Essential (Oligo Basics, Cascavel, Brazil) is a feed additive that contains cashew nut shell liquid (CNSL) and castor oil (CO) as active ingredients. CNSL and CO are considered functional oils (FO) due to their anti-oxidative and gastro-protective properties. This study aimed to compare the milk production and feed efficiency of cows fed FO or monensin (MON) during the summer season in Pirassununga, Brazil. Thirty-six cows (201 ± 63 DIM, 599 ± 78 kg BW, and 28.7 ± 3.92 kg/d milk yield) were used in a randomized complete block design experiment. Cows were allocated in a barn containing forced ventilation with 25.0 ± 0.83°C temperature, 81.5 ± 4.67% of relative humidity, and 75 ± 0.8 ITU during the 5 wk of experiment. Cows within block were assigned to one of treatments: control (CON); FO, 500 mg/kg DM of Essential; or MON, 22 mg/kg DM of MON (Rumensin, Elanco Animal Health, Greenfield, IN). Additives were provided mixed into the concentrate of a TMR fed twice daily. Samples of ingredients and orts were collected weekly to determine DM intake. Cows were milked twice daily, being the milk samples collected weekly during 6 consecutive milkings to determine the contents of fat, protein and lactose by infrared method. Data were analyzed as repeated measures using the PROC MIXED of SAS, and differences were calculated by PDIFF test. Data from one week before the start of experiment were used for covariate adjustments. FO-fed cows had greater milk fat concentration and DM intake than MON (Table 1). In addition, FO numerically increase milk fat concentration and had similar DM intake in relation to CON. FO may improve milk fat concentration of cows in summer. Table 1.

Item	Treatment			SEM	P-value
	CON	MON	FO		Diet	Time
Production, kg/d
Milk	29.2	29.5	29.1	0.41	0.786	0.003
3.5% FCM	28.7	28.3	29.9	0.43	0.153	<.001
Milk concentration, g/kg
Fat	34.6ab	32.5b	36.5a	0.51	0.021	0.021
Protein	30.7	30.9	30.7	0.09	0.412	<.001
Lactose	45.9	46.2	46.3	0.06	0.323	<.001
DMI, kg/d	24.6a	23.0b	24.5a	0.46	0.021	0.158
Milk yield/DMI	1.21	1.30	1.20	0.03	0.092	0.386

Diet by time (wk.) interaction effect >0.10 for all variables.