This work was carried out, using 15 dairy cows (Holstein x Gyr) full fed isoproteic diets with 60% silage and 40% concentrate in the dry matter (DM) basis and 0, 0.7, 1.4, and 2.1% urea, correspondent to 2.08, 4.01, 5.76, and 8.07% of crude protein levels in the form of non protein nitrogen compounds (NNP), with the objectives to evaluate the intakes and the apparent digestibilities of dry matter (DM), organic matter (OM), crude protein (CP), ether extract (EE), total carbohydrates (CHO) and neutral detergent fiber (NDF) and the total digestible nutrients (TDN) intakes. Possible changes on the milk production and composition were also evaluated. Cows averaging initial live weight of 511.8 kg were randomized allotted to the treatments. The experimental period last 90 days for each cow, starting immediately after calving. The indigestible acid detergent fiber (IADF) was used as marker to determine the nutrients apparent digestibility. The results were interpreted by variance and regression analyses. The addition of increasing NNP to the diet showed decreasing linear effect on the intakes of DM, OM, NDF, CP, EE, CHO and TDN. NNP levels did not affect the apparent digestibilities of DM, OM, CP, CHO and NDF. There was quadratic effect for EE, and the maximum digestibility of 89.12% was obtained with the estimated NNP content of 4.73% in DM. The maximum milk production, of 20.10 kg/day, was obtained with the estimated NNP content of 4.79%, or 0.7% urea in dietary total DM. Linear reduction of fat content was observed as the dietary NNP contents increased, while there was a quadratic behavior for protein, with the maximum content of 3.4% estimated with 3.88% NNP.
intake; Non-protein nitrogen (NPN)
