Effectiveness of Selection for Efficiency of Gain in Duroc Swine

Abstract

Results are presented from selecting for high and for low individual feed requirements per pound of gain in two strains of Duroc swine for five generations, beginning with a common foundation stock in 1937. All pigs from each litter were full-fed a mixed ration in individual pens with concret floors, from 72 days of age to a live weight of 225 pounds. To minimize inbreeding, one boar and gilt were usually selected from each of the 8 litters in each strain every year, and sib matings were avoided. Thus, selection was based largely on differences between litter mates. These differences were found to be about 24² percent heritable, judging by the amount of selection actually practiced and the resulting mean difference between unselected progeny of the two strains. Heritability of differences between non-sibs was estimated from regression of progeny on mean of the parents within strain and year. In percent, the estimates were: 26³ for feed requirements, 43² for daily gain and 47² for length of feeding period, from 72 days to a live weight of 225 pounds; 9 for weight at 72 days and −23 for weight at birth. Regression of progeny on sire and dam, separately, revealed that the lower heritability of feed requirements relative to daily gain was due to a much stronger negative correlation between the dam's transmitted and environmental influences on the feed requirements than on the growth rates of her pigs. Such a negative correlation for prenatal growth rate may explain why heritability of birth weight was significantly negative among sires, but positive among dams, since the sires that were heavier at birth would be from dams which provided good uterine nutrition but transmitted a slower growth impulse. Regressions of one character in the progeny on another character in the parents indicated that the same genetic factors tend to increase 72-day weight and the rate and efficiency of gain thereafter. They also provided further evidence for incompatability of good suckling ability with inherently lower feed requirements and less noticeably, with inherently rapid growth. The correlation between litter mates, among progeny of the same strain and year of dam and progeny, was .23² for feed requirement, .37² for daily gain, .42² for length of feeding period, .40² for 72-day weight and .36² for birth weight. The lower litter mate correlation for feed requirements is attributed to the opposite effects of the dam's transmitted and nutritional influences. Similarity in maternal environment was largely responsible for the large litter mate correlations for weight at birth and at 72 days, whereas likeness in genetic constitution accounted for more of the litter mate correlations for growth rate and feed requirement after 72 days. The best estimates obtained for genetic correlation were those for the combined sire and dam influence as follows: feed requirement was correlated −.78² with daily gain, .83² with feeding period and −.54² with 72-day weight; daily gain was correlated −.96² with feeding period and .65² with 72-day weight; and feeding period was correlated −.77² with 72-day weight. These are generally higher than the corresponding total correlations of −.66², .67², −.19², −.95², .44² and −.60² respectively. Evidently,hereditary and maternal influences on performance were more constant throughout the growing period than intralitter environmental influences. These results indicate that selection based on rate of grain from weaning to market weight would be nearly as effective in improving economy of gain as selection based directly on individual feed requirements. The genetic factors that lower feed requirements, without increasing rate of gain, also apparently impair the suckling ability of sows. This indirectly increases post-weaning feed requirements enough to offset the extra reduction in the pig's own inherent feed requirements. Selection based on individual and litter weights at weaning would be moderately effective in improving the suckling ability of sows but relatively ineffective in increasing the rate and efficiency of post-weaning gains. Copyright © . .