Zebras vs. cattle: Not so black-and-white

African ranchers often prefer to keep wild grazers like zebras off the grass that fattens their cattle.

But a new study by Kenyan and University of California at Davis researchers shows that grazing by wild animals doesn't always harm, and may sometimes benefit, cattle.

The results are published in this week's issue of 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 and a co-author of the paper.

“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,” said Truman Young, a plant scientist at UC Davis and lead author of the paper.

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

“The finding that wildlife has a positive effect on cattle growth and production during times of plenty adds new insight into the role that facilitation plays in natural communities,” said Saran Twombly, program director in the National Science Foundation's Division of Environmental Biology, which funded the research.

“Results from this study also speak directly to the critical importance of biodiversity in maintaining natural ecosystems,” Twombly said.

The researchers enclosed 10-acre plots of savanna rangeland with fences to exclude wild animals–principally zebras.

Then they weighed the cattle grazing either with or without wild grazers to measure how much weight they put on at different times of the year, which is the bottom-line for ranchers.

Odadi and colleagues found that during the dry season, grazing by wild animals reduced weight gain by cattle, 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.

“From a management perspective, the positive effect of wildlife on weight gain by cattle during the wet season suggests that wildlife conservation is not necessarily detrimental to, and can be compatible with, cattle production,” Odadi said.

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

Other co-authors of the paper are Moses Karachi, Egerton University, Kenya, and Shaukat Abdulrazak, National Council for Science and Technology, Nairobi, Kenya.

The work also was funded by the Smithsonian Institution, the National Geographic Society, the African Elephant Program of the U.S. Fish and Wildlife Service, and the International Foundation for Science.