COMPARISON OF THE PROTEIN EVALUATION SYSTEMS OF FEEDS FOR DAIRY-COWS

Four different feed protein evaluation systems were compared on the ba sis of 157 treatment means obtained from 25 productions trials. Compar isons were made between the British metabolizable protein (ARC, 1984; AFRC, 1992), the French PDI (Verite et al., 1979; Verite and Peyraud, 1989) and the Nordic AAT-PBV systems (Madsen, 1985). Protein values fo r feedstuffs were calculated using effective protein degradability (EP D) estimated by the nylon bag method and to estimate energy supply for rumen microbial protein synthesis either feed table values or determi ned diet digestibilities were used. Protein supply (S), requirement (R ), SIR ratio and utilization of absorbed amino acids available for mil k protein production were calculated using treatment means. The accura cy of the systems to predict differences in dietary protein value was studied by one-way analysis of variance using trial as a factor. Coeff icient of variation (C.V.) in SIR ratio and utilization of amino acids for milk protein production were smallest with the AAT-PBV system ind icating that this system was the most accurate in predicting differenc es in dietary protein value. Accuracy of the AAT-PBV system was furthe r improved by reducing the original passage rate of 0.05 h(-1) to 0.02 h(-1) for forages, 0.03 h(-1) for energy supplement and 0.04 h(-1) fo r protein supplements in calculating EPD values. Estimating microbial protein synthesis from digestible carbohydrates (DCHOs) and rumen degr adable protein (RDP) rather than from DCHO alone also improved accurac y. A value of 179 g per kg DCHO + RDP was used to estimate microbial p rotein synthesis. Since 1995, a modified AAT-PBV system has replaced d igestible crude protein for feed evaluation in Finland. Regression ana lyses showed that the utilization of AAT for milk protein production i mproved with increasing production level. In feeding recommendations t his was taken into account by reducing AAT requirement per kg of corre cted milk with increasing production level. (C) 1998 Elsevier Science B.V.