OmniGen-AF (OMN; Phibro Animal Health, Teaneck, NJ) is an immune stimulating feed additive composed of a blend of ingredients including silicon dioxide, aluminosilicate, brewers dehydrated yeast, dried Trichoderma longibrachiatum formation products, and B-complex vitamins. This experiment aimed to determine whether OMN influence milk yield and composition of cows under a semi-intensive management. The experiment was conducted in a commercial farm in Brazil (77.8 temperature-humidity index). Thirty-six multiparous Holstein cows (136 ± 52 DIM, 23.8 ± 4.96 kg/d, and 512 ± 65 BW, at the start of experiment) were used in a completely randomized experiment, being assigned to control diet or top dressed OMG (50 g/d, equal amounts in each meal) during 84 d. Cows were maintained in pasture of Panicum maximum (cv. Mombasa), excepted for periods of milking and meals (4 h/d). Cows received feed in individual bunks consisting of fixed amounts of corn silage (6.2 kg/d DM basis), and grain mixture adjusted according to NRC (2001). Cows were milked and fed twice daily (0500 h and 1600 h), being the milk weight registered and samples taken weekly. Milk samples were analyzed to contents of fat, protein, lactose, casein, urea N, and somatic cell count (SCC) at Clínica do Leite (University of Sao Paulo, Piracicaba, Bazil). Milk urea N concentration was determined by enzymatic-colorimetric method. Data were analyzed as repeated measures using the PROC MIXED of SAS 9.3. OMN-fed cows had greater yield of milk, 3.5% fat-corrected milk, and milk fat content than those fed control (Table 1). Further, cows fed OMN showed lower milk urea N concentration and SCC than control-cows. OMN improved milk yield and SCC of dairy cows in a semi-intensive management. Table 1.

Item	Treatment		SEM	P-value
	CON	OMN		Trt	Wk	Trt by wk
Production (kg/d)
Milk	20.3	21.0	0.38	0.043	0.001	NS
3.5% FCM	19.6	21.4	0.37	0.002	0.001	NS
Fat	0.67	0.74	0.01	0.007	0.001	NS
CP	0.67	0.68	0.01	NS	0.002	NS
Lactose	0.91	0.95	0.01	NS	NS	NS
Casein	0.50	0.53	0.01	NS	0.321	NS
Content (%)
Fat	3.32	3.53	0.03	0.003	0.021	NS
Protein	3.28	3.25	0.02	NS	NS	NS
Lactose	4.51	4.50	0.01	NS	NS	NS
Casein	2.47	2.51	0.02	NS	NS	NS
MUN (mg/dL)	18.5	17.9	0.42	0.012	NS	NS
SCC (×1,000/mL)	119	92	6.65	0.048	NS	NS