Milk yield and butterfat percentage of twin and single-born cattle under experimental and field conditions

Abstract

1. A comparison has been made of 305-day lactation milk yields and butterfat percentages of twin and single-born cattle under experimental and field conditions. Experimental two-egg twins, as judged from within-pair variances, were four times as variable as one-egg twins. Two-egg twins in the experimental herd were more alike than twins in dairy herds in fat percentage due to more frequent butterfat testing. Variance of milk yield within pairs was the same in both experimental and farm twins.2. Twins and half sisters in dairy herds had similar within-pair variances for milk yield and fat percentage. Full sisters, of necessity non-contemporary, differed more in yield than twins and half sisters; significantly so in the latter comparison.3. The between-pair variance components for experimental twins were high, relative to twins in herds. In experimental twins, breed differences, lack of culling and uniform treatment were probably contributing causes, and also more frequent observation of fat percentage during lactation. Twins n i dairy herds had greater between-pair variances than full sisters.4. Experimental MZ twins had an intra-class correlation for milk yield of 0·88 and DZ twins of 0·67 which was much higher than for twins on farms. Farm twins and half sisters had correlations for milk yield of 0·37 and 0·31, to be compared with 0·20 in full sisters. Corresponding values for fat percentage were 0·40, 0·35 and 0·25 on farms as against 0·88 and 0·65 for MZ and DZ twins at the experiment station. These differences between experimental and commercial pairs were largely attributable to differences in between-pair variance.5. There is no evidence from experimental or commercial data that twins produce less milk than single-born cows.6. Differences between farms increased the within-pair variance in yield and decreased the intra-class correlation of twins and half sisters but had less effect on full sisters.