Grass-Cast: A precision ag tool for livestock owners

Cattle gather in a pasture.

Up until recently, most precision ag tools have been for those producing crops, but a coordinated effort with several groups has resulted in a precision ag forecasting tool that will help cattle producers predict the forage production on their range or grasslands.

Grass-Cast, a grassland productivity forecast, uses relationships between historical weather and grassland production. It combines current weather data and seasonal climate outlooks from the NOAA Climate Prediction Center with a well-trusted grassland model to predict total biomass in pounds per acre for individual counties, compared to their 38-year average, according to Bill Parton, the lead researcher at Colorado State University.

Parton has found that precipitation data can be used to estimate actual evapotranspiration (iAET), which correlates well with the vegetative growth index (iNDVI). The iNDVI, in turn, is a predictor of above-ground primary productivity (ANPP) – a measure of plant biomass.

By using this tool, Parton’s research team combines observed daily precipitation with seasonal precipitation outlooks from NOAA’s National Weather Service-Climate Prediction Center to predict iAET. The final steps involves transforming the predicted iAET into iNDVI and then into ANPP for the growing season.

The resulting grassland productivity forecast map, or Grass-Cast, shows forage production forecasts for 382 individual counties in the Great Plains region. The map actually reports the average of 10 different ANPP forecasts based on various combinations of recent weather data and seasonal precipitation outlooks.

It is an optional tool that managers can use to form an educated guess about the upcoming growing season. It can also help inform the design of proactive drought management plans, trigger dates, stocking dates and grazing rotations, according to those who have put the tool into practice. In short, it helps producers in the Great Plains reduce the uncertainty of this important economic source – forage production.

Forecast maps

Grass-Cast provides three maps for ranchers and rangeland managers to consider. The maps show how well rangeland vegetation is expected to grow if precipitation during the rest of the growing season (through July 31) is above-average; how well rangeland vegetation should grow if precipitation during the rest of the season is near-normal; and how well vegetation should grow if precipitation is below-average.

Below the maps, a statement indicates if the maps are equally likely, or if one map is thought to be more likely than the others, based on NOAA’s Seasonal Precipitation Outlook. Within an individual map, each county is assigned a color, which indicates whether rangeland vegetation is expected to be certain percent higher or lower than that county’s 34-year history.

Not the complete answer

Producers should not rely on Grass-Cast as a sole source for making management decisions, nor should they look at Grass-Cast just once during the growing season. The accuracy of Grass-Cast improves with time as the growing season unfolds. Ideally it should be consulted every two weeks when it is updated with newly observed weather data.

Additionally, Grass-Cast should not be used as a sole source of information for things such as setting stocking rates, determining turnout dates, or other aspects of lease agreements, allotments or permits.

In addition to Parton, those who helped develop this tool include: Melannie Hartman of Colorado State University, Justin Derner with USDA-ARS, Brian Fuchs of the National Drought Mitigation Center, Bill Smith from the University of Arizona, Dana Larsen with USDA-NRCS and Dannele Peck of the USDA-Northern Plains Climate Hub.

To get the forecast maps during the growing season visit