In vitro evaluation of slow-release urea compounds.

The objective of this study was to evaluate the effects of different slow-release urea (SRU) compounds on ruminal fermentation, nutrient degradation, and nitrogen (N) utilization in a dual-flow continuous-culture system (experiment 1), as well as ammonia-N (NH-N) release rate in a batch culture system (experiment 2). In experiment 1, 8 fermenters were used in a replicated 4 × 4 Latin square design with 4 treatments and 4 experimental periods. The treatments were formulated to contain the same amount of N, differing in the source of nonprotein N, such as control (CON), with noncoated urea at 0.62% DM; partial inclusion of SRU compound 1 (SRU1) at 0.51% DM; partial inclusion of SRU compound 2 (SRU2) at 0.51% DM; and partial inclusion of SRU compound 3 (SRU3) at 0.51% DM. Each period lasted 10 d. The last 3 d of each period were used for sample collection. Samples were collected for pH, lactate, VFA, NH-N kinetics, nutrient degradability, and N metabolism. In experiment 2, a batch culture incubation was conducted as a complete randomized block design, using 3 Erlenmeyer flasks per treatment in 3 runs. Each treatment contained 1 of the 4 NPN sources used in experiment 1, (CL, S1, S2, S3) or without any NPN (blank, BK), and samples were collected at different time points for NH analysis. All flasks, except for BK, contained equal amounts of 21.56 mg N, and all flasks were inoculated with 260 mL of a 1:2 mixture of ruminal fluid and nutritive solution. Data of both experiments 1 and 2 were analyzed using the MIXED procedure of SAS. In experiment 1, there were no effects of treatment on pH or NH-N kinetics. There were no effects of treatments on lactate kinetics; however, there was an interaction between treatment and time. For 24-h VFA pool, there were treatment effects on acetate, propionate, acetate:propionate ratio (A:P), and branched-chain (BC)VFA proportion. Compared with CON, SRU1 had lower A:P ratio and acetate proportion, with greater proportion of propionate, which could represent a favorable fermentation partner compared with CON. Treatment SRU1 had lower BCVFA proportion than the other treatments, which indicates less protein degradation. There were no treatment effects for nutrient degradability and N flow. Based on observations in experiment 1, SRU1 could have the potential of improving ruminal fermentation and N utilization. Moreover, the different SRU compounds had different fermentation patterns according to their VFA profiles. In experiment 2, there were significant effects for treatment and time, and a tendency for treatment by time interaction effect. The N release rate of S1 was similar to CL and faster than S2 and 3, and the differences in N release rates could be detected as early as at 0.75 h of incubation. Thus, S1 may not be as slow degrading when compared with S2 and S3. In conclusion, no effects were found on nutrient degradability and N utilization. However, the different SRU compounds had different N release rates, which could affect ruminal fermentation pattern in different diets.