Investigating effects of increasing levels of slow release urea on feed intake, nutrient digestibility and rumen microbial fermentation of ewe’s fed low quality forage

Introduction: Energy and protein sources are of prime importance for ruminants as they stimulate microorganisms in the rumen and enhance the productive functions of the animals. Many species of rumen microorganisms can use ammonia as a nitrogen source for their growth and formation of microbial biomass that is key to meet the amino acid needs of the host animal. Because low-quality forages (<7% CP) are often limiting in protein, a positive relationship exists between ruminally degradable protein (RDP) supplementation and forage utilization (Koster et al., 1996). In contrast, it has been shown that forage intake did not affect mature ewes fed low-quality grass hay in response to increasing levels of supplemental RUP (Swanson et al., 2000). Similarly, Salisbury et al. (2004) reported no difference in roughage intake in wethers consuming low-quality hay supplemented with high or low RUP. However, those researchers suggested that RDP from forage could have been adequate to support ruminal fermentation. Urea is the main NPN source used in ruminants diets. However, there are still concerns regarding its use due to the rapid release of ammonia (NH3–N), which can be faster than its use by microorganisms for protein synthesis. The efficiency of protein synthesis from urea depends, among other factors, on energy availability in the rumen (Russell et al., 1992). Despite evidence of positive effects of RUP supplementation of fiber digestion, little information has been published observing the influence of slow-release urea (SRU) supplementation on the nutrient digestibility and DMI of ewe’s fed by low-quality forage. Also, feed sorting behavior that could be related to fiber digestibility in the rumen, has not been investigated in ruminant animals fed by increasing levels of slow-release coated urea. Therefore, the objective of this study was to determine how urea supplementation affects feed intake, nutrient digestibility, ruminal NH3–N, blood urea nitrogen concentration and feed sorting behavior in ewes. Material and methods: A total of 28 Arabian ewes (40±2 kg live weight) were used in a completely randomized design with four treatments for 60 days. Experimental diets included 1- control (no urea), 2-1.2% conventional urea, 3- mixture of conventional and coated urea (Optigen®) (50:50 mixture; 0.6% and 0.675% ration DM respectively), and 4- coated urea (1.35% ration DM). Ewes were housed individually in pens (1.3 m×1.5 m) in an open shed building and were allowed ad libitum access to feed and water throughout the trial. All diets contained 60% forage (wheat straw) and 40% concentrate (60 : 40; forage : concentrate). The ingredients and chemical composition of the rations fed to ewes are shown in Table 1. The ewes were fed the total mixed rations ad libitum twice daily at 0800 h and 1700 h and had free access to fresh water at all times. Feed offered and refusal of each lamb were recorded daily. Digestibility was measured by the total collection of feces during a 5-d period. Samples of ruminal fluid were collected from each ewe 0 (before morning feeding), 3, 6 and 9 h post-feeding using a stomach tube attached to an Erlenmeyer flask and vacuum pump. The first 50 ml of collected rumen fluid was discarded to avoid saliva contamination. The remaining was filtered through four layers of cheesecloth. Rumen fluid pH was measured using a portable pH meter. Blood samples were taken from each lamb at the same time as ruminal fluid sampling by venipuncture of the jugular vein in 10-mL tubes treated with sodium heparin. Samples were centrifuged in a refrigerated centrifuge at 850 × g for 30 min within 30 min of sampling, and the plasma was frozen at −20 °C until used. The sorting index was calculated as the ratio of actual intake to the expected intake of particles retained on each sieve of the PSPS (Leonardi and Armentano 2003). The predicted intake of an individual fraction was calculated as the product of the DMI of the total diet multiplied by the DM percentage of that fraction in the fed TMR. A sorting index of 100, >100 and <100 indicated no sorting, sorting for, and sorting against respectively. Data were analyzed using a GLM procedure of SAS 9.2 (SAS Institute, Inc., Cary, NC). Comparisons between treatments were completed with Tukey’s test. Treatment effects were declared significant at P≤0.05. Results and discussion: The results showed that the experimental treatments had no significant effect on intake of dry matter, organic matter, neutral detergent fiber and acid detergent fiber. Crude protein intake was lower in the control treatment relative to the other treatments (P <0.05). This finding was expected urea-containing diets designed to have higher CP content. Apparent digestibility of dry matter, crude protein, neutral detergent fiber and acid detergent fiber was not affected by the treatments. Apparent digestibility of organic matter increased in animals who received coated urea relative to the control treatment (P < 0.05). The ruminal pH of ewes was not affected by dietary treatments before morning feeding and 9 hours post-feeding. An increase in rumen pH was observed in coated urea treatment relative to the control group, 3 and 6 hours post-feeding (P < 0.05). Animals fed coated urea also had higher rumen pH compared to those fed common urea and a mixture of coated urea and common urea (P < 0.05). Rumen ammonia nitrogen concentrations were higher in all urea-containing treatments compared to the control treatment, 3 hours post-feeding (P = 0.002). At 9 hours post feeding, coated urea treatment had higher ammonia nitrogen concentration than common urea and mixture of coated urea and common urea treatments and lowest concentration observed in the control treatment (P = 0.009). Blood urea nitrogen was also higher in 3 and 6 hours post feeding in all urea-containing treatments than the control treatment (P <0.05). Ewes fed a mixture of conventional and coated urea sorted in favor of medium particles relative to other treatments (P = 0.03). Regardless of treatments ewes sorted against the longest ration particles (>19 mm) and sorted for fine particles (<4 mm) dietary particles. Conclusion: Results of this study showed that increasing levels of coated urea in diets containing low-quality forage have the potential to keep higher rumen ammonia nitrogen concentration for a longer period but this increase didn’t have a positive effect on nutrients digestibility compared to conventional urea.