Effect of forage type and supplementation on microbial synthesis and efficiency in the rumen of dairy cows

A wide range of different forages were either fed as the sole feed or supplemented with a fixed amount of concentrate and fed to dairy cows in four separate experiments. Rumen microbial AAN synthesis was calculated based on content of RNA in isolated rumen liquid associated bacteria and in duodenal chyme. Efficiency was calculated based on apparent total tract digested carbohydrates, apparent and true rumen digested OM and true rumen digested fermentable OM. Microbial AAN synthesis varied from 47 g/d for unsupplemented late cut grass silage to 174 g/d for supplemented lucerne hay, and seemed to increase when forages were supplemented. Intake of OM, true rumen digested fermentable OM and total digested carbohydrates could explain 79%, 70% and 72% respectively of the variation in microbial AAN synthesis, whereas fractional rate of passage of liquids and particles only could explain 36% and 6% respectively. Efficiency was clearly not constant as assumed in many protein evaluation systems. Efficiency varied from 15 to 26 g AAN/kg rumen truly digested fermentable OM, and only little of the variance in efficiency could be described from fractional passage rates of liquid and particles and feed characteristics.