The effects of urea and artificial saliva on rumen bacterial protein synthesis in sheep receiving a high‐cereal diet

Abstract Four sheep fitted with cannulas in the rumen and duodenum and given a basal diet of 18% chopped hay, 41% rolled barley and 41% flaked maize, known to be associated with low rates of bacterial protein synthesis in the rumen, were used in a 4 × 4 Latin Square experiment to study the effects of continuous intraruminal infusions of water (2 litres day−1; control), urea solution (7.5 g urea litre−1, 2 litres day−1; urea), artificial saliva (4 litres day−1; saliva) and artificial saliva with added urea (3.75 g urea litre−1, 4 litres day−1; saliva+urea) on rumen ammonia‐nitrogen concentration, pH, liquid clearance rates and bacterial protein synthesis. Rumen ammonia‐nitrogen concentrations for control, urea, saliva and saliva + urea treatments were 81, 158, 38 and 151 mg litre−1, respectively. Corresponding mean values for rumen pH were 6.18, 6.09, 6.37 and 6.41 units and for rumen liquid clearance rates 0.082, 0.057, 0.062 and 0.051 h−1. The mean rate of bacterial protein synthesis in the rumen, estimated from the duodenal entry of α‐ϵ‐diaminopimelic acid, for the control treatment was 163 g crude protein (CP) kg−1 organic matter (OM) apparently digested in the stomach. Corresponding rates for the saliva and saliva + urea treatments were 169 g CP kg−1 OM apparently digested and 215 g CP kg−1 OM apparently digested, indicating responses in protein synthesis to the urea+saliva treatment but not to the saliva alone treatment. Infusion of urea solution increased the mean rate of protein synthesis to 204 g CP kg−1 OM apparently digested in the stomach but the responses were variable between animals and in three of the animals were small. It is concluded that with the type of diet used an enhanced entry of saliva into the rumen is necessary to ensure that supplementary urea produces a consistent improvement in ruminal protein synthesis.