Abstract
A model to predict daily milk yield and body weight change was built based on a metabolizable energy (ME) system. The data needed for using the model are the daily DM intake and energy concentration (TDN or ME) of the feed consumed by cows, and body weight at calving. A key assumption of the model is the potential production in calorie bases for milk (pM) and growth (pG) proposed by Bruce et al. (1984), but includes some modifications. The real production in calorie bases for milk (rM) and growth (rG) were related to the potential production ; (pM-rM)/(pG-rG)=a constant. And to modify the daily potential milk production (pM) expressed in Wood's lactation curve, a parameter (p(t), t=days from calving) was assumed to be affected by a shortage of daily energy intake (p(t)=-d∫(pM-rM) dt, where d was a constant and P(0)=1.0). When the model was applied to two different experimental data obtained by high-producing cows, one was overestimated in milk yields, and the other was overestimated in body weight changes. For grazing cows with supplementary feed, the model outputs in milk yield showed good agreement with the experiment, but did not simulate the great fluctuation in body weight changes observed. The constant value of a converting factor from TDN toME and/or the constant value of gross energy concentration without consideration for the feed composition may cause a discrepancy between model outputs and experimerits. Another possibility is the value that converts energy gain to live weight gain because of large variation in the proportion of gut contents to live weight gain.
