Factors Affecting Whole- and Part-Lactation Milk Yield and Fat Percentage in a Herd of Holstein Cattle

Relationships among milk and fatcorrected milk yield, fat percentage, weight change, body weight, and days open were studied in 1,004 lactations collected over a 14-year period in a herd of Holstein cattle. Least-squares analyses of 31 to 60-day, 121 to 150-day, and of 181 to 210-day part lactations and total lactation were performed on an intra-sire basis. The regressions of milk and fat-corrected milk on weight change were consistently large, negative, and linear. The regressions of yield on average lactation body weight were significant and eurvilinear; production increased up to average or above-average weight, then declined. Effects of the number of days open were small, accounting for less than 2 % of the variance in all cases. Years and age were the factors most closely associated with variation in fat percentage. O . O ~1 cO Cq Cq