The objective of this study was to evaluate the effects of dietary supplementation of malate
(MAL) and active dried yeast (ADY) on feed intake, rumen fermentation parameters, rumen
microbial populations, selected blood metabolites, and milk production during a sub-acute ruminal
acidosis (SARA) challenge in primiparous lactating dairy cows. Six rumen-fistulated Holstein dairy
cows (body weight: 630 ± 55 kg, 110 ± 25 days in milk, mean ± SD) were assigned to the following
treatments in a 3 × 3 Latin square design: 1) control TMR (CON); 2) a TMR supplemented with 80
g of sodium-calcium malate/head per day (MAL); and 3) a TMR supplemented with 10 g of active
dried yeast providing 20 × 109 CFU of Saccharomyces cerevisiae/head per day (ADY). Each
experimental period consisted of 14 d of adaptation to the experimental treatments, 4 d of SARA
challenge, and 10 d of rest. Dry matter intake (18.4 vs. 19.8 kg/d), and milk yield (29.3 vs. 30.4
kg/d) were depressed during SARA compared with adaptation. Malate and ADY had no effect on
DMI and milk yield during the adaptation and SARA phases. Malate and ADY had no effect on
ruminal pH characteristics during adaptation. During SARA, maximum and mean ruminal pH was
not affected by supplementation, but minimum ruminal pH tended to be higher for ADY compared
to CON and MAL. Time spent with ruminal pH