Methods of estimating energy intake by production traits and body weight

Relationships among three estimates of energy intake of individual lactations were measured. The study was of 167 Holstein heifers in first lactation fed 1.8 kg of alfalfa hay and one of the two mixed rations of concentrate, corn silage, and grass silage ad libitum. Estimate 1 (Megacalories of net energy for lactation) was calculated from lactation milk yield, mean fat test, mean body weight, change in body weight, and net energy requirements for production, maintenance, and growth from the National Research Council. Estimates 2 and 3 were computed from feed intake of the individual and the National Research Council's estimates of net energy content of individual components of feed. Dry matter for each component of Estimate 2 was constant for the duration of the study, whereas Estimate 3 accounted for weekly variation in dry matter of feeds. Estimate 1 explained 88 and 91% of the variation in Estimates 2 and 3, indicating that accounting for weekly variation in dry matter improved the relationship only slightly. Regressions of Estimates 2 and 3 on 1 were 1.17 and 1.14. An empirical estimate of the net energy requirement for milk protein was 9.7 Mcal of net energy for lactation per kilogram of protein when data were adjusted for year-season of calving. Addition of protein yield to regression models of Estimates 2 and 3 on individual production and weight variables increased coefficients of determination by a maximum of 3%.