I own a Hanoverian broodmare (10 years old, 1050 lb or 475 kg) that is 300 days in foal. She spends most of her time outside on a pasture with no fescue. She has timothy/alfalfa hay available to her at all times. She gets 1/3 quart of a complete feed and 1 quart of a 30% supplement, plus Thyro-L, a thyroid supplement. Given the variable selenium levels in the region where I live, is my mare consuming sufficient selenium, copper, iron, and other minerals for her last trimester of pregnancy?
MicroSteed is a program that allows the nutritionists at Kentucky Equine Research (KER) to combine all the components of the diet in the amounts they are supplied with the information on the horse (specific breed, age, weight and reproductive state or activity) to see how well the diet meets the recommended daily allowances. I used average values for pasture and mixed hay and nutrient values from the feeds you’re giving your mare.
For the horse, I used the information you gave us in your request, but I used 1350 lb (612 kg) for your mare as a typical weight for a Hanoverian, especially one that is pregnant. MicroSteed requires that we enter the weight of the feeds and you gave me a volume value (quart), so I used an approximate weight of 1 lb (0.45 kg) for each quart.
Using the results from the MicroSteed analysis, I don’t think you need to be concerned about a lack of the minerals you mention (selenium, copper. and zinc); in fact, your mare is getting a lot more than she may need. What might be of concern is oversupplementation, especially vitamin D. From my calculations, she would be meeting her recommended daily allowances with just 1 lb per day of the 30% supplement. This is why it would be advisable to weigh the feed to know what you are feeding. Once the foal is born, she will need 1.5-2 lb (0.68-0.90 kg) of the 30% supplement to meet her increased needs for lactation, but only if you do not need to feed her a more calorie-dense feed to keep her from losing weight.