Effects of dietary hydrochloric acid on voluntary food intake and metabolism of sheep in relation to the use of mineral acids as silage additives.

1. In Expt I, a pelleted grass-meal diet was supplemented with hydrochloric acid, added to the grass pellets before feeding, at five levels from 0 to 628 mmol/kg dry matter (DM). Each diet was offered ad lib. for 21 d to five sheep in a 5 x 5 Latin-square design. 2. Voluntary food intake decreased rectilinearly with increasing HCl supplementation (P less than 0.001), to 42% of the control value for sheep on the high-HCl diet. The decrease in food intake was related both to dietary pH and to the extent of metabolic acidosis induced by the HCl treatment. Although the pH of rumen fluid decreased slightly with increasing HCl supplementation, effects of the HCl treatment on volatile fatty acid concentrations in rumen fluid were not significant (P greater than 0.05). 3. In Expt 2, palatability and metabolic effects of dietary HCl were studied by comparing its effect when mixed into the pelleted grass meal before feeding, with and without a supplement of an equivalent amount of sodium bicarbonate given intraruminally, or when HCl was given intraruminally while the sheep consumed pelleted grass meal alone. Each of the three treatments was given at two levels of HCl, 280 and 560 mmol/kg DM. At each level of dietary HCl supplementation, the three treatments and the control diet (pelleted grass meal alone) were each given to four sheep, in a Latin-square design, for 11 d. 4. At the low level of supplementation, HCl, when mixed into the pelleted grass meal, reduced food intake by 17%, this effect was not altered by NaHCO3 supplementation, but when HCl was given intraruminally food intake was not reduced. At the high level of HCl supplementation, food intake was reduced by about 40% by each method of HCl supplementation; and NaHCO3 supplementation did not appreciably alter the effect of HCl on food intake, but prevented metabolic acidosis associated with the HCl treatments. Food intakes for the low-HCl treatments were significantly higher than those for the high-HCl treatments (P less than 0.01) and the level of dietary HCl x treatment interaction was also significant (P less than 0.01). DM digestibility, and the pH and volatile fatty acid concentrations of rumen fluid were not significantly affected by the different treatments. 5. It is concluded that at a low level of HCl supplementation the adverse effects of dietary HCl on voluntary food intake of sheep is determined by palatability associated with low dietary pH, whereas at a high level of HCl supplementation the effect is determined by palatability and by a metabolic response.