Extension factors for part-lactation in Churra sheep breed

Extension factors of incomplete lactation were developed for Churra sheep breed. These factors were estimated according to the methodology proposed for dairy cows, which is based on the prediction of the unknown portion of the lactation starting from the last available monthly record. Extension factors (FEMi) were obtained by estimating the regression of milk production on a given control until 120 days with regard to the number of remaining days from this control until 120 days. The flock production level, type of birth, parity and age of dam at lambing were included in the model as fixed effects. The genetic correlations between standardized and extended lactations for two or three monthly controls were high, 0.997 and 0.999 (± 0.001) respectively, whereas heritabilities for standardized and extended lactation considering the daily production in two and three monthly controls were 0.240, 0.211 and 0.237 (± 0.02), respectively. These results could suggest that both, standardized and extended lactation are measures of the same trait. Spearman correlations coefficients between the breeding values of sires, from the standardized and extended lactations were 0.963 and 0.977, respectively. This would indicate that it is convenient to include extended lactations when estimating the breeding value of sires, for it would help to increase its precision as the tendency to reject low producing animals would be avoided.