Written By Dr. Kris Hiney

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).

Table 4.  Percent total protein required in the diet on an as-fed basis depending on the total consumption of the horse per day.

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.