In other cases it fails to concentrate and the foal requires an alternate supply. Keep in mind that the cheapest, and best way of getting an adequate IgG into a foal is for it to nurse good colostrum early on. You then have to wait to see if the foal develops an adequate level. Meanwhile the opportunity for oral supplementation is dwindling. By 24 hours for sure, and often much earlier, the foal loses its ability to absorb the IgG in the colostrum intact.
Transfusions are very expensive, and time consuming.
Some vets don't like to do them in the field, in which case the foal would have to be transported. This only adds another source of risk, expense, and trouble. This is far cheaper, and better for the foal, than taking the risk that the mare will fail to reconcentrate her colostrum, so that you are forced to supplement via the intravenous route. If you decide to bank some colotrum from your mares for future use, do take the trouble to have it tested. Testing will reveal two important things.