EFFECTS OF 3 PREPUBERTAL BODY GROWTH-RATES ON PERFORMANCE OF HOLSTEINHEIFERS DURING FIRST LACTATION

The effects of body weight (BW) gain, different sources of protein dur ing the prepubertal period (90 to 320 kg of BW), and the performance o f Holstein heifers during their first lactation were studied. Heifers (n = 273) were assigned to one of three dietary energy treatments that were designed to achieve average daily gains of 0.6, 0.8, and 1.0 kg/ d. Within each energy treatment, different protein sources (plant prot ein and urea or both plant and animal proteins) were imposed. Actual a verage daily gains by heifers on each energy treatment were 0.68, 0.83 , and 0.94 kg/d for heifers that were fed diets formulated for average daily gains of 0.6, 0.8, and 1.0 kg/d, respectively, which allowed th e following ages at first calving: 24.5, 22.0, and 21.3 mo. Breeding w as initiated when heifers weighed approximately 340 kg. Protein source s did not affect average daily gain or milk yield. Analysis of the pre planned comparisons of actual 305-d and 4% fat-corrected milk yields i ndicated that yield was significantly reduced for heifers grown at 0.9 4 kg/d (9387 and 8558 kg, respectively) compared with that of heifers grown at 0.68 kg/d (9873 and 9008 kg, respectively). However, further regression analysis of fat-corrected milk and residual milk from a tes t day model on prepubertal BW gain only explained 8 and 2% of the vari ation in milk yield, respectively. Postcalving BW and body condition s core were different among treatments. Posttreatment factors, such as p ostcalving BW, accounted for more of the variation in milk yield than did prepubertal BW gain. Prepubertal BW gains, when evaluated on a con tinuum from 0.5 to 1.1 kg/d, explained little of the variation in milk yield; therefore, BW gain during the prepubertal period did not signi ficantly affect milk yield during first lactation.