Previously we have discussed important concepts in protein nutrition concerning amino acids, digestibility, site of digestion as well as the requirements for several classes of horses. However, we have not yet looked at the young growing horse. This month we will discuss the protein needs of horses from weaning to two years of age, and examine some typical equine diets to determine if they fulfill a young horse’s protein requirements.
The protein source used to meet our horse’s amino acid requirement is especially important in the growing animal. Young horses are usually the model used to test protein sources, as researcher’s can monitor the average daily gain of the horses. Ideally, the amount of calories and protein does not different between the horses, only the protein source. These diets are referred to as iso-nitrogenous and iso-caloric. The horses which are able to achieve greater rates of growth are doing so presumably because the amino acid profile of a particular protein source more closely matches the needs of the young horse’s body for protein synthesis. In fact, in 4 month old horses, milk protein supported greater rates of growth than did other sources including linseed meal, soybean meal or barley. This would certainly make sense, as one would expect that the amino acid profile in milk designed to support foal growth would do so better than plant proteins!
With young horses, it is especially important that we try to eliminate deficiencies of amino acids which limit growth. Again, those amino acids which are deficient in the diet are refereed to as the liming amino acids. For horses (along with many domestic livestock species) the most important limiting amino acid is lysine. For young horses, it is recommended that lysine make up 4.3% of the total protein consumed in the diet, or alternatively, that the young horse between 4-10 mo of age receive 33-42 g of lysine per day. The young horse may even need to consume less total protein, if key amino acids are supplemented in the diet. Threonine has also been shown to limit young horse growth, and supplementation of this amino acids as improved growth rates, as well as lowering serum urea nitrogen. A decrease in serum urea nitrogen indicates that the animal is undergoing less catabolism or breakdown of amino acids, and using them instead for protein synthesis. If an animal is fed a poor quality protein, with a bad amino acid composition, the horse will still be able to use those amino acids, but only for energy or storage as fat. As part of this process, the nitrogen of the amino acid is removed and incorporated into other amino acids, or into urea for later excretion. Thus when an animal has a higher blood urea nitrogen, it indicates poor protein utilization.
In Table 1, the amount of crude protein needed per day is given for horses up until 18 months of age. For simplicity's sake, ages of horses are grouped, rather than each month’s requirements listed. As such the higher value for grams of crude protein is listed for the age range. This was preferred rather than taking an average value, and underfeeding protein. However, you can see that the young growing horses’ protein requirements begins to decrease as it reaches its yearling year. If we relate that to the increase in size of the young horse, the concentration of protein needed in the diet decreases as well. It is the early rapid growth that requires the greatest amount of protein that the horse will ever need through its lifetime. Table 2 illustrates the amount of crude protein necessary in the total diet in order to reach the young horse’s requirements. As the horse matures, the amount of crude protein needed in the diet declines. It is also easy to see that allowing the foal to ingest greater amounts of feed, requires a lower concentration of protein needed in the diet, and a more conservative approach to protein intake.
Expected mature weight (lbs)
Table 1. Protein requirements (g/d) for young horses based on their expected mature body weight.
% of Bwt consumed
Table 2. The total percent crude protein needed in the diet for a growing horse.
These values are based from the total intake on a dry matter basis. The change in body weight of the foal is taken into account. For each age grouping, the smaller weight of the foal (i.e., a 4 mo old foal would weigh less than a 6 mo. old foal) is used in order to ensure adequate protein intake.
When feeding your young horses, it is always important to start with a good quality hay. Ideally you are using a legume hay or at least a legume grass mix. If the young horse has access to good quality, growing pasture, this also supplies an excellent source of protein. However, this does entail pasture maintenance. When a plant is in a younger stage of maturity, or actively growing, its protein content will be higher. If the foal is forced to graze mature stands of grasses, or even weeds, the protein content will be lower. Let’s work through a few examples in order to demonstrate the type of diet the foal will need.
