Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 29, 1996
Publication Date: N/A
Interpretive Summary: Increased frequency of twinning can increase economic efficiency of beef cattle. The frequency of twinning in a herd selected for twinning at the U.S. Meat Animal Research Center has increased to nearly 30% as compared to rates of about 1% in most breeds of beef cattle. One reason for the effectiveness of selection has been the use of repeated measures of ovulation numbers in heifers before being bred for the first time. The small estimates of heritability for both ovulation rate and twinning rate suggested that dominance effects may be important. The results of analyses of measures of ovulation and twinning rates, however, indicate that dominance effects are not important for ovulation rate. Thus, crosses among independent lines selected for ovulation rate would not be expected to express heterosis for ovulation rate. The results for twinning rate which is a product of ovulation rate, conception rate, and survival rate are ambiguous although the analyses of first parity twinning rates suggest there may be dominance effects involved which could be due to dominance for conception rate and(or) embryo survival. If that is true, crosses of lines selected independently for twinning rate would be expected to express heterosis for twinning rate.
Estimates of variances due to additive and dominance genetic and permanent and temporary environmental effects were obtained for ovulation and twinning rates from a composite population selected for twinning rate. Measures of ovulation rate after 11 months of age on 2,317 heifers with a total of 19,209 measures were used. Twinning measures were on 1,522 first parity cows, 1,311 later parity cows with a total of 3,571 measures, and 1,704 all parity cows with 5,100 measures. Models included fixed effects of year-season-age at calving for twinning, and year-season of birth, age in months and calendar month of measurement for ovulation rate. Four analyses were done for each sample; combinations of models with and without dominance effects and with and without covariates for 7 foundation groups. Various samples of data were analyzed. Variance components as fractions of phenotypic variance for combined analysis of ovulation rate measures were .076, .045, and .000 for additive, dominance, and permanent environmental effects with no groups in the model and .069, .050, and .000 with groups in the model. For sums of 8 measures, the estimates were .287 and .000 for additive and dominance effects with groups in the model and .316 and .000 with groups ignored. For twinning rate for first parity, estimates were .126 and .209 for additive and dominance effects and for later parities were .045 and .035 for models including foundation group effects. The results suggest lack of dominance effects in expression of ovulation rate and the possibility of dominance effects for embryo and(or) fetal survival or conception rate because twinning rate is the product of ovulation, conception and embryo and(or) fetal survival rates.