Relationships between Age, Body Weight, and Yield of Dairy Cows

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Abstract
Comparisons were made of the relative effects of age and weight on yield. Yield components studied were milk, butterfat, total solids, fat-corrected milk (FCM), solids-not-fat, protein, and lactose. There were 216, 305-day lactation periods of Holstein, Jersey, and Guernsey cows maintained as one experiment station herd. On an intra-breed basis FCM yield increased 570 lb./100 lb. in weight (significant), but when the effects of age were removed the increase was 236 lb. (not significant). Similarly, butterfat changes were 22 lb. (significant) and 9 lb. (not significant). On an intra-breed basis the relationship between yield and age was more than 2 times as important as weight as determined by the standard partial regression coefficients. Breeds for the various comparisons were strikingly different. Effects of weight on yield are interpreted too highly if differences due to breed and age are not removed.