Let’s begin with a foal that we expect to mature out to 1100 lbs. He is currently 6 months of age, so we know that he should be receiving 676 g of protein per day. At this age, the foal should weigh 473 lbs. We have a grass legume hay mix which supplies 16% crude protein. If we look at table 2, we can see that our foal should receive enough protein if he is fed at 2.5% of his body weight per day, or 11.8 lbs of hay per day. If he eats more, he will definitely meet his protein requirements. But let’s make this a little more complicated. We decide to only feed him 2% of his body weight in hay per day. He now receives 9.5 lbs of hay per day.
Doing the math, our hay provides: 9.5 lbs /2.2lbs/kg = 4.3 kg
4.3 kg x 16% = 688 g of crude protein.
That meets his requirements as listed in the table above. Why is that? Again, for simplicity's sake, the table uses the lowest weight possible for each group of horses. Therefore, the actual total protein needed in the diet is slightly less than for the 4 month of foal. Essentially, if receiving a good quality legume hay, your foal will be adequate in protein. However, what if the hay has a greater proportion of grass, or the hay was cut at a later stage of maturity? To explore this possibility, we will feed a hay that only contains 13.5% crude protein.
Following the same procedure as above:
4.3 kg x 13%= 559 g of crude protein.
We are now deficient in protein. We also might need to be concerned that our amino acid profile may be poor for a young growing horse. So let’s look at two different alternatives.
We can use a commercial feed that supplies 15% crude protein to our horse on an as-fed basis. Previously we were calculating our feed values on an as-fed basis. We will continue with the concentrate by staying as-fed, as is seen on the feed tag. How much grain will we need to supply the foal as we are only deficient by 120 g of crude protein?
If we divide the amount of protein needed by the percent protein in the feed : 120g /16% ; we need 750 g of the feed. Converting that to pounds, and our horse needs to eat 1.7 lbs of grain per day. That certainly does not seem like an excessive amount of grain for our young horse per day. In fact, he is still below the total 2.5% of his body weight. So what does this mean overall? Choosing higher protein hays will ensure your foal has the adequate amounts of protein for higher growth. If your hay offers less protein, a commercial feed designed for young horses will typically easily meet the deficiency in that hay. Additionally, when examining these feed tags, you will often see that some of the key amino acids are supplemented in that feed. This ensures that your foal will grow optimally, provided nothing else is going wrong!
This month we will continue our protein nutrition series with the classes of horses having the most demanding protein needs: the broodmares. It would make sense that this group of horses is the most sensitive to changes in protein nutrition, as they are continually supplying nutrients for the growing fetus or foal. Shortchange your mare and you may be short changing your future generation. But let’s take a look at what these animals need, so that we can avoid any potential pitfalls in our feeding strategies.
Before we start feeding a mare for gestation, we at least need to get her pregnant first. Therefore proper nutrition of the broodmare does start before conception. As long as the mare is fed adequate amounts of protein (no higher than required by a maintenance horse) there should be no nutritionally related reproductive issues. However, mares which are deficient in protein are slower to begin cycling normally and have a higher rate of early loss of pregnancy. Short changing the mare on the feed bill will definitely not save money in the long run. Always begin the breeding season with a proper feeding program for optimal results.
After the mare has conceived, her early pregnancy requirements are not much different from when she was open. Just like with her energy needs, her changes in protein requirements are really quite minimal at the beginning of her pregnancy. As the rate of growth of her developing fetus increases, she must have a greater supply of amino acids in her diet. Looking at Table 1, you can see that the greatest increase in her protein requirements occurs in the last two months of pregnancy. Table 2 places these daily requirements into a simpler expression of the total percent protein that your mare will need depending on the amount of feed she is consuming. Just as you can imagine, the more the mare consumes, the lower the total percentage of crude protein needed in the diet. In fact, this is usually what happens. Most mares will voluntarily consume more feed as her energy needs go up, thus also meeting her protein requirements.
