Harper Adams University researchers investigated the effects of dietary crude protein content and supplementing a low crude protein diet with dietary starch or rumen-protected Met (RPMet) on dairy cow performance, metabolism, and nitrogen use efficiency when fed red clover and grass-based silage. The research included 56 Holstein Friesian dairy cows that were randomly assigned to one of four diets throughout a 14-week feeding period. The diets were designed to provide comparable metabolisable protein content, with crude protein concentrations of 175 g/kg dry matter (CON), 150 g/kg dry matter (LP), or LP supplemented with extra barley as a source of starch (+64 g/kg dry matter; LPS) or RPMet (+0.3 g/100 g MP; LPM).
Following the 14-week feeding period, 20 cows (5 per treatment) were given the same diets for an additional 6 days, and total urine output and faecal samples were collected. The researchers discovered that dietary treatment had no effect on dry matter consumption, but there was a diet-week interaction, with intake greatest in cows given LPS in week 4 and CON in weeks 9 and 14.
The research discovered that lowering the crude protein content of red clover and grass silage-based diets from 175 to 150 g/kg DM while maintaining MP supply did not effect performance, but did decrease urinary nitrogen excretion and increase nitrogen usage efficiency. Supplementing with extra starch or RPMet had minimal impact.
