Interactions Between Energy and Protein (Amino Acids) in Lactating Dairy Cows

摘要

Energy is required for cows to efficiently convert amino acids (AA) into milk protein, and crude protein (CP) is needed by cows to efficiently convert gross energy into net energy for lactation (NEL). In early lactation when cows are in negative energy balance, milk protein yield is likely to increase when supply of the proper AA is increased independent of energy; however, in later lactation, response to AA (or CP) supplementation is dependent on energy supply. In later lactation, if energy allowable milk is approximately equal to actual milk, increasing supply of AA or CP will not greatly affect milk protein yield; however, if energy allowable milk exceeds actual milk, milk protein yield should respond to improved protein nutrition. Because of both economic and environmental reasons, lower concentrations of dietary CP in lactation diets are often encouraged, and in many situations, they have been implemented successfully. However, reducing dietary CP concentrations can reduce NEL intake via reduced digestibility and DM intake. Reducing CP via reducing rumen degradable protein (RDP) appears to have the greatest negative effect on digestibility, even when the resulting RDP concentration still appears adequate (e.g., -10% of diet DM). Therefore, RDP should be maintained in lower protein diets. The negative effects of reducing dietary protein also likely depend onwhat nutrient or nutrients replace the CP that is being removed from the diet. If CP is replaced with forage NDF, intake often will decrease. In higher starch diets, replacing CP with starch may reduce digestibility, and in lower starch diets, replacingCP with byproduct NDF likely will reduce digestibility.