The objective of the present study was to evaluate the effect of virginiamycin (VM) inclusion on milk yield and composition in Holstein cows maintained under commercial conditions in Mexico. Four pens of lactating cows (approx. 100 cows each) were blocked based on lactation number (LN), DIM, and milk yield (2.1 and 2.3 LN; 190 and 115 DIM; and, 47.3 and 48.5 L/d, for blocks 1 and 2 respectively) and assigned to 2 treatments. Control cows (CT) were fed a diet balanced to meet or exceed their nutrient requirements (NRC, 2001); Treated cows (TR) were fed the CT diet plus 500 mg/cow/d of VM. Diets were randomly assigned within each block. Diets were based on corn silage and steam-flaked corn. Feed offered and refused was weighted and analyzed daily to determine pen DMI. The study lasted for 42 d; d 1–19 for adaptation, and d 20–42 to measure milk yield and composition. Individual milk records were obtained daily. Additionally, one pooled milk sample per pen was collected for milk fat and protein determination using a MilkoScan Mars equipment (FOSS, Hillerød, Denmark). A mixed model analysis with repeated measures was conducted at the end of the trial. Pen was the experimental unit. The model included treatment as fixed effect and block as random effect. No cows were diagnosed with clinical acidosis during the course of trial and no day × treatment interactions were observed (all P > 0.10). However, milk yield, milk fat, and ECM were significantly greater in the TR group (P < 0.01; Table 1). TR cows tended to have greater DMI but milk:feed ratio was not affected by treatment (P > 0.10). Milk fat:protein ratio was significantly increased (P < 0.01) suggesting the effects on milk yield and composition may have been related to VM preventing the development of SARA and other related digestive disorders. Table 1. Milk yield, composition, ECM yield, and DMI during the measurement period

Item	CT	TR	SEM	Treatment effectP-value
Milk yield, L/d	42.7	43.3	0.25	<0.01
Milk fat, %	3.43	3.49	0.120	<0.01
Milk protein, %	3.30	3.28	0.004	<0.01
ECM1, kg/d	42.8	43.7	0.16	<0.01
Fat:Protein	1.04	1.06	0.003	<0.01
DMI, kg/d	27.4	27.8	0.20	0.08
Milk:DMI	1.57	1.56	0.011	0.67

¹ECM, lb = 0.327 × Milk yield, lb + 12.95 × Fat yield, lb + 7.65 × Protein yield, lb.