Shopping for Feeds
It might be surprising to look at the values in Table 2 compared to the typical amount of protein present in a commercial feeds. Most feeds designed for broodmares range between 14 and 16% crude protein, yet the overall protein percentages needed in the diet are much lower. So why do feed manufacturer’s offer such high levels of protein in their products? For one, most of the feeds are really designed to meet the needs of the lactating mare, rather than the gestating mare. Additionally, you may remember from earlier articles that forage protein is not digested as fully as the protein which comes from concentrates. While protein requirements do try to account for some of this variance, an average digestibility value combining both forages and concentrates is used to calculate the total amount needed in the diet. Furthermore, the guidelines for protein intake are based largely from research with horses consuming mixed diets usually offering an equal proportion of concentrates and forages. Thus if more of your horse’s diet is forage, you must consider that when selecting an appropriate concentrate. Remember as well, that legumes are digested to a further extent than grass hays, thus also offering not only more protein on a concentration basis, but also being absorbed more thoroughly by the horse. A general guideline would be, that if feeding high quality legume hays, the amount of protein required in the concentrate could be much lower (as low as 10-12% CP). If feeding primarily grass hays, then you should select a concentrate closer to 14% CP.
Wt of horse (lb)
Table 1. Protein requirements (g/d) for a gestating mare based off her initial non-pregnant weight.
% of Bwt consumed
Table 2. The total percent crude protein needed in the diet for a pregnant mare throughout gestation. These values are based from the total intake on a dry matter basis. However, percent total protein is represented on an as fed basis, as is represented on feed tags.
Protein requirements for Lactation
The lactating mare will consume more feed than the typical maintenance horse as her energy demands have increased greatly. Just like with energy, it is the lactating mares who really have the biggest nutritional demands of any of our horses. Compare the grams of protein needed per day in Table 3 to Table 1. You can see that her protein needs have more than doubled. If mares are deficient in protein, they will be unable to produce as much milk as those at an adequate plane of nutrition, and may end up losing weight. This is certainly undesirable, as this is also the time period when most mares are being rebred as well. Mares do a fairly good job of producing milk though certainly not equivalent to a Holstein cow! Mare’s milk production usually averages about 3% of her body weight, with that value tapering off to about 1.9% of her body weight during late lactation. The protein concentration in the milk is the highest during the first 22 days of lactation (when foal growth is very rapid) and thereafter plateaus throughout the rest of her lactation.
Wt of horse (lb)
Table 3. Crude protein requirements in grams per day for the lactating mare.
Last month we learned that meeting a mature idle horse’s protein requirements are surprisingly easy. If a horse is provided with good quality hay at 2% of its body weight it can easily consume enough protein even without eating concentrate. However, if forage quality is low, adding a supplemental designed to provide essential amino acids can easily make up the difference.
But my horse works hard!
But what about if your horse has more of a job to do than just stand in his pasture and eat? Many people automatically reach for a higher protein feed once their horse goes to work, but is that really the right thing to do? Of course protein requirements of a working horse do go up due to the increased tissue turnover and repair associated with exercise. Further, horses also lose nitrogen through sweating and increase muscle mass with training. However, the increase in protein required pales in comparison to the increase in calories needed. High protein diets increase the need for horses to excrete urea (the form in which excess nitrogen is removed from the body) and may alter their acid base balance. While horses seem to be able to handle the increased need to remove nitrogen from higher protein diets quite easily, it will result in more urine excretion. Thus more ammonia may build up in poorly ventilated buildings and bedding costs will go up. In fact, it may be beneficial to feed a lower total protein amount to the horse while providing key amino acids. In one study, horses fed a lower protein diet but supplemented with lysine and threonine had higher blood pH values after exercising compared to horses on a higher protein diet (Graham-Theirs et al., 2001). When horses exercise intensely they produce lactic acid. Lactic acid drops the pH in the blood and can contribute to the onset of fatigue. Therefore this lowered protein diet may protect against a drop in blood pH and therefore allow the horses to exercise longer or recover faster. However it should be noted that the lower protein group was also supplemented with fat as well, clouding interpretation of results.
So how much do they really need?
Table 3 shows the total amounts of protein needed, while Table 4 again expresses this on a % protein basis. You can see that most performance horses will do quite well if you select a feed between 10-12% crude protein. Remember that when selecting a feed, you must consider your forage source first! For example, if your performance horse was eating a primarily alfalfa hay with a value of 16% crude protein, his protein needs would already be met! Selecting a concentrate then would primarily serve to supply any additional energy needs the horse may have.
