The proteins which contain much of the nitrogen consumed by dairy cattle are comprised of individual amino acids. The proteins in the body are synthesized from a specific pattern of amino acids, and many of these cannot be synthesized readily in the body. If such an essential amino acid is not supplied adequately in the diet, protein synthesis is arrested and growth or milk production are reduced. Therefore, it is economically advantageous to feed excess protein including essential amino acids to prevent a loss in production.
For simple stomached animals (e.g. swine), there is a clear requirement for certain amino acids. For example, lysine is deficient in corn-based products and methionine is deficient in legume-based feeds. Because corn and soybean meal are the cheapest and most common feeds for animals, their diets are typically first-limiting in lysine or methionine. Feed companies have supplemented their products with these amino acids to reduce the cost of feeds and the amount of protein required. This technology is cost effective and has been implemented for swine and poultry. For example, adding lysine to finishing diets for pigs reduces the total protein feeding by 15 to 20%, and the nitrogen excretion by 30 to 40%.
Ruminants (cattle, sheep) complicate protein nutrition because they have pre-stomach chambers where digestion occurs. In the first two chambers, the rumen and the reticulum, a population of symbiotic bacteria and protozoa ferment the feeds and grow from non-protein nitrogen sources like ammonia or urea. These bacteria can digest fiber in plants enabling cattle to obtain energy from these feeds. They also synthesize protein from inexpensive byproducts. The microbial population continuously washes out of the rumen to the true stomach where it is digested to supply amino acids to the cow.
The microbial population alone cannot supply all of a productive dairy cow's nutrition. There is a limit to how much microbial protein can be synthesized. Although it is imperative to feed the bacteria in the rumen, the cow must also be provided with feed protein which escapes degradation in the rumen and provides protein that is digested in the small intestine. Although the bacteria have an excellent amino acid composition, the ruminally undegraded feed protein is usually deficient in lysine or methionine. Therefore, feeding rumen protected sources of lysine or methionine can result in reducing the total amount of protein needed to balance the amino acid requirement.
Although supplementing the diets of simple-stomached animals with amino acids is a common agricultural practice, doing the same for cattle is a more complex process. If free amino acids are fed directly to cows, the rumen microbes destroy them before they reach the true stomach. Recently, several companies have developed amino acid supplements that are chemically protected from degradation in the rumen. These supplements are expensive and their effectiveness must be studied carefully because of the complexity of determining amino acid requirements in the rumen, and the degree that the supplements can escape degradation. In theory, protein feeding could be reduced for dairy cows by feeding protected amino acid supplements (Dinn, et al., 1996).
Typical dairy cow diets are limiting in the amino acids lysine and methionine. Previous research has shown that milk production will increase from adding up to 12 g of rumen protected lysine and methionine when feeding below the animal's protein requirements. Alternatively, adding feed protein will also increase the availability of these essential amino acids to cause a production response. However, feed protein provides an excess of other amino acids and more nitrogen to the animal than needed to meet requirements. For example, lysine comprises 6.1% and methionine comprises 1.5% of the protein in expeller soybean meal. Half of the expeller soybean meal amino acids would be digested by rumen microbes, and of that remaining, 20% would be indigestible and excreted in feces. Therefore, 12 g of methionine and lysine could replace up to 1000 g of the soybean protein that is not degraded in the rumen, or a third of the total protein requirement for most dairy cows.
The successful substitution of protected methionine and lysine for feed protein would reduce nitrogen intake of a typical lactating cow from 1.1 lbs per cow per day to .72 lbs per cow per day (a 35% reduction). As a result, manure N would be decreased from 0.75 to 0.39 lb per cow per d (a 48% reduction). In practice, there is a risk that other amino acids are likely to be co-limiting at such low levels of protein feeding and the amount of protein that can be substituted for protected amino acids may be much less. Nonetheless, a substantial reduction in protein feeding should be possible when feeding protected amino acid supplements, but the actual amount of the required protein that can be substituted with protected amino acids is not known.
Despite the theoretical benefits of using protected amino acids, in practice, further work is needed to improve our understanding of animal amino acid requirements and availability in the animal. Research is needed to improve diet formulation models to balance for amino acid requirements. Feeding trials are needed to confirm and to quantify the potential to reduce protein feeding when using protected amino acids. Most of the previous research on using protected amino acids has aimed to increase milk protein production rather than to substantially decrease protein feeding.
Because the cost of these products is very high, they have been difficult to justify based on the expected milk production response. The cost of protected lysine and methionine together is greater than the cost of the obtaining these amino acids from sources of feed protein like heated soybean meal. In contrast, the least expensive source of available methionine is from protected methionine. Therefore, once the extent and occurrence of methionine limitation is understood and adequately predicted, methionine supplements will be cost effective -- especially on legume-based diets rather than corn-based diets. The environmental benefit from supplementing methionine alone has not been well established, but the benefit is likely to be less than half of that from supplementing both lysine and methionine.
Table 1. Cost of lysine or methionine from rumen protected amino acids or common feedstuffs: heated soybean meal (SBM) or corn gluten meal (CGM).*
Ingredient % % avail. $/lb % % avail. $/lb Lys-Met 13,700 40.00 40.00 17.12 15.00 15.00 45.67 Met 13,700 0 0 -- 70.00 70.00 9.79 SBM-heated 300 2.25 1.08 13.89 0.53 0.25 60.00 CGM 350 1.03 0.45 38.89 1.49 0.66 26.51
*Available lysine or methionine is determined as that which would escape digestion in the rumen and be absorbed in the small intestine.
Dinn, N. E, L. J. Fisher, J. A. Shelford, and J. Paul. 1996. The use of the Cornell Net Carbohydrate and Protein System and rumen-protected lysine and methionine to reduce nitrogen excretion of lactating dairy cows. J. Dairy Sci. 79: Suppl.1 p. 129.
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