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

Separate experiments were conducted in 2 zebu-derived herds grazed at pastu re in the dry tropics of Central Queensland, to determine genetic and envir onmental effects on temperament scores. At Duckponds, temperament of crossb reed steers and heifers from 2 calf crops was assessed using an objective f light speed score and 2 subjective scores (a visual flight speed score and a crush score) to determine whether the subjective tests were useful as sel ection criteria for on-farm selection programs. At Belmont, temperament of bull and heifer calves from 7 calf crops was mea sured by recording objective flight speed scores of individual animals at w eaning (6 months), 12 months, and 18 months. At Duckponds, heritabilities o f average objective flight speed, visual flight speed, and crush scores wer e 0.35, 0.08, and 0.30, respectively. Phenotypic and genetic correlations b etween the different measures of temperament were moderate to low. Based on low correlations and re-ranking of fixed effect classes that occurred betw een subjective tests, it was concluded the subjective tests were not suffic iently correlated with the objective measure to justify their use. The obje ctive flight speed score is recommended as the test of choice for use in br eeding programs using Bos indicus or Bos indicus derived animals. At Belmon t, even a single flight speed score was moderately heritable, but use of th e average of 2 or 3 repeated flight speed scores substantially increased he ritability. The realised heritability of average flight speed score was 0.4 8, almost identical to the estimated heritability of average flight speed s core of 0.50. Direct responses to a single generation of divergent selectio n for single flight speed scores at different ages demonstrated that select ion for flight speed was effective in modifying flight speed scores of prog eny. Correlated responses to divergent selection for flight speed scores in dicated that, under extensive grazing systems, the economic value of temper ament arose mainly through a reduction in production costs. There were significant differences in temperament scores between sire breed s used in the crossbreeding experiment at Duckponds, but there were no diff erences between lines at Belmont. From these results, it is possible that C ontinental breeds either have specific, negative combining ability when cro ssed with Brahmans, or themselves have temperaments that are no better than those of Bos indicus. Treatment to control gastrointestinal parasites (wor ms) had a significant (P< 0.001) negative effect on flight speed.