A genetic analysis of complete lactation milk production in Improved Awassi sheep

Monthly test-day records of milk production comprising 3740 lactations, collected between 1993 and 1999 from 1360 fully pedigreed Improved Awassi ewes of the Kibbutz Ein Harod Israel, were analyzed. In this flock, ewes were milked from lambing until milk production dropped to 0.5 l/day, with no suckling period. Lactation curves were fitted and a number of parameters of the lactation curves were calculated. Genetic models were fitted to the different lactation parameters. Additive genetic, permanent environmental, maternal genetic and residual variances were estimated and used to calculate the heritability, repeatability and maternal components of 13 traits of lactation. The heritability of total milk yield throughout the lactation (TMY) was 0.10 +/- 0.036 and the repeatability 0.46 +/- 0.042. The repeatability was reduced to 0.44 when lactation length was used as a covariate. A maternal genetic effect of 4% (TMY) was found and the heritability of TMY of first lactation records alone was 0.13 +/- 0.081. For other lactation parameters, the heritability and repeatability were: 0.03 and 0.08 (day of peak yield), 0.12 and 0.36 (peak yield), 0.09 and 0.16 (increase in milk production per day) and 0.11 and 0.15 (persistency of lactation), respectively. The level of the additive genetic variance for TMY and the other lactation parameters was low in this improved Awassi population. Permanent environmental effects, which may include dominance and epistasis, have a profound contribution to the total variance of milk yield traits in this population. (C) 2001 Elsevier Science B.V. All rights reserved.