Metabolic Profile of F1 Holstein × Zebu Lactating Dairy Cows Under Grazing Supplemented with Two Non-Protein Nitrogen Sources

Pastures dedicated to cattle production in Venezuela have a low crude protein concentration. One of the nutritional strategies to correct the forage protein deficit is using non-protein nitrogen in the diet. To evaluate the effect of two non-protein nitrogen sources on the metabolic profile of lactating dairy cows, 31 F1 Holstein × Zebu multiparous cows averaging 419 ± 46 kg body weight and 17.1 ± 3.3 kg milk·animal·d-1 were randomly assigned to two treatments for 91 days: 1) 59 g·animal-1·d-1 of urea and 2) 64 g·animal-1·d-1 of Optigen II® as slow-release urea (SRU). The animals were grazing (2.12 AU·ha-1) on Urochloa decumbens, U. humidicola, and U. brizantha (7.98-9.61 % CP) and supplied with 4 kg·animal·d-1 of concentrate (15 % CP), and 108, 60, and 108 g·animal·d-1 of mineral, molasses, and bypass fat, respectively. Glucose, total protein, albumin, cholesterol, urea, and β-hydroxybutyrate concentrations, aspartate aminotransferase, and gamma-glutamyl transferase activities were determined in blood serum samples every 28 days. ANOVA analyzed data in a completely randomized design with repeated measures. There were no differences among treatments in the parameters investigated; however, collecting time influenced all of them. No treatment × time interactions were found except for albumin concentration, higher in urea at day 0. The results indicate that under the conditions assayed in the present work, Optigen II® offers no advantage over urea concerning blood indicators of energy and protein metabolism and hepatic functionality in dairy cows.