Investigations on the effect of the ruminal N-balance on rumen metabolism, urea content in blood serum and milk as well as some liver parameters of lactating cows

Abstract

Seven dairy cows fitted with ruminal and duodenal cannulae were used to investigate the influence of the amount of ruminally available N (Ruminal N-Balance, RNB) on the rumen metabolism and to answer the question on the lowest N-amount in the rumen, without negative effects on the fermentation. Animals were fed a ration on the basis of 7.9 kg corn silage and 7.2 kg concentrates related to dry matter, intended to meet the animals mean NEL and protein requirements. 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 by urea supplements in the concentrates. The increase in RNB resulted in higher NH3-N concentrations in the rumen fluid and in the duodenal digesta and higher urea concentrations in the blood and milk. The significantly highest amount of protein at the duodenum was detected when RNB showed an equilibrium (RNB = 0). The efficiency of microbial protein (MP) synthesis (g MP/kg fermented organic matter) was the same, g MP/d and g MP/MJ ME were significantly lower with RNB = −0.6 g/MJ ME as compared to RNB = 0. The group with the lowest RNB showed the highest level of feed protein degradation as well as the lowest organic matter, NDF and ADF fermentation. An effect on cholesterol, total bilirubin and γGT due to different RNB was not detected. The activities of GLDH and AST were highest when the RNB was −0.6 g/MJ ME. From the results, it can be concluded that significantly negative effects on rumen fermentation occur when RNB-values are below −0.3 g/MJ ME. However, a positive RNB did not increase the degradation and synthesis capacity of the rumen micro-organisms as compared to RNB = 0.