African ranchers often prefer to keep wild grazers like zebra off the grass that fattens their cattle. But a new study by UC Davis and Kenyan researchers shows that grazing by wild animals doesn’t always harm — and can sometimes benefit — cattle. The results are published Sept. 23 in the journal Science.

“Although savanna rangelands worldwide are managed on the premise that cattle and wildlife compete for food, there is little scientific information to support this assumption,” said Wilfred Odadi, a researcher at the Mpala Research Centre in Kenya.

“Our findings provide further evidence that biodiversity conservation and economic development can be simultaneously achieved in human-occupied savanna landscapes,” he said.

Truman Young, a professor of plant sciences at UC Davis and senior author of the study, said the interaction between cattle and wildlife is more complicated than has been appreciated.

“When we look at the effect of wildlife on cattle, we find that they sometimes do suppress weight gain by cattle, but also sometimes enhance it,” Young said.

“Generally the decision has been to exclude wild animals, but we’re saying that things are not that simple,” he said.

The researchers enclosed 10-acre plots of savanna rangeland inside fences to exclude wild animals (principally zebra). Then they weighed the cattle grazing inside and outside the fences to measure how much weight they put on at different times of the year.

The research team found that during the dry season, cattle that grazed with wild animals had reduced weight gain — the bottom line for ranchers. But in the wet season, cattle actually put on more weight when they grazed alongside wildlife.

The explanation is that during the wet season, grass can grow long and become rank, inaccessible and poor in nutritional value.

“When the grass grows very fast and is at risk of becoming rank, having zebras is beneficial,” Young said. “They are more than willing to knock back the rank grass.”

That means higher-quality, fresher grass for the cattle.

It’s not yet clear whether there is a net benefit over a whole year or series of years, Young said, because conditions can vary considerably from year to year. Ranchers are beginning to explore additional ways to control rank grass, such as controlled burns.

The other co-authors of the study are Moses Karachi, Egerton University, Kenya; and Shaukat Abdulrazak, National Council for Science and Technology, Nairobi, Kenya. The work was funded by the Smithsonian Institution, the National Geographic Society, the National Science Foundation, the African Elephant Program of the U.S. Fish and Wildlife Service, and the International Foundation for Science.