Protein level for alfalfa and corn silage-based diets: II. Nitrogen balance and manure characteristics

This N balance study was completed with 48 multiparous Holstein cows ( body weight [BW] = 653 kg; days in milk = 89) blocked by calving date and assigned to a 2 x 2 factorial arrangement of dietary treatments. The total mixed ration included alfalfa silage ( AS) or corn silage ( CS) as the primary forage source ( 41 and 14% vs. 14 and 41% of diet dry matter (DM), respectively) and were formulated for recommended ( RP) or excessive ( HP) amounts of rumen degradable protein (RDP) and rumen undegradable protein (RUP) according to the guidelines of the National Research Council (NRC). Crude protein (CP) averaged 16.5, 18.0, 16.4, and 17.3% for the AS-RP; AS-HP; CS-RP; and CS-HP diet, respectively ( DM basis). Regardless of primary forage source, the reduction in dietary CP to the NRC guidelines tended to improve milk yield (43.4 vs. 41.0 kg/d) but did not alter 3.5% fat-corrected milk (37.0 kg/ d) or milk true protein yield ( 1167 g/d). In this trial, cows fed the CS- based diets consumed less DM than those fed the AS-based diets in part because of rumen acidosis. The adverse effect of low rumen pH was accompanied by an increase in urinary N( UN) as a percentage of N intake, but did not alter milk yield. Notwithstanding partial confounding, fecal N (FN) was 49 g/d lower ( 213 vs. 164 g/d), UN was unchanged ( 229 g/d), but milk N tended to be higher ( 194 vs. 206 g/d) when cows were fed the CS- based diets compared with AS-based diets. Compared with the HP diets, cows fed the RP diets had similar FN ( 189 g/d) and milk N ( 200 g/d), but UN and urine urea N were reduced by 41 g/d ( 249 vs. 208 g/d) and 40 g/d ( 210 vs. 171 g/d), respectively. Fecal N concentration was higher for CS- based diets, but urinary N concentration was higher for AS-based diets. The reduction in dietary CP did not influence these concentrations but lowered urine volume. The metabolic relationships between energy and protein in determining the fate of excess dietary N ( primarily in the form of excess RUP in this trial) was illustrated by a 17% increase in the UN to FN ratio for cows fed AS-HP compared with the AS-RP diet and a 42% increase in the UN to FN ratio for CS-HP compared with CS-RP diet, when cows' energy status was compromised because of rumen acidosis. In this trial, UN ranged from 150 to 320 g/d, and was best predicted as UN (g/d) = 0.0283 x BW ( kg) x milk urea N (mg/dL). The NRC protein guidelines should not be exceeded to avoid unnecessary losses of manure N and, in particular, urine urea N.