Limiting Amino Acids (AA)
In ruminants, AAs are supplied by ruminally synthesized microbial protein, rumen-undegradable protein (RUP), and to a lesser extent, endogenous protein. Microbial protein typically supplies a majority of the AA. However, RUP may supply more than 50% of the absorbed AA in high-producing cows fed a high-concentrate diet. The quantity of AA provided by endogenous protein secretions is smaller, assumed to account for less than 10% of total absorbed AA (NRC, 2001 and H. Lapierre, personal communication).
How efficiently the digestible protein is utilized depends directly on the AA profile of the feed. If one AA is in shortage relative to its requirements, the other AA cannot be used and therefore will be in excess (figure 1). As a result, they will be destroyed, which increases the nitrogen burden in the dairy cow and has a non-optimum impact on animal performance and the environment.
Figure 1. Limiting amino acid concept following Liebig’s law of the minimum
Lysine Essential Nutrient
Dairy cows require amino acids to make milk and muscle protein, to make the protein in a growing fetus, and to make the proteins they need to maintain themselves (such as enzymes required to digest feeds). For this reason, a cow will require different amounts of each of the essential amino acids depending on her stage of lactation, growth, and pregnancy. Without proper supplementation of the main amino acids such as lysine (Lys) and methionine (Met), it's hard to reach nutritional goals. This is also an indication that other important proteins production in the body may be compromised (Figure 2).
Figure 2: List of the main proteins present in the body of the animal
Lysine Deficient Diets
Methionine (Met) and lysine (Lys) have been identified most frequently as the two most limiting AA for lactating dairy cows fed corn-based rations (NRC, 2001). Research conducted since the publication of NRC (2001) has confirmed these findings. Met and Lys are the first two limiting AA in most feeding situations, it is not surprising given their low concentrations in most feed proteins relative to concentrations in rumen bacteria and in milk and tissue protein (Graph 1).
Graph 1. A comparison of lysine (Lys) and methionine (Met) concentrations in crude protein CP of lean tissue, milk, rumen bacteria and common feedstuffs (NRC, 2001).
SmartMILK: Balancing for AA has, without question, been a contributing factor to higher milk yields, higher milk component levels, and greater herd profitability for many dairy producers.
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