Hot summer temperatures are a highly limiting factor for milk production of dairy cows. To maintain body temperature, cows reduce intake, which decreases the heat increment that results from rumen fermentation. In order to improve milk production under these conditions it is necessary to restore intake, and/or improve nutrient digestibility. Since forage fiber requires more time for its degradation in the rumen, it produces more heat of fermentation than concentrates.
Less fiber in the diet however, results in less chewing, reduced saliva production, more grain fermentation, and ultimately a more acidic rumen pH. These changes shift the pattern of fermentation from predominantly acetate to propionate→lactate, risking the appearance of sub-clinical or clinical rumen acidosis. Past research has demonstrated that feeding live yeasts or yeast cultures increases milk production of cows under heat stress.