Brood mares are often left out in the paddock until it’s time for the stallion or semen delivery and left again until it gets near the time for foaling. Since they are the mother of your new addition their health will reflect in the health of the foal. The growing foal will start to draw on the mare’s resources in the third trimester of pregnancy so good supplementation is really important, for example the demand for calcium will be drawn out of the mare’s cells if insufficient is arriving via the placenta to the foal.
A mare in her third trimester and also when lactating to provide milk for the foal until its weaned is “working” as hard as a race horse or performance sport horse in full training. A good formula of minerals is required as even good pasture will not provide enough, extra vitamins are not required in normal grazing conditions except for Vitamin E, and again this is supplemented as it will pass through the placenta to the foal.
As the conditions get warmer then electrolytes should be added to a small balanced feed of fibre and fat and carbohydrates.