Two experiments were conducted during the summer in an individual feeding barn. Cows fed with a 15% CP, 33% RUP, were divided into two nutritional treatments. Control (CON) cows received 18% true NDF whereas treatment cows received 13% true NDF. In the first experiment low forage fiber diet was achieved by replacing 2 kg of wheat hay by soy hulls; In the second experiment 2 kg of whole cottonseeds replaced 2 kg of concentrates. In both experiments, feed intake and milk yield were reduced during the experimental period. In both trials heat production tended to be higher during the cooler daily hours, and average heat production, intake and milk yield were similar between groups. Metabolizable energy intake was higher in the experimental diets. Changes in body weight, as well as blood metabolites concentration indicate that cows were in positive energy balance, while milk energy utilization was apparently not improved by both treatments. It was concluded that reduction of true forage fiber cannot serve as a means for improved diet efficiency in the summer. Yet, as whole cottonseed diet consumption was increased during the hottest day hours suggest an advantage of fat supplementation under summer conditions.