Working somewhat like vacuum cleaners, horses could pick up the strongyles waiting deep in the grass for a cow as a host—and vice versa. Because “cow strongyles” can’t survive in horses, and “horse strongyles” can’t survive in cattle, each species can theoretically deworm the pasture for each other, said Felix Heckendorn, PhD, of the Research Institute for Organic Agriculture, in Frick, Switzerland.

Last year, French researchers investigated fecal egg counts in horses grazed with cattle on French horse farms. They found these horses had significantly lower worm burdens than those grazing on pastures used exclusively for horses.

Meanwhile, Heckendorn and his fellow researchers decided to test how worm burdens changed in young cattle when they grazed land previously grazed by horses in the Swiss Jura Mountains.

They started by taking two side-by-side pastures and filling them with either five young Red Holstein/Holstein cattle or three Franche-Montagne horses, letting the animals graze in their respective pastures for two weeks. Then they removed the horses and cattle and left the pastures empty for six weeks. At this point, they dewormed the cattle and divided them into two groups of five: one group in each of the test pastures. They let the animals graze down the pastures to a height of 2.5 inches and then moved them to a new, previously ungrazed pasture.

Three weeks later, when any swallowed worms would have had time to grow and start reproducing, the researchers ran fecal egg count testing on the cattle. They found that worm egg excretion was significantly lower in those that had grazed on the pasture where horses had been, Heckendorn said.

The researchers noted another finding: The cattle on the horse-grazed pasture gained more weight than the other five, although the difference was not statistically significant.

Strongyles can generally wait for a host in the lower grass near the ground for about six weeks or more, depending on weather conditions, said Heckendorn. So property owners could choose to empty a pasture long enough for the worms to die out on their own. However, that can waste pasture, he said. Alternatively, the grass could be cut to make hay—with a drying process the worms wouldn’t survive.

Sharing fields between grazing species is a good management option because the other species can use the pasture right away, and the grazing and fertilizing help maintain the pasture (as long as it’s not overgrazed), said Heckendorn. It might also be a better way to clear out the strongyles than waiting, as the worms get removed from the pasture as they’re consumed into the wrong host, where they die.

The results are promising and seem to confirm what many farmers have suspected for hundreds of years. However, further studies with more animals are needed to understand the benefits of alternate grazing between cattle and horses, Heckendorn said.