The objective of the present study was to investigate whether feeding barley grain steeped in lactic acid (LA) and treated with heat would affect rumen fermentation patterns, milk production and composition in lactating dairy cows. Eight rumen-cannulated Holstein cows were assigned to a 2 x 2 crossover design and fed once daily a total mixed ration containing rolled barley grain (319.6 g/kg, dry matter basis) steeped for 48h in equal quantity of tap water (CTR) or in 10 g/L of LA and heat at 55 degrees C (LAH). Results demonstrated that cows fed the LAH diet had rumen pH values above 5.8 during most intensive fermentation phases (i.e., 8-12 h) post-feeding (P<0.05). Feeding cows the LAH diet did not affect (P>0.05) postprandial concentrations of volatile fatty acids (VFA) but lowered (P<0.05) their preprandial concentrations in the rumen fluid. Results of diurnal fermentation patterns indicated that molar proportions of acetate and butyrate were greater (P<0.01), whereas propionate was lower in the LAH group (P<0.01). Data also demonstrated increased milk fat content (P<0.01), and milk fat yield (P<0.05), as well as an increase in the fat: protein ratio in treated cows (P<0.05). In addition, the LAH group showed better milk energy efficiency than CTR cows (P=0.02). There were tendencies for greater milk protein (P<0.10), energy-corrected milk (P<0.10), and lower milk urea nitrogen (P<0.01) in cows fed the LAH diet. In conclusion, results showed that treatment of barley grain with LA and heat had multiple beneficial effects compared to the CTR group of animals. Further research is warranted to explore the benefits of this new emerging technology in early lactating dairy cows and to further optimize the most suitable concentration of LA and steeping time for barley grain treatment. (C) 2011 Elsevier B.V. All rights reserved.