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Age of dam and sex of calf adjustments and genetic parameters for gestation length in Charolais cattle¹

Agriculture and Agri-Food Canada Research Centre, Lethbridge, Alberta T1J 4B1 Canada

² Corresponding author: dcrews{at}agr.gc.ca

To estimate adjustment factors and genetic parameters for gestation length (GES), AI and calving date records (n = 40,356) were extracted from the Canadian Charolais Association field database. The average time from AI to calving date was 285.2 d (SD = 4.49 d) and ranged from 274 to 296 d. Fixed effects were sex of calf, age of dam (2, 3, 4, 5 to 10, 11 yr), and gestation contemporary group (year of birth x herd of origin). Variance components were estimated using REML and 4 animal models (n = 84,332) containing from 0 to 3 random maternal effects. Model 1 (M1) contained only direct genetic effects. Model 2 (M2) was G1 plus maternal genetic effects with the direct x maternal genetic covariance constrained to zero, and model 3 (M3) was G2 without the covariance constraint. Model 4 (M4) extended G3 to include a random maternal permanent environmental effect. Direct heritability estimates were high and similar among all models (0.61 to 0.64), and maternal heritability estimates were low, ranging from 0.01 (M2) to 0.09 (M3). Likelihood ratio tests and parameter estimates suggested that M4 was the most appropriate (P < 0.05) model. With M4, phenotypic variance (18.35 d²) was partitioned into direct and maternal genetic, and maternal permanent environmental components ( = 0.64 ± 0.04, = 0.07 ± 0.01, r_d,m = –0.37 ± 0.06, and c² = 0.03 ± 0.01, respectively). Linear contrasts were used to estimate that bull calves gestated 1.26 d longer (P < 0.02) than heifers, and adjustments to a mature equivalent (5 to 10 yr old) age of dam were 1.49 (P < 0.01), 0.56 (P < 0.01), 0.33 (P < 0.01), and –0.24 (P < 0.14) d for GES records of calves born to 2-, 3-, 4-, and 11-yr-old cows, respectively. Bivariate animal models were used to estimate genetic parameters for GES with birth and adjusted 205-d weaning weights, and postweaning gain. Direct GES was positively correlated with direct birth weight (BWT; 0.34 ± 0.04) but negatively correlated with maternal BWT (–0.20 ± 0.07). Maternal GES had a low, negative genetic correlation with direct BWT (–0.15 ± 0.05) but a high and positive genetic correlation with maternal BWT (0.62 ± 0.07). Generally, GES had near-zero genetic correlations with direct and maternal weaning weights. Results suggest that important genetic associations exist for GES with BWT, but genetic correlations with weaning weight and postweaning gain were less important.

Key Words: beef cattle • Charolais • genetic parameter • gestation length