The effect of systematic factors on 4% fat-corrected milk yield in Simmental cows

Abstract

The effect of systematic and continuous environmental factors on 4% fat-corrected milk (FCM) yield in 2805 Simmental cows over whole and standard lactations was evaluated using the general linear model. The test systematic or fixed factors included the effect of farm or breeding area, calving season, year of birth, season of birth, lactation group and interactions between year of birth and season of birth, breeding area and calving season, and breeding area and lactation group. The continuous factor analysed was the effect of age at first conception. The effect of breeding area, lactation group and calving season on 4%FCM yield over whole and standard lactations was found to be very highly significant (P<0.001). The interactions of fixed i.e. systematic factors involved in the model, including those between year and season of birth, breeding area and calving season, and farm and lactation group had a very highly significant effect (P<0.001) on 4% FCM yield over both whole and standard lactations. The age at first conception, as a continuous factor, showed a very highly significant effect (P<0.001) on 4% FCM yield over both whole and standard lactations. The model used to correct the 4%FCM yield over whole and standard lactations for the effect of systematic factors was very highly significant (P<0.001). The variance of the model accounted for 15.93% and 21.55% of the total variance in 4% FCM yield over whole and standard lactations, respectively. The resulting coefficient of determination (R2) in 4% FCM yield was 0.159 over whole lactations and 0.216 over standard lactations.