The Effect of Applying Sodium Fertilizer on the Rate of Digestion of Perennial Ryegrass and White Clover Incubated in Rumen Liquor, with Implications for Ruminal Tympany in Cattle

A high herbage K:Na ratio increases the risk of ruminal tympany in cattle, which may relate to digestion rate. Experiment 1 examined whether in vitro digestibility of ryegrass was affected by NaCl fertilizer or by Na concentration in artificial saliva. Fertilizer Na increased grass digestibility, but Na in artificial saliva decreased it, probably due to the energy cost of sodium exclusion from bacteria. Increased herbage digestibility with fertilizer Na is therefore not due to additional Na, but may relate to increased water-soluble carbohydrates. Experiment 2 examined whether NaCl fertilizer applied at 35 or 70 kg Na ha(-1) to ryegrass and white clover affected in vitro gas production. Sodium fertilizer increased maximum gas output from grass and rate of production, confirming the increase in grass digestibility recorded previously but in clover it had the opposite effect, thereby potentially reducing ruminal tympany in cows fed a high legume diet. (C) 2001 Harcourt Publishers Ltd.