Feeding the nursing broodmare is really about feeding two horses: the mare, so that she can prepare to be rebred and support the new embryo, and the nursing foal, that will derive most of its nutrients exclusively from the mare, particularly in the early weeks after birth.
A nursing mare can produce up to 20 litres of milk per day, much of which is water, so she will need free access to clean water at all times. Providing a mare with sufficient calories to meet the demands of nursing is also critical. A good way to judge if the mare is meeting her caloric needs is to watch her body condition score. Mares will use their own body reserves (such as fat) to put towards milk production and will lose weight and condition before the milk production or quality will decrease. Therefore, if you can maintain the mare in good weight and condition by providing adequate energy intake, milk production should also be satisfactory.
Monitoring the growth rate of the foal is another tool to ensure the mare is meeting her nutritional needs to provide sufficient nutrients in her milk. Protein, vitamin, and mineral needs will also increase during lactation. A lactating mare has calorie requirements almost two times the amount of non-pregnant mares at maintenance, while protein and calcium requirements are almost three times higher.
As mares tend to increase their feed intake during lactation anyway, offer her free choice forage –as much as she will eat – supplemented with a commercial concentrate feed formulated to meet the needs of nursing mares. Forage should be high quality, ideally with some legume such as alfalfa mixed in with a grass species (orchardgrass, timothy, etc.). This will provide an excellent source of calories to the mare, and the inclusion of legume in the hay will have the added benefit of providing additional protein and calcium. The mare might eat as much as three per cent of her body weight in forage – or about 15 kg of hay per day.
Concentrate feed should be fed in the amounts indicated on the label at the very least to guarantee sufficient vitamins and minerals are provided. They can be increased as needed to maintain body weight, providing the mare is already consuming all of the forage she can eat – remember, forage first! Additional calories can also be provided through the inclusion of beet pulp, rice bran, or vegetable oil.
It is a good idea to try to keep a broodmare at a body condition score of approximately 6 (“moderately fleshy”) on the 1-9 Henneke scale. Having some extra condition will ensure she has additional reserves to draw upon if needed, and will help the mare with rebreeding. Research has shown that higher body condition scores are associated with a higher rate of reproductive efficiency (as measured by pregnancy rates, fewer days to foal heat, etc.). However, it is not recommended to keep a mare too fat (>7 on the scale) as the negative health risks associated with obesity, such as laminitis, joint issues, etc., outweigh any potential improvements on reproductive efficiency. Furthermore, excessive energy intake and obesity is actually associated with decreased milk production.
It should be noted that during the last three months of gestation and while lactating, mares should not be fed fescue hay/pasture. Most fescue is infected with an endophyte fungus that produces compounds (ergot alkaloids) that decrease milk production. If a mare does not produce milk and endophyte-infected fescue is suspected, she can be treated with medication such as domperidone to stimulate prolactin secretion and milk production.