BISMARCK, N.D. — As ranchers are considering how long to leave the cows on pasture this fall, an online tool might help them determine how much more grass production might remain

Grass-Cast was created by the U.S. Department of Agriculture’s Agricultural Research Service, the University of Nebraska-Lincoln’s National Drought Mitigation Center, Colorado State University and the University of Arizona.

Dannele Peck, director of the USDA’s Northern Plains Climate Hub, said that in the past, climate scientists would tell ranchers the precipitation outlook for the next few months. But telling them that there was a 40% chance of below average precipitation wasn’t overly helpful.

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“The immediate reaction I would get is, ‘Well thanks, but what does this really mean for me as a rancher?’” she said. “Kind of, ‘so what?’ or ‘what’s next?’ And I never had a good answer for them.”

Grass-Cast takes those precipitation forecasts and translates them into an estimate of how many pounds of forage are going to grow per acre compared to the historic average.

Using 30 years of historical data about weather and vegetation growth, Grass-Cast presents three different scenarios: how much forage would be produced if there is above-average precipitation, below-average precipitation or average precipitation from that point forward.

Peck said the three maps — rather than just one map based on the latest forecasts — came about through input from ranchers who said they would be unlikely to just trust one forecast. With the three maps, the ranchers can think through the different possibilities, look at precipitation forecasts and make management decisions, such as stocking rates or rotational patterns, for themselves.

Peck had Grass-Cast on display at the America’s Grasslands Conference Aug. 20-22 in Bismarck. A computer in Peck’s Northern Plains Climate Hub allowed people to see and play with Grass-Cast, including zooming into the 6-mile-by-6-mile block that includes their own pasture to see the precipitation

Grass-Cast was developed in 2017, and the first maps were released in May 2018. The product now covers the entirety of the Great Plains, and Peck hopes that the southwestern U.S. will be added by 2020.

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The first maps of the season come out in late April or early May and then are updated every two weeks, Peck said. As the season goes on, the maps become more and more accurate. So Grass-Cast may be most helpful for making late-season grazing decisions.

“Most of the grass growth has already happened. There’s only maybe a little bit left that’s uncertain,” Pecks said. “Don’t just check it once. Go back and check it every two weeks.”

There are limits to what Grass-Cast can predict; the website’s Frequently Asked Questions section points out that it does not account for damages from flooding, like soil erosion, plant die-offs due to lack of oxygen or contaminants in the water, so Grass-Cast may overestimate how well forage will grow in particularly wet areas.

“This is a perfect example of why Grass-Cast should be combined with a landowner's intimate knowledge of their landscape, including its unique characteristics and conditions at the local scale and how these differ from those at the larger landscape scale,” the website says.

Peck said her team worked closely with the USDA’s Natural Resources Conservation Service and Extension Services from universities across the region.

“So far, we’ve gotten really good feedback,” she said.

They want to get more feedback on the accuracy of Grass-Cast and how it could be improved. Comments can be sent through the Contact Us section of the website.

Grass-Cast is available at http://grasscast.agsci.colostate.edu.

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