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 i n a single flock, over the years 1977-96, were used to estimate geneti c parameters for total milk yield (TMY), standardized milk yield (SMY) and lactation length (LL). Estimates for heritability and genetic cor relations among TMY, SMY and LL studied in this work were obtained emp loying multitrait analysis with a repeatability animal model. The mode l 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 corresp onding figures for repeatabilities were 0.41 +/- 0.013, 0.38 +/- 0.013 and 0.040 +/- 0.010, respectively. Genetic correlation between TMY an d SMY was 0.99, whereas genetic correlations of LL with TMY and SMY we re 0.61 and 0.62, respectively. Phenotypic correlations were 0.97, 0.1 0 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 correla tions between genetic evaluation of sires based on TMY with that based on SMY records were 0.85 and 0.84, respectively. As expected, these c orrelations were lower than the true genetic correlation between TMY a nd SMY mainly due to the accuracy of breeding value predictions for bo th traits. Results indicate heritabilities in the range found in the l iterature for TMY and SMY. The low heritability estimate found for LL confirmed that this trait is mainly affected by environmental conditio ns. Because of the low heritability estimate found for LL, the lower r esidual correlation between SMY and LL, both close to zero, and the eq ual genetic correlation of LL with both TMY and SMY, it seems better t o use SMY than TMY for selection programs of Churra dairy sheep. (C) 1 998 Elsevier Science B.V.