One of the limitations of conventional urea usage in ruminant diet is rapid hydrolysis and increase in ruminal ammonia concentration and its lack of synchrony with carbohydrate degradation and microbial growth in the rumen. An alternative strategy is continuous supply of nitrogen using slow release urea (SRU) and gradual release of ammonia in the rumen. Sugar-containing liquid feeds can increase the energy density of the diet, stimulate dry matter intake (DMI), and serve as carriers for fat, non-protein nitrogen (NPN), and other ingredients. Sugars can change the ruminal fermentation pattern, typically decreasing ruminal NH3 concentration and increasing ruminal butyrate concentration. The purpose of this study was to investigate the effects of SRU in comparison with conventional urea, with or without the addition of molasses on the growth performance and rumen fermentation of fattening lambs.
The experiment was conducted in a completely randomized design with 5 treatments including control, two sources of non-protein nitrogen (conventional urea and SRU) with or without molasses (0 and 20 % of ration DM) and 7 replicates using 35 Arabian lambs. Forage to concentrate ratio was 30 to 70. The length of the fattening period was 105 days, including 15 days for adaptation and 90 days of data collection. At the beginning of the main period of fattening and then once a month, weighing the lambs was carried out with 14 to 16 hours of starvation. The lambs had free access to water during the test period. The lambs were fed the total mixed ration ad libitum twice daily at 0800 and 1600 hours. The digestibility coefficients of dry matter (DM), organic matter (OM), crude protein (CP), neutral detergent fiber, (NDF), acid detergent fiber (ADF) and ether extract (EE) were determined by the total fecal collection method. Daily feed intake, daily weight gain, feed conversion ratio, carcass compositions, meat colour, rumen and blood parameters were recorded. Data were analyzed as a completely randomized design using the General Linear Model (GLM) procedure of SAS.
Replacement of urea with SRU and the addition of molasses did not affect dry matter intake, daily weight gain, feed conversion ratio, final body weight, cold and hot carcass weight, carcass efficiency and components, meat colorimetric parameters and nutrient digestibility. The lack of difference between control and treatment groups in the growth performance could be due to sufficient supply of the microbial protein production in the rumen. Addition of SRU and molasses did not have a significant effect on dry matter intake and digestibility of nutrients. The hydrolysis rate of SRU may not be reduced to the extent that it could improve the efficiency of rumen microorganisms in utilizing the available energy source of molasses. Propionate and NH3-N concentration in the rumen increased in animals provided with NPN in comparison with the control diet. The concentration of acetate in rumen of animals fed diets containing NPN sources was lower than those on the control diet. The concentration of total volatile fatty acids in SRU diets and also diets containing conventional urea was higher than control diet. Molasses addition in diets, increased butyrate and total volatile fatty acids and decreased blood urea nitrogen concentrations, compared to control diet. Animals fed NPN diets showed higher rumen pH than the control diet. The ruminal pH decreased in diets containing molasses. Reducing the saliva secretion, due to the physical and chemical properties of molasses and the high amount of fermentable sugars supplied by molasses in the rumen, may possibly explain the decrease in the rumen pH. Animals that had received urea containing diets showed higher rumen pH presumably due to the buffering capacity of urea. An increase in ruminal ammonia concentration due to urea hydrolysis can increase the pH of the rumen by obtaining H+ by ammonia and turning it into NH4+. However, it is likely that the result reflects the higher grain content of the control diet relative to NPN containing diets.
In general, the results of the present study showed that although replacement of soybean meal with conventional urea and/or SRU, with or without molasses, in high concentrate diets, influenced some rumen fermentation and blood metabolites, but these changes were not large enough to improve fattening performance, carcass characteristics and nutrient digestibility.
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