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Increasing Utilization of Urea in Ruminant Feeding
Urea contains 42–46% nitrogen. If 70% of this nitrogen is utilized by synthetic bacterial protein, 1 kilogram of urea can be converted into a protein equivalent of about 4.5 kilograms of soybean meal after ruminal transformation in ruminants. According to foreign studies, adding 1 kilogram of urea to a protein-deficient diet can increase milk production by 6–12 kilograms or lead to weight gain of 1–3 kilograms. It may also result in an additional 50–150 grams of net hair growth. In domestic trials, it has been reported that 3.6–4.6 kilograms of milk can be produced per kilogram of urea used.
Rumen bacteria in ruminants use urea as a nitrogen source and soluble carbohydrates as a carbon and energy source to synthesize bacterial proteins. These bacterial proteins are then digested and absorbed by the animal, similar to regular feed proteins.
However, not all of the ammonia from urea is used in bacterial protein synthesis. The effectiveness of urea depends on several factors. To improve its utilization and avoid ammonia toxicity, the following guidelines should be followed:
First, the diet supplemented with urea must contain sufficient easily digestible carbohydrates. During the process of synthesizing bacterial proteins, rumen bacteria require both energy and carbon sources. Crude fiber alone as an energy source results in low urea utilization (around 22%). When starch and fiber are combined, urea utilization can increase to over 60%. This is because starch degrades at a rate similar to urea, allowing for synchronized release of nitrogen and energy. Therefore, when adding urea to roughage-based diets, it's recommended to add 1 kg of easily digestible carbohydrates (2/3 starch and 1/3 sugar) per 100 grams of urea.
Second, the protein level in the diet should be appropriate. If the protein content exceeds 13%, urea utilization decreases, and there’s a risk of ammonia poisoning. On the other hand, if the protein level is below 8%, bacterial growth and reproduction may be affected. It is generally advised to keep the protein level between 8–13% before supplementing with urea.
Third, ensure adequate mineral supply. Cobalt is essential for vitamin B12 synthesis, which plays a key role in protein metabolism. A deficiency in cobalt can hinder bacterial use of urea. Sulfur is necessary for the synthesis of sulfur-containing amino acids like methionine and cysteine. The ideal nitrogen-to-sulfur ratio for urea-containing diets is 10–14:1, and the nitrogen-to-phosphorus ratio should be around 8:1. Other minerals such as calcium, phosphorus, magnesium, iron, copper, zinc, manganese, and iodine are also important for microbial activity and overall urea utilization.
Fourth, control the amount of urea fed. Urea should make up about 20–30% of the crude protein in the diet, or no more than 1% of the dry matter. Adult cattle typically receive 60–100 grams per day, while adult sheep get 6–12 grams. Calves under 2–3 months old should not be given urea due to underdeveloped rumens. If the diet already contains high levels of non-protein nitrogen, such as silage, urea should be halved to avoid waste and toxicity. Overfeeding can lead to ammonia poisoning, which occurs within 30 minutes to an hour after feeding. Symptoms include muscle tremors, difficulty breathing, and foaming at the mouth. Without treatment, death can occur within 2–3 hours.
Fifth, pay attention to feeding methods. Urea should be mixed evenly with roughage or diluted in concentrate feed. It can also be added to silage. A common practice is to mix 4 kg of urea and 2 kg of ammonium sulfate into 1 ton of corn silage. Ruminants should be gradually introduced to urea over 5–7 days, and daily amounts should be divided into multiple feedings. Never feed raw urea alone or mix it with feeds containing urease, such as barnyard grass. Urea should not be fed in water; instead, animals should have access to water 3–4 hours after feeding.
Lastly, consider using high-efficiency urea additives. To slow down urea breakdown in the rumen and improve safety, several techniques can be used: adding urease inhibitors like oxalic acid or fatty acid salts, coating urea with cooked grains, making urea granules with gelled starch, producing urea blocks, or using urea derivatives like urea phosphate or biuret. These methods help control the release of ammonia and enhance bacterial protein synthesis.