Weight Loss

Right about this time of year Buzz seems to lose an awful lot of weight
He gets fed the same all-year round and in winter when the grass grows he gets let out, but because there is still grass he is still getting some the same thing happened last year and he picked up in a few weeks but I was wondering what would cause it??

What exactly is he being fed year around? Is he just getting forage of what is left in the paddock or does he have hay/feed that you give him daily?