Protein Nutrition VI: The Growing Horse

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Written By Dr. Kris Hiney

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)
4-6 mo.
7-8 mo
11-12 mo.
13-15 mo
16-18 mo
Table 1. Protein requirements (g/d) for young horses based on their expected mature body weight.
% of Bwt consumed
4-6 mo.
7-8 mo.
9-10 mo
11-12 mo
13-15 mo
16-18 mo.
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!


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