Genetic variability in milking speed of dairy goats

The present investigation deals with the genetic variability of milking speed, measured as the volume of milk collected during the first minute of milking (MD1), and its association with dairy traits. Data originated from 2589 lactations of 1421 Alpine goats, sired by 93 bucks, measured between 1985 and 1997 at the Moissac Goat Experimental Station (Lozère, France). Two genetic analyses were carried out. Firstly, a polygenic model was used to estimate genetic and phenotypic parameters among milking speed and dairy traits using a multiple-trait animal model. Secondly, segregation analysis was used to test the hypothesis of mixed model inheritance (polygenes + major gene) for MF1. Heritability and repeatability of MF1 under the polygenic model were high (0.65 and 0.82, respectively). Estimated genetic and phenotypic correlations between milking speed and dairy traits were low, positive for yields and negative for contents. Segregation analysis yielded a highly significant likelihood ratio, confirming the segregation of a major gene with two alleles with partial dominance. The difference between the mean values of the two homozygotes was around 2.3 phenotypic standard deviation units of the trait. The major gene explained more than 60% of the estimated total genetic variance. The estimate of the 'residual' heritability, after taking into account the effect of the major locus, was 0.30.