Table 3. Crude protein requirements for work (g of CP/d).
Wt of horse (lb)
Table 4. Percent total protein required in the diet on an as-fed basis depending on the total consumption of the horse per day.
% of Bwt consumed
Let’s do math!
Now let’s put this together in a practical problem. We will feed an 1100 lb horse 2% of his body weight in grass hay. Our grass hay has 9% crude protein value on a DM basis. We weigh out 22 lbs of hay for our horse per day but we weigh it on an as-fed basis (meaning what it weighs on a scale that day).
First we will convert our weight of hay to the weight of our hay on a dry matter basis. We will assume the hay is 85% dry matter.
22 lbs x .85(% dry matter) = 18.7 lbs of hay on a dry matter basis
Then we will convert our lbs to kilograms.
18.7 lbs /2.24 = 8.3 kg of hay
Multiply that by our percentage of protein.
8.3 kg x .09 = 747 g of CP.
Let’s check this horse’s lysine requirements as well. Remember that the only value for amino acids required by the horse is for lysine. The current available knowledge suggests that horses need 4.3% of their protein to come from lysine. Typically grass hays are fairly low in lysine compared to legume hays. An average grass hay harvested at a mature stage is 0.38 % lysine. Again we multiply the amount of hay fed 8.3 kg x .0038 = 31.5 grams of lysine. Our maintenance horse only needs 32 grams of lysine. We therefore have met his requirement by feeding this hay.
Even if our 1100 lb horse is in moderate work we are short by only 7 grams of CP. This can easily be met by any additional concentrate or by simply eating more hay. However, if we move him up in work, we become much more deficient in protein as well as lysine. Let’s assume he is now in heavy work and deficient by 100 g of protein. We want to add 3 lbs of concentrate (which isn’t very much) to his diet.
3 lbs /2.24 = 1.3 kg of feed
We need our 1.3 kg to supply 100 g of CP. So our feed needs to be 100g/1300 g of feed = 7.7 % CP on a dry matter basis. On an as fed basis, this would be 9% CP. Almost every commercial feed will contain this level of crude protein. Hopefully we have now illustrated that there is no need to feed a high protein feed designed for growing horses or broodmares to our exercising horses.
In summary, protein requirements for maintenance horses or even those at work are fairly easy to meet by a normal horse diet. If feeding a poor quality hay, you may have to supplement your horse’s diet. If so, then choose a feed that contains legumes (like alfalfa meal) or a concentrate that contains a high quality protein like soybean meal. While no clear amino acid recommendations are available for working horses, there appears to be some benefits of feeding lower total quantity of protein while supplementing with key amino acids. This certainly does appear to be the future of equine research concerning protein nutrition.
Next month we will address the protein needs of the groups of horses which need the most attention: the growing horses and the broodmares.
In the last two articles we discussed the importance of protein quality, not only in terms of site of digestion in the horse, but also the amino acid composition of that feed. Now we will try to simplify these concepts into selecting appropriate feeds for horses.
The first task in developing a feeding program for your horse is to identify what class of horse you have. (For a review revisit: Equine Energy Requirements and Energy Requirements for the Working Class) The horses with the lowest protein requirements relative to their body size are the mature horses not in work, or maintenance horses. This does not include geriatric horses which may have altered protein needs due to the lowered efficiency of their digestive system. We also must assume that the horse is receiving enough calories in the diet. A horse fed a diet sufficient in protein, but low in calories will lose weight (makes sense right?) but a horse which has adequate calories but not enough protein can also lose weight. We also make some consideration for the activity level of the horse as well. Remember the difference in energy requirements between the couch potato Quarter Horse and the active Thoroughbred mare (Equine Energy Requirements)? Well a similar relationship exists with protein requirements. Essentially the more active horse would have more lean tissue/muscle to support than the lazy horse. Table 1 lists the actual protein requirements for a maintenance horse depending on activity level and their body weight. Remember that this does assume a quality protein source. Lower quality protein (less digestible or poor amino acid profile) can adjust these figures upwards.
Table 1. Crude Protein requirements for maintenance (grams of CP/d) based on average activity level.
Wt of horse (lb)
What percent protein do you need?
