MILK YIELD AND COMPOSITION IN BEEF COWS AND THEIR EFFECT ON COW AND CALF PERFORMANCE IN TWO ENVIRONMENTS
Authors: RAHNEFELD G. W. , WEISS G. M. , FREDEEN H. T. 
Published: March 2010

Relationships of milk yield and composition of 10 specific F1 crosses of beef cows with preweaning growth of their calves and with changes in the weight and subcutaneous fatness of the cows during lactation were examined over 2 yr for a population of 120 cow-calf pairs each year at two climatically distinct locations. The crosses were represented by the Hereford × Angus (HA) and nine crosses produced by mating Charolais (C), Simmental (S), and Limousin (L) sires to cows of Hereford (H), Angus (A), and Shorthorn (N) breeding. Environmental differences were provided by two climatically distinct locations (Brandon, Manitoba; Manyberries, Alberta) with two treatments (drylot confinement, T1; pasture, T2) imposed at each location. Milk was measured at 8-wk intervals commencing 1 June and terminating 1 October (weaning). Cow weight and fat at calving were positively associated (P < 0.0001) at both locations. Weight of the cow at calving was positively associated with average milk yield and calf growth rate at Manyberries (P < 0.0001), but at Brandon the association was positive but not significant (P > 0.05). Weight of cow at calving was positively associated with calf birth weight at Manyberries (P < 0.02). Calf preweaning growth rate was directly related to birth weight (P < 0.009). Calf sex had the expected effect on calf growth rate throughout lactation (M > F) but milk yield of the cow was not influenced by the sex of calf reared. Percentages of the milk constituents (butterfat (BF), solids-not-fat (SNF), protein (PT), declined as milk yield increased but the reduction was significant only for PT (P < 0.0005). Changes in cow weight and subcutaneous fat cover during lactation were inversely related with milk yield, the reductions in weight being significant (P < 0.008) at both locations and the loss of subcutaneous fat cover significant (P < 0.0001) only at Manyberries. Calf growth rate increased as milk yield increased with the largest regression coefficients at each stage of lactation provided by the estimate of average milk production for the complete lactation; however, this difference was not significant (P > 0.05). The statistical model with the covariate average milk production accounted for approximately 58% of the variance for calf gain during the lactation period. The covariate alone contributed 36% of this total R2 value at Brandon and 48% at Manyberries. Simmental cross cows produced the fastest growing calves while HA and L cross cows produced the slowest growing calves at both locations. The highest milk yields were recorded for the S cross cows at both locations while HA cows were lowest at Brandon and LH the lowest at Manyberries. At both locations, the SN cows recorded the lowest values for % BF and % SNF, and the SA cows recorded the highest values for % PT. For % BF and % SNF, the HA cows were the highest at Brandon while at Manyberries, the LH cows recorded the highest values. Average milk yield (AMY) was the best milk production variable explaining variance associated with calf growth. The R2 values were higher at Manyberries than Brandon and the R2 values decreased as the lactation season progressed. The addition of any of the milk constituent variables added little to the R2 values for AMY. Key words: Milk yield, milk composition, calf growth, cow weight change, cow subcutaneous fat change