Influence of the ruminal N-balance on rumen fermentation, microbial protein synthesis, amount of utilisable crude protein and N-excretion

Five dairy cows alloted to four groups were used to investigate the influence of the ruminal N-balance (RNB) on rumen fermentation, microbial protein synthesis (MP), utilisable crude protein (uCP) amount and N-excretion. Animals were fed a ration of cornsilage and concentrates (50% : 50% on dry matter basis) which was formulated to meet the requirements of NEL and uCP. RNB amounted to -0,6 g/MJ ME in the basis ration. The other 3 rations were adjusted to RNB-values of -0.3, 0 and 0.3 g/MJ ME which was accomplished by urea supplements. The RNB increase resulted in a higher NH3-N concentration in the rumen fluid (1,6-28,5 mg/100 ml) and in the digesta (2,9-11,3 mg/100 ml). Additionally, urea concentration in the milk was increased (13,6-63,0 mg/100 ml). However, the difference of RNB did not show effects on pH-value, volatile fatty acids concentration in the rumen fluid as well as on ruminal digestibility of the organic matter (47,5-50,5%) and on efficiency of MP synthesis (7,7-8,3 g MP/MJ ME; corresponding to 144-148 g MP/kg fermented OM). The group with the lowest RNB (-0,6 g/MJ ME) showed the highest level of feed protein degradation (UDP= 338 g/d versus 464 g/d, which was the average of the other three groups). Additionally, the amount of fermented organic matter (FOM) was found with 9.2 kg/d to be significantly lower as compared to group 0 (9,9 kg/d). The amount of MP which was reduced in this way and the lower amount of UDP resulted in a reduced amount of uCP, which was significant between RNB-group -0.6 and group 0. As compared to the other two groups the difference was nearly 200 g.