Written By Dr. Kris Hiney

 

 

Last month we took a more in depth look at the calcium requirements for horses. This month we will look at phosphorous needs in the horse, and then put it all together in formulating some diets for horses.

 

 

 

Many horse owners know that P is important for bone growth and skeletal health in their horses. While Ca is the major player, P makes up 14-17% of the mineral component of a horses skeleton. But that is not all it does. Phosphorous is vital in energy transfers (a little molecule called ATP – the P stands for phosphate!), DNA and RNA synthesis, cell membranes etc. Your horse would be going no where without P! Therefore, we shouldn’t forget P when looking at our horse’s diet.

 

 

 

 

 

 

For many horse owners, P is usually not to close to the forefront of our minds as most diets for horses will be adequate. However, the same is not true if you raise other livestock species. Phosphorous runoff is an environmental concern, and not something that even horse owners should completely ignore. Researchers have shown that high calcium diets may lower the ability of the horse to absorb P. This does not hurt the horse, as they are still absorbing a sufficient amount to meet their needs. But when we decrease absorption from the gut, you increase excretion in the form of feces. Now horse manure has a lower proportion of water soluble P in it compared to other species, and thus is less of a threat of contributing to run-off, especially in a pasture management situation. This may not eliminate the need for concern for larger stables with more concentrated numbers of animals. It may not be too long until the horse industry also comes under the scrutiny of the EPA. Now don’t panic! The point is that we simply want to avoid the random supplementation of minerals to horses without full consideration of the diet.

Phosphorous deficiencies and excessPhosphorous deficiencies are typically not seen in mature horses, even when exercising. However, just like with Ca, special attention needs to be paid to the broodmares and babies, who are both busy forming new bone! Certainly inadequate P results in a slowing of the growth rate of young horses, and can lead to improper bone formation.

 

 

 

However, excess P can be more of a concern. As stated last month, excess P can inhibit Ca absorption, which is why we always check the Ca:P ratio. Prolonged conditions of excessive P can lead to the development of secondary nutritional hyperparathyroidism. So what is the limit? Horses should not be fed more than 1% of their diet as P (and this is assuming a correct ratio). Typically this is not an issue unless the feed source is predominantly cereal grains (like wheat bran or oats) which are high in P. Conceivably this could happen on all grass forage (ie not legumes) with a high grain intake containing no mineral supplement as a balancer.

RequirementsAs the tables below show, maintenance horses will be fairly easy to meet their P requirements. In exercising horses, most of the requirements were determined using young horses who were also concurrently laying down more bone. However, again, the P requirements for mature exercising horses are estimated to be higher, more as a margin of safety.

 

 

 

Gestating broodmares and lactating mares again have higher P requirements, which you can see in Table 5. And just like last month, while the babies requirements might look low, they are listed in grams needed per day. That means that overall they need to have a higher concentration of P in the diet. Babies that are being fed for rapid growth without properly balanced P levels in the diet can certainly lead them to develop joint disease.

Weight (lbs.)	Maintenance	Light work	Moderate work	Heavy work
900	11	14	17	23
1000	12	16	19	26
1100	14	18	21	28
1200	15	19	23	31
1300	16	21	24	34

Table 4. Phosphorous requirements (in grams/day) for adult horses at maintenance or work. To determine which class your horse fits into, read ENERGY REQUIREMENTS FOR THE WORKING CLASS HORSE.

Weight (lbs)	Month of Gestation			Month of Lactation
–	6th	7th-8th	9th-11th	1st	2nd	3rd	4th
900	11	16	21	31	31	29	21
1000	12	18	23	34	34	32	23
1100	14	20	26	38	37	35	26
1200	15	21	28	41	41	39	28
1300	16	23	30	44	44	42	30

Estimated Mature Weight	4 – 7 month	8-14 month	15-24 month
900	17	17	16
1000	19	19	18
1100	21	20	20
1200	23	22	22
1300	25	25	24

Table 6. Approximate P requirements (g/d) for growing foals based on their estimated mature weight.

 

Before we start formulating some diets, it might be helpful to look at some typical feed stuffs that we use to create diets for horses. The following values are estimates only, and remember that forages grown on mineral deficient soils may have lower values. Typically, most grains are going to be higher in P than Ca. This should not pose a problem as concentrates should be fed to provide the extra energy or protein the horse needs, rather than making up the majority of their diet. The exception in the table is beet pulp, which isn’t a cereal grain at all. When looking at some typical hays, we can see that legumes (the alfalfa and red clover) provide much greater amounts of Ca than do the grass hays. This makes them ideal choices for broodmares and babies. Even most grass hays have Ca in the correct proportion to P, which makes feeding an imbalanced ratio hard to do. Note however, that orchard grass may be this exception.

Concentrate	% Ca	% P
Beet Pulp	.89	.09
Barley	.06	.39
Cracked Corn	.04	.30
Rolled Oats	.11	.40
Rice Bran	.07	1.78
Wheat Bran	.13	1.18

Table 7. Common concentrates fed to horses. All % are on a dry matter basis.

