Calving day and age at first calving in Angus heifers1

Because of difficulties in data collection and analysis, in most breeds there have been limited ways to evaluate fertility in females on a between- herd basis. The objective of this study was to determine the heritabilities (direct and maternal) for CD (calving day) and AFC (age at first calving) in American Angus heifers and to evaluate the potential for using these traits in genetic improvement of female fertility. Records (n = 2,082) from 2 herds were used. Calving day was defined as the calving date of a heifer minus the first calving date in her contemporary group. To avoid bias, noncalving heifers were assigned a penalty record CD of 30, 60, and 90 d after the last CD in that contemporary group. These assigned CD were also used to give open heifers a predicted AFC. Data were analyzed by MTDFREML using a general linear animal model. Fixed effects included herd-year, service sire of the heifer, and age of dam, and a covariate of age of the heifer at the start of the breeding season (for CD only). A model including a maternal effect was also analyzed. Heritabilities for CD using a direct model were 0.07 ± 0.04, 0.10 ± 0.05, and 0.11 ± 0.05, for each penalty adjustment, respectively. Average, minimum, and maximum estimated breeding values (in days) for sires of heifers for the 3 adjustments were -0.7, -10.6, and 9.8; -1.1, -17.2, and 16.5; and -1.6, -22.6, and 19.5. The estimates of heritability for AFC using a direct model did not differ for the different adjustments for penalty records and were 0.28 ± 0.06. Average, minimum, and maximum estimated breeding values (in days) for sires of heifers for the 3 adjustments were -0.6, -46.6, and 45.9; -1.2, -50.1, and 51.6; and -1.7, -52.9, and 56.7. In a direct-maternal model, direct heritabilities for CD decreased slightly, and for AFC increased to 0.66 ± 0.14. The maternal heritabilities and direct-maternal genetic correlations were 0.08 ± 0.05 and -0.18 ± 0.58 for CD, and 0.32 ± 0.08 and -0.85 ± 0.06 for AFC. Although AFC had a greater heritability and a wider range of breeding values than CD, the negative direct-maternal genetic correlation indicated that selecting on AFC may favor heifers that are themselves born later in the season. Therefore, CD may be more useful than AFC in selecting for female fertility in beef cattle.s [PUBLICATION ABSTRACT]