Genetic variation in testicular diameter in a random-breeding Merino flock

Abstract

This paper reports estimates of genetic parameters for testicular diameter (TDM) from a random-breeding Merino flock maintained at Trangie, N.S.W. Over a period of 4 years, TDM was measured, generally monthly, on 800 rams from four Merino strains and 14 lines within these strains between weaning (4 or 5 months) and 12 months of age. In 2 of the 4 years TDM was again measured at age 19 months. The between-year source of variation was a highly significant component for all measures of TDM except at 4 and 19 months of age. Differences between strains were significant at some ages with changes of ranking occurring was rams approached 12 months of age. Lambs born as twins and triplets had significantly smaller testes up to 9 months of age while date of birth (6–7 week lambing period) had a significant effect up to and including 19 months of age. Rams born later had smaller testes. Paternal half-sib estimates of heritability of TDM between 6 and 12 months of age were all high, ranging between 0.49 ± 0.14 and 0.69 ± 0.14. AT 4, 5 and 19 months of age lower estimates were obtained (0.45 ± 0.19, 0.28 ± 0.13 and 0.22 ± 0.17, respectively). Genetic correlations between these monthly measures of TDM were generally high (> 0.75) except for those involving TDM at 4 months of age. Although the genetic relationship between weaning liveweight and TDM at 5 months of age was high (0.84 ± 0.18), weaning liveweight was lowly correlated with TDM at other ages (0.11–0.26). Correlations between 12-month liveweight and TDM at other ages were higher (0.51–0.74). AT 12 months of age liveweight and TDM were also highly correlated ( 0.80 ± 0.15).