Thursday, December 29, 2016

Analyzing Nimo's Diet: Protein Requirements

In the fourth installment of my series on equine nutrition, let's turn our attention to the final source of energy:  protein.  It is at this point that something useful finally comes out of Nutrient Requirements of Horses in terms of a starting place, but it is only that, a starting place.  Because it turns out that not all protein is equal.  Horses don't really have a requirement for protein in general; rather, they have a requirement for amino acids.  Not all sources of protein have all the essential amino acids in the right amounts.  Plus, there is the matter of the digestibility of the protein as well as its bioavailability.  And, of course, it is difficult to measure the protein in food, so we don't always really know what we're feeding.

The best measure of protein for the average horse person is probably Crude Protein (CP).  You'll see this number (usually as a percentage) on feed and supplement labels.  CP tells you the level of total protein in the food, but it doesn't tell you how much of that protein the horse will be able to use or how complete it is (i.e. how many or which amino acids it has).  It does, at least, give you a maximum number, though.  If your feed has a CP of 8%, you'll know that it isn't possible for your horse to get more protein out of that feed than 8%.  And, according to some of the studies identified in the book, it's more likely that the true digestible protein will be somewhere between the 40 and 90% of whatever percentage is listed on the bag, depending on the source of the protein if it is a feed or the type of grass/legume if it is a hay.  So, if the feed has an 8% protein, but only 50% of that is digestible, now you're only feeding 4% protein, which is probably pretty low.

But there are formulas in this section, so I can at least get a baseline for Nimo.  Here's the one I choose for Nimo:

CP (g/d) = (BW(kg) x 1.44) + (BW(kg) x 0.354)

This formula includes the maintenance requirement at the elevated level (for working horses) plus the additional protein needed to support a horse in very heavy work.  If we plug the numbers in, we get:

CP (g/d) = (680 x 1.26) + (680 x 0.354) = 1,220 g

Thus, 1,220 g of protein is the amount of crude protein I should be looking for in Nimo's diet as a starting point.

Aside from this formula, I found a couple of other noteworthy pieces of information in this section of the book.  First, horses that are fed a ration that completely meets their caloric requirement but is deficient in protein will lose weight (see p. 58).  So if you have a horse that should be getting enough food based on your calculation of DE (that means Digestible Energy), but is still losing weight, it's probably a good idea to check not just the level of protein, but also the quality of protein.  But what is a good quality protein, you ask?  Nutrient Requirements stops short of giving us any ideas for actual food products, but it does tell us what the ideal amounts of amino acids are on p. 65.  Presumably, a food source with ratios close to these would be a great source of protein for the horse.  These amounts are based on the ratios of the amino acids in the muscle of the horse.  Lysine is set at a value of 100 and the other amounts are as compared to lysine:

Methionine: 27
Threonine: 61
Isoleucine: 55
Leucine: 107
Histidine: 58
Phenylalanine: 60
Valine: 62
Arginine: 76
Typtophan: Unknown

I'm not sure why lysine is used as the base for the ratios, but we do, apparently, have a good idea of what the lysine requirement is for horses.  It is 4.3% of the CP requirement (see p. 58).  So for Nimo, that would be 52 g/d.  The book notes that if the source of protein doesn't have close to the ideal relationship of lysine to CP, you may need additional sources of protein (see p. 60).

One thing that is addressed is excess protein.  I can't remember how many times I've been told that if you feed a diet too high in protein, it causes the horse to drink more to excrete the extra protein and it can even stress the kidneys due to the extra water and protein that have to be processed.  Nutrient Requirements, however, admits that "not much evidence exists concerning the effect of excess protein consumption" (see p. 65).  There may be an issue for growing horses in terms of reduced growth and increased calcium and phosphorus loss, though.  Additionally, one study found that excess protein "may interfere with acid-base balance during exercise (Graham-Thiers et al., 1999, 2001)" (see p. 65).  (Note:  Acid-base balance is actually a really important concept for endurance riders.  The best place that I know to look for information is on Mel's blog using this link:  http://melnewton.com/?s=acid+base.  Mel did a multi-part series on acid-base balance that is very informative.)  So, it's hard to know if excess protein really does strain the kidneys, and it's also hard to know exactly what constitutes excess protein.  That said, there doesn't appear to be a good reason to go overboard with protein either, so especially for growing and performance horses, more careful monitoring of protein is probably a good idea.

It does appear that exercising horses do need extra protein based on several studies that in particular found nitrogen loss during exercise through sweat.  (I didn't realize until recently that nitrogen is an important component of amino acids, so its loss apparently indicates protein loss.)  I already added the extra protein required for Nimo in my formula above, but if you are interested in knowing how much your horse's protein requirements increase based on level of work, check out p. 64.

So we know protein is really important for horses and we also know that horses need a certain composition of amino acids to get the best use of the protein they ingest.  But what sources of species-appropriate food contain a good ratio of amino acids?  Are there any particularly good sources of protein that we should be feeding?  Once again, I must throw my hands up in frustration.  Nutrient Requirements cannot recommend even one good source of protein for horses.  It talks a lot about swine and there was apparently one study done on the presence and ration of amino acids in mare's milk.  (Unsurprisingly, mare's milk fits the amino acid profile established as ideal...)  But there is absolutely not a single recommendation on where the wondrous food might be that you could feed your horse.

Just for fun, I googled the amino acid composition of duckweed.  The composition can vary depending on the water in which it is grown, but here are the values reported by one study in grams of amino acid per 100 grams of dried duckweed (Lemma gibba).

Methionine: 0.64 g
Threonine: 1.68 g
Isoleucine: 1.66 g
Leucine: 2.89 g
Histidine: 0.73 g
Phenylalanine: 1.75 g
Valine: 2.12 g
Arginine: 2.14 g
Typtophan: 0.40 g
Lysine: 1.85 g

Unfortunately, it's hard to compare these to the ratio reported by Nutrient Requirements as optimal for horses.  (It's late at night and my brain isn't working too well.)  So you don't have to scroll up and look, here are the values for horses again:

Methionine: 27
Threonine: 61
Isoleucine: 55
Leucine: 107
Histidine: 58
Phenylalanine: 60
Valine: 62
Arginine: 76
Typtophan: Unknown
Lysine: 100

However, my initial (and not super mathematical) assessment is that while duckweed could be a component of a horse's diet, it does not have the optimal ratio of amino acids.  If we use lysine as the benchmark, then we would expect all the amino acids except leucine to be a little to a lot below the level of lysine, and we don't see that in the analysis for duckweed.  It looks like the ratio of lysine to methionine may be pretty good in duckweed, but other ratios are not in alignment.  So there goes my theory of growing the perfect protein source for horses in my aquarium:) 

Here is Nutrient Requirements' conclusion:  "Several factors can affect amino acid digestion in horses...these include site of digestion, feedstuff variation, biological value of protein, protein intake, amount consumed, and transit time through the digestive tract" (pp. 65-66).  So your guess is as good as mine in terms of how you should assess protein in your horse's diet.

Going forward, I will turn my attention to vitamins and minerals, but I'm rapidly losing faith that all this reading and research is yielding any real benefit in terms of how I feed Nimo.  That said, I have been concerned about the quality of protein he has been getting in the primary source of his hay, which is why I've been supplementing with alfalfa and alfalfa/grass mix hay for many years.  I'm still a ways off from being able to do a true assessment of how necessary that supplementation is, but it is on my list of things that I hope to accomplish through this series of posts.

So next up in the series is Calcium.  If my memory is correct, there is some tangible information in the book about this particular nutrient, so let's hope that I can find something helpful:)

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