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Some Relationships of Body Weight and Age with First-Lactation Yield¹

Department of Dairy Science, University of Illinois, Urbana

ABSTRACT

The data were from the Illinois crossbreeding experiment which included Holsteins, Guernseys, and three intermediate crossbred groups. One set of analyses indicated that on a within season-sire-breed group basis: 1) 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) ; 2) body weight taken soon after calving, included with DCC as the only additional covariate, was highly significantly associated with yields of milk, milk fat, and FCM; 3) 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; and 4) 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.

¹ Data for this study came from a cooperative project between the Illinois Agricultural Experiment Station and the Dairy Cattle Research Branch AHRD, ARS, USDA. The project is a contributing project to the North Central Regional Project NC-2, The Improvement of Dairy Cattle Through Breeding.

² Present address: Regional Swine Breeding Laboratory, Iowa State University, Ames, Iowa.