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

A selection experiment to change reproductive traits in Angus cattle has be en maintained for 14 years, with first calvings in 1985. Alongside an unsel ected control line, three lines were established, selected for increased ag e at puberty in heifers (AGE+ line), reduced age at puberty in heifers (AGE - line), or increased scrotal circumference (SC line). The last two lines w ere merged at the 1992 matings, with the revised objective of applying furt her selection in both sexes to reduce age at puberty in heifers. Dates at p uberty 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 subsequ ent 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. Gene tic 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 P R over seven years (AGE- mean minus AGE+ mean), excluding yearling heifer m atings, was 4.7 +/- 2.1% (P < 0.05). Mean CD in the AGE- line was 3 days ea rlier than in the AGE+ line (P < 0.09). These correlation and selection-lin e results suggest that selecting for higher SC and earlier heifer puberty l eads to earlier CD and higher PR.