Forage	% Ca	% P
Coastal Bermuda	.19	.27
Alfalfa	1.27	.24
Brome Grass	.29	.28
Red Clover	1.38	.24
Fescue	.41	.30
Orchard Grass	.27	.34
Timothy	.48	.23

Table 8. Common forages fed to horses. All hays are assumed to be harvested at the midbloom stage. Remember that soil type as well as stage of maturity can alter your hays nutritional content.

Now, let’s put some of these numbers together. For simplicities sake, we will work with a generic 1100 lb horse, and then compare the numbers we get with the table values for different classes of horses. For our first example, we will feed this horse two different diets, one solely Bermuda Coastal grass hay, and one of alfalfa. We will feed him at 2% of his body weight per day. All values above are in dry matter, so we won’t have to convert those values.

First, we determine how much the horse will eat.
1. 1100 lbs x .02 = 22 lbs
Now remember all of our Ca and P requirements are in grams, so lets’ convert lbs to kg.
2. 22 lb x 1 kg/2.24 lbs= 9.82 kg of hay
So we know our horse will eat 9.82 kg of our hay per day.

For the grass hay, we will multiply the amount fed by the percentages of Ca and P in that hay.
1. 9.82 kg x 0.0019 =0.0187 kg of Ca
For that to make sense, convert kg into grams.
2. 0.0187 * 1000 = 18.7 g Ca
Now for P.
3. 9.82 kg x 0.0027 x 1000 g/kg = 26.5 g P
Finally, calculate your Ca to P ratio.
4. 18.7/26.5 = 0.71 to 1

So, what does this tell us? First of all, we probably need to have a mineral supplement for our horses to avoid the inverted Ca to P ratio. Alternatively, we could add some legume hay to its diet. When we look at simply meeting the requirements of our 1100 lb horse, we can see we are deficient in Ca if it is a working horse and certainly very low if that was all we fed to a gestating or lactating mare.

Now, what if we fed alfalfa hay instead? Use the same calculations, but insert the new % of Ca and P for alfalfa.
1. 9.82 kg x 0.0127 x 1000 g/kg =124 g Ca
Now for P.
2. 9.82 kg x 0.0024 x 1000 g/kg = 23 g P
Finally, calculate your Ca to P ratio.
124/23 = 5.4:1

Now we can see that our Ca to P ratio is more desirable. Looking at the horses’ requirements, we can see we have more than met the Ca requirement for all classes of mature horses, and are adequate for P for all working horses except those in heavy work. For mares, we are good until the last part of gestation and through lactation. That shouldn’t surprise you, as broodmares should be fed a better quality diet than our other horses.

Lastly, let’s see what happens if we decide to add 6 lbs of oats to this 1100 lb horse’s diet. Note: I am doing this solely for the purpose of calculations. There should always be some rationalization for why we add concentrate to our horse’s diet. For my mythical generic horse in this example, we don’t even know what class it is in or what its body condition score is.

For this example, we will change hays and feed red clover and orchard grass hays.
Let’s begin with the red clover.
1. 9.82 kg x 0.0138 x 1000 g/kg =135 g Ca
Now for P.
2. 9.82 kg x 0.0024 x 1000 g/kg = 23 g P
We will now calculate the contribution from our oats.
3. 6 lbs x 1 kg/2.24 lbs = 2.7 kg oats
4. 2.7 kg x 0.0011 Ca x 1000 g/kg = 3 g Ca
5. 2.7 kg x 0.0040 P x 1000 g/kg = 11 g P
Add the two values together for hay and oats
6. 135 g Ca from hay + 3 g Ca from oats = 138 g Ca
7. 23 g P from hay + 11 g P = 34 g P
Calculate your ratio
8. 138/34 =4.1:1

Looks good!!!

Now again, compare across our class of horses. Calcium is adequate for all classes, and our P requirements are met for all horses except the lactating mares.

Lastly, we will try adding oats to our orchard grass hay. 1. 9.82 kg x 0.0027 x 1000 g/kg =26.5 g Ca
Now for P.
2. 9.82 kg x 0.0034 x 1000 g/kg = 33.4 g P
Our oat values will remain the same as above.
Add the two values together for hay and oats
3. 27 g Ca from hay + 3 g Ca from oats = 30 g Ca
4. 33 g P from hay + 11 g P = 44 g P
Calculate your ratio
30/44 =0.68:1

Doesn’t look like something we should do!!!

To wrap up, remember that these diets were a simple exercise in calculating the contribution of calcium and phosphorous from different feed sources. These are not recommendations for actual diets, as we made no attempt to adjust amount fed, supplements added, or appropriate concentrates selected. Next month we will add in a more sophisticated approach to balancing diets inconsideration of type of horse, energy requirements, and growing horses. Until then, have fun practicing with your calculators!