Cubic-spline interpolation to estimate effects of inbreeding on milk yield in first lactation Holstein cows

Milk yield records (305d, 2X, actual milk yield) of 123,639 registered first lactation Holstein cows were used to compare linear regression (y = β0 + β1X + e), quadratic regression, (y = β0 + β1X + β2X2 + e) cubic regression (y = β0 + β1X + β2X2 + β3X3 +e) and fixed factor models, with cubic-spline interpolation models, for estimating the effects of inbreeding on milk yield. Ten animal models, all with herd-year-season of calving as fixed effect, were compared using the Akaike corrected-Information Criterion (AICc). The cubic-spline interpolation model with seven knots had the lowest AICc, whereas for all those labeled as “traditional”, AICc was higher than the best model. Results from fitting inbreeding using a cubic-spline with seven knots were compared to results from fitting inbreeding as a linear covariate or as a fixed factor with seven levels. Estimates of inbreeding effects were not significantly different between the cubic-spline model and the fixed factor model, but were significantly different from the linear regression model. Milk yield decreased significantly at inbreeding levels greater than 9%. Variance component estimates were similar for the three models. Ranking of the top 100 sires with daughter records remained unaffected by the model used

Effects of inbreeding and age at first calving on milk yield and failed services in first lactation of bST and non-bST treated Holstein cows

Production and reproduction records on first lactation Holstein cows were used to study effects of inbreeding (F) and age at first calving (AFC) on milk yield and failed services (FS). For milk yield (305d), records of 123,639 registered Holstein heifers were used to compare linear regression, fixed factor, saturated and cubic spline (CS) for modeling effects of F and AFC under bST and non-bST treatments. Thirty seven models were compared using Lack of Fit test and Akaike Information Criteria while fixing the animal genetic variance. Four models were detected as best fitting the data: CS for F at 4, 5, 6 and 7 knots with CS for AFC at 5 knots. There were no significant differences in effects of F and AFC under different bST treatments. Cows treated with bST produced 434 kg more milk than non-treated cows. Inbreeding greater than 6.25% significantly reduced milk yield. Milk yield significantly increased with increases in AFC. Heritability of milk yield was estimated to be 0.33. For FS, treated as a binary trait, reproduction records of 121,518 first lactation cows were used. Cows with at least one bST treatment prior to first service were considered bST treated animals. Cows with three or more FS received a score of 1 indicating reproductive problems. Effects of F and AFC were not significantly different under different bST treatments. Odds of three or more FS were 11 times greater in bST treated cows. For F, odds of three or more FS were highest for inbreeding between 12.5% and 18.75%. For AFC, heifers calving between 22 and 26 months had significantly smaller odds of three or more FS than heifers calving at earlier or later ages. Heritability was 0.37 on the underlined normal scale and 0.158 on the observed scale