Now Keep in mind this is only what I do and my opinion. I have learned that if you are going to go through the trouble of selecting stallions and mares and investing in breeding quality horses that you are going to show and sale you need to be a responsible breeder. It is more than just buying a mare bred because you want a baby. This can get very expensive and that is why my horses are not the cheapest for sale. I do not go overboard but I do what I feel is important for the health and welfare of the animal.
I check all my foals after foaling within a 6=8 hour time frame. I do not check mares milk because even though the mare may produce a large bag that is dripping wet with colostrum that does not mean the resulting foal has absorbed the required amount to sustain a healthy start. Nor does it mean that the mare has even produced good colostrum. There is no real way of telling and if you wait for the foal to start showing signs of illness, it could be to late.
When I have a foal born, I start right away monitoring its temp. this is always one of the first signs of the foal not getting the needed ingredients to a good and healthy start. Normally an infection, most commonly in the joints, is the first indication that you are in trouble. This type of infection can be extremely expensive to battle and the results are not always positive. An ounce of prevention.
I have seen the kits out on the market for testing the foal for IgG levels but I have found the best way for ME to do this is through my vet. Most test require blood plasma that is not an easy task to get from a foal. The results are instant and the vet I use has ordered plasma in small bags just for me and my minis. If you are into the breeding and your vet is new into minis, ask them to order small bags of plasma so that the cost will be kept way down. Having to give plasma is expensive, but for me it is worth it.
I have in the past not tested my foals IgG levels because I thought they just LOOKED to healthy to be sick, only to get up a few day after foaling and finding my little ones to weak or having to ill of joints to even get up. It can cost into the triple digits to save a foal from this type of infection, you need to decide what is right for you, your foal and your pocket book, before you breed.
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It normally cost me around $150 - $250 for the syrum. The is also IgG syrum that you can buy and give orally as a safety measure, I keep that on hand as well, but keep in mind, there is nothing better than mares colostrum and there is no substitute only supplements and those supplements will NOT give them the needed antibodies. Hope this has been helpful.