Predicting maternal protein and fat balances of growing and mature dry cows

To examine factors affecting maternal protein and fat changes of dry cows, we fed a diet of silages at varying intakes. Conceptus-corrected balances (fat and protein) were computed and predicted from ration protein and energy densities (digestible, metabolizable, or total digestible nutrients), predry body weight, dry matter intake as percents of body weight or predry body weight, and square or natural log of each. Maximum daily dry matter intake was 1.8% of body weight. Observed energy density did not change with changing intake. Proportion of ingested energy apparently stored increased with dry matter intake/predry body weight, primarily at the expense of heat production. Partition of energy and nitrogen were not significantly affected by this intake expression. Cows in first and subsequent dry periods fed low intakes had positive maternal fat balances at energy intakes equal to or below established maintenance requirements. Body condition score of primiparous cows did not increase and their growth was related to dry matter intake, predry weight, total digestible nutrients, and protein percentage of the diet; daily maternal fat balance followed protein balance (2:1 ratio) up to energy intake 14% above maintenance, thereafter increasing to 4:1 ratio. In older cows, maternal daily fat balance varied primarily with digestible energy density of ration, intake, and to a lesser extent with predry weight and ration protein; protein balance was not closely related to factors studied.