Part 10: Body Conditioning Score Series: Urea as a protein supplement
For most cow-calf producers, forages are the primary component of the cow’s diet, therefore supplements should only be used when they increase the nutritional value of the forage. A common form of protein supplementation used by cow-calf producers when cattle are on a primarily forage-based diet are non-protein nitrogen (NPN) based supplements. Non-protein nitrogen supplements supply nitrogen, most commonly in the form of urea, to the microbes in the rumen. There are two problems commonly associated with relying on urea to meet the protein demands of cattle on a low-quality forage diet: 1.) urea cannot be converted to usable protein if adequate energy is not present in the diet, and 2.) protein-starved cattle can overconsume urea supplements and experience toxicity.
Molasses-based lick tubs, liquid feed supplements, and some range cubes commonly include NPN as a feed ingredient. Urea-based lick tubs and liquid feeders have grown in popularity due to a perceived convenience that the producer doesn’t have to pour out feed each day. In order for the microbes in the rumen to use urea to build protein, there must be adequate digestible energy in the cow’s diet. Energy is most commonly measured by Total Digestible Nutrients (TDN) in forage-based rations. Rations high in TDN will allow maximal utilization of urea, while diets low in TDN (i.e. poor-quality forages) do not provide adequate energy to allow the microbes in the rumen to use urea to build protein. If energy is limited in the diet, such as when cattle are on poor-quality forage, cattle performance and BCS will not improve because urea cannot be efficiently converted to protein in the rumen. Therefore, in many situations, “lick tubs” are not an appropriate option for protein supplementation when cattle are fed a low-quality grass hay diet.
In addition to problems with the nutritional value of urea for cows fed primarily grass hay, urea may also cause toxicity issues in protein-starved cattle. If too much urea is consumed at one time by the cow (which can frequently occur due to large amounts of molasses used in “lick tub” type products to make them more palatable) the urea is rapidly converted to ammonia, and large amounts of ammonia enter the bloodstream, causing ammonia toxicity. This commonly occurs when cattle are deprived of protein on low quality forages for long periods of time, and NPN supplements are suddenly introduced. Hungry cattle overconsume the liquid or solid molasses-based NPN supplement, and can die from urea/ammonia toxicity. Ammonia toxicity may also occur even when cattle are acclimated to the NPN feed if the liquid feeder is allowed to go dry, or if the cattle go for a period of time without “lick tubs” available.
Feedstuffs such as soybean hulls, corn gluten feed, cottonseed meal, whole cottonseed, and dried distiller’s grains are high in digestible fiber. This digestible fiber is a source of energy for the microbes in the rumen, and can improve utilization of urea supplements. When combined with a source of protein (e.g. soybean meal) that allows for microbial growth and function, these feedstuffs provide fiber that is readily broken down by the microbes and do not compete with other fiber supplied from the hay in the diet of the cow. These feeds do not lower the pH of the rumen, and allow the cow to utilize the fiber in hay more efficiently to produce energy.
The information in this article as well as additional information on protein supplements can be found at the following sources:
Olsen, K.C., Cow Supplementation: Getting the Best Bang For your Buck. Proceedings: The Range Beef Cow Symposium XXIV. 2015. https://beef.unl.edu/documents/2016-RBCSymposium/RBCS-2015-Cow-Supplementation.pdf
Parish, Jane A., Rhinehart, Justin D., Protein in Beef Cattle Diets. Mississippi State University Extension Service. Publication 2499. 2018. https://extension.msstate.edu/sites/default/files/publications/p2499.pdf