The objectives of the present study were (i) to investigate the changes in ruminal E_h and pH of lactating cows during a dietary transition from a low to a higher level of readily fermentable carbohydrates (RFC), and (ii) to compare the daytime and 24-h measurement of these 2 parameters. The experiment lasted 37 d. Eight early (averaged 47 DIM) lactating Holstein cows fitted with ruminal cannulas were fed a basal diet (67.7% maize silage, 10.8% alfalfa hay, and 21.5% concentrate, DM basis) with low level of RFC (% DM) (1.4% of soluble sugars, 18.2% of starch) for 21 d. Thereafter, they were fed 3 successive diets (containing 3.5%, 5.6% and 8.6% soluble sugars; 16.4%, 17.7%, 19.4% starch, respectively) at d 22, d 27 and d 32 to manage a progressive transition. Diets were offered ad libitum in equal amounts twice daily. The DMI and milk production were recorded individually. Ruminal E_h and pH were continuously measured for 3 d at the end of each dietary treatment, by using a ruminal submersible data logger (Dascor, Escondido, CA). The E_h and pH data were summarized as mean E_h and pH over daytime (from 1 h before morning feeding to 8 h after) and over 24 h. Dry matter intake (P = 0.361) and milk yield (P = 0.868) did not change during the dietary transition: in average 18.2 kg DM/d and 32.6 kg/d respectively. Increasing proportions of dietary RFC increased significantly E_h (+ 56 mV) and decreased pH (- 0.32). Compare with mean daytime pH, mean pH over 24 h allows a better distinction between treatments (Table 1). In conclusion, a long-term continuous 24-h measurement shows an effect of increased proportions of RFC in the diet on the diurnal pattern of ruminal E_h and pH. Table 1. Ruminal E_h and pH in lactating dairy cows fed increasing proportions of readily fermentable carbohydrates

	Treatment				SEM	P-value
	Basal diet	Diet 1	Diet 2	Diet 3
Daytime ruminal Eh, mV	−317.7a	−292.7ab	−280.8b	−261.0b	42.9	<0.001
24 h Ruminal Eh, mV	−318.3a	−291.6ab	−282.7b	−261.9b	42.3	<0.001
Daytime ruminal pH	6.25a	6.16b	6.13b	5.95b	0.17	<0.001
24 h Ruminal pH	6.25a	6.14b	6.09bc	5.93c	0.17	<0.001