Prediction of ruminal pH from pasture-based diets.

This study identified suitable predictors of ruminal pH and identified relationships between ruminal pH and animal measures for diets based on fresh pasture. Animal and dietary variables (121 treatment means from six countries) were collated from 23 studies of lactating dairy cows fed pasture. Mean daily ruminal pH ranged from 5.6 to 6.7 across studies. Within studies, a low ruminal pH was associated with higher (P < 0.05; r2 > 0.40) microbial N flow from the rumen, total and individual volatile fatty acid concentrations, milk and milk component yields, and dry matter intake, and with lower (P < 0.05; r2 > 0.30) concentrations of milk fat, fat:protein, and acetate:propionate. Large variation between studies meant that these ruminal and production variables could not be used to make reliable predictions of ruminal pH in future pasture-based studies or feeding scenarios. Ruminal pH was positively related (P < 0.05; r2 < 0.15) to forage neutral detergent fiber (NDF) and NDF content within study, and negatively related (P = 0.001; r2 = 0.14) to nonstructural carbohydrate across studies. No single dietary variable, or group of variables, could be used to make a reliable prediction of ruminal pH. Estimates of effective fiber for diets containing only pasture were made using the Cornell Net Carbohydrate and Protein System ruminal pH equation. Mean effectiveness of fiber in pasture was 43% of NDF, and ranged from 17 to 78% across studies. High flows of microbial nitrogen, milk, milk fat yield, and dry matter intake suggested that the performance of cows fed high quality pasture was not limited when mean ruminal pH decreased to 5.8.