But let’s put these numbers into something more people are familiar with, percent of the diet. Table 2 provides the percent protein of the total diet a horse would need to consume to meet their protein requirements. Looking at Table 2 shows how easy it is to meet a maintenance horse’s protein requirement. You can also see that as total consumption goes up, the percent of the protein needed in the diet goes down. Conversely, if you fed less you would need to increase the percent protein in the diet. Horses will usually consume between 1.5 and 2.8% of their body weight per day on a dry matter basis. Typically you will see horses lower their consumption of less palatable hay which often equals poor quality. However, this can largely be based on the individual, as some horses compensate by lowered feed quality by increasing intake (Edouard et al., 2008). It turns out that horses are much more variable in their voluntary intake than other domestic species are! Just like a horse to always want to be unpredictable. If you notice your horse picking through its hay and leaving a good proportion of the hay untouched it may be wise to select a supplement designed to provide amino acids but not to greatly increase the calorie consumption by the horse. Alternatively it may be time to find a new hay supplier (see Selecting Forages).
Table 2a. Percent total protein required in the diet on a dry matter basis depending on the total consumption of the horse per day.
% of Bwt consumed
Table 2b. Percent total protein required in the diet expressed on an as fed basis assuming an average dry matter content of 85%. Note: this can change with the feed fed and is only representative of harvested feeds, not pasture or grasses.
% of Bwt consumed
Protein content of common horse feeds
Now let’s look at some typical protein values for feeds. To be sure of your own feed ideally have your forage tested as well as examine your feed tag. Corn ranges between 8-9 % CP on a DM basis, oats 12-13%, soybean meal – 43-49%, grass hays – 10-18%, and legumes between 18-25%. With the range of protein content in forages, one can see how important it is to have knowledge of your nutrient content prior to selecting your concentrate. Even with these ranges, most maintenance horses will easily meet their protein requirements by forage alone. If you look at the range of percent protein needed by the maintenance horse in their total diet, it compares quite well with grass hays. If you are feeding your horse and he is maintaining weight, he should easily be meeting his protein requirements at the same time. Remember, we assume the horse is receiving good quality hay. If you are worried about the horse meeting its amino acid needs, many feed companies make supplements specifically designed to be fed with a strictly forage diet, rather than greatly increasing the concentrate intake. For example, many feed companies offer protein supplements in the range of 30-35% crude protein. These are designed to be fed at a minimal rate (only 1-2 lbs per day) in order to simply balance out any deficiencies from an all forage diet. Clearly not all horses need the extra calories that come from feeding higher levels of concentrates. This provides a convenient, easy way to ensure that your horse’s nutritional needs are being met.
Next month we continue with protein nutrition in the exercising horse.
Edouard et al.2008. Animal:An international journal of animal biosciences. 102:10:1526-1533.
In the last article, we introduced the idea of examining the protein in our horse’s diet beyond just the mere percent crude protein on the feed tag or a forage report. We discussed the concept of protein digestibility and the importance of the site of digestion. As there is only limited evidence that uptake of amino acids takes place in the hind gut of the horse, we prefer to feed proteins which are digested and absorbed in the small intestine of the horse. Typically concentrates offer more pre-cecal digestibility of protein than do forages. Therefore site and extent of digestion are key components of protein quality. However, there is another equally important factor in protein quality, and that is the amino acid profile.
Last month the amino acids that horses could synthesize on their own were listed, as well as the amino acids that are essential to be supplied by the diet. Horses must receive the proper balance of amino acids in order to synthesize the many complete proteins in their body. When they do not receive enough of a particular amino acid, protein synthesis is limited. That amino acid is then referred to as the limiting amino acid. Typically, lysine is the most common limiting amino acid in the horse’s diet, or the one in shortest supply. It does not mean it is the most abundant amino acid found in the horse’s body, rather the one that is most commonly deficient in feeds. Now remember, every protein in the body is coded for by the DNA that provides a blue print to build that protein. The DNA provides the proper sequence of amino acids that must be linked together to form the protein. Let’s say that to build one molecule of actin (a protein found in muscle cells responsible for their contractile activity) there are 25 lysines, 30 threonines and 46 alanines. (The actual polypeptide chain of actin is over 300 amino acids long.) If the diet contained only enough for 20 lysines to be added to the peptide chain, protein synthesis would halt, even if you had 35 threonines. If you added those lysines back in through the diet, protein synthesis could continue. Now, this is an arbitrary example, instead try to think of protein synthesis occurring throughout the body, constantly adding amino acids that have come from the diet, synthesized by the horse, or that have been recycled by tearing down old proteins. Obviously the more protein the horse is synthesizing (think young growing horses) the more critical the amino acid profile of the diet.
