How do i get rid of the extra winter hair on my horse?

I have tried brushing and brushing it for hours but it just is not coming out! We got my mare in January (she was in a heated barn) and brought her out to my barn which is not heated but we had a blanket on her for the rest on the winter. I don'tt know what to do and am going to resort to body clipping as a last optionsince my horse HATES clippers. At the horse show the other day somebody told us to use 1/8th cup of corn oil but it has not taken affect yet. PLEASE help!!!