The objective of this research were to detect how many days where necessary to animals to adapt to diets, to measure intake and milk production; also, to evaluate the effect of four levels of crude protein in the non protein nitrogen (NPN) based compounds (2,22; 4,18; 5,96 e 8,09%) on the milk composition and production, the intake and apparent digestibilities of dry matter (DM), organic matter (OM), neutral detergent fiber (NDF), ether extract (EE), total carbohydrates (CHOT) and crude protein (CP) and the intake of total digestible nutrient (NDT). Sixteen Holstein cows were allotted to four balanced 4 x 4 Latin square design in accordance with the lactation period. The four experimental diet were formulated to contain 60% of corn silage and 40% of corn meal, soybean meal, urea and mineral mix misture. Crescent levels of urea in the concentrate (0,0; 0,7; 1,4 e 2,1%) for the treatment 1, 2, 3 and 4, respectively, were used. All diets were isoproteics, approximately 14% CP. There were a decrease in the intake of DM, OM, CP CHOT and TDN; and EE linearly increased as the NPN increased in the diets. While, the intake of NDF, expressed as kg/day and %LW, presenting a quadratic response, with maximum intake values of 6,99 kg and 1,56% for 4,54 and 4,26% NPN, respectively. The total apparent digestibilities of DM, OM, NDF, EE, CP and CHOT were not influenced by the NPN level in the diets, with observed means of 69,45; 70,71; 62,26; 88,34; 70,57 and 70,91%, respectively. The milk production corrected or not for 3,5% fat, the fat and milk protein yield and milk protein content linearly decrease while the fat content and feed efficiency, expressed as kg milk/ kgDM were not influenced by the NPN levels in the diet.
non protein nitrogen (NPN) compounds; intake; digestibility; milk production; lactating cows
