This question brings up a few good points;
1) Although colostrum is quite valuable, I would discourage milking a mare for it unless she was already dripping or a foal was in need of it. There is a wax plug in the orifice of the nipple that protects the gland from infection. Removing it to just collect colostrum simply isn't valuable enough in my opinion.
2) All mammal species produce colostrum for 3 days, the neonate is only capable of absorbing the vital antibodies in it for the first 24 hours.
3) Mares store very little milk, so if colostrum harvesting is going to occur then optimal collection occurs if milked every 2 hours.
4) Quality of colostrum goes up with age of the dam and the number of babies she has had as well as vaccination status. But, the quality of colostrum cannot be determined by 'looking' at it. A colostrometer can be purchased that will accurately measure the quality.
5) It does not need to be collected in a sterile manner, simply run it through a sieve if there are large particles of debris then either freeze it in ice cube trays (put the cubes in sealed bags for long term storage) or plastic pop bottles.
6) Colostrum must be warmed in hot water for use, it cannot be microwaved or put on direct heat as these will ruin the antibodies (but the colostrum is still useable as milk).
Dr Taylor