Effect of substituting different levels of biuret instead of urea on nutrient intake, growth performance, nutrient digestibility, blood metabolites, and feeding behavior of Afshari fattening lambs

Introduction: Protein is one of the most expensive nutrients used in ruminant diets and is considered one of the limiting factors in feed intake in ruminants. The various protein sources used in ruminant nutrition include plant protein sources (oilseed meal, including soybean meal, rapeseed meal, etc.), animal protein (including meat meal), marine products (fish meal), and non-protein nitrogen (NPN). However, the apparent shortage of protein sources required by ruminants has become a global problem and has significantly increased production costs. Therefore, finding alternative sources to soybean meal as a standard protein source seems essential. NPN sources are nitrogen-rich compounds that can partially replace real protein in the diet of ruminants and include urea, biuret, thiouret, dicyandiamide, thiourea, hydrazine, and several ammonium salts such as ammonium acetate, ammonium succinate, ammonium carbonate, ammonium bicarbonate, ammonium phosphate, and ammonium lactate. Low supply cost, high nitrogen content, and easy availability are some of the advantages of using NPN sources in ruminant nutrition. Urea is the most widespread source of NPN, which is an organic compound soluble in water and alcohol with the chemical formula CO(NH₂)₂ and contains 46% nitrogen, which is equivalent to 287.5% crude protein (CP). In Iran, there is a relatively large capacity for urea production and the possibility of making slow-release nitrogenous compounds from it, while, unfortunately, the use of NPN in livestock nutrition has not been given due attention, and these compounds are also imported from abroad in slow-release form. Excessive use of urea in ruminant diets, due to its extremely high solubility in rumen fluid, increases the concentration of blood urea nitrogen (BUN) and causes ammonia poisoning. Slow-release urea production technology with controlled release of nitrogen in the rumen ensures that rumen bacteria have continuous and gradual access to the nitrogen source. To overcome this problem, several slow-release urea products have been developed and evaluated in the past. Another source of slow-release urea is biuret (carbamyl urea, alphanamides) with the chemical formula 2(NH₂CONHCONH), which is produced by combining two urea molecules at high temperatures, and as a source of NPN, it has good potential in feeding ruminants. The cost of producing biuret is lower than other slow-release urea sources. Biuret is less soluble in water and is converted to ammonia in the rumen at a much slower rate than urea. Also, biuret is relatively non-toxic compared to urea and does not negatively affect the palatability of the diet. Biuret can be used in ruminant nutrition at much higher levels than urea. Biuret is very safe compared to urea and can be used in beef cattle diets at a dose 20 times higher than the toxic dose of urea. So far, few studies have been conducted on the effects of biuret feeding in ruminants, especially determining the optimal level of biuret in the diet. Therefore, the present study aimed to investigate the effects of replacing different levels of biuret in the diet with urea on nutrient intake, growth performance, nutrient digestibility, and feeding behavior of Afshari fattening lambs.
Materials and methods: In this study, 28 Afshari fattening male lambs with an average age of 135±15 days and average live weight of 34±3.49 kg were used in a completely randomized design with four experimental treatments and seven replications. The experimental diets included a control treatment (100% urea on a dry matter (DM) basis) and the inclusion of levels of 34, 67, and 100% biuret based on dietary DM instead of urea. First, the experimental diets were incubated in an in vitro test to determine the concentration of ammonia-N, and then the diets were fed to the experimental animals for 70 days.
Results and discussion: Using biuret in the diet did not affect nutrient intake, including DM, organic matter (OM), CP, and neutral detergent fiber (NDF), and feeding behavior traits compared to the control treatment. With increasing the level of biuret in the diet, daily weight gain, total weight gain, and feed conversion ratio improved linearly compared to the control treatment (P<0.05). The improvement in growth performance traits in biuret-fed lambs can be explained by higher feed intake compared to the control (urea only) treatment. One possible reason may be the better efficiency and retention of nitrogen in biuret due to the lower rumen degradation rate compared to urea. The slower release of ammonia from biuret as a source of NPN compared to urea probably makes biuret more efficiently utilized by rumen microorganisms. As the level of biuret increased in the diet, the digestibility of NDF increased linearly compared to the urea-containing treatment (P<0.05), although OM digestibility was not affected by the experimental diets. In the present study, it was expected that the improvement in NDF digestibility with increasing biuret levels in the diet would be accompanied by an increase in nutrient intake, which was not the case. The reason for the increase in fiber digestion with biuret supplementation compared to urea may be due to the greater availability of ammonia to rumen microbes, especially cellulolytic bacteria, as ammonia is known to be the most important source of nitrogen for the growth and proliferation of fiber-digesting bacteria in the rumen. With increasing the amount of dietary biuret, the concentration of blood total protein increased linearly, but the concentration of blood urea nitrogen decreased linearly compared to the treatment containing urea (P<0.05).
Conclusions: The results of the present study showed that the use of biuret compared to urea in the diet caused a decrease in ammonia-N content in the rumen and improved growth performance and feed conversion ratio. Therefore, it is recommended to use biuret instead of urea up to the full replacement level in the diet of fattening lambs.