Abstract

Direct and correlated responses in a 23-year selection experiment to change puberty in heifers are reviewed. Selection was for age at first oestrus (AFO). It was applied in both sexes, using the genetic correlation (-0.25 ± 0.09) between scrotal circumference in juvenile males and AFO in female relatives. The main breeding lines were those selected for early AFO (AGE-), late AFO (AGE+), or unselected (Control). A 69-day difference (19% of the herd mean) was achieved between the two AGE lines, with a 3.0 cm (9.9%) difference in scrotal circumference. Relative to the AGE+ line, the AGE- line means were +10.8 ± 3.1 kg (3.9%) in yearling weight, -6.0 ± 3.0 kg (1.3%) in cow mature weight, -47 ± 6 kg (16%) in heifer weight at first oestrus, and +3.7 ± 1.5 percentage points for mixed-aged cow pregnancy rate. There were significantly fewer sub-fertile yearling bulls in the AGE- line (P <0.001), and maternal effects on autumn and winter weights on calves of this line were positive. For carcass composition, only depot fat differed significantly between lines (AGE- mean < AGE+ mean). No major unfavourable traits were observed in the AGE- line in this study.

CA, Morris, and NC Amyes

Proceedings of the New Zealand Society of Animal Production, Volume 70, Palmerston North, 202-205, 2010