Energy metabolism of lactating dairy cows treated with prolonged-release bovine somatotropin or energy deficiency.

Abstract

Two experiments were carried out with lactating Simmental x Red Holstein Friesian dairy cows. In the first experiment, 8 cows each received six injections at 4-wk intervals of either saline or 640 mg of prolonged-release bST. In the second experiment, 14 of 20 cows received 20 MJ NEL less than their daily requirement for 3 wk. Digestible and metabolizable energy were not affected by the treatments. Mobilization of body energy compensated the bST-induced increase of milk energy and supported milk production of underfed cows by two-thirds of the net energy deficit. The utilization of mobilized body energy and metabolizable energy from the diet for maintenance and milk production was unchanged. The bST and energy deficiency increased N utilization for milk production, because bST increased milk N secretion at the same N intake, and energy deficiency partly compensated the reduced N intake by N mobilization. Milk yield oscillated between the bST injections. Mean milk yield increased by 2.8 kg at constant milk composition. During energy deficit, milk yield dropped by 1.7 kg with decreasing milk protein content.