During a five-generation selection experiment, in which litter size at 3 wk (F line) and September weight (BS line) were selected for, reciprocal crossings between lines were performed in Generations 3 and 4 (1987 and 1988) and after terminating the experiment (1990). Furthermore, one line was subjected to combined selection for litter size and body weight (I line), and a randomly mated control was maintained. In 1987 and 1988, only 2-yr-old animals from the F and BS lines were used for crossings, but in 1990 bom yearlings and 2-yr-old animals were crossed. Crossings using F females gave the best reproductive result. The reproductive output of separate selection followed by crossing greatly exceeded that of me I line. The BS females seemed to have a poor maternal ability mat concealed any expression of kit heterosis. Body size was also improved by crossing. The results indicate mat crossings should be performed with yearling 
females to maximize reproductive output.

G. Lagerkvist, N. Lundeheim

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 19. Selection and quantitative genetics; growth; reproduction; lactation; fish; fiber; meat., , 280–283, 1994
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