Responses in growth rate, food conversion efficiency, carcass composition, residual food intake, meat quality and reproduction traits to selection in pigs for four years, for high or low post-weaning growth rate on a fixed but restricted ration were measured in progeny grown on either restricted, ad libitum individual or ad libitum group feeding regimens. Genetic and phenotypic correlations among the traits were also examined on animals whose full pedigrees comprised a total of 5324 animals. Substantial divergent response was achieved from selection for high vs. low growth rate on restricted feeding, with the genetic means of daily gain diverging by 81.25 g or 2.78 genetic standard deviations (SigmaG). The high and low growth rate lines also differed in ultrasonic backfat and food conversion efficiency by 1.12 SigmaG and 0.84 SigmaG, and by 0.88 SigmaG, 1.10 SigmaG, and 1.14 SigmaG for carcass weight daily gain, carcass fat and lean percentage, respectively. These responses were consistent with the moderately to highly, favourable genetic correlations among and between performance and carcass composition traits. Genetic changes in growth rate, ultrasonic backfat, food conversion ratio, carcass daily gain, carcass fat and lean percentage when measured on ad libitum individual feeding were all economically beneficial. The high growth line had a significantly lower voluntary daily food intake and residual food intake than the contemporaneous low line, indicating a reduced energy requirement for maintenance in the high relative to the low line. Genetic correlations between either growth rate or backfat and carcass traits were favourable but unfavourable between daily food intake and carcass fat or lean percentage, implying that selection strategies which place emphasis on increased leanness and decreased fatness would result in a decline in voluntary food intake of the animals. The descendants of breeding pigs selected for high post-weaning daily gain on restricted feeding showed significantly increased growth rates, slaughter weights and carcase lean percentages and reduced fatness when grown in group housing with ad libitum feeding. This suggests that restricted feeding is an effective performance testing approach for seedstock lines supplying commercial regimens with group housing and ad libitum feeding. Carcase lean pH values measured 24 hours post-mortem (pH24) were not statistically different between the selected lines, suggesting that an increase in basal metabolic rate due to increased rate of lean deposition in the high relative to the low line may have been compensated for by a reduction in heat production from other sources such as physical activity. Such a reduction was evidenced by a lower residual food intake in the high than the low growth lines. This supports the hypothesis that selection for high growth rate on restricted feeding does not have detrimental effects on pork quality. The sows of the high growth line produced more piglets with significantly heavier weights at birth, giving a potential for better post-natal growth and higher yield of pork weaned per sow per year than the low growth sows. Body weights of the high line sows prior to mating were also significantly higher than those of low line sows suggesting earlier age of sexual maturity. A non-significant tendency for high line sows to have a lower voluntary food intake than low line sows pointed to the possibility that their lower maintenance requirement as growers was carried over to the lactational phase of life. It is concluded that selection for high growth rate on a restricted ration of fixed amount will produce pig strains with a high genetic potential for lean growth, reduced energy requirement for maintenance and improved reproductive performance, and which will be suitable for growing under a wide range of commercial feeding environments.
| Identifer | oai:union.ndltd.org:ADTP/291105 |
| Creators | Nguyen Hong Nguyen, Nguyen |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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