Abstract
The effects of genetic factors on the lactation traits of the Azawak cattle breed are estimated from 11,998 monthly milk records from 1275 complete lactations from 471 Azawak cows bred at the Toukounous experimental centre (Niger), using a multi-trait animal model based on the REML method. The results show that heritability was moderate for persistency (h(2) = 0.23), peak lactation (h(2) = 0.34), milk yield at 305 days (h(2) = 0.30), daily milk yield (h(2) = 0.33) and total milk yield (h(2) = 0.35). In addition, very high repeatability estimates ranging from 0.50 to 0.58 were associated with the last four traits. Significant phenotypic correlations varying from 0.23 to 0.40 existed between the two reproductive traits (age at first calving and calving interval) and the lactation traits. Similarly, the significant genetic correlations between the traits of milk production and reproduction traits were unfavorable, varying according to the lactation traits considered from 0.32 to 0.87 for age at first calving and from 0.48 to 0.97 for calving interval, indicating that selection for milk yield only should result in a longer calving interval and a later age at first calving. Estimates of the heritability of lactation traits are moderate, as are those of many functional reproductive traits, so the genetic gain from selection on milk production traits alone would be rapid, but antagonistic with reproductive performance.