Amino Acids Are Required Nutrients
There are over 700 AA that exist in nature. Twenty serve as building blocks for protein. Of these 20 “protein” AA, 10 are classified as nutritionally essential (indispensable), meaning they cannot be synthesized in the body and must be provided by the diet and absorbed in the amounts needed.
Figure1.: The role of methionine in the cow’s metabolism (Luchini and Loor, 2014)
Protein AA are needed for the synthesis of hundreds of different tissues, regulatory, receptor, blood, protective, and secretory proteins. Protein synthesis is a genetically determined event, and as a consequence, the AA composition of each protein, while having its own unique AA composition, is the same every time it is synthesized. In addition to their role in protein synthesis, which affects virtually every aspect of metabolism in every living cell (e.g., all enzymes are proteins), free AA (both protein and non-protein AA) are also key regulators of various pathological and physiological processes, including immune responses. They are also used for the synthesis of the other N-containing compounds in the body, which includes dozens of compounds such as hormones, neurotransmitters, nucleotides (RNA and DNA), histamine, polyamines, etc.
Table1. A comparison of lysine (Lys), histidine (His) and methionine (Me)t concentrations in crude protein (CP) of lean tissue, milk, rumen bacteria and common feedstuffs.
Limiting Amino Acids
In ruminants, AAs are provided 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 that is balanced to meet requirements for RDP and RUP. 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 amino acid (AA) profile of the feed (table 1). If one AA is in shortage relative to its requirements, the other AA cannot be used and therefore will be in excess. 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 2. AA Barrel
Methionine and lysine 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.
Benefits Of Amino Acid Nutrition
The benefits of AA balancing, with the focus being almost entirely on Lys and Met thus far, are well known and have been summarized. These benefits include reducing the risk of cows experiencing an AA deficiency, optimizing transition cow health, increasing milk and milk component yields, and feeding less RUP to post-transition cows. Feeding less RUP not only decreases feed costs, it also allows for increased carbohydrate feeding. The consequence is increased synthesis of microbial protein, a protein of high quality, and increased synthesis of volatile fatty acids, important substrates for lactose and fat synthesis. The benefits of AA balancing are clearly the most noticeable in transition and early lactation cows (Schwab, 2012, Osorio et al., 2014); however, benefits of reduced feeding of RUP and milk protein concentrations exist throughout the rest of the lactation.
Figure 3. 4 key roles of amino acids in the cow
Amino acid balancing can have profound effects in early lactation cows. Experiments continue to show the importance of adequate Met supplementation of transition cows. Two interesting findings have been made. The first is that plasma Met concentrations were significantly lower in cows with fatty livers than in cows classified as healthy (Pechova et al., 2000; Shibano and Kawamura, 2006). The second is that there is now growing evidence that RP-Met supplementation of transition cows may reduce post-calving metabolic disorders, including ketosis, and that these effects are likely mediated not only by enhanced protein synthesis, but also by enhanced liver function and reduced inflammation and reduced oxidative stress (Osorio et al., 2013, 2014 and Zhou et al., unpublished). In other words, achieving more optimal supplies of Met, relative to Lys, improves both health and production and at least part of that benefit is due to a better immune-metabolic status.
Conventional wisdom would indicate that any ration manipulation that reduces post-calving metabolic disorders and improves the overall health of early lactation cows may have a positive effect on reproduction (Santos, 2005). Robert et al. (1997) observed a better uterine involution (percentage of animals whose uterus has regressed to normal size at 45 days post calving). This was associated with a reduced number of inseminations needed per conception; however, neither effect was significant. They also measured milk progesterone levels every three days for the first 112 days of lactation to follow the cyclicity. They were able to show that the cows receiving a ration balanced for Lys and Met had higher progesterone levels pre successful ovulation than control animals. This is considered to potentiate a strong ovulation.
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.