Just curious how many people do this. I think I remember reading selenium levels can affect fertility...Please do correct me if I am way off track on that. I need a refresher on this subject.
You need to check with your vet about what region you are in with regard to selenium levels in your soil. Some regions actually have too much selenium, while most of the northwest has too little and does require both selenium in our vitamin/mineral supplements and a selenium shot to our newborns.
There can be physical problems with both too little AND too much selenium.
Its an ingrediant in most mixed feeds. I would suggest you check with your feed company about what minerals etc are already being added before giving your animals any added supplaments.
I totally agree with MiniV -- do not add any selenium until you have checked with your local extension office to see what the soil conditions are for your area, or the area where you buy your hay. You could be overloading your horse on selenium if you aren't careful. A simple phone call is all that it takes.
You do need to check your actual property AND where you hay is grown. We live on hills that are not selenium low, but the neighboring areas that much of our hay is grown in is low.
We use to supplement with an annual VitE/Selenium shot for all the horses - mares a month out from foaling, all others in the Spring.
We have switched over to offering the Selenium mineral block along with the white salt blocks and with the supplemental grain, it seems to be enough.
I would check with your vet - the VitE/Selenium shots aren't hard to give, but they're thick, so a bit on the painful side going in!