Lactating Mare and Garlic

I have a lactating mare who has a cough this spring - I do believe it is a result of dusty hay allergy. I do not have the facilites to soak her hay, nor to isolate her from the rest of the herd. I had been feeding her some MSM supplement in her feed, which seemed to help. However, we have a terrible tick and mosquito problem here, and I read that garlic also has a naturally occurring MSM in it, and is said to repel blood sucking pests. Will garlic harm the mare's milk or the foal's system if introduced gradually?

To start, other things beyond soaking the hay to minimise dust might be to ensure there is a good supply of fresh air in the stable, lots of exercise and to purchase dust-free vaccuum packed haylage instead of regular hay. Other thoughts to reduce mucous (and hence the cough) are to be sure she feeds from the floor (to drain mucous) and to avoid mucous producing foods like barley.
Garlic should indeed help repel biting flies and reduce mucous but it can taint the milk of a lactating mare which in turn can upset the intestinal balance of a new foal. You can try to start the garlic off very slowly but I cannot guarantee that it will not cause problems in the foal's delicate digestive tract. However, give it a try and let me know! Typical recommended doses for a non-lactating horse taking garlic are: 6 to 8 crushed cloves per day. Other herbs to help with hayfever/excess mucous production are: Aniseed (one handful of seeds per day) and Marshmallow (20 grams of cut root daily, in feed).
[Follow up from Rebecca:]

Thanks for your response – we have been feeding Bug Off Garlic from Springtime to this mare. We started very gradually, and have had no problems with the foal. In fact, now that the foal is eating her own grain, she prefers the grain that has garlic added to it to the grain without.