RESPONSE TO SELECTION FOR AGE AT PUBERTY IN AN ANGUS HERD

摘要

Direct responses and some correlated responses to selection for age at puberty in heifers were estimated in an Angus herd established in 1984/85. Lines were selected for early or late age at first behavioural oestrus (AFO) alongside an unselected Control, and evaluated up to 2004. Breeding values for AFO were calculated for both sexes and used for selection, from data on AFO and age-adjusted scrotal circumference or size (SS) evaluated monthly from 7 to 13 months; no liveweight selection criteria were included. Realised heritabilities for standardised AFO and single-record SS were 0.27 +- 0.03 and 0.41 +- 0.04, respectively, with a genetic correlation of-0.25 +- 0.09 between them. The direct response in AFO (difference between the 'early' and 'late'lines) was 62 days, 16% of the mean, and the correlated response in mean SS was 3.1 cm, 12% of the mean. There was no selection-line difference in weaning weight and only a 2.7% difference between the lines in yearling live weight (P < 0.05). The 'early' and 'late' selection lines differed in yearling-heifer pregnancy rate by 29 percentage points (P < 0.001), and there was a trend for an associated pregnancy-rate difference in mixed-aged cows between lines (91.8 vs 86.1%, respectively; P < 0.10). Therewas a significantly greater incidence of subfertile yearling bulls in the 'late' than in the Control or 'early' lines (P< 0.001).