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Connecting AG to Climate: Grass-Cast Translates Weather Forecasts into Grassland Forecasts for Ranchers

by Wyoming Livestock Roundup

Ranchers across the Great Plains are in a never-ending game with Mother Nature when it comes to weather, especially when trying to predict how much grass will be available during the grazing season. 

Precipitation on Western rangelands varies greatly from year to year, only increasing the challenge producers face when trying to match livestock demand with available forage supplies. 

Adding to the challenge of making plans for the summer grazing season, weather forecasts might not provide enough information for ranchers to base their stocking decisions on.

With this in mind, a team of researchers from the U.S. Department of Agriculture, Colorado State University, University of Arizona and the National Drought Mitigation Center set out to provide producers with a forecast more directly relevant to stocking and grazing decisions. 

The Grassland Productivity Forecast (Grass-Cast) was created to help ranchers reduce the economic uncertainty related to forage availability for livestock. 

Grass-Cast estimates the productivity or the future forage growth of native rangelands throughout the Great Plains and Southwest, which can help inform stocking rates, grazing decisions and proactive drought management.  

The science behind the tool

Grass-Cast combines almost 40 years of historical and current weather observations from the National Oceanic and Atmospheric Administration (NOAA) with a well-trusted grassland growth model. 

Information from the model about soil moisture and water loss through evapotranspiration are correlated with greenness measured across rangelands, which is then translated into the amount of available forage forecasted for the upcoming grazing season. 

This forecasted amount of forage is compared to the 36-year average for the local area.

Because weather in the Great Plains and Southwest is so hard to forecast during the growing season, Grass-Cast provides users with three different “what-if” scenarios – what if an area receives above-normal, near-normal or below-normal precipitation for the rest of the growing season through August? 

The three Grass-Cast maps below show forecasted vegetation growth under each precipitation scenario. 

The accuracy of Grass-Cast improves as the growing season unfolds, so producers and managers are encouraged to consult the tool every two weeks when it is updated with newly-observed weather data.

Interpreting Grass-Cast maps

After looking at the three Grass-Cast maps for an area, one might wonder which of the three maps is most likely. 

To answer this question, visit the NOAA website at and view seasonal precipitation outlooks for spring and summer months to see if the odds are leaning towards above-, near- or below-normal precipitation for the area. 

This will tell individuals if the odds are leaning towards Grass-Cast’s left, middle or right map. If NOAA indicates equal chances, then all three Grass-Cast maps are equally likely.

While Grass-Cast is a useful tool for informing grazing decisions, it is important to know its limitations. Producers and land managers should not rely upon Grass-Cast as their sole source for setting stocking rates, determining turnout dates or other important grazing decisions. 

Ranchers will get the most value from the forecast when they combine it with their knowledge of local soils, plant communities, grazing history and any other tools typically used in their decision-making process. 

Keep in mind, Grass-Cast provides a forecast of total production, not grazeable production. So, if it says an area might have 20 percent less pounds per acre than usual, and producers follow the “take-half, leave-half” rule of thumb, then this could translate into a 40 percent reduction in grazeable forage. 

To view the most recent maps, zoom into a local area, watch a tutorial video or learn more about the science and team behind the tool, visit 

Averi Reynolds is an ORISE science communications fellow for the U.S. Department of Agriculture Northern Plains Climate Hub, serving Wyoming, Montana, Colorado, Nebraska, South Dakota and North Dakota. The USDA Northern Plains Climate Hub strives to provide unbiased information about adaptation and mitigation strategies for ranchers, farmers and foresters to help increase their operations’ resilience to weather variability and a changing climate. For more information on the Northern Plains Climate Hub, visit

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