Young male sheep were selected on an index of testis diameter adjusted for body weight. There was a direct response to selection with a realised heritability of 0.53. Sheep in the High-selected line became earlier maturing but had a smaller mature weight than those in the Low line. Ovulation rate did not respond to selection. However, due to higher fertility and lamb survival, High line ewes produced more lambs per ewe mated and reared more lambs to 210 days. This, combined with the changes in growth patterns, leads to a greater predicted production efficiency of ewes in the High line compared to those in the Low line.

G. J Lee, Chris S Haley

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume XVI. Poultry, fish and horse genetics and breeding, growth and reproduction, immune response and disease resistance., , 370–373, 1990
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