Harrowing is often suggested as a preventative measure for worm control but in fact, it can actually increase worms if done in the wrong way.
Horses with plenty of room on which to graze will divide their grazing into two distinct areas, 1) roughs and 2) lawns.
They do nearly all their droppings in the rough area, an area with overgrown grass and weeds, an area which horses will not graze down.
The lawns are the areas which horses do graze, and on which they rarely do droppings. Clearly then, most of the worm eggs are in the roughs.
This is a natural system for horses to keep down their worm burdens.
Harrowing, mowing or dragging increases worm levels by transferring the high levels of worm eggs from the ‘roughs’ to the ‘lawns.’
However, in cooler regions, harrowing at the end of the grazing season has been shown to reduce the survival of infective stages of worms over winter.
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