VARIATIONS IN MILK OUTPUT AND MILK PROTEIN-CONTENT IN RESPONSE TO THE LEVEL OF ENERGY SUPPLY TO THE DAIRY-COW - A REVIEW

The purpose of this review was to determine with accuracy the effect of the lactation stage, length of under/over-feeding, age and production potential of the animals on the milk output and milk protein content response to variations in the energy supply. Sixty-six feeding trials on dairy cows producing 9 to 29 kg/day (6 to 30 cows/experimental group) and including at least three levels of energy supply were compared. The energy supply was expressed as an energy unit effectively available to the animal (Feed Unit for Lactation, UFL). In early lactation, the milk yield response to energy supply was linear and large (+ 0.9 kg of milk/UFL). In mid-lactation and in long-term trials this response was curvilinear. When the supply was below the requirements, the response was higher in long-term trials than in short-term trials performed in mid-lactation (2.2 kg/UFL versus 1.3 kg/UFL. P < 0.01). The protein content response to energy supply was linear whatever the stage of lactation or the length of the trials. In early lactation, it was half that in mid-lactation or in long-term trials (0.6 g per kg/UFL). The relation between variations in milk yield and in milk protein content was curvilinear and depended on the stage of lactation and on the level of feeding proportional to the energy requirements. It was not possible to demonstrate any significant effect of the production potential on milk yield and milk protein content responses to variations in energy supply.