ORIGINAL ARTICLE Effect of monensin and vitamin E on milk production and composition of lactating dairy cows SH. Khodamoradi 1 , F. Fatahnia 1 , K. Taherpour 1 , V. Pirani 2 , L. Rashidi 3 and A. Azarfar 4 1 Department of Animal Science, Ilam University, Ilam, Iran, 2 Office of Veterinary Organization, Ilam, Iran, 3 Department of Food and Agriculture, Institute of Standard and Industrial research of Iran, Karaj, Iran, and 4 Department of Animal Science, Lorestan University, Khorramabad, Iran Keywords dairy cow, monensin, vitamin E, milk production, milk composition, milk fatty acid profile Correspondence F. Fatahnia, Department of Animal Science, Faculty of Agriculture, University of Ilam, Pajoohesh Bullvar, Ilam, Iran. Tel: 0098 841 2227015; Fax: 0098 841 2227015; E-mail: ffatahnia@yahoo.com Received: 14 January 2012; accepted: 12 March 2012 Summary Feeding unsaturated oils to lactating dairy cows impair ruminal biohy- drogenation (BH) of unsaturated fatty acids (USFA) and increase rumi- nal outflow of BH intermediates such as trans-10, cis-12 CLA that are considered to be potent inhibitors of milk fat synthesis. Supplementing lactating dairy cow’s rations containing plant origin oils with monensin and/or vitamin E may minimise the formation of trans-10 isomers in the rumen, thereby preventing milk fat depression. Therefore, this study was conducted to evaluate the effects of monensin and vitamin E sup- plementation in the diets of lactating dairy cows containing whole cot- tonseed, as the main source of FA on feed intake, milk production and composition, milk fatty acid profile, efficiency of nitrogen (N) utilisation, efficiency of net energy (NE) utilisation and nutrients digestibilities. Four multiparous Holstein lactating dairy cows (86 ± 41 days in milk) were assigned to a balanced 4 · 4 Latin square design. Each experimen- tal period lasted 21 days with a 14 days of treatment adaptation and a 7 days of data collection. The control diet was a total mixed ration (TMR) consisted of 430 g/kg forage and 570 g/kg of a concentrate mix- ture on dry matter (DM) basis. Cows were randomly assigned to one of the four dietary treatments including control diet (C), control diet sup- plemented with 150 mg of vitamin E/kg of DM (E), control diet supple- mented with 24 mg of monensin/kg of DM (M) and control diet supplemented with 150 mg of vitamin E and 24 mg of monensin/kg of DM (EM). Dry matter intake (DMI) ranged from 19.1 to 19.5 kg/d and was similar among the dietary treatments. Dietary supplementation with vitamin E or monensin had no effect on milk production, milk fat, pro- tein and lactose concentrations, efficiency of utilisation of nitrogen and net energy for lactation (NE L ). Digestibility of DM, organic matter (OM), crude protein (CP) and ether extract (EE) was not affected by the die- tary treatments. Digestibility of neutral detergent fibre (NDF) was higher in cows fed with the M and EM diets in relation to those fed the C and E diets. The concentrations of C4:0, C6:0, C8:0, C10:0, C12:0, C14:0, C15:0, trans-10-16:1, cis-9-16:1, 17:0, 18:0, trans-11-18:1, cis-9-18:1, cis-9, trans-11 conjugated linoleic acid (CLA), trans-10, cis-12 CLA, and 18:3n-3 FA in milk fat were not affected by the dietary supplementa- tions. While feeding the M diet tended to decrease milk fat concentra- tion of C16:0, the milk fat concentration of C18:2n-6 FA tended to be DOI: 10.1111/j.1439-0396.2012.01307.x Journal of Animal Physiology and Animal Nutrition ª 2012 Blackwell Verlag GmbH 1