Feeding Urea to Cows
There are lot of misunderstanding on feeding of urea to cows. Urea is a cheap source of nitrogen for ruminants. Since microbes in complexed stomach cows can capture released nitrogen and convert into microbial proteins. These microbes are then digested in abomasum broken into simple peptides and used for body functions. But this conversion of urea into urea and microbial proteins take place under certain circumstances.
Age of the cow: A major factor to consider is age of the cow. The new-born calf is not born with a functional rumen which develops over time. Two practices are important: (a) how fast solid feed is introduced and (b) exposure to the cow and barn environment which facilitates picking up of cud inoculate which stimulates rumen development and function. Instead of age a general rule is to hold off using urea as a protein supplement for cattle until they weigh around 250 kg as rumen development has been found to be closely associated with body weight.
Fermentable Carbohydrates: For capturing nitrogen from urea microbes require fermentable sugar which cows can get from starch, pectin or fibre since these can be degraded and used by microbes as source of energy. Addition of sufficient quantity of fermentable carbohydrate and urea has been shown to result in synthesis of high quality protein. But if in feed fibre portion or neutral detergent fibre (NDF) fraction is high urea utilization is limited since these degrades very slow. Thus, a good quantity NFC must be present to utilize added urea properly. In general nutritionists advise use of urea to add crude protein to poor quality forage or straw but also advise caution that it should be done only when readily available carbohydrate is available as energy source. Excess of urea must be excreted into manure in order to prevent toxicity and this also requires energy. Urea could be used in this situation quite effectively though if a small quantity of grain or molasses would be added. When grain mix is sued urea can be included at the rate of 1.5 to 3% depending on the energy value of feed. Farmers should be aware that most of the concentrate feed available in the market contain urea up to 3%.
Other Proteins in Diet: Urea in feed is also classified as non-protein nitrogen (NPN) source. A large portion of plant source proteins is also degraded in rumen to ammonia and nitrogen which is then captured ad utilized by rumen microbes. This a class of plant proteins are in fact source of nitrogen like urea. While supplementing urea as source of protein the quantity to be added should also account for nitrogen pool generated from protein sources. Thumb rule is that urea should be just enough for rumen conversion and no excess ammonia should pass to circulation as its clearance from body is energy expensive.
Rations: Rations containing high levels of grain or good quality corn silage, corn stalks or sorghum can utilize urea quite well since these forage tends to have a higher NFC level and lower rumen degradable protein fraction. It is also necessary to consider nitrate content of water as these could also be source of nitrogen. Below are a couple examples of what the above article tried to convey.
Finally, urea can be used in the ration up to the point where the rumen degradable intake protein (DIP) meets 100% of the available carbohydrate. If the DIP level exceeds 100%, no urea or NPN source should be used.
For farmers feeding corn silage a ration consisting of 25 kg silage per day can be added with 200 – 225 g urea. For hay rations alone urea should not be used unless around 1.75 Kg dry corn is not added. In this case urea is allowed @ 155 g to balance the ration.
In India urea molasses blocks are also available as lick. In this product urea and molasses are packed with straw. Since cows lick this whole day small amount of urea and molasses as source of soluble carbohydrate is available. In few products mineral is also added to these blocks converting these into multi-nutrient blocks for cows. It must be reiterated that if properly followed urea supplementation in ration is a valuable measure to reduce cost of feed. Moreover urea is also regarded good for rumen health. Most of the accidents occur when urea is not labelled and stored properly.
Urea Poisoning: Urea poisoning can occur when cows have accidental access to large amount of urea, beyond the capacity of rumen to capture and convert to microbial protein. It can also occur when urea is suddenly introduced in the feed. When cows consume large amount it could also be fatal. Signs of poisoning can include twitching of ears and facial muscles, grinding of the teeth, frothy salivation, bloat, abdominal pain, frequent urination, forced rapid breathing, weakness, staggering, violent struggling and bellowing, and terminal spasms. Often, animals are found dead near the source of the urea supplement. Dead animals show lots of foam and bloat, opening of rumen would release ammonia. The rumen fluid and respiratory discharges smell of ammonia. There is no good laboratory test to diagnose urea poisoning. In sick animals if blood can be collected in EDTA and transferred to laboratory on ice within 30 minutes of collection blood ammonia level could be of diagnostic value. This test however should not be done in blood drawn from dead animals.
Treatment is rarely effective. However, if cattle can be handled, a stomach tube can be passed to relieve the bloat and then used to drench the animal with around 45 L of cold water followed by 2-6 L of 5% acetic acid or vinegar. This dilutes rumen contents, reduces rumen temperature and increases rumen acidity, which all help to slow down the production of ammonia. Treatment may need to be repeated within 24 hours, as relapses can occur. Rumenotomy and removal of rumen contents can be done in valuable animals.