Composição do leite, pH do fluído ruminal e perfil metabólico de vacas leiteiras suplementadas com uréia de liberação lenta / Milk Composition, Ruminal pH and Metabolic Parameters of Dairy Cows Supplemented with Slow-Release Urea

ABSTRACT

Background: The protein supplements can represent up to 50% of the costs in the concentrated fraction of the diet in dairy cows. A possible alternative that meets the requirements regarding protein levels and can reduce the costs are the nonprotein nitrogen (NPN) supplementst. For these reasons, this strategy it is widely used in low milk production systems, where cost of production has a higher impact. In this scenario the use of NPN aims to increase production without altering the economic balance of the system. Recently, a supplement consisting of urea coated with a biodegradable polymer was developed. This system promotes the controlled release of urea that can reach up 36 h after ingestion. The aim of this study was to evaluate the effects of feeding slow release urea formula on the composition and milk production, metabolic parameters and pH of the ruminal fluid of dairy cows. Materials, Methods & Results: This study was conducted over a period of 60 days, using 20 multiparous cows with 90 ± 18 days in milk. The animals were divided into two groups according to the type of NPN 1) conventional urea (CG) and 2) slow release urea (SRG). Each group was composed of six Holstein cows (average body weight 528.7 ± 51.9 kg) and four Jersey cows (average body weight 389.3 ± 33.3 kg). Cows in the CG received a diet containing 80g of conventional urea/cow/day, and SRG cows were fed a diet containing 88g of slow release urea/cow/day, both diets were kept isocaloric and isonitrogenous between groups. Weekly weighing and sampling was performed for evaluation of milk production, composition, somatic cell count (SCC) and urea levels in milk. Blood samples were taken weekly to measure the levels of γ-glutaryl transferase, urea, and albumin. Biweekly ruminocentese was performed for collection of ruminal fl uid and pH measurement. Among the metabolites examined the only difference between groups was for milk urea, with SRG having higher milk urea than CG cows (P < 0.04). No differences were observed for milk yield corrected to 3.5% fat, milk composition and ruminal pH (P > 0.05). However, a breed effect was observed, with the Jersey cows having the highest milk fat content (P < 0.04; 4.4 ± 0.27% vs 3.6 ± 0.22%), total solids (P < 0.01; 13.22 ± 0.43% vs 11.74 ± 0.35%) and urea (P < 0.002; 52.31 ± 1.43 mg/dL vs. 46.12 ± 1.17 mg/dL) compared to Holstein cows, respectively. Discussion: Milk production was not different between groups supplemented with different sources of NPN, similar to the described by other studies. Previous studies that found differences in milk production were probably due to the with drawal of the true protein fraction of the diet, which allowed the synchronization of the degradation of carbohydrates and nitrogen release by the slow release formula. Serum urea concentration remained within the physiological levels for both groups, demonstrating that the diet was maintained in equilibrium. The enzyme γ-glutaryl transferase did not show any difference between groups, indicating that no systemic damage occurred. The results of this experiment indicate that supplementation with slow release urea at 1.8% of the dry matter intake did not influence the parameters studied, behaving like conventional urea. Thus the slow release urea can be used as an alternative protein source without damage to the metabolism, however for low producing dairy systems, characterized as semi-skilled, no benefits are observed since it has a higher cost.