Every spring, ranchers face the same difficult challenge of trying to guess how much grass will be available for livestock to graze during the upcoming summer.

Now, an innovative new Grassland Productivity Forecast (“Grass-Cast”) has published its first forecast to help producers in the northern Great Plains reduce this economically important source of uncertainty, according to the U.S. Department of Agriculture's Agricultural Research Service (ARS).

The new experimental grassland forecast is the result of a collaboration among ARS, the National Drought Mitigation Center, Colorado State University and the University of Arizona.

Grass-Cast uses more than 30 years of historical data about weather and vegetation growth — combined with seasonal precipitation forecasts — to predict if rangelands in individual counties are likely to produce above-normal, near-normal or below-normal amounts of vegetation for grazing, ARS said.

According to ARS economist Dannele Peck, director of the USDA Northern Plains Climate Hub, the accuracy of Grass-Cast depends on how far into the future ranchers try to look. The program’s accuracy improves as the growing season unfolds, so it should be consulted more than just once during the growing season.

The Grass-Cast forecasts are updated every two weeks to incorporate newly observed weather data and emerging trends in grazing conditions, such as changes caused by the flash drought in the western Dakotas and in eastern Montana in 2017.

Grass-Cast also provides ranchers with a view of rangeland productivity in the broader region to assist in larger-scale decision-making and to determine where grazing resources might be more plentiful if their own region is at risk from drought.

Grass-Cast provides ranchers and land managers with an indication of what productivity is likely to be in the upcoming growing season relative to their own county’s more than 30-year history. Ranchers and land managers will need to combine the forecast information with their knowledge of local soils, plant communities, topography and other conditions as part of their decision-making process.

Since Grass-Cast cannot tell the difference between desirable and undesirable forage species, it is important for producers to know what proportion of a pasture is occupied by weeds and how well those weeds respond to rain (or lack of rain) compared to the desirable species, ARS explained. Producers should monitor these different vegetation types to see if one is responding to the weather better than the other and adjust the Grass-Cast productivity estimates accordingly.

Producers should not rely on Grass-Cast as a sole source for making management decisions, ARS noted. Similarly, public land managers should not use Grass-Cast as the sole source of information for setting stocking rates, determining turnout dates or for other aspects of lease agreements, allotments or permits.

The first Grass-Cast was released in May 2018. Updates will be released on the Grass-Cast website, on the USDA Northern Plains Climate Hub’s website or on Twitter (@PeckAgEc).

Funding for this project came from ARS, USDA’s Natural Resources Conservation Service and the National Drought Mitigation Center.