There is a reason the horse evolved to foal in the Spring. The rich, tender, high moisture young vegetation provides the energy and protein dense nutrition the mare needs to make sufficient milk.
In peak lactation the mare needs 1.8X more calories than when she is not milking. That’s the easy part. She also needs 2.5X more protein and 3X more lysine, calcium and phosphorus. If her diet was only providing enough protein, lysine and minerals to support adult, nonpregnant body condition, simply feeding enough to meet calorie needs is going to leave sizeable deficits in protein/lysine, calcium and phosphorus. Even diets adjusted for late pregnancy will not have a sufficient concentration of protein, lysine and minerals.
Nature gives precedence to providing for the foal. If the mare’s diet is deficient in protein she will begin to break down her own muscle tissue to obtain amino acids for the milk. This leads to both loss of muscle mass and weight loss in general. Milk production also suffers with insufficient protein. To get the minerals she needs she will leach calcium and phosphorus from her bones, trace minerals as needed from any stores she has in the liver and kidneys. This is not a sustainable situation for her future health and fertility.
What she needs will, as always, depend on what is in the base diet. If the mare is turned out on sufficient high quality young pasture, calories, protein, lysine, vitamin and essential fatty acid requirements will be met. The only hole will be minerals. Working with a professional will help you be more precise but generally these mares do very well with a carrier feeding of 1 lb each alfalfa pellets and plain oats with 1.5 to 2 times the regular dose of a vitamin/mineral supplement with a calcium:phosphorus ratio of 1.2:1.
If the mare is on hay with a protein level of at least 11%, her crude protein needs will be met but she will need mineral supplementation as above plus key amino acids of lysine, methionine and threonine, double the usual dose of 10-5-2 grams per day. Mares not on fresh pasture also need vitamin E and essential fatty acids from flax or chia.
If the hay is below 11% protein, substitute up to 2 pounds of a 25% protein vitamin and mineral supplement for the plain minerals above. If this isn’t enough to make up the protein deficit, add a 40% protein supplement to avoid excess minerals or too much bulk. Decrease the triple essential amino acid supplement to one dose and finish off with the vitamin E and flax.
One more thing extremely important for lactating mares is salt. Add 2 oz directly to food or sprayed on hay and keep loose salt available in a small feeder.
As the foal grows and eats more solid food, milk production will drop and you will see the mare start to gain weight. When this happens, begin decreasing how much she is fed and cut back on all supplements by the same percentage – e.g. 25% drop in food and 25% drop in supplements.
You can get away with a lot when feeding lactating mares but the rewards of doing it correctly are immediately obvious in the condition and health of both the mare and her foal.
Eleanor Kellon, VMD