Genetic correlations and heritabilities for milk yield and lactation length of dairy sheep

A total of 6917 lactation records for 1569 Churra dairy sheep raised in a single flock, over the years 1977-96, were used to estimate genetic parameters for total milk yield (TMY), standardized milk yield (SMY) and lactation length (LL). Estimates for heritability and genetic correlations among TMY, SMY and LL studied in this work were obtained employing multitrait analysis with a repeatability animal model. The model accounted for year-season effect, parity and type of birth as single or multiple lambings. Heritabilities for TMY, SMY and LL were 0.27 +/- 0.036, 0.25 +/- 0.034 and 0.015 +/- 0.010, respectively. The corresponding figures for repeatabilities were 0.41 +/- 0.013, 0.38 +/- 0.013 and 0.040 +/- 0.010, respectively. Genetic correlation between TMY and SMY was 0.99, whereas genetic correlations of LL with TMY and SMY were 0.61 and 0.62, respectively. Phenotypic correlations were 0.97, 0.10 and 0.25, respectively. Residual correlation between TMY and LL was 0.21 and only 0.01 between SMY and LL. Product moment and rank correlations between genetic evaluation of sires based on TMY with that based on SMY records were 0.85 and 0.84, respectively. As expected, these correlations were lower than the true genetic correlation between TMY and SMY mainly due to the accuracy of breeding value predictions for both traits. Results indicate heritabilities in the range found in the literature for TMY and SMY. The low heritability estimate found for LL confirmed that this trait is mainly affected by environmental conditions. Because of the low heritability estimate found for LL, the lower residual correlation between SMY and LL, both close to zero, and the equal genetic correlation of LL with both TMY and SMY, it seems better to use SMY than TMY for selection programs of Churra dairy sheep. (C) 1998 Elsevier Science B.V.