Validation of protein evaluation systems by means of milk production experiments with dairy cows

Abstract

Protein evaluation systems (crude protein (CP), digestible crude protein (DCP), protein digested in the intestine (PDI), amino acids truly absorbed in the small intestine (AAT), absorbed protein (AP), metabolizable protein (MP), crude protein flow at the duodenum (AAS) and digestible protein in intestine (DVE)) were validated using data from 15 production experiments with dairy cows, carried out in the Netherlands. Only treatments that were deficient in protein according to at least one system were selected. Average yield was 31.2 kg of fat and protein corrected milk daily and 989 g of milk protein daily. The observed milk protein production was compared with milk protein production predicted from the protein supply and requirements in each system. The difference between observed and predicted milk protein production expressed as the absolute and relative prediction error was smallest for the DVE-system (-2 g/day; 5.7%) and increased in the order CP (-22 g/day; 6.7%), PDI (-19 g/day; 7.8%), DCP (-44 g/day; 8.8%), AP(-37 g/day; 9.3%), AAS (100 g/day; 11.7%), AAT (112 g/day; 13.4%) and MP system (204 g/day; 22.9%). Predictions can be improved when a variable efficiency of milk protein production is used. In the DVE-system the observed efficiency decreased with increasing protein to energy ratio in the diet and milk production level. It was concluded that under Dutch conditions the prediction of milk protein production decreased in the order DVE, CP, PDI, DCP, AP, AAS, AAT and MP system.