Heterosis in crosses of inbred lines of Duroc swine.

Abstract

The litter weights and number of live pigs per litter for 371 litters of Duroc swine at birth, 21, 56, and 180 days of age were studied. The same items were studied on 73 additional litters at birth and 56 days. All litters were farrowed at the Oklahoma Agricultural Experiment Station during the period from 1939 to 1949 inclusive. The study included inbred litters from seven different lines, as well as two-line-cross, three-line-cross, and outbred Duroc litters. Intra-season weighted mean differences were obtained for number of pigs per litter and litter weights at birth, 21, 56, and 180 days for five different comparisons. The comparisons made were: I, two-line-cross litters with inbred litters within line of dam; II, two-line-cross litters with the average of the two parental inbred lines; III, three-line-cross litters with two-line-cross litters; IV, three-line-cross litters with outbred Duroc litters; V, two-line crosses with outbred Durocs. The data on litters produced by sows over one year of age were corrected to a gilt-equivalent age. Hybrid vigor was evident both in number of pigs per litter and litter weights at birth, and increased as the litters became less dependent upon the direct mothering ability of the dams. This vigor was expressed to a greater extent in the increased viability of the pigs and productivity of two-line-cross gilts than in the increased growth rate of the individual pigs. There was no significant difference in the performance of two-line-cross litters raised by inbred sows and the performance of outbred Duroc litters which were produced in the same season. Three-line-cross litters, raised by two-line-cross dams and sired by inbred boars of a third line, were superior to both two-line-cross and outbred Duroc litters with which they were compared. The differences were not statistically significant for all items. Increased number of pigs per litter in most cases was sufficient to account for a large percentage of the increase in total litter weight. When initial weights of the pigs were the same, advantages of two-line-cross pigs over inbreds with the same line of dam in post-weaning rate of gain and efficiency of gain were relatively small and inconsistent. Because heterosis was expressed in both number of pigs which survived and the growth rate of the individual, total weight of the litter seems to be the one best over all measure of performance for comparison of lines or crosses.