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.

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 |

Table 5. Calcium requirements (g/d) for gestating and pregnant mares. Remember to use Table 5 from *BROODMARES AND BABIES* to determine how much your mare should weigh.

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.

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!

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