Rumen pH as low as 5.6 has been reported in cows fed highly digestible pasture, however, incidences of sub-acute acidosis have not been well documented and there are no easy or reliable means for farmers to measure ruminal pH. This study collated animal and dietary parameters (81 treatment means), from 20 studies of lactating dairy cows fed pasture, to evaluate the relationship between ruminal pH and animal parameters, and identify suitable predictors of ruminal pH. Mean ruminal pH (5.6-6.7) was negatively correlated (P<0.05) with total volatile fatty acid concentration, milk yield, microbial N flow from the rumen, milk protein yield, milk fat yield, total non-structural carbohydrate content and acid detergent fibre content of the diet. No single dietary parameter, or group of parameters, was able to adequately predict ruminal pH. The increase in microbial nitrogen flow, milk, and milk fat production as ruminal pH decreased below 6.2 suggested that the performance of cows fed high quality pasture was not limited when mean ruminal pH decreased to 5.8. This implies that dairy cows fed pasture can tolerate a lower ruminal pH before sub-acute acidosis occurs than when forage-concentrate diets are fed.
Proceedings of the New Zealand Society of Animal Production, Volume 61, Christchurch, 241-243, 2001
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