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 temporarily 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.
For more in-depth information buy the book “What You Don’t Know About Worms Will Suprise You” by Dr. Ann Nyland which covers how to harrow and rotate pastures effectively.
This book is a clear easy-to-understand guide to horse worms and dewormers. Although written in easy language for the layperson, the book is heavily referenced to scientific academic journals.
Today, the problem horse worm is the small strongyle yet the vast majority of advice given today for worming horses is still aimed at the old methods of eradicating the large strongyle. Rotation is no longer advocated by equine parasitologists.
The book cuts through the claims about worming products, both chemical and natural / herbal, and presents the scientific evidence. When to deworm, resistance, rotational deworming, harrowing, are all covered in this book.