Urea Recycling Saves Money and Helps the Environment

If you haven’t visited with your nutritionist lately, now is the time. Most rations have room for improvement, and simple alterations can increase how well feed proteins are utilized for milk production. Improving the efficiency of conversion of feed protein to milk will mean that we will get more milk with lower feed cost. An added bonus to improving the conversion of feed protein to milk is that nitrogen (N) excretion will decrease reducing the environmental impact of dairying. Since much of the nitrogen excretion that leads to air and water pollution is from urea-N in the urine my Ph.D. research focused on how we can reduce urea-N excretion by altering the ratio of rumen degradable protein (RDP) to rumen undegradable protein (RUP).

Rumen degradable protein is broken down in the rumen by the rumen microbes resulting in microbial protein production. Microbial protein is an excellent protein source to support milk production. Therefore, it is important that we maximize microbial protein production. Rumen undegradable protein, on the other hand, passes through the rumen relatively untouched and is broken down in the small intestine supplying specific proteins to the cow to support high levels of milk production. However, through the process of urea-N recycling RUP may be able to supply protein to support microbial protein production.

Urea-N recycling is a process where blood urea is transferred to the rumen instead of being excreted in the urine or milk. Since rumen bugs can readily utilize urea to produce microbial protein, recycled urea-N can serve as an RDP source much like urea in the feed. This blood urea pool is formed from excess protein in the rumen being converted to ammonia, which is then absorbed into the blood and converted to urea by the liver. The production of urea from ammonia also requires the input of N from amino acids (building blocks of proteins), which is how N in the absorbed amino acids from dietary RUP can actually return to the rumen a second time. Although this process has been studied extensively in other animals there is very little data on how much urea is coming back to the rumen in lactating dairy cows, and no one has looked at how it is used by the microbes once it gets there.

In order to get a handle on urea-N flows in lactating cows (Figure 1) and how much it is contributing to microbial protein production we created different diets each with low or high RDP or RUP. We found that cows on a lower RDP diet had increased urea-N recycling compared to cows on higher RDP diets. The idea is that urea-N is recycled to the rumen and used by the rumen bugs to produce microbial protein represents improved protein utilization efficiency, and improved efficiency equates to lower feed costs. We also found that as the protein concentration of the diet increased, the amount of N excretion in the urine increased. As the RUP concentration increased the amount of N excreted in the feces also increased. Not only is excess excretion a problem from an environmental standpoint, but from an economic standpoint, we saw a decrease in feed efficiency (lb of milk per lb of feed) and no change in milk components.

It is important to monitor and balance dairy rations to optimize the cow’s utilization of the feed provided. Feeding protein above the cow’s requirement only led to manure with higher N content without any production benefits. This is only one study, but something to keep in mind when trying to keep your future feed costs in check.

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Sarah Dinh, Dairy/Environmental Extension Educator, Lancaster County