RESPONSES IN THE VOLUNTARY INTAKE OF HAY OR SILAGE BY LACTATING COWS TO INTRARUMINAL INFUSIONS OF SODIUM-ACETATE OR SODIUM PROPIONATE, THE TONICITY OF RUMEN FLUID OR RUMEN DISTENSION

Rumen-fistulated lactating cows were individually fed on hay or silage and intakes were monitored during 3 h treatment periods and for 2 h a fter. Each experiment used five, six or seven animals and the treatmen ts were applied in a Latin Square design. Sodium acetate infusions of 1.8-11.0 mol in 4.5 litres water caused a dose-related depression in h ay intake, the extent being 82 g dry matter (DM)/mol infused (P < 0.01 ). Sodium acetate infusions of 6-0-15.0 mol in 4.5 litres water caused a dose-related depression in silage intake of 118 g DM/mol infused. R umen fluid pH for both diets was unaffected by treatment. Acetate and Na concentrations were increased and significantly negatively correlat ed with intake of both diets. Infusions of 2-8 mol sodium propionate c aused a dose-related depression of hay intake which was significant wh en cow and day effects were accounted for. Sodium propionate infusions of 4-8 mol significantly depressed silage intake by 140 g DM/mol infu sed (P < 0.001). Rumen fluid pH was unaffected by treatment while prop ionate and Na concentrations were elevated and significantly negativel y correlated with intake for both diets. Inflation of a rubber balloon in the rumen with 12.5-20 litres warm water resulted in a dose-depend ent depression in hay intake of 66 g DM/l distension (P < 0.05). There was significant overeating during the 2 h following the 20 litre trea tment. With silage, 15-25 litres of balloon distension for 3 h resulte d in a dose-dependent depression in intake of 28 g DM/l distension (P < 0.001). There was no significant overeating during the 2 h following distension. When given in physiological amounts, at the lower end of the range used in these experiments, acetate, propionate and distensio n of the rumen did not significantly affect hay intakes. However, in e ach case the linear relationship between intake depression and level o f treatment suggested that these factors could contribute to the contr ol of feed intake.