A comparison of microbial protein synthesis in beef steers fed ad libitum winter ryegrass or fodder beet

Microbial protein synthesis of steers (liveweight 286±9 kg) was compared with diets of ad libitum winter grass and fodder beet with 1 kg DM of lucerne silage. The experimental design was a four-by-two treatment comparison (experiment 1 and 2), using 11 day individual pen trials with total faecal and urinary collection. The trials assessed in vivo digestibility of diets, and diurnal variations in rumen parameters (pH volatile fatty acid, ammonia and urea concentrations) and urine and faeces production and N excretion. Microbial protein production was estimated from purine derivatives determined from total urine collection. Dry matter digestibility, voluntary dry matter intake and microbial protein supply to the steers, and the mean diurnal rumen pH, were higher in the steers fed the fodder-beet treatment than in those fed the winter-grass treatment. Fodder beet supplied almost twice as much microbial protein as winter grass despite a lower crude protein concentration of the diet. Total rumen VFA and ammonia concentrations were lower for the fodder-beet treatment than for winter grass but the rumen urea concentration was higher in the fodder-beet treatment. The observed high microbial protein supply with the fodder beet diets is likely the result of ruminal and extra-ruminal adaptations to greater nitrogen recycling in low dry matter diets of high energy density and low crude protein concentrations.