DNA molecule which contains the information to build every protein in your horses body.
So what is the amino acid profile? Simply put, it is the percentage of each amino acid that appears in the diet, or even in a horse’s tissue. For example, in equine muscle tissue, lysine is set at a relative value of 100%, while other amino acids such as arginine, leucine, phenylalanine and threonine appear at 76, 107,60 and 61% respectively of the amount of lysine present in muscle(Bryden, 1991). In other animal species such as swine and poultry, nutritionists try to match the amino acid profile of the diet to the amino acid profile of the actual animal. In this manner, the least amount of amino acids are wasted. Instead, they are incorporated into the animals’ body to allow for growth, reproduction etc. One of the goals of many animal nutritionists is to achieve something called zero nitrogen balance in the animal. That means the amount of N going into the animal matches the amount of N going out of the animal. Or we are replacing the amount of N that is being lost by the animal through normal tissue turnoever. If we feed protein beyond what the animal requires, the animal will still digest and absorb those amino acids. However, if they exceed the horse’s requirements to synthesize protein, the horse will instead catabolize those amino acids for fuel, and excrete the nitrogen in the urine as urea. If you have ever walked into a poorly ventilated barn with horses that were fed high protein diets, you probably have smelled the ammonia that comes with excessive protein feeding. Furthermore, feeding excess N just adds to the N being added back to the ground through runoff from facilities.
Protein quality and it’s amino acid profile can alter how much protein the horse actually requires. While we often just discuss protein requirements generically as a percentage, in fact horses can be fed a lower total amount of protein if it is of higher quality. For example, in young horses, the lysine requirement is 4.3% of their crude protein requirement. The higher amount of lysine in the feed, the less of that feed will need to be fed. Let’s take a 4 month old foal which requires 669 grams of CP and 28.8 g of lysine. We then feed our horse either a 16% crude protein feed of high or low quality. We feed him at 2.5 % of his body weight and he weighs 370 lbs. That provides an intake of 9.25 lbs per day. If his feed provides 16% protein, he gets 660 grams of protein. We have almost completely met his protein requirements. But what if one of our feed sources contained only 2% lysine? That means that the foal would be deficient by 15.8 grams (the feed would provide 13 g of lysine) and thus his growth rate would be limited. Therefore the foal would have to eat much more of that diet (more than he can consume) in order to consume the correct amount of lysine. Our foal on the high quality diet would receive 28 g of lysine, meeting his requirements, and allow his body to grow normally. In older horses whose protein requirements are easier to meet, we can actually lower the total amount of protein in the diet provided it is of a high quality. In fact, lowering the total protein in the diet while supplementing key amino acids has been proven effective in both growing horses (Graham et al., 1994, Stanier et al, 2001) and in exercising horses (Graham-Thiers et al, 1999, 2001)
The future of protein nutrition in the horse may very well focus on identifying the correct amino acids needed in the horse’s diet, and moving to a lowering of the absolute % CP in the diet, therefore minimizing waste, and decreasing the amount of N added back to the environment. While nutritionists still have much to learn, the goal when feeding protein is to feed just the right amount the horse needs, and not to overfeed needlessly.
Next month, we put the theories into practice and discuss protein requirements for various classes of horses.
This month we begin a new series focused on protein requirements for our horses, and the various feeds which provide protein. Protein is commonly the first concern of many horsemen when selecting their feeds, and the most frequently discussed. In the equine industry, there tends to be a common fallacy that if a product costs more, it clearly is a more desirable product to feed. As feedstuffs which contain more protein are often more costly, this tends to make higher protein feeds more attractive to the consumer. However, that higher protein content may not be necessary to feed to your horse! Before we can delve too deeply into the proper amounts of protein to feed to the various classes of horses, we should back up and break down what protein does for the animal, and examine how the horse digests this very important nutrient.
