I have a pregnant mare, and I’ve read five books on nutrition and all of your articles. It’s still unclear what my mare needs in her last trimester and during lactation. The math just confuses me. She is in her sixth month, and her diet is: 24/7 salt, grass pasture (safe fescue), two pounds twice a day of a commercial grain mix, one cup ground flax seed and two cups stabilized rice bran. I plan to increase this mix to eight pounds into three feedings the last three months of the pregnancy and during lactation. I will provide free-choice alfalfa and her pasture. Do I need additional vitamins' Should I increase her calcium and phosphorus'
Vitamins aren’t the problem. It’s the minerals. You need to pay attention to more than just calcium and phosphorus. At eight pounds per day, you’ll have all the calcium she needs for late pregnancy from the grain, but not enough phosphorus, even factoring in the flax and rice bran. The trace mineral intake is close but a bit low. For lactation, it doesn’t meet her needs for any mineral.
If you feed her enough grain, you’d put her over the top on calories. Plus, it still wouldn’t mean she’s getting a properly balanced diet because you’re ignoring minerals coming from the hay and pasture. Fescue tends to have a low calcium:phosphorus ratio, which would help with your phosphorus shortage during pregnancy. However, it’s low in zinc.
Giving the mare free-choice alfalfa will put your calcium:phosphorus ratio out of line again, and alfalfa also falls short on zinc. Zinc and copper are important for normal development of bone and joints.
Her protein intake during pregnancy would probably be OK with the diet you mentioned, but she’ll likely fall short after foaling, even with the alfalfa (although this depends on the protein level in the alfalfa and how much she actually eats.)
To really do this right, you should analyze the hays/pasture and consult with a nutritionist. But, using ballpark figures, you could feed her five pounds each of alfalfa and plain oats with 10 lbs. of fescue hay and have a good calcium:phosphorus ratio overall. However, it would fall short for total amounts and the trace minerals and protein during the highest demand, which is the first three months after foaling. This could be easily rectified by feeding her a 20-30% protein/mineral pellet with a calcium:phosphorus ratio of between 1.2:1 and 2:1, and a trace-mineral profile that contains more zinc than it does manganese.
With free access to young pastures in the spring, she won’t need any additional protein, so you can switch to a straight mineral supplement. If she needs a denser source of calories, you can add feeds such as you mentioned in this basic balanced diet.
If you want to stick with your current plan, look at how our table compares the horse’s peak requirements of nutrients vs. what she’ll get from her grain and what she’ll get from 15 lbs. of fescue or the equivalent in grass, using some average analysis figures for fescue.
Basically, you’re covered for protein, calcium, phosphorus and calories. Trace minerals are also covered or at least close to where they should be, but you would run into problems if the hay is high manganese. It might be wise to get it tested.