Four continuous culture fermenters were used to determine the ruminal pH required for the optimal digestion of pasture. Spring pasture was fermented at four controlled levels of ruminal pH (5.4, 5.8, 6.2 and 6.6) according to a 4x4 Latin square design. Decreasing pH from 6.6 to 5.4 reduced dry matter digestion by 14%, neutral detergent fibre digestion by 11%, total non-structural carbohydrate digestion by 5% and microbial nitrogen flow by 18%. A quadratic relationship was observed between ruminal pH and the true ruminal digestibility of dry matter (R²=0.94, SE=2.06) and microbial nitrogen flow (R²=0.66, SE=0.05). The ruminal digestibility of pasture dry matter was optimised at pH 6.35, and synthesis of microbial protein was optimised at pH 6.13. This experiment indicated that digestion was largely insensitive to pH across a broad range of pH (5.8 to 6.6), but that a large reduction in digestion occurred when mean pH was less than 5.8.
Proceedings of the New Zealand Society of Animal Production, Volume 59, , 66-69, 1999
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