INBRED LINES FOR HETEROSIS TESTS? 



333 



in litter size and growth rate in 49 inbred lines from five projects with an 

 average of 9 seasons per line (see also Dickerson, 1951). In Figure 21.1 the 

 average actual linear time trend (solid line) is negative for both litter size at 

 weaning and for pig weight at 154 days of age. An estimate of the eflfective- 

 ness of selection was made by adjusting the time trends for the effect of the 

 increased inbreeding, using corrections derived from the intra-season com- 

 parison of inbreds and linecrosses from the same inbred lines involved in the 

 time trends. The adjusted time trend (dashed line) indicates that selection 



TIME IN YEARS 



Fig. 21.1 — Linear time trend within mildly inbred strains for pigs weaned per litter and 

 154-day weight per pig. Solid line is actual trend, dashed line is trend adjusted for effect 

 of inbreeding trend to non-inbred basis, and the top broken line indicates mean superiority 



of selected parents. 



has failed to improve genetic merit for litter size and has allowed growth 

 rate to decline, although selection of parents per year has averaged about 

 .6 pigs for size of litter weaned by the dam and sixteen pounds for pig 

 weight at 154 days (top broken line). 



These results must be accepted with caution, because time trends can be 

 influenced by trends in nutrition, parasites, disease, management, or other 

 factors. Also, the correction for inbreeding effects may have been underesti- 

 mated. It seems clear that improvement has been at best only a small frac- 

 tion of what would have been expected from the heritability of these traits 

 and the amount of selection practiced for each. Evidence from comparison of 

 intra -breed linecrosses with representative purebreds is meager but does not 

 suggest any major improvement. Intra-herd comparisons of viability and 

 growth rate of progeny from inbred and from representative purebred boars 



