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Hybrid Vigor in Single Crosses between Inbred Lines of Poland China Swine¹

United States Department of Agriculture and Iowa Agricultural Experiment Station

Abstract

To measure the effect of the inbreeding of the pigs on their performance, single crosses between inbred lines of Poland Chinas were compared with inbreds by the same boar. The 28 intra-sire comparisons included 56 inbred litters from 11 lines and 60 single-cross litters representing 43 different combinations. These were fall litters from 1942 and 1943. Inbreeding of litters averaged 42 percent for inbreds and 6 percent for single crosses; corresponding averages for inbreeding of dams were 34 and 28 percent and for age of dams were 18 and 17 months. Performance of both groups was adjusted to the same age and inbreeding of dams.

Hybrid vigor of the pigs or its converse, the effect of inbreeding, was greater in viability than in rate of growth, although the two are related in that the faster growing pigs are more likely to survive. Mortality was lower among crosses than among inbreds, both before and after birth, until at five months of age crosses exceeded inbreds by 1.4 live pigs per litter, or by 42 percent. Crosses were little heavier than inbreds at birth and 21 days, while both groups were suckling inbred sows, but crosses exceeded inbreds by 3.4 pounds or 12 percent at 56 days and by 25 pounds or 21 percent at 154 days. In total litter weight at 154 days, crosses exceeded inbreds by 290 pounds or 72 percent. Although they grew more rapidly, crosses required as much feed as inbreds for 100 pounds of gain from 84 days to 225 pounds. Slaughter data suggested, but did not establish, that crosses had a lower dressing percentage, less fat, but plumper hams than inbreds.

These results indicate that in the development of inbred lines more careful selection is necessary to maintain litter size and viability than to hold rate of growth. The performance of these single crosses, out of dams inbred 28 percent, averaged about the same as published data on outbred Poland Chinas in Experiment Station herds. This fact suggests that the selection practiced in the development of these lines has effected considerable improvement, and that using the best of such improved lines in topcrossing on outbred sows or in triple crosses offers opportunity for surpassing the performance of outbreds.

¹ Journal paper No. J-1306 of the Iowa Experiment Station, Ames, Iowa, Project No. 32 and 402 in cooperation with the Regional Swine Breeding Laboratory, Bureau of Animal Industry.

² Grateful acknowledgement is made to Professor B. R. McClurg for taking and interpreting data on the carcasses.