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 (NH₃-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, NH₃-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 NH₃ 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 NH₃-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.