High-energy diets increase the risk of Sub-Acute Ruminal Acidosis (SARA) in ruminants. The behaviour of ruminants is modified under SARA however the impact on behaviour of a high-energy diet and those of SARA per se are often confounded. We aimed at distinguishing the effects of the diet and of SARA on cow behaviour. We fed 28 Holstein cows a low-starch diet (low starch diet, 10.5% starch) or a high-starch diet (high starch diet, 31.5% starch). Control cows (n = 14) received the low starch diet for 60 days, Challenge cows (n = 14) received the same diet except for a 2-week period during which they received the high starch diet and the 10 preceding days to ensure the transition between diets. We monitored ruminal pH and activity of cows thanks to sensors, dry matter intake (DMI), and milk yield of each cow on a daily basis. The SARA status was defined according to the relative decrease in ruminal pH and to pH variability. High starch diet induced SARA more often than low starch diet (81% days when cows received high starch diet vs. 8% when they received low starch diet. High starch diet also decreased milk yield and made cows spending less time eating but eating more quickly (Challenge vs. Control cows during the challenge period: milk yield, 20.0 vs. 18.2 L/d; % time spent eating, 22.5 vs. 27.6; eating rate, 77.1 vs. 69.6 g DMI/min; P (diet x period) < 0.001 in all cases). A decrease in milk yield, a decrease in the time spent eating and an increase in eating rate were also observed to a lesser extent in cows experiencing SARA during the transition or the challenge periods, no matter the diet (Challenge and Control cows: milk yield, - 0.5 and -0.3 L/d, P(SARA) = 0.03; % time spent eating, - 1.4 and - 0.84, P(SARA) = 0.02; eating rate, +4.9 and +3.2 g DMI/min, P(SARA) = 0.06; P (SARA x diet) > 0.50). In conclusion, an increase in eating rate, especially combined with a decrease in milk yield should alert farmers to the risk of ruminal acidosis.