Synthesis of microbial protein in the rumen. I. Influence of the level of nitrogen intake

Abstract

Direct measurement has been made of the daily production of microbial protein in the rumen. When sheep were fed on a virtually protein-free purified diet, increases in the intake of nitrogen from 2 to 9 g/day increased linearly the production of protein in the rumen from 32.5 to 50.0 g/day. There was no further increase in protein production when the nitrogen intake was raised to 16 g/day. The amount of total nitrogen flowing out of the rumen showed a net increase over that ingested at daily nitrogen intakes of 2 and 4 g, no change at 9 g, and a substantial net loss at 16 g. At the lowest nitrogen intake at least 4 g recycled nitrogen was utilized by the rumen microorganisms daily. The yield of protein from the ruminal fermentation increased from 9.1 g/100 g organic matter digested in the rumen when nitrogen was most limiting, to 13.3 g/100 g when nitrogen was in excess of requirements. It was calculated that protein production in the rumen at the highest nitrogen intake was less than the potential production from the energy supplied to the microorganisms. Possible limiting factors are discussed.