Hello Dr. Getty,

I have a question concerning our pregnant mare (8th month of gestation) and phosphorus. She weights approximately 450kg.

I use NRC’s Nutrient Requirements of Horses as my guide in determining her intake requirements of Digestible Energy, protein and minerals.

Concerning minerals, I multiply NRC’s minimum recommendations by 150 percent, except for sodium, chloride, potassium and iron, which I leave at NRC’s recommendation.

Using that formula she would need 37.5g of calcium (25g, which is NRC recommendation x 150% = 37.5g) and 27g of phosphorus (18g x 150%).

In her diet now, which is an all forage diet, she is getting 50g of calcium (well above the 37.5) and 19g of phosphorus, which is more than the NRC minimum of 18g but 8g short of the 150% mark of 27g.

At current levels, the ratio is 2.6:1. If I added the 8g of phosphorus, the ratio would be 1.8:1, if my math is correct. Both of these are acceptable, in my opinion.

Would you add the 8g phosphorus or leave as is?

I know that as she progresses in her gestation, all requirements change.

Thank you,

Billy

Where are you from? North Florida

A tag along question: Since this post I have been trying to educate myself about the relationships among minerals. I just read where excess calcium can interfere with the absorption of zinc. If this is true, should I be concerned with the excess calcium in this mare's diet? Thank you.

Where are you from? North Florida

Sorry to keep adding to this post, but the mare is about to enter her 9th month, and according to NRC, Ca and P requirements are going to change. She still will be getting enough calcium. But phosphorus requirements are now 21g x 150%= 31.5 grams, or 12.5 grams more than she is getting in her diet. Appears to me like a phosphorus supplement is now needed.

What do you suggest, wheat/rice bran or a commercial supplement?

Thanks

Where are you from? North Florida

Hi Billy,

Thank you for your patience during the holiday season.

Exact balancing of calcium and phosphorus is tricky since not all of the calcium in the diet is well absorbed. Young horses may absorb around 70% but as horses age, their calcium absorption rate declines (see page 71 of Nutrient Requirements of Horses, 6th edition).

In addition to age, the amount of phytate (found in forages) and oxalates in the diet can further decrease calcium absorption.

And additionally, as phosphorus increases, calcium absorption decreases.

For this reason, I am hesitant to supplement phosphorus. The risk of adding too much phophorus in relation to the actual amount of Calcium reaching the cells, is too high.

During the last three months of gestation, calcium requirements increase, as you mentioned. Use an absorption efficiency of 50% when doing your calculations. According to the NRC, the optimal dietary calcium concentration is 0.55% of the diet. You can mix grass and legume hays together to come up with an amount close to this percentage.

Regarding phosphorus, it, too, is poorly absorbed. But, as long as it is adequate in relation to calcium, your mare will be fine. The goal, as you know, is for phosphorus to not exceed calcium. Adding rice bran, for example, can be done, but I would opt for a calcium to phosphorus ratio between 2.5 and 3.0 to 1. Take into consideration all of the grain she is getting, as well, since cereal grains are high in phosphorus.

Regarding interactions with zinc... yes, too much calcium can interfere with zinc absorption, and vice versa. This is why I prefer to feed whole feedstuffs, rather than supplement individual minerals.

Bottom line... I would improve the phosphorus content through bran and/or grain supplementation only to the extent that the requirement is met. As long as phosphorus is adequate, your mare can safely tolerate 5 or even 6 times more calcium.

Happy Foaling! And a very Happy New Year,

Dr. Getty
Author of Feed Your Horse Like A Horse

Where are you from? Bayfield, CO