Proteins are actually complex molecules that are comprised of a series of amino acids joined together by peptide bonds. All proteins are built from a specific series of the 20 most common amino acids. While many more amino acids are found in nature, we typically limit our discussion of amino acid nutrition to these basic 20 amino acids. Of these amino acids, some are characterized as dietary essential amino acids or non-essential amino acids. The dietary essential amino acids include lysine, methionine, phenylalanine, the branched chain amino acids (valine, leucine and isoleucine), tryptophan, threonine, histidine and arginine. The distinction between these two classes is that mammalian systems lack either the ability to synthesize some amino acids (they lack the appropriate synthetic enzymes) or they cannot produce enough of that amino acid to support normal bodily function. The non-essential amino acids can be produced in the animal’s diet from other amino acids. In contrast, bacteria can produce all amino acids. This is what gives the ruminant animals such as cattle, sheep and goats, such a unique advantage. They can rely very heavily upon bacterial synthesis of amino acids to meet their requirements.
Figure 1. The levels of protein structure from primary (just the linear chain of amino acids) up to the completely folded, functional proteins.
All animals, in addition to plants, bacteria and fungi produce a wide array of proteins necessary for normal function. These include structural proteins found in muscle, bone, cartilage etc., enzymes, hormones, antibodies, and cell signaling molecules. The specific make up of a protein is coded for by the organisms DNA. The DNA contains billions of individual genes that contain the information on how to build each specific protein found in the organism. Different tissues will turn on or express different genes, thus making liver produce the necessary proteins for its function, and the heart a separate set of proteins. However, every cell will contain the genetic information to produce every protein. That is why you can use a skin cell to clone an entirely new animal. All the necessary information to make a new horse is contained in that one cell. While this information may not seem germane to feeding horses, it is important to understand why the animal must receive particular amino acids in the diet. Without the right amount of essential amino acids, the animal is unable to synthesize protein correctly.
When we feed protein, we need to be concerned with more than just the amount of protein contained in the feed. Feed tags will provide us with the percent of crude protein present in the feed. This number is really based on the percent of nitrogen found in the feed. That number is then multiplied by 6.25 to achieve the amount of crude protein. That is based on the fact that the average protein contains 16% nitrogen. However, there may be non-protein nitrogen present in the feed which may or may not be useful to the animal. While all protein contains nitrogen, not all nitrogen is protein. For example, in ruminant animals, non-protein nitrogen is often fed in the form of urea, due to the unique makeup of the animals digestive system which allows it to utilize these feed sources. But what the crude protein percentage does not provide us with is information about how useful is that protein to the animal. As horseman, we need to be concerned not only with the quantity of the protein we feed, but more importantly, with the quality.
The quality of the protein can be thought of in two different ways. One indicator of protein quality is how digestible is that protein to the animal. A feedstuff which has a high percentage of protein that is unavailable to the animal, or is unable to be absorbed, is essentially useless. So let’s take a moment and explore protein digestion in the equine. Protein digestion begins in the stomach of the horse, where it is exposed to both inorganic acids (hydrochloric acid) and proteolytic enzymes – pepsin. These two digestive secretions begin the breakdown of protein by initiating the unfolding of the protein structure. While proteins are coded for by DNA in a linear form, the particular amino acids present in the protein cause the peptide chain to fold and wrap around itself to form its unique functional shape. In fact, one misplaced amino acid can render an entire protein useless. This is often the basis of many genetic diseases, a mutation which causes the wrong amino acid to be added to the peptide chain. Pepsin begins the disruption of the polypeptide chain by cleaving after specific amino acids, primarily tryptophan, tyrosine and phenylalanine. While gastric digestion does not completely break protein down to a point it can be absorbed by the animal, it shortens the polypeptides and allows more access to the enzymes which will be present in the small intestine
As the dietary protein enters the small intestine, it will be digested much further by a series of proteolytic enzymes which arise from the pancreas. These include chymotrypsin, trypsin, elastase, carboxypeptidase etc. These enzymes essentially complete the breakdown of protein into small enough pieces that can be absorbed including single amino acids, dipeptides and tripeptides. Once these are absorbed into the enterocyte, the dipeptides and tripeptides are further hydrolyzed into single amino acids. These then enter the blood stream of the animal where they can be delivered to various tissues to be used for protein synthesis. Non-protein nitrogen can also be absorbed out of the small intestine of the horse. Feeding urea causes an elevation of blood urea nitrogen,as well as urinary nitrogen, as the animal flushes the nitrogen out of the animal’s system. There is no evidence to support the usage of feeding nonprotein nitrogen to the horse, as it is unable to be incorporated into bodily protein.
