The influence of the dietary balance between energy and protein on milk urea concentration. Experimental trials assessed by two different protein evaluation systems

Twenty-three dairy cows were fed rations with different proportions of energy and digestible crude protein (DCP). When the ration was balanced for energy and DCP according to Swedish standard the cows' milk urea concentration was 4.66-4.92 mmol/l (95% CI of mean). With increasing intakes of DCP, fed together with standard levels of energy, the mean milk urea concentration increased in proportion to the surplus of DCP. In contrast, the concentration of urea decreased when the cows were overfed with energy at the same time as they were underfed with protein. When the rations were recalculated in accordance with the AAT/PBV system for dietary protein evaluation the 95% CI for the mean milk urea concentration of the cows receiving a balanced ration was 3.76-4.56 mmol/l. The concentration of urea was dependent primarily on the PBV. When the 2 protein evaluation systems were compared there was a strong correlation between PBV and DCP. Ammonia was the only constituent of the rumen whose concentration was strongly correlated with the milk urea concentration. Taken together with earlier data the present results suggest that a milk urea concentration between 4.0 and 5.5 mmol/l should be regarded as normal at least when cows are fed conventional feedstuffs.

[ATP in the metabolism of ruminants]

The ATP yield from the carbohydrates of anaerobically living microorganisms in the rumen amounts to only 5-10% of the ATP yield of the intermediary metabolism in the presence of oxygen. Vital functions and thus microbial protein synthesis are due to protein degradation in the rumen. The ATP yield in the intermediary metabolism of ruminants is mainly achieved from propionate and microbial protein by means of gluconeogenesis because the absorption of glucose from digested starch is very low. The relationships between ATP yield in the rumen and the processes of glucose provision for the production of lactose as well as the protein content of the milk are shown. As important processes of ATP production in microorganisms from easily soluble carbohydrates take place in silage preparations before feed intake, the corresponding consequences for the metabolism of high-performance cows fed with silage are shown.