Responses to increased dietary contents of non-structural carbohydrates in herbage-based diets for dairy cows: Between- and within-cow ruminal pH variation

While dietary factors, especially high starch intake, are known to promote subacute ruminal acidosis (SARA), individual cows may differ markedly in their susceptibility and response - yet these inter- and intra-animal variations remain poorly understood, limiting targeted management. We investigated the effects of herbage-based diets with moderate (M-TNC) and high (H-TNC) levels of total non-structural carbohydrates (TNC) on ruminal fluid composition, pH, and performance of dairy cows, and their variation between and within animals. Three consecutive crossover experiments were conducted involving 14 animals. In experiments 1 and 3, cows were fed hay and concentrates (experiment 1: 23 (M-TNC) and 35 (H-TNC) % TNC; experiment 3: 18% and 36%). In Experiment 2, cows received either M-TNC (12% TNC) or H-TNC (23% TNC) fresh herbage without concentrate. The overall effects of the H-TNC diet were a greater TNC intake, a shift toward a propionate-enhanced ruminal fermentation profile, and a lower minimum ruminal pH compared to the M-TNC diet. A great between-animal variation in area under the curve at pH <5.8 (AUC_pH<5.8) was observed: two-thirds of cows showed increased AUC_pH<5.8 with the H-TNC diet, and one-third showed decreased or similar values. Half of the cows included repeatedly in at least two experiments consistently had a greater AUC_pH<5.8 with the H-TNC diet than with the M-TNC diet, while the other half showed alternating reactions. Ruminal pH reactions to great dietary TNC contents seemed to vary with diet, individual cow, and time; however, larger studies are needed to confirm these interactions.