Carcass composition and growth in Angus cattle genetically selected for differences in pubertal traits

Abstract

A selection experiment to change reproductive traits in Angus cattle was established in New Zealand in 1984/85. Alongside an unselected control line, three lines were set up, selected for increased age at puberty in heifers (AGE+ line), reduced age at puberty in heifers (AGE‐ line), or increased scrotal circumference (SC line), with the last two lines being merged at the 1992 matings (forming a new AGE— line, with continued heifer puberty selection). The heritability of the direct response (age at puberty) through to 2004 was 0.27 ± 0.04. The purpose of the present study was to investigate correlated responses in carcass composition and growth traits, in response to pubertal selection. Traits measured included weaning, yearling, and breeding‐cow weights, carcass weight, dressing percentage, the weights of pericardial, omental and kidney fats, and the percentages of meat, bone, and trimmed fat. The carcass study included selection‐line and control animals (n = 185; calf crops born in 1997–2002), and also back‐crosses to both lines (n = 294; calf crops born in 2000–02). The divergence in age at puberty in heifers from the AGE+ and AGE‐ lines was 62 days (16% of the mean). For carcass traits, the only consistent line effects across both the AGE+ and AGE‐ lines and the back‐cross lines were in pericardial fat weights of post‐pubertal bulls (P < 0.05). However, where significant line differences in kidney fat weight or fore‐quarter fat trim percentage occurred, they were also in the same direction as for pericardial fat (AGE+ animals fatter than AGE‐ animals). For heifers and steers, none of the line contrasts for fat traits was significant. The selection‐line effects for weaning, yearling, and pre‐calving mature cow weights (AGE+ minus AGE‐ means) were 2.5 ± 2.1, ‐7.3 ± 3.0 (P < 0.05), and 8.2 ± 3.7 kg (P < 0.05), 1.5, ‐2.7, and 1.8% of their respective means. The interpretation of these results was that earlier sexual maturity in heifers (AGE‐ line) was associated with a steeper juvenile growth curve through puberty, but ultimately the cows were lighter than in the AGE+ line and not different in fatness. Earlier sexual maturity in bulls was also associated with a steeper growth curve, but there was the suggestion from some fatness traits that the AGE— line were leaner than the AGE+ line when compared at the same age.