Some Relationships of Body Weight and Age with First-Lactation Yield

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Abstract
The data were from the Illinois cross-breeding experiment which included Holsteins, Guernseys, and 3 intermediate crossbred groups. On a season-sire-breed group basis, age at freshening, when included with days carried calf (DCC) as the only additional covariate, was significantly associated with first lactation yields of milk fat and fat-corrected milk (FCM). Body weight taken soon after calving, included with DCC as the only additional covariate, was significantly associated with yields of milk, milk fat, and FCM. When both factors were included as additional covariates, weight retained its importance, but the independent association of yield with age became trivial; lactation milk yield increased 395 kg with each 100-kg increase in body weight. Neither factor was significantly associated with per cent milk fat. In a separate analysis, lactation yields of milk, milk fat, and FCM varied approximately as the .6 power of body weight at calving. Expressing first-lactation yield as a function of body weight was proposed as a measure of performance. A final analysis indicated that body weights estimated directly from a tape were consistently high and often unreliable.
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