Graduate Thesis Or Dissertation
 

Relationship of inbreeding to heterosis in swine

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  • Records of 889 litters of three populations of swine farrowed between 1962 and 1973 were analyzed by least squares procedures. Estimates were obtained for the effects of age and inbreeding of dam, inbreeding of litter, inbreeding of dam's dam and inbreeding of the sire of the litter on litter size traits and weight at birth and at weaning. Heterosis was calculated for maternal performance of crossbred dams and for crossbred litters from purebred dams. Positive significant effects of age of dam were found on total number born at birth in all three populations, on average pig birth weight in Yorkshires and in Composites, and on number of pigs born alive in Composites. Inbreeding of dam showed negative significant effects on litter size at birth in Yorkshires and in all of the litter traits except average pig birth weight in Composites. The only positive significant effect of inbreeding of dam was on average pig birth weight in Berkshires. Inbreeding of litter had no significant effect in Composites, negative significant effects on number of pigs at weaning, percent survival and average birth weight in Yorkshires, and on all the litter traits except total number born and average pig birth weight in Berkshires. Inbreeding of dam's dam had no significant effect on any litter trait except a highly positive one on number of pigs weaned in Yorkshires. Inbreeding of sire of litter showed a highly positive significant effect on average pig birth weight in Berkshires, a negative significant effect on litter size in Yorkshires and on average weaning weight in Composites. Crossbred dams showed more heterosis than crossbred litters from purebred dams for litter performance traits. The comparison between purebred and crossbred litters from purebred dams with various degrees of inbreeding were studied. Heterosis was greatest for number born alive and weaned; it tended to increase as purebred dams became more inbred. Adjustment for inbreeding decreased heterosis to levels comparable to that in non-inbred dams.
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