GENETIC-PARAMETERS FOR TESTOSTERONE RESPONSE TO GNRH STIMULATION AND SCROTAL CIRCUMFERENCE IN TROPICAL BEEF BULLS

Genetic relationships between plasma testosterone levels (T) in response to stimulation with gonadotrophin releasing hormone (GnRH), scrotal circumference (SC), growth, resistance to ticks, worms and buffalo flies and heat tolerance were studied in zebu cross cattle at 9 and 18 months of age in a tropical environment. Mixed linear models were fitted to the data and variance components were estimated using Restricted Maximum Likelihood methods. Age, genotype, year, lactational status and age of dam, and selection line were all found to have significant influences on T and SC. Heritabilities of T at 9 and 18 months of age were 0.42 +/- 0.28 and 0.55 +/- 0.15 and for SC were 0.36 +/- 0.12 and 0.28 +/- 0.10, respectively. Corresponding coefficients of variation were 0.38, 0.33, 0.10 and 0.08. Thus T would respond more rapidly to selection than SC. Estimates of genetic correlations indicated that selection for T would increase growth but would not change parasite resistance or heat tolerance. Selection for SC would also increase growth but in general would decrease parasite resistance. Correlated responses to single generation selection experiments verified these conclusions. The implications of using T and SC as indirect selection criteria with a view to improving cow and bull fertility are discussed.