Many of you will have noticed that calf prices have been under pressure this fall in part due to higher than expected feed prices. Both feed grains and many traditional protein sources (i.e. distillers grains, pulse screenings, commercial supplements) are trading at prices higher than a year ago. In the face of higher protein prices, I have been getting questions regarding the use of urea in various feeding situations, including the use as a protein supplement for cows on low-quality forage and for backgrounding and finishing cattle. In order to answer these questions, we need to explore some basic concepts regarding urea feeding.
First let’s review how cattle meet their protein needs. Feed consumed by the animal enters the rumen and is subject to fermentation by rumen bacteria. This fermentation results in the breakdown of the carbohydrate and protein fractions of the feed. Fermentation of the carbohydrate fraction (i.e. starch, fibre) provides energy to the animal and concurrently to the rumen bacteria. The protein fraction is broken down to its constituent parts, specifically amino acids. These released amino acids are used by the bacteria to synthesize bacterial protein or are further broken down to ammonia, which in turn is used by the rumen bacteria to resynthesize amino acids, which are then incorporated into bacterial protein.
Critical to the use of ammonia for bacterial protein synthesis is an available energy supply, which as discussed above is derived from fermentation of the carbohydrate fractions of the ingested feed. If this energy is lacking or available at the wrong time, the efficiency of bacterial protein synthesis drops dramatically.
Finally, when the bacteria flow out of the rumen, they enter the small intestine of the animal and are subject to normal digestive processes. In particular, the bacterial protein and the high-quality amino acids it contains become available to the animal for its protein needs.
How does urea feeding relate to this process? First, remember that urea is strictly a nitrogen source; it is not a source of pre-formed amino acids as is the case with feed protein. The nitrogen content of urea is approximately 45 per cent. This equates to a crude protein value of approximately 280 per cent, based on a conversion factor of 6.25 (i.e. the nitrogen content of feed protein). While 280 per cent crude protein seems impressive, remember it is solely a nitrogen source. Urea is rapidly broken down in the rumen to ammonia. As with ammonia released from the fermentation of feed protein, ammonia released from urea breakdown needs to be converted to bacterial amino acids and then into bacterial protein to be used by the animal. If not used in this fashion, it is wasted.
The key to efficient use of urea-nitrogen by rumen bacteria is feeding a ration that is high in available energy, typically grain-based rations. These rations are readily fermented in the rumen, supplying both the animal and the bacteria with a readily available energy supply. In the case of the bacteria, this energy is used to incorporate urea-nitrogen into amino acids and ultimately bacterial protein. In situations where urea is fed without a readily available source of energy (i.e. poor-quality forage-based diets), much of the urea-nitrogen is wasted as there is no available energy for the bacteria to incorporate it into microbial protein.
With this background, let us try to answer the questions raised by producers regarding urea feeding. First, let’s look at using urea as the primary protein source in supplements (i.e. pelleted supplements, loose minerals or tubs) for mature cows fed poor-quality forage (i.e. poor hay, straw). It should be obvious from the above discussion that unless the supplement is fortified with an energy source such as molasses or the ration with grain, the urea in these supplements will be poorly utilized, as the forage consumed by these animals does not supply sufficient energy for the rumen bacteria to efficiently synthesize protein.
A similar situation applies to backgrounding rations. Such rations are primarily forage-based with a relatively low grain content. In such cases, protein/mineral supplements where urea supplies most, if not all of the protein should be avoided, as there is not sufficient energy released from the fermentation of these high-forage diets to make efficient use of the urea-nitrogen.
In contrast, urea can be used very effectively when high-grain rations are fed as the fermentation and release of energy from these diets nicely matches the release of ammonia from the urea. Such synchronization optimizes the ability of rumen bacteria to incorporate urea-nitrogen into protein. A good rule of thumb to follow in these high-grain feeding situations is that protein from urea comprises no more than one-third of the total ration crude protein.
In addition to ensuring the right “feeding situation,” there are several other important considerations when using urea as a protein source. Most importantly, keep in mind that urea can be toxic if fed in excess. As such, ensure that urea-based supplements are fed at recommended levels and thoroughly mixed in the ration. Do not feed multiple sources of urea. As well, rations need to be balanced for all essential nutrients, including the macro and micro minerals. Finally, from both a palatability and efficiency-of-use perspective, urea should not be fed to light-weight calves.