Ruminal pH Dynamics and Milk Production Response to Concentrate Supplementation in Pasture-Based Dairy Cows

The risk of subacute ruminal acidosis (SARA) in grazing dairy cows remains uncertain, particularly when concentrates are supplemented. This study evaluated the effects of concentrate supplementation on the evolution of diurnal ruminal pH and its relationship with production, nutrient utilization, digestive indicators, and health status. Eight ruminal-fistulated multiparous Holstein–Friesian cows were assigned to a 2-period crossover design comparing a pasture-only diet (PO) and the same pasture supplemented with 6 kg/d of grain-based concentrate (PC). Each 28 d period included 21 d of adaptation and 7 d of measurements. Cows were fed freshly cut perennial ryegrass. Ruminal pH was recorded at 0, 2, 4, 6, 8, 10, 12, and 18 h relative to feeding. Concentrate supplementation slightly decreased minimum ruminal pH (5.97 vs. 6.15) but remained above the SARA threshold. Total volatile fatty acids (VFAs) increased, and acetate molar percentage decreased with supplementation. Total dry matter intake increased by 3.5 kg/d, increasing milk yield by 3.85 kg/d, and improving milk protein concentration, while milk fat and fatty acid profile, ruminal ammonium, and clinical indicators were unaffected. Ruminal pH was higher and VFA concentrations lower late in the season. Overall, concentrate supplementation was associated with improved productivity without compromising ruminal stability under the conditions of this study; however, responses were influenced by seasonal variation in pasture characteristics.