Correlated changes in growth patterns and overall production efficiency following selection for testis size adjusted for body weight in young male lambs

Two divergent lines of sheep were produced by selection on an index of testis diameter adjusted for body weight at 6, 10 and 14 weeks of age. There were correlated changes in growth patterns, such that animals in the high and low lines were of a similar weight at birth, but thereafter, the low line became progressively heavier than the high line. After 11 years of selection there was a 7-kg difference between the mean body weight of females in the two lines at 1.5 years of age and a 12-kg difference at 2.5 years. There was little evidence of any accompanying changes in body shape or in carcass composition. High-line females of all reproductive ages had a significantly higher fertility. High-line lambs had a significantly lower mortality and, at the end of selection, had an estimated probability of survival to 210 days of 0.25, 0.13 and 0.03 higher than in lambs from the low line, in lambs from 1-, 2- and 3-year-old dams respectively. The estimated number of lambs alive at 210 days per ewe mated was 0.30, 0.38 and 0.34 higher in the high line for ewes of 1, 2 and 3 years of age, but low-line lambs were 3 kg heavier on average at 210 days. The combined impact of improved ewe performance and changes in growth patterns led to a predicted greater production efficiency in the high line. It was estimated that, after 11 years of selection on the index of weight adjusted testis diameter, 0.62, 0.71 and 0.74 kg of high-line ewe at 1, 2 and 3 years of age, respectively, would be needed to produce the same weight of lamb at 210 days as 1 kg of low-line ewe. Improved ewe performance had a much larger impact on the changes in comparative efficiency than changes in growth patterns, but the contribution of the latter increased with age of ewe. Further experimental verification of the line differences in reproduction and lamb survival and study of their causes are needed. Nevertheless, these results indicate the promise of using indices of body weight and testis size to increase the efficiency of lamb production.