Genetic and environmental factors affecting temperament of zebu and zebu-derived beef cattle grazed at pasture in the tropics

Abstract

Separate experiments were conducted in 2 zebu-derived herds grazed at pasture in the dry tropics of Central Queensland, to determine genetic and environmental effects on At Duckponds, temperament of crossbreed steers and heifers from 2 calf crops was assessed using an objective flight speed score and 2 subjective scores (a visual flight speed score and a crush score) to determine whether the subjective tests were useful as selection criteria for on-farm selection programs. At Belmont, temperament of bull and heifer calves from 7 calf crops was measured by recording objective flight speed scores of individual animals at weaning (6 months), 12 months, and 18 months. At Duckponds, heritabilities of average objective flight speed, visual flight speed, and crush scores were 0.35, 0.08, and 0.30, respectively. Phenotypic and genetic correlations between the different measures of temperament were moderate to low. Based on low correlations and re-ranking of fixed effect classes that occurred between subjective tests, it was concluded the subjective tests were not sufficiently correlated with the objective measure to justify their use. The objective flight speed score is recommended as the test of choice for use in breeding programs using Bos indicus or Bos indicus derived animals. At Belmont, even a single flight speed score was moderately heritable, but use of the average of 2 or 3 repeated flight speed scores substantially increased heritability. The realised heritability of average flight speed score was 0.48, almost identical to the estimated heritability of average flight speed score of 0.50. Direct responses to a single generation of divergent selection for single flight speed scores at different ages demonstrated that selection for flight speed was effective in modifying flight speed scores of progeny. Correlated responses to divergent selection for flight speed scores indicated that, under extensive grazing systems, the economic value of temperament arose mainly through a reduction in production costs. There were significant differences in temperament scores between sire breeds used in the crossbreeding experiment at Duckponds, but there were no differences between lines at Belmont. From these results, it is possible that Continental breeds either have specific, negative combining ability when crossed with Brahmans, or themselves have temperaments that are no better than those of Bos indicus. Treatment to control gastrointestinal parasites (worms) had a significant (P < 0.001) negative effect on flight speed.