Genetic parameters for growth, puberty, and beef cow reproductive traits in a puberty selection experiment

Abstract

A selection experiment to change reproductive traits in Angus cattle has been maintained for 14 years, with first calvings in 1985. Alongside an unselected control line, three lines were established, selected for increased age at puberty in heifers (AGE+ line), reduced age at puberty in heifers (AGE‐ line), or increased scrotal circumference (SC line). The last two lines were merged at the 1992 matings, with the revised objective of applying further selection in both sexes to reduce age at puberty in heifers. Dates at puberty in heifers from an average of 8 to 16 months of age and SC in bulls were recorded in all lines, along with the pregnancy rates (PR) and subsequent calving dates (CD) in heifers and all cow‐age groups. Heritabilities of single‐record SC, standardised age at first behavioural oestrus (SFO), and single‐record CD were 0.41 ± 0.04 (repeatability 0.70 ± 0.01), 0.27 ± 0.04, and 0.09 ± 0.04 (repeatability 0.19 ±0.03), respectively. Genetic correlations of SC with SFO and CD were ‐0.25 ± 0.09 and ‐0.23 ± 0.15, respectively; SFO with CD 0.57 ± 0.17; and PR with SFO and SC‐0.36 ±0.13 and 0.12 ± 0.11, respectively. The selection‐line difference in PR over seven years (AGE‐ mean minus AGE+ mean), excluding yearling heifer matings, was 4.7 ± 2.1% (P < 0.05). Mean CD in the AGE‐ line was 3 days earlier than in the AGE+ line (P < 0.09). These correlation and selection‐line results suggest that selecting for higher SC and earlier heifer puberty leads to earlier CD and higher PR.