Were she my horse I would not just feed vitamins by hand "hit and miss".
Unless you have your pasture grass and hay sent out for testing, you have no way of knowing what vitamin and minteral deficiencies your horse has.
Because of testing, I know that my pastures and my hay that is grown five miles away, are very high in iron, therefore copper and zinc deficient.
Copper and zinc are needed for the immune system and crucial to insulin resistant horses because it helps balance the glucose.
Which segways into my next thought. If she is that easy a keeper and getting hefty off the grass, she may be predisposed to metabolic issues.
Being out on pasture 24/7 may not be the best thing for her this time of year. Evening/night time grass is the best for these types of horses.
In the stall or drylotted during the day, or even wearing a muzzle during the day could help a lot. Most horses won't keep a muzzle on though.
If the horse is predisposed to metabolic issues, it would be in her best interest to put out a white salt block rather than a mineral block because of the iron in the mineral block. Iron blocks the absorption of copper and zinc
If that sounds complicated, it is
I have one horse that has been I-R for three years and another that is most certainly predisposed to it. It is neither fun nor cheap to deal with
At any rate, feeding vit/mins on a "whenever" basis is not good for any horse. It's not too expensive to have your pasture grass tested.
Equi-Analytical Labs in New York do a bang up job for not much money, and will e-mail you the results in around ten days