Abstract
The objective of this study was to evaluate alternatives for modeling the interaction between age of dam at calving (AOD) and the dam genetic group (DGG) on the weaning weight (W225) of Charolais-Zebu (Ch-Z) crossbred calves. Data from 56,965 crossbred calves were analyzed by the least square method. Regression coefficients for age of dam at calving were estimated nested into each class of the dam genetic group (CLA model); for age of dam at calving × dam Charolais percentage (age of dam at calving × FCh) and age of dam at calving × dam heterozygosity (age of dam at calving × FH) (FChFH model); for age of dam at calving × dam Charolais percentage (FCh model); for age of dam at calving × FH (FH model); or only for age of dam at calving (NINT model). Segmented polynomials were used to model the general shape of the age of dam at calving effect and its interaction with dam genetic group. The knots were at 6.33 and 10.66 years of age of dam at calving and general linear and quadratic coefficient regression and specific quadratic coefficient regression after each knot were fitted. The regression coefficients were estimated nested within sex of the calf in all situations. According to the F test for sum of squared residuals differences, the inclusion of the age of dam at calving × FH interaction did not improve the fit of the model and the CLA model provided the best fit. However, the estimates of the age of dam at calving and dam genetic group interaction from the CLA model for dam genetic group × sex of the calf classes with few records were not appropriate, but the estimates of the age of dam at calving and dam genetic group interaction from the FCh model for those classes were appropriate. The differences were small in the estimates of the age of dam of calving and dam genetic group interaction from the CLA or FCh models for dam genetic group × sex of the calf classes with many records.
Highlights
Differences in weaning weight in beef cattle can be attributed to genetic and environmental factors
Regression coefficients for age of dam at calving were estimated nested into each class of the dam genetic group (CLA model); for age of dam at calving × dam Charolais percentage and age of dam at calving × dam heterozygosity (FChFH model); for age of dam at calving × dam Charolais percentage (FCh model); for age of dam at calving × FH (FH model); or only for age of dam at calving (NINT model)
From the total observations used, 57% were derived from Canchim × Canchim mating, 19% Charolais sire with 5/16 Ch dam and the others were distributed in 12 combinations (Table 2)
Summary
Differences in weaning weight in beef cattle can be attributed to genetic and environmental factors. Additive and non-additive genetic effects can be sources of significant variation on milk production of the dam (Alencar et al, 1996; Cerdótes et al, 2004), they might influence calf weaning weight. The age at calving significantly influences the milk production of the dam (Teodoro et al, 2000), significant effects are expected from this factor on the weight of the calves at weaning. Significant interactions between age of dam at calving and the dam genetic group (DGG) have been reported for the weaning weight in populations of crossbred beef cattle (Massey & Benishek, 1981; Elzo et al, 1987; Gregory et al, 1991; Alencar et al, 1999b). To fit the age of dam at calving effect to each dam genetic group separately is complicated because there are several genetic groups, and not considering the effect of the age of dam at calving x dam genetic group interaction on the genetic evaluations can damage the prediction of the genetic values
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