Giving a pasture a "break" from grazing helps to keep the grass varieties that you want, more healthy. Most horses pick through and "over graze" on the plants they like, and leave the weeds and other stuff. So the pasture can appear lush thanks but not really be providing much grazing for them, becuase they will continue to munch on the same plants over and over. When the "good stuff" is gone, they will resort to what's left in the field, sometimes eating unsavory, toxic weeds, but that's usually only in REALLY poor pasture and the horses are starving, but I digress. As an area gets overgrazed, the hooves start to destroy the roots when they trample over the same spot over and over.
Taking the horses compeletly off the pasture for a week-3 at a time and putting them on a different section to "rest' the pasture will keep the best grasses growing and prevent root damage from the hooves. Also, it's a good idea to keep them off the pasture when it's SUPER muddy to prevent root damage, if you can.
Also, rotating the pasture DOES help with parasite control, breaking up the worm cycle, esp if you spread the manure piles out to dry in the sun (sunlight helps kill some worm eggs).
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