Nutrient metabolism in the rumen and milk production in cows fed on grass-silage and fresh grass based diets

Abstract

The main objective of this experiment was to quantify and compare the flows of nitrogen (N), microbial N (MN) and amino acids (AA) to the small intestine of lactating dairy cows on fresh grass and grass-silage based diets. Four cows fitted with rumen and duodenal cannulae were subjected to the following four treatments in a Latin Square design, (1) grass offered ad libitum (G), (2) grass offered ad libitum+4 kg of a by-product based concentrate (C) per head per day (G4), (3) wilted silage offered ad libitum+C (W4) and (4) unwilted silage offered ad libitum+C (UW4). The double marker technique was used to measure flows to the duodenum with Yb and Co as the solid and liquid phase markers, respectively. Purines were used as microbial markers. Total dry matter (DM), organic matter (OM) or N intakes did not differ significantly between treatments. Flows of N, NAN, MN, lysine or methionine were not significantly different between G and G4. Flow of N was significantly higher on G4 compared to W4 ( P<0.05) and UW4 ( P<0.001). Flow of N on W4 was significantly higher than on UW4 ( P<0.01). Non-ammonia N (NAN) and non-ammonia non-microbial N (NANMN) flows were higher on G4 compared to W4 ( P<0.05) and UW4 ( P<0.001), and the flow of NAN was significantly higher on W4 compared to UW4 ( P<0.01). There was no significant difference between treatments in the quantity of MN flowing per kg of organic matter apparently digested (OMAD) or organic matter truly digested (OMTD) in the rumen but flow on UW4 was numerically lower than on the other treatments. Flows of total AA were significantly higher on G4 compared to UW4 ( P<0.01). Lysine and methionine flows as a proportion of total AA were not significantly different between G4, W4 and UW4 or between G and G4 ( P>0.05). Propionic acid concentrations were significantly higher, while acetic acid concentrations were significantly lower on G4 compared to W4 and UW4 ( P<0.01) while concentrations were not significantly different between G and G4 ( P>0.05). Ammonia concentrations in rumen fluid on W4 and UW4 were substantially higher than the concentrations on G4. It is concluded that in general flows to the duodenum of N and AA are higher on a fresh herbage based diet than on an unwilted grass-silage based diet. Wilted silage results in improved flows of MN compared to unwilted silage.