The effects of monensin sodium, administered by intraruminal controlled-release devices at a rate of 320 mglday, on milk and milk solids production and bloat score were examined in a trial with 90 lactating dairy cows given only mixed ryegrasslwhite clover pasture. Monensintreated cows had significantly greater yields of milk and milk protein than control cows (1397·2 (s.e. 22·9) v. 1296·9 (s.e. 16·3) kg milk, 49·6 (s.e. 0·9) v. 46·5 (s.e. 0·6) kg protein) over the 14-week period of treatment (P < 0·01). Yield of fat was similar in monensin-treated and control cows (62·9 (s.e. 1·5) v. 63·4 (s.e. 1·1) kg). Live weight, condition score and bloat score were not influenced by treatment but pasture conditions were not conducive to severe bloat challenge.
