|dc.description.abstract||Microbial protein synthesis of steers of mean starting liveweight 286kg was established on a control diet of ad libitum winter grass and a treatment diet of ad libitum fodder beet with 1kg DM of lucerne silage as a fibre source. The experimental design was a four steer by two diet treatment comparison (experiment 1 and 2) with five sub-trials in each experiment. These series of trials assessed in vivo digestibility of diet energy and protein, nitrogen balance, and diurnal variations of rumen parameters (pH, redox, and acid, ammonia and urea concentrations) and urine and faeces production and parameters (pH, nitrogen concentration) in housed individual metabolism crates for 11d. Microbial protein production was estimated from purine derivatives determined from total urine collection for 9d of this period. Outdoors, under industry standard grazing conditions, rumen evacuations (10am, 6pm and 2am) and serial placement of in sacco nylon bag (0, 2, 4, 6, 12, 24, 48h) methods were used to assess rumen fluid passage rates, pool changes and feed dry matter and nitrogen degradability for both treatments.
Voluntary dry matter intake and microbial protein supply to the steers, and the mean diurnal rumen pH, was higher in the fodder beet treatment than in the winter grass treatment. Fodder beet supplied almost twice as much microbial protein as winter grass despite a lower crude protein content of the diet. Total rumen VFA and ammonia concentrations, and mean redox, were lower for the fodder beet than winter grass but the rumen urea concentration was higher in the fodder beet treatment. The winter grass treatment rumen nitrogen pool size was only greater than the fodder beet at the 6pm rumen evacuation. The initial 0h DM and nitrogen % disappearance of fodder beet bulb was extremely high as assessed by serial nylon bag placement and removal. However, the rate of DM and N disappearance (%/h) of fodder beet leaf after 6h incubation was higher than that of the fodder beet bulb and the winter grass. The higher voluntary intake, DMD% and rate of disappearance of fodder beet bulb and leaf compared with winter grass did not result in lower rumen pH or higher redox as would be expected by the current understanding of rumen function. The lower crude protein content and very low concentrations of rumen ammonia were not observed to reduce microbial protein production as would be expected by the current understanding of nitrogen kinetics in the rumen. The observed high microbial protein supply with fodder beet diets may be the result of ruminal and extra-ruminal adaptations to greater nitrogen recycling in low dry matter diets of high energy density not well described at present.||en