Nutrition for Advancing Pregnancy

In the last half of pregnancy the foal grows from the size of a small dog to an average 100 lb foal. Nutritional needs ramp up significantly in late pregnancy and calories are the least of the concerns.  This means keeping the mare at a good weight can still result in important deficiencies.

The mare in late pregnancy needs 28% more calories but 41% more protein. Calcium and phosphorus requirements are increased 80%, vitamin A 100%, vitamin E 60%. If you keep her diet the same as when not pregnant, just increase calories by that 28%, she will end up short on all the other nutrients.

How are you faring with providing for your pregnant mare? The answer lies with the quality of your hay but few people focus on that so let’s look from another angle. Most people describe their feeding program in terms of their bagged feed. If you are feeding 5 lbs/day (roughly 5 quarts by volume) of a supplemented and balanced 14% protein feed you are providing roughly 35% of your mare’s late pregnancy protein and mineral needs if her prepregnancy weight was around 1100 lbs.

If you add 1 lb/day of an at least 25% protein/mineral supplement she will be getting about half of the protein she needs and as much as 75% of minerals (depending on the product) from the combination of grain and protein/mineral supplement. This is a good start on minerals but quite a way to go yet on protein. This mare will also be eating 10 to 15 pounds of hay, which should more than fill in the gaps in minerals but will still leave her short on protein unless it is at least 11% protein.

While it is very common to build the diet for a pregnant mare on fairly high grain feeding as above, it’s expensive and there is another way. A good quality grass hay, with a calorie level of 0.9 Mcal/lb and 11% protein can support an 1100 lb mare through pregnancy at only 24 lbs/day even in the last month. Calorie needs are met and protein actually exceeds requirements. An equine nutritionist can advise on mineral supplementation needed based on the hay analysis.

Feeding a pregnant mare nothing but good quality hay or pasture may seem impossible because we are so conditioned to think of the horse’s diet in terms of the bagged feed, “necessary” for protein, vitamins and minerals. However, because grains are so calorie dense they need to provide at least 2.5 times the protein present in hay to just break even. If a pound of a bagged feed = 2.5 lbs of hay on a calorie basis, it would have to be 25% protein to provide as much as the 2.5 lbs of a 10% protein hay. Even making allowances for better digestibility of protein in the grain mix, they still come up short.

What your mare lacks in her diet she will take from her body. This is a nice fail-safe for the developing foal but not at all good for the mare long term.

What if your hay changes too frequently to analyze and you are not sure if the quality is adequate? Add a protein/mineral supplement, 1 lb/day for the first half of the pregnancy, 1.5 to 2 lbs/day after that for the average size mare.

When feeding this it is unnecessary to feed a supplemented bagged feed and actually causes excessive mineral intake and potentially toxic combined levels of vitamin A and D. If she is eating sufficient hay – 24 to 30+ lbs/day in late pregnancy – there is no need to feed grain. Just feed the protein/mineral supplement. If she is not eating enough hay, you can make a simple feed mixture that will be balanced for major minerals and cost you far less. Examples include:

  • 4 oz soybean meal, 1 lb each beet pulp and oats
  • 4 oz soybean meal, 2 lbs oats, 1 lb alfalfa pellets
  • 4 oz soybean meal, 1 lb each alfalfa pellets and wheat bran
  • 4 oz soybean meal, 1 lb alfalfa pellets, ½ lb rice bran (without calcium added)

As always, introduce diet changes gradually to avoid digestive upset. Finish off the diet with 1.5 to 2 oz of salt/day and 2000 IU of vitamin E in oil.

Mare on pasture? You can’t measure how much she is eating but you can keep a close eye on body condition. Skin is stretched thin over the ribs in a heavily pregnant mare so this area cannot be reliably used. Instead, pay attention to the fullness of the rump, the neck and the topline.

Eleanor Kellon, VMD

About Dr. Kellon

Graduate of University of Pennsylvania Veterinary School. Owner of Equine Nutritional Solutions,, industry and private nutritional consultations, online nutritional courses. Staff Veterinary Expert at Uckele Health and Nutrition.
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