The effect of slow-release urea sources on digestibility of nutrients and growth performance of fattening lambs fed rations containing low-quality forage

 The main factors limiting the consumption of low-quality fodder is their low nitrogen. Therefore, it is often difficult to provide sufficient nutrients, especially energy and protein for ruminants, which use these resources. Deficiency of nutrients in low quality forage also affects the supply of nutrients for growth of ruminal microorganisms and leads to a decrease in the ruminal digestion rate of these forages; this in turn reduces the feed intake and performance of the animal. Providing the sufficient nitrogen to ruminal microorganisms is important to increase carbohydrate digestibility and microbial protein production. Therefore, to achieve optimal performance, it is important to provide appropriate nitrogen supplements in areas where a significant portion of the ruminant diet consists of low-quality forage. One of the characteristics of ruminants is the ability to use non-protein nitrogen sources to synthesis of microbial protein in the rumen. Non-protein nitrogen, mainly urea, has been considered as a substitute for part of true protein due to its low price per unit of nitrogen compared to other true protein sources. The problem with using common urea is its rapid hydrolysis in the rumen and its conversion to ammonia. Most ruminal ammonia enters the bloodstream and causes destructive effects, from reduced feed intake and animal function to death from ammonia poisoning. The various slow-release urea compounds are made with the aim of continuously supplying nitrogen in the rumen. However, a part of the slow-release urea compounds may leave rumen without being converted to ammonia, reducing their performance for microbial protein production. Therefore, different sources of slow-release urea may have different effects. Processed or slow-release urea reduces ammonia poisoning and nitrogen wastage by reducing the rate of nitrogen release and improving feed costs. Therefore, the aim of this study was to investigate the effects of two sources of slow-release urea and compare them with diets without urea or containing common urea in the fattening lambs fed with low quality forage, to ultimately reduce production costs.

 The present experiment was conducted at Agricultural Sciences and Natural Resources University of Khuzestan. Four experimental treatments were including 1- control treatment (without urea) and three treatments containing three different sources of non-protein nitrogen including 2- 1.8% slow-releasing urea-slowgen 3- 1.69% slow-release urea-optigene 4- 1.6% common urea. Twenty-four male Arabic lambs with (a mean weight of 25.65 ± 0.6 kg and 5± 1.0 months old were assigned to four experimental treatments with six replications. The nutrients digestibility was measured during seven days. The feed orts and feces were daily weighted and about 10% of them were kept in the plastic bags at -20˚C. At the end of this short period, the orts and feces samples were mixed and one representative sample obtained. The samples were oven-dried and grounded using 1 mm mesh screen. The chemical composition of rations, feed orts and feces, including dry matter, neutral detergent fiber, acid detergent fiber, organic matter, and crude protein were measured with standard methods. The dry matter intake, initial weight, every two weeks weight, final weight was recorded and feed conversion ratio and feed efficiency were calculated. Protozoa population, ammonia nitrogen, and pH of the rumen liquor were measured with standard procedure. The blood glucose, cholesterol, triglyceride, high density lipoprotein (HDL), low density lipoprotein (LDL), and BUN, were measured by spectrophotometric procedure. The data was analyzed using the GLM procedure of SAS (version 9.4). The differences among treatments were evaluated using Duncan’s adjustment (P < 0.05).

 Compared to the control group, the use of slow-release urea sources had no effect on feed intake and digestibility of dry matter, NDF, ADF, and organic matter. Crude protein digestibility was highest in the control treatment and lowest in common urea treatment (P <0.05) and the control was not different from the slowgen treatment. Feed intake tended to increase in treatments containing urea (P = 0.07). Due to urea malnutrition, the use of urea sources may reduce feed intake, but in the present experiment this did not occur and even a slight increase was observed. This may be due to the presence of more molasses in urea-containing treatments; because molasses, due to its palatability, can increase the dry matter consumption of livestock. The growth performance of lambs in the whole period including final weight, average daily weight gain, total gain, feed conversion ratio, and feed efficiency were not affected by experimental treatments. The experimental treatments had no significant effect on the concentration of ruminal ammonia nitrogen, volatile fatty acids, protozoa population, pH, and blood parameters such as glucose and urea nitrogen. The diets containing common urea and slowgen had better benefits than control diets. In general, the findings of the present experiment showed that the results of treatments containing non-protein nitrogen sources were competitive with protein sources (control diet). In addition, there was no obvious difference between two sources of slow-release urea with each other, control, and common urea diets; but the slowgen worked better.

 Finally, according to the results of the present experiment, related to the concentration of ruminal parameters in the present experiment, it can be concluded that the use of non-protein nitrogen compounds such as normal urea or slow-release as a substitute for conventional protein sources such as soybean meal did not have negative effects on ruminal fermentation. so, due to the lower price of urea compounds compared to soybean meal and other real nitrogen sources, although no difference was observed between slow-release urea sources and common urea; it can be stated that production productivity has also improved economically, therefore, the use of urea sources for fattening periods is recommended.