As the cost to feed cows continues to climb, and as environmental concerns continue to mount over nutrient pollution coming from dairy farms, much more attention is being directed toward researching and defining the proper amino acid balance for both milk cow growth and health as well as maximum milk production and optimal milk components.
The primary focus of properly balancing amino acids in milk cow diets is to produce milk more efficiently and make milk more valuable through increased components while minimizing contamination of soils and water sources with dairy waste products.
All proteins are constructed of specific amino acid sequences, and there are about 20 amino acids that contribute to the formation of proteins. Milk volume, fat, protein and lactose in milk are all synthesized by different enzymes.
Those enzymes, being proteins, are made up of specific amino acids. When there’s not enough of a specific amino acid to make a protein, the amino acid is referred to as “limiting.”
Diets that are deficient in a specific amino acid, preventing the synthesis of a protein, oftentimes will limit growth in calves, have a negative impact on the immune system in calves, heifers and cows, as well as reduce reproductive efficiencies and (most importantly for dairy producers) limit milk production and components.
The challenge for dairy scientists is to identify those limiting amino acids most critical to health and production, establish their requirements and introduce them into diets in ways that are both affordable and effective.
While rumen microbes provide much of the metabolizable protein to the small intestine, those microbes also have a nitrogen requirement that must come from dietary protein. The fiber-digesting microbes essentially break the amino acids apart, take the nitrogen they need (much in the form of ammonia) and discard the rest.
What’s left of this dietary rumen-degradable protein (RDP) then becomes part of the rumen-undegradable protein (RUP) pool and proceeds to the small intestine for absorption if it is needed.
A diet that is deficient in RDP will impede rumen function. It’s important to recognize that protein coming from the diet generally provides only a fraction of the metabolizable protein directly to a cow, and the RUP passing to the small intestine often does not have a desirable amino acid profile.
Research has shown that milk production and milk components respond favorably when the amino acid profile of RUP closely matches that of the amino acid profile of microbial protein.
For lower-producing cows, the endogenous microbial protein leaving the rumen may supply most of the cow’s metabolizable protein needs; adding expensive RUP is often not necessary.
For high-producing milk cows, the microbial protein must be supplemented with additional sources of RUP. Therefore, additional sources of essential amino acids must be supplied in the diet and make it past (bypass) the rumen and into the small intestine.
Comparing the amino acid profiles of rumen bacteria, milk and lean tissue to many common feedstuffs has shown that the amino acid lysine is deficient in nearly all commodities fed to dairy cows with the exception of soybean meal and animal proteins such as meat, blood and fish.
Because the lysine level in corn and corn byproducts is low, and the majority of milk cow diets in the U.S. are heavy on corn usage, increasing and balancing for metabolizable lysine levels in milk cow diets receives much attention.
After many years of research, there are a number of companies offering an affordable rumen-protected lysine product that can deliver a predictable level of lysine to the small intestine.
The amino acid methionine is also considered limiting in many diets and has been available in rumen-protected forms for many years as well. Balancing lysine and methionine and their symbiotic relationship has been a main focus in diet formulation, especially for cows in early lactation where more metabolizable amino acids are necessary for milk production.
Studies have also shown that histidine may be the first limiting amino acid in diets high in grasses or legumes. Still others have suggested that arginine may also be limiting in some situations.
In reality, it remains challenging to peg just which amino acid may be limiting at any given time due to variations in feedstuffs, feed intakes and overall rumen activity. Fluctuations in microbial population will have a profound effect on the amount of microbial protein getting to the small intestine.
In a perfect world, we could have identical corn silage month after month, the same hay crop month after month, as well as the same commodities, so that the amino acid profiles and absorption rates will remain constant. Such is not the case.
Even though we may know the approximate levels of amino acids in many feeds, there is still the question of how much of a given protein in a feedstuff is degraded in the rumen and how much is left to bypass the rumen at any given time.
Commodity byproducts and forages that undergo heating have large variations in RDP levels. Variations in forage quality – protein and fiber digestibility – will affect the microbial population in the rumen on a daily basis with frequent changes in forage. Further complicating the issue is the fact that feed intakes vary from day to day depending on feed quality and weather.
Dairy farmers and nutritionists should consider it money wasted if attempting to balance dairy cow diets for amino acids when feed intakes are not known or are inaccurate.
It’s also counterproductive to try to balance diets for single-TMR groups when there’s a significant range of milk production and days in milk. Rumen-protected amino acids are probably best used on cows in early lactation.
It should also be understood that making sure that other dietary factors, such as effective fiber, adequate energy, proper types of carbohydrates and adequate RDP, should be properly balanced before investing in rumen-protected amino acid products.
Quite often, significant improvement to milk production and components can be accomplished merely by getting other basic dietary requirements in line.
While protein and amino acid nutrition continues to represent a sizable portion of total dietary costs on dairy farms, properly balancing amino acids can result in production efficiencies that more than cover the investments.
The objective of all dietary changes or adjustments to dairy diets should be to increase net milk revenue through a combination of improved milk production efficiencies or reductions in net feed costs.
Amino acid balancing has come a long way in recent years and has been proven to add value to milk production. Much of the decision to use rumen-protected amino acids should be based upon how much milk a dairy farmer is looking for the cows to produce.
The economics of whether to use protected amino acid products needs to be left to the dairy farmer and his management team to figure out, especially when milk prices are low or there may be weaknesses in overall nutritional management.
Even though a couple of pounds of extra milk or a couple points of protein will improve the milk check, carefully consider the cost of amino acid supplementation and spend it when and where it makes the most sense. PD
ILLUSTRATION: Illustration by Sarah Johnston.
-
John Hibma
- Nutritionist
- Central Connecticut Co-operative Farms Association
- Email John Hibma
