SED 0.155) in CFT and 1.24% and -1.27% (SED 0.150) in LEAN. It was concluded that selection for increased post-weaning daily gain on a ration of fixed amount reduces the age at slaughter and the level of backfat and increases the efficiency of food utilisation, weight and leanness of pig carcasses.

The effects of selection for lean growth and the halothane allele on growth performance and mortality of pigs in a tropical environment

The effects of the halothane allele and other genes for rapid lean growth were examined in pigs grown in an environment of high temperature/transport stress. The halothane allele n was segregating with its normal alternative N in a line which had been selected for rapid lean growth and in an unselected control line. Pigs of the three halothane segregants, NN, Nn and nn from both lines were grown on ad lib. feeding from 25 to 90 kg liveweight. Relative to the control, the selected line had faster, more efficient growth with lower 90 kg backfat thickness. Least square means in the selected and control lines were 0.87 and 0.77 ± 0.01 kg for daily liveweight gain, 2.63 and 2.98 ± 0.03 for food conversion ratio and 13.1 and 18.7 ± 0.3 mm for backfat. The halothane allele reduced appetite, growth rate, food conversion ratio and backfat. Means of the NN, Nn and nn segregants were 2.46, 2.31 and 2.10 ± 0.04 kg for daily food intake, 0.86, 0.84 and 0.78 ± 0.01 kg for daily liveweight gain, 2.89, 2.77 and 2.74 ± 0.03 for food conversion ratio and 16.7, 15.8 and 15.1 ± 0.4 mm for backfat. The average effect of the n allele on food intake and growth rate was higher in the selected than the control line. Mortality rates were increased by the halothane allele. Its effect on mortality was greatest in the selected line in summer while pigs were on road to the abattoir. Mean mortality rates were 1.4 ± 0.4, 2.6 ± 0.4 and 13.7 ± 1.1% for the NN, Nn and nn segregants.

The effect of selection for rapid lean growth on the dietary lysine and energy requirements of pigs fed to scale

A line of pigs (S line) selected for weight of ham lean, a measure of lean growth, was compared with an unselected control line (C line) of common origin on a series of food regimens ranging in average daily intake from 23.7 to 27.2 MJ digestible energy and from 13.3 to 23.4 g total lysine. The comparison was made over a 12-week test period starting at 25 kg liveweight and measurements were made of growth rate, fat depth by ultrasonics and, from these, predicted weight of lean in the ham at the end of test. As energy and lysine in the diet were increased, growth rate and ham lean rose at rates and reached limits which were higher in the S than the C line. As a result of 4.4 standard deviations (SD) of selection differential accumulated over five generations of selection, the superiority of the S over the C line in ham lean ranged from 0.5 (SD) on a low energy-lysine diet to 2.7 SD on a high energy-lysine diet. Maximum growth rate and ham lean were reached in the S line on a diet which provided 1 MJ day-1 more digestible energy and 3 g day-1 more total lysine than the diet at which the maxima were reached in the C line. Increasing dietary energy raised fat depths in the C line and increasing lysine lowered fat depths in the S line. Pigs from both lines were most profitable on diets lower in energy and lysine levels than those which gave maximum growth. Net monetary returns were most responsive to changes in energy in the C line and to changes in lysine in the S line.

A comparison of the effects of porcine somatotropin, genetic selection and sex on performance, carcase and meat quality traits of pigs fed ad libitum

A comparison was made of the effect of recombinant porcine somatotropin (PST) and genetic selection on performance, carcase and meat quality traits of entire male and female pigs grown from 50 kg to 90 kg liveweight and fed ad libitum. Pigs of each sex were drawn from a selected and an unselected control line of common genetic origin and subjected to daily intramuscular injections of either 90 g PST or saline per kg body weight. The selected line had undergone a period of selection for low backfat (P2) depth and high growth rate. The effects of PST and selection were additive for all traits. For performance traits, growth rate was increased 17% by PST and 22% by selection, food conversion ratio was reduced 20% by PST and 14% by selection. For carcase traits P2 fat depth was reduced 15% by PST and 14% by selection. Both PST and selection caused a 1.3% reduction in killing out. For chemical composition of soft tissue, fat was reduced 9.1% by PST and 2.4% by selection, water was increased 6.9% by PST and 2.2% by selection, and protein was increased 2.1% by PST. For lean quality traits, PST had the slightly adverse effect of increasing paleness, cooking loss and firmness of certain muscles, particularly in males, but selection had no adverse effect. The changes brought about by PST could be accounted for by the repartitioning of metabolisable energy away from fat and toward protein whereas both repartitioning and increased appetite accounted for the effects of selection.