The aim of the present study was to evaluate the effect of virginiamycin (Vm) plus monensin (Mn) on milk production and composition of grazing dairy cows. Eighty dairy cows, with an average of 2.5 lactations, 129 d in milk and a milk yield (MY) of 27.7 kg per day were randomly distributed into 2 groups; Group 1 (n = 40): 300 mg/cow/d Mn (M), and Group 2 (n = 40): 300 mg/cow/d Vm and 300 mg/cow/d Mn (V). Cows were grazed in a rotational grazing system of 2 grass paddocks per day. The cows were milked twice a day and received 4 kg of a pelleted concentrate and mineral supplement in the parlor at each milking. Both groups were also fed once a d 7 kg corn silage, 6 kg grass haylage, 2 kg corn grain, and 2 kg wheat middlings (all quantities as fed). Milk yield and composition were measured on d 0, 17, 34, 52, 66, 78, 90, 101, 111, 120 and 129, and body condition score (BCS) was measured on d 0, 52 and 129. Four-percent fat corrected milk (FCM) was estimated using the following formula MY × (0.383 × % fat + 0.242 × % protein + 0.7832)/3.1138. Data were analyzed using a mixed model with repeated measures, using the cow as random effect, and treatment, time and their interaction as fixed effects. The treatment × time interaction was not significant in all evaluated variables (Table 1). However, on FCM a trend for time and treatment interaction (P = 0.14) was approached: V cows had greater FCM on d 52, 78 (P < 0.1) and 101 (P < 0.05) than M cows. The V treatment group had numerically greater MY, milk fat and protein content, and BCS at the end of the trial, but differences were not statistically significant (Table 1; all P > 0.15). In conclusion, the addition of Vm in diets containing Mn increases FCM during peak- to mid-lactation. Table 1. Milk yield, composition, and FCM in grazing cows supplemented with Mon (M) and Mon+Vm (V)

Item	Treatment		P-value
M	V	Trt	Time	Trt × Time
MY, kg/d	27.6	28.3	0.46	<0.01	0.64
Milk fat, %	3.34	3.46	0.15	<0.01	0.47
Milk protein, %	3.21	3.24	0.26	<0.01	0.72
FCM, kg/d	25.0	26.1	0.14	<0.01	0.14
d 0 FCM	27.1	26.8	0.80
d 34 FCM	25.8	26.8	0.33
d 52 FCM	27.4	25.7	0.06
d 78 FCM	28.3	26.5	0.07
d 101 FCM	26.7	24.6	0.04