Sidebar: Foal management. The exception to protein digestion occurs right after birth. The foal is born with a naïve immune system and must receive antibodies from the dam. These come from ingestion of colostrum or the first milk a mare produces. The colostrum contains a trypsin inhibitor which prevents the antibodies which are present in the milk from being digested.The foal also helps out the process by having an “open gut”. The proteins initially presented to the gut can be absorbed intact through the process of pinocytosis, or engulfing of the entire protein by the gut cell membrane. However, there is a finite capacity of the gut to perform this action. The most efficient uptake of intact proteins takes place within the first 12 hrs, with relatively little to no absorption past 24 hrs. Additionally, there is a limit to how much protein can be absorbed. If a foal was first provided milk replacer prior to the administration of colostrum, they may not be able to absorb the important antibodies if fed later with the appropriate colostrum. So always make sure your foal receives high quality colostrum in the first 24 hours of life. Don’t skip straight to the milk replacer in an orphaned foal!
Any protein which escapes digestion in the small intestine is then delivered to the hind gut of the horse, where microbial fermentation can take place. The microbes are quite adept at further degrading feedstuffs, and producing compounds that are of value to themselves, as well as to the horse. The bacteria in the hindgut of the horse are quite capable of synthesizing all of the amino acids, however there is little evidence this is useful to the horse. In contrast to the ruminant animal, whose site of fermentation and microbial protein synthesis is prior to the site of enzymatic digestion and protein absorption in the hindgut, the horse is unable to absorb these proteins. Unless the horse practices copraphagy, or the ingestion of feces, this protein is believed to be lost to the animal. In horses infused with lysine either in the stomach or in the cecum, only gastric infusion resulted in an elevation of plasma lysine, indicating the inability to absorb lysine out of the hindgut. However, there has been some recent evidence that some absorption of amino acids may occur in the hindgut. Messenger RNA coding for specific amino acid transporters have been isolated from the hindgut of the horse. This indicates a potential ability of the horse to absorb at least some specific amino acids from the hindgut. However, the transporter proteins themselves have yet to be isolated, or the disappearance of specific amino acids from the hind gut to be proven.
Finally, let’s compare different feedstuffs in the extent and site of their digestion in the equine. Protein digestibility differs according to both type and amount. It has been repeatedly documented across a wide number of studies that increasing the amount of protein in the diet, also increases the percentage of that protein which will be digested. Perhaps this is due to an adaptation of the digestive system to increase the synthesis of proteolytic enzymes when presented with larger amounts of protein. Total tract digestibility of protein in forages ranges from 73-83% for alfalfa, 57-64% for coastal Bermuda grass hay, and 67-74% for other grass hays. Total tract digestibility for grains or concentrates is much higher, ranging between 80-90%. If however, the site of digestion is take into account, prececal digestion of forages ranges between 25-30% while concentrates are digested much more extensively in the small intestine, up to 70-75%. So what does this mean for the horse owner? Ideally when selecting protein sources for horses, both the digestibility as well as the site of digestion must be considered. Ideally,protein should be digested in the small intestine rather than the hindgut, to optimize amino acid absorption. However, there may be some evidence that this species which does rely so heavily on hind gut fermentation may be capable of some absorption out of the hind gut. The extent of that is yet to be proven.
For next month, we will look at the second piece of protein quality, the amino acid profile of the diet.