Effects of the composition of grass silages on milk production and nitrogen utilization by dairy cows

Sixteen varied grass silages metabolizable energy (ME): 9 . 76 to 11 . 99 MJ/kg ethanol-corrected toluene dry matter (TDM); crude protein (CP: 149 to 211 g/kg TDM; lactic acid: 3 . 5 to 134 . 7 g/kg TDM; butyric acid 0 . 4 to 46 . 7 g/kg TDM) were offered ad libitum to early-lactation daily cows (12 per experiment) along with a fixed allocation of 7 kg/day of a standard concentrate. Four silages were offered in each of four incomplete change-over design experiments with three 21-day periods. This design meant that each cow tons allocated to receive three of the four silages evaluated in that experiment. ME intake ranged from 108 to 262 MJ/day (mean 177 (s.d. 30 . 2)). Similar variation was obtained with milk yields (mean 26 . 5 (s.d. 4 . 36) kg/day), fat content (mean 37 . 7 (s.d. 5 . 60)g/kg) an_d protein content (mean 29 . 0 (s.d. 2 . 36)g/kg). Urinary purine derivative/creatinine ratio (PD/C), an index of microbial protein measured in spot samples (two per day) averaged 2 . 92 (s.d. 0 . 757) mol/mol. Allantoin made up an almost constant molar proportion of PD excretion (mean 0 . 876 (s.d. 0 . 0377)), with a small but significant (P < 0 . 001) decline of 0 . 0132 (s.d. 0 . 003) per unit increase in PD/C. Maximal utilization of silage nitrogen occurred with silages having higher ME and lower CP concentrations. Urinary PD/C suggested that microbial protein yield varied in a way which would not be predicted in current schemes and that it was a major source of variation in milk protein yield under the conditions of the present experiment. Principal components regression confirmed independent effects of ME supply and MP supply (indexed by urinary PD/C) on milk protein yield. Further work should pursue the possibility of using the urinary PD/C technique to refine protein feeding at the farm level.