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It is an exciting time in the world of dairy nutrition with the recent update to the nutrient requirements of dairy cattle by the National Academies of Science, Engineering and Medicine (NASEM, 2021). For protein nutrition, NASEM (2021) introduced numerous updates. We are now looking at multiple essential amino acids (more than methionine and lysine) as well as highlighting that cows have flexibility in amino acid metabolism and not strict amino acid requirements. This metabolic flexibility includes cows possessing the ability to match amino acid metabolism to dietary supply. By understanding a cow’s metabolic flexibility, we can develop targeting amino acid feeding strategies.
What is metabolic flexibility of a cow, and how do we describe this? When an amino acid is absorbed by a cow’s digestive tract, this amino acid has one of three fates (Figure 1).
- Milk protein, pregnancy or growth
- Necessary maintenance functions such as secretion in urine, feces (digestive enzymes, intestinal cells) and scurf (skin and hair)
- Metabolized and converted to non-essential amino acids or energy
The first fate generates income on our dairy, and hence we want to maximize this. Although the second fate does not generate income, this fate is necessary to keep cows alive and healthy. The final fate is a non-productive use of amino acids and one we want to minimize for economical and environmental reasons. While the maintenance fate for amino acids is essentially fixed and a function of bodyweight and dry matter intake (DMI), the milk or metabolic fate is dynamic. This is one way in which a cow demonstrates her metabolic flexibility.
If the supply of a given amino acid supply is low relative to productive needs, then metabolism will decrease and the utilization efficiency of that amino acid for milk and maintenance will be high. If the supply of a given amino acid is high relative to productive needs, then a cow will metabolize a larger proportion of that amino acid and have a low efficiency. In other words, excess intake of a given amino acid relative to productive needs will result in that amino acid being burned up and a low efficiency.
How can we utilize amino acid efficiency in practice? Based on metabolizable amino acid supply, milk protein yield and maintenance use (function of bodyweight and DMI), we can calculate amino acid efficiency. It is simply milk protein plus maintenance amino acid needs divided by metabolizable supply. If this calculated efficiency value is high, then there is a high likelihood that increasing supply of that given amino acid will increase milk protein yield. If this value is low, then cows are not very likely to increase milk protein yield in response to increased supply of this amino acid. In fact, you might want to feed less of that amino acid if the efficiency is low. These values are much like a college football quarterback. At the end of the fourth quarter, and if the game is on the line, do you want a quarterback that has a high or low passer efficiency rating? In this case, higher is better. Just like quarterbacks, the amino acid with high observed efficiency is the one you want to put money into (increase supply) to give you the best chance for a positive return (milk protein or a touchdown).
What is a high or low amino acid efficiency value? Although this seems like a relatively straightforward question, it is not. NASEM (2021) provides target efficiency values for nine essential amino acids. Target values along with mean, minimum and maximum estimated efficiency for five key amino acids are listed in Table 1. These values provide a reference a nutritionist can use to determine whether amino acid efficiency is high or low and hence whether to increase, decrease or maintain supply of a given amino acid.
Based on research data, amino acid efficiency ranges from approximately 30% to over 100%. However, where exactly one should be on this range of efficiency is as much an economic question as it is a nutrition question. Ideal amino acid efficiency depends on the price of a given amino acid, objectives of a given farm, group of cows and the value of milk protein. If milk protein price is high and the cost of an amino acid is low, then ideal efficiency may be lower than the NASEM target values, as higher supply likely will result in more milk protein. The opposite is true when milk protein price is low and the cost of an amino acid is high. Additionally, the cost to increase the supply of some amino acids is relatively inexpensive due to their concentration in feeds (corn protein for leucine and soy protein for isoleucine), while other amino acids can be more expensive (synthetic methionine and lysine in rumen-protected form). There is certainly a place for the latter in dairy diets, but economics should be considered to ensure a positive return.
Amino acids also likely have a positive effect on other outcomes besides just milk protein synthesis. Recent university research has demonstrated that amino acids can stimulate milkfat synthesis. In the NASEM model, two amino acids are part of the milkfat prediction response equation, illustrating this link of amino acids to milkfat production. In addition to milkfat synthesis, amino acids supply may improve health, reproduction and fetal development. However, these factors are not considered in NASEM or other nutrition models when trying to determine ideal amino acid supply. To date, responses are not well defined, and return for the non-milk factors can be challenging to determine. More work is certainly needed in this area.
Cows have metabolic flexibility to adapt to a wide range in amino acid supplies and combinations. The new NASEM publication highlights this metabolic flexibility and provides additional tools that can be used to determine whether to increase or decrease supply of an individual amino acid. If the utilization efficiency for milk and maintenance is high, then there is likely opportunity to increase supply to increase milk protein yield. If efficiency is low, then milk protein yield is likely maximized, and there may be opportunity to decrease supply and ration cost. The ideal efficiency should maximize income over feed cost and thus depends on the price of given amino acid and the value of milk protein.
See previous article, Our understanding of amino acid nutrition for lactating cows is changing for more information.
