Effect of non-protein nitrogen source in high-protein diet and feeding frequency on growth performance, rumen fermentation parameters, and the activity of microbial enzymes in fattening lambs

Introduction: The lack of animal feed resources and the increase in animal feed costs relative to the total production costs have created challenges in supplying society with animal protein. Therefore, correctly estimating the nutritional value of feed, especially protein sources, can be an important step in meeting the needs of livestock and reducing production costs. Protein is the most important and expensive nutrient used in ruminant diets. Using non-protein nitrogen (NPN) sources such as urea or its products can be considered a cheap alternative source in animal feeding. Iran is one of the most important urea-producing countries in the world; therefore, using urea or its slow-release products as a feed additive in animal nutrition can be beneficial. In the past years, various sources of slow-release urea have been produced and used in animal feeding. Biuret (heated urea) is one of the slow-release urea compounds, and its production cost is lower than other slow-release urea sources. Compared to urea, it has less solubility in water and is converted to ammonia in the rumen at a much slower rate. It is also less toxic than urea and has a lower negative effect on feed palatability, so it can be used to a greater extent than urea in the diet of ruminants. Biuret is very safe and it can be included in the diet of animals in an amount of 20 times higher than the toxic dose of urea. Biuret usually contains 248.5% crude protein (39.8% nitrogen content), which is a little less than urea. On the other hand, increasing the daily feeding frequency is one of the management methods that may improve the productive performance of livestock, as well as the quantity and quality of the carcass. Increasing the feed frequency may improve nitrogen retention and reduce body fat. It is believed that increasing the frequency of daily feeding will keep the feed fresh, reduce the amount of feed waste, and improve livestock performance. Therefore, this study aimed to investigate the effect of NPN source, in high-protein diets, and feeding frequency on growth performance, rumen parameters, and the activity of rumen microbial enzymes of fattening lambs. Materials and methods: Twenty-eight male Lori-Bakhtiari fattening lambs (age: four to five months; live weight: 36.1±3 kg) were assessed for 60 days using a 2×2 factorial experiment in a completely randomized design with four treatments and seven replications. The experimental high-protein diets (16% of DM) contained urea or biuret, which were given to the lambs in the form of a total mixed ration two or three times a day. The lambs were kept in 1×1.5 m2 individual stalls. Nutrient intake, growth performance, and ruminal parameters [on day 45 of the experiment at fasting (before morning feeding), three, and six h after feeding] including pH, ammonia and volatile fatty acid concentrations, and microbial enzyme activity were measured. Results and discussion: Results showed that intakes of dry matter, organic matter, crude protein, neutral detergent fiber and ether extract, final weight, and total weight gain were not affected by NPN type or feeding frequency (P<0.05). Compared to urea, lambs consuming biuret had higher average daily gain and better feed conversion ratio (P<0.05). The ruminal pH was not affected by the NPN source, feeding frequency, and their interaction (P<0.05). Rumen ammonia nitrogen concentration at three and six hours after morning feeding was significantly higher in lambs fed the diet containing urea than in biuret (P<0.05). The effect of NPN source, feeding frequency, and their interaction was not significant at 0 and six h after feeding on the ruminal concentration of total volatile fatty acids (VFA), acetate, propionate, butyrate, valerate, iso-valerate, and acetate to propionate ratio (P<0.05). However, feeding a diet containing biuret and given three times a day increased total VFA and acetate concentration at three hours post-feeding (P<0.05). The ruminal activity of carboxymethyl cellulase at 0, three, and six hours after feeding increased in animals fed biuret-containing diet compared to urea (P<0.05). However, rumen protease activity significantly increased in lambs fed a urea-containing diet than biuret-containing diet (P<0.05). Feeding three times a day increased the activity of carboxymethyl cellulase at three hours post-feeding (P<0.05). Filter paper degrading activity at three and six hours after feeding in lambs fed biuret was significantly higher than those fed urea (P<0.05). The activity of microcrystalline cellulase and α-amylase was not affected by the NPN source, feeding frequency, and their interaction (P<0.05). Conclusions: Using biuret compared to the urea, in high-protein diets, and feeding diets three times a day, compared to two times a day, improved rumen fermentation parameters and performance of fattening lambs.