Some genetic and environmental factors affecting performance and carcass measurements in swine Public Deposited


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  • Data from 592 litters of pigs farrowed and raised at the Oregon Agricultural Experiment Station at Corvallis from the spring of 1961 through the spring of 1968 were analyzed for effects of year and season of birth, breed, sex, inbreeding and age of dam on measures of fecundity, growth rate and carcass merit. Hierarchal analysis of variance was done to find the effects of population structure and inbreeding on the components of phenotypic variance and heritability for measures of litter size and average daily gain. The Oregon State University swine herd is composed of a partly closed line of Berkshires, a partly closed herd of Yorkshires, reciprocal crosses between these two breeds and a line developed from inter-se mating of F₁ and later generations. Significant differences among the various breed groups were found for number of pigs born alive, number born alive plus dead, number born plus mummified fetuses, number weaned per litter, average daily gain, carcass length, average backfat thickness, loin eye area and ham weight. Differences among breeds were not significant for loin weight. The level of performance did not increase, over the period studied, for any of the traits studied except loin eye area. The lack of increase in level of performance may be due to a lack of sufficient selection pressure, detrimental environmental effects or the depression in performance due to inbreeding. Fall-farrowed pigs had lower average daily gains than spring-farrowed pigs but, in general, had more desirable carcasses. The differences in performance and carcass traits are attributed to the smaller proportion of males performance tested from fall-farrowed litters. Males gained 0.096 pounds per day faster than females but carcasses were 0.264 inches shorter, had 0.067 inches more average backfat, had ham and loin weights that were 0.798 and 0.516 pounds less respectively and had loin eye areas that were 0.382 square inches smaller. Negative regressions for the effects of inbreeding of the dam on litter size at birth and weaning and positive regressions for the effect of inbreeding on litter size at birth were found but most effects were not statistically significant. The number of mummified fetuses and stillbirths was not affected by inbreeding but mortality between birth and weaning increased significantly when inbreeding of the dam rose above a threshold of ten percent. Both linear and quadratic components of age of dam had highly significant effects on the litter size at birth and on the number of pigs weaned with a peak in sow productivity occurring at about three years of age. Age of dam effects were thought to be due almost entirely to differences in ovulation rate or early embryo survival as the number of mummified fetuses, stillbirths or pre-weaning deaths were not related to age of dam. Environmental variation of litter size and average daily gain increased with increasing levels of inbreeding, indicating that inbred individuals were unable to buffer themselves against fluctuations in environment. Heritability of average daily gain decreased with inbreeding because of lower genetic variance and higher environmental variance. Heritability estimates of litter size were quite variable due to the limited number of sire groups available but, in general, were quite low and generally not significantly different from zero. Estimates of heritability of average daily gain were more than twice as high as most estimates reported in the literature. These high estimates were thought to be biased due to the cumulative effects of a number of characteristics of the population and of the data. A line developed from a Berkshire-Yorkshire crossbred foundation should have more alleles segregating and therefore should have a higher additive genetic variance than the parental breeds. However, no differences between breed groups were observed in the magnitude of genetic variance, environmental variance or heritability estimates.
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