INTRODUCTION
Proteins provide the amino acids needed for maintenance of vital functions, reproduction, growth and lactation.
Non ruminant animals need pre-formed amino acids in their diets, but ruminants can utilize many other nitrogen sources because of their rare ability to synthesize amino acids and protein from non-protein nitrogen sources. This ability is associated with the presence of the microorganisms in the rumen. In addition, ruminants possess a mechanism to spare nitrogen. When a diet is low in nitrogen, large amounts of urea (which is normally excreted in the urine) return in the rumen where it can be used by the microbes. In non-ruminants,urea is always entirely lost in the urine. Research showed that it is possible to feed cows with diets containing non-protein nitrogen as the only nitrogen source and still obtain a production of 580 g of high quality milk protein daily and 4000 kg milk throughout the lactation.
PROTEIN TRANSFORMATION IN THE RUMEN
Feed proteins are degraded by microorganisms in the rumen via amino acids into ammonia and branched chain fatty acids (Figure 1). Non-protein nitrogen from the feed and the urea recycled into the rumen through the saliva or the rumen wall contribute also to the pool of ammonia in the rumen. If ammonia levels in the rumen are too low there will be a nitrogen shortage to bacteria and feed digestibility will be reduced. Too much ammonia in the rumen leads to wastage, ammonia toxicity, and in extreme cases, death of the animal.
The bacterial population uses ammonia in order to grow. The extent to which ammonia is used to synthesize microbial protein is largely dependent upon the availability of energy generated by the fermentation of carbohydrates. On the average, 20 grams of bacterial protein is synthesized per 100 grams of organic matter fermented in the rumen. Bacterial protein synthesis may range from less than 400 g/day to about 1500 g/day depending primarily on the digestibility of the diet. The percentage of protein in bacteria varies from 38 to 55% (Table 1). However, when cows eat more feed, the bacteria contain more protein and pass from the rumen to the abomasum more rapidly.
Usually, a portion of the dietary protein resists ruminal degradation and passes undegraded to the small intestine. The resistance to ruminal degradation varies considerably among protein sources and depends upon many factors. Usually, the proteins in forage are degraded to a further extent (60 to 80%) than proteins in concentrates or industrial by-products (20 to 60%). A portion of the bacterial protein is broken down within the rumen, but the majority flows to the abomasum attached to feed particles. The strong acids secreted by the abomasum stop all microbial activity and the digestive enzymes start breaking down the protein into amino acids.
Approximately 60% of the amino acids absorbed through the small intestine is from bacterial protein, and the remaining 40% is from ruminally undegraded dietary protein. The amino acid composition of bacterial protein is relatively constant regardless of the composition of dietary protein. All amino acids, including the essential ones. are present in bacterial protein in proportion that is fairly close to the proportion of amino acids required by the mammary gland for milk synthesis. Thus, the conversion of dietary protein to bacterial protein is usually a beneficial process. The exception occurs when high quality protein is fed and the ammonia produced in the rumen cannot be utilized due to a lack of fermentable energy.
PROTEIN IN FECES
About 80% of the protein reaching the small intestine is digested, but the remaining is passed into the feces. Another major source of nitrogen in the feces comes from digestive enzymes secreted into the intestine and the rapid replacement of intestinal cells (fecal metabolic protein). On the average, for every increment of 1 kg of dry matter ingested by the cow, there is an increase of 33 g of body protein lost in the intestine and excreted in the feces. Ruminant feces is good fertilizer because it is rich in organic matter and is particularly rich in nitrogen (2.2 to 2.6% nitrogen or the equivalent of 14 to 16% crude protein) as compared to the feces of non-ruminant animals.
LIVER METABOLISM AND UREA RECYCLING
When fermentable energy is lacking or when crude protein in the diet is excessive or highly degradable, not all the ammonia produced in the rumen may be converted to microbial protein. Excess ammonia cross the ruminal wall and is transported to the liver. The liver converts the ammonia to urea which is released in the blood. Urea in the blood can follow two routes:
1) It can return to the rumen through the saliva or through the rumen wall.
2) It can be excreted into the urine by the kidneys.
When urea returns to the rumen, it is converted back to ammonia and can serve as a nitrogen source for bacterial growth. Urea excreted in the urine is lost to the animal. With rations low in crude protein, most of the urea is recycled and little is lost in the urine. However, as crude protein increases in the ration, less urea is recycled and more is excreted in the urine.
MILK PROTEIN SYNTHESIS
During lactation, the mammary gland needs large amounts of amino acids to syntesize milk protein.
The metabolism of amino acids in the mammary gland is extremely complex. Amino acids may be converted into other amino acids or oxidized to produce energy. Most of the amino acids absorbed by the mammary gland are used to synthesize milk proteins. Milk contains about 30 g of protein per kg, but there are important variations between cows within a breed and among breeds.
About 90% of the protein in milk is casein. There are many types of casein (Table 2) and they contribute to the high nutritive value of many dairy products. Whey proteins are also synthesized from amino acids in the mammary gland. The enzyme ?-Lactalbumin is essential for the synthesis of lactose and ??lactoglobulin is important in curd formation during cheese production. Some proteins found in the milk (immunoglobulins) play a role in passing disease resistance to the newborn calf. The immunoglobulins are absorbed directly from the blood and not synthesized within the mammary gland, so their concentration in the colostrum is high. Milk also contains non-protein nitrogen compounds in very small amount (e.g., urea: 0.08 g/kg).
PROTEIN AND NON-PROTEIN NITROGEN IN DAIRY RATIONS
Recommendations for the crude protein concentration in dairy cow rations vary from 12% for a dry cow to 18% for a cow in early lactation. As long as the diets of cows producing about 20 to 25 kg of milk contains about 16% crude protein, most forage and concentrates are adequate protein sources. However, as milk production increases, bacterial protein synthesis in the rumen may become insufficient, and protein sources resistant to ruminal degradation may be needed to supply the required amount of amino acids. Typical sources of proteins resistant to microbial degradation in the rumen include brewer’s grain, distiller’s grain and proteins of animal origin (slaughter house byproducts, feather meal and fish meal).
On the other hand, non-protein nitrogen sources may be used, especially when a ration contains less than 12 to 13% crude protein. Urea is probably the most popular source of non-protein nitrogen in dairy rations. However, it must be used with caution because its excess may rapidly lead to ammonia intoxication. Feeds most successfully supplemented with urea are high in energy, low in protein, and low in non-protein nitrogen. A partial list of such feeds includes cereal grains, molasses, sugar beet pulp, hay of mature grasses and corn silage. Urea should not be used with feeds rich in rapidly available nitrogen. Such feeds include oilseed meals (e.g., soybean, canola), legume forage and young grasses. In addition, urea should be limited to no more than 150 to 200 g/cow/day, thoroughly mixed with other feed to improve palatability and added progressively in the ration to allow adaptation by the cow.
