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Want to Know How Much Forage You’ll Have This Summer? Check Out Grass-Cast

Every spring, ranchers face the same difficult challenge—trying to guess how much grass will be available for livestock to graze during the upcoming summer. Now an innovative new Grassland Productivity Forecast or “Grass-Cast” has published its first forecast to help producers in the Northern Great Plains reduce this economically important source of uncertainty.

This new, experimental grassland forecast is the result of a collaboration between the USDA Agricultural Research Service (ARS), Colorado State University, the National Drought Mitigation Center (NDMC), USDA Natural Resources Conservation Service (NRCS),and the University of Arizona. While researchers continue to improve Grass-Cast, they decided to release it this spring because predictions for some parts of the Northern Great Plains looked quite grim and they wanted producers to be able to use it to help make important decisions

How Does It Work?

Grass-Cast uses over 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. Forecasts are updated every two weeks to incorporate newly observed weather data and emerging trends in grazing conditions, such as changes caused by flash drought, like what happened in 2017 in the western Dakotas and eastern Montana.  Its accuracy depends on how far into the future researchers try to look and improves as the growing season unfolds. So consulting it more than once provides the best results.

To see how this works, check out the progression of forecasts below. On the left you’ll see predictions for the amount of forage a county could have if it receives above average precipitation in June and July. In the middle is what you might have with just average precipitation. On the right is what happens with below average precipitation. What each set of maps is predicting is how much total forage you will have on July 31, based on historical weather and vegetation growth data combined with current weather forecasts. The key to the right tells you what the colors mean from red, indicating you can expect at least 30% less forage than average if not worse, to dark blue meaning you can expect at least 30% more forage than average if not better. The middle color of light green means you can expect about average amounts of forage, plus or minus 5%. (For counties in white, no forecast is available due to insufficient data or weak statistical relationships. If you live in one of those counties you’ll have to wait until further data is accumulated.)

Here’s May 1, 2018

Here’s May 8, 2018

Here’s May 21, 2018

Here’s June 4, 2018

As you can see, as spring has worn on, predictions for forage production have increased in some counties, and decreased in others, based on how the weather has unfolded through the spring.

But which of the three scenarios is most likely to happen?

The NOAA Climate Prediction Center has so far indicated equal chances (33% each) of precipitation in May-June-July being above/near/below normal for this region, so each of the maps above are equally likely. When I spoke with Dannele Peck, ARS economist and Director of the USDA Northern Plains Climate Hub, she said, “If you’re hoping for that miracle rain, and thus making decisions based on the map on the left, remember that while you have a 33% chance of that happening, you also have a 66% chance that what the other two maps show will happen instead.”

What Do the Researchers Think?

Looking at the maps, here’s what the researchers concluded:

  • For much of North Dakota, producers should expect below-normal (yellow and orange) rangeland production (lbs/acre), particularly if rainfall in June and July is only near or below normal. If rainfall is above-normal, it is possible that some counties could have near-normal production (light green). In western North Dakota, rainfall during the last two weeks (late May, early June) boosted our rangeland production estimates for that region.
  • For central South Dakota, even if rainfall in June and July end up being above-normal, we expect rangeland production (lbs/acre) to only be near-normal (light green) or slightly below-normal (yellow). Conditions in western South Dakota are slightly better, with some possibility of above-normal production, but only if they get above-normal rainfall in June and July.
  • For much of Nebraska, rangeland production (lbs/acre) should be near-normal (light green) or above-normal (shades of blue) no matter how much rainfall they receive in June and July. But if rainfall is below-normal, the southern-most counties in Nebraska (bordering Kansas) could see below-normal production.
  • For southeastern Colorado, producers should expect below-normal (yellow or orange) rangeland production (lbs/acre) no matter how much rainfall arrives in June and July.
  • For much of northeastern Colorado, Wyoming, and Montana, rangeland production (lbs/acre) is expected to be near-normal (light green) or above-normal (shades of blue) regardless of how much rain falls in June and July. In eastern Montana, rainfall during the last two weeks (late May, early June) boosted our rangeland production estimates for that region.
  • For some north-central Montana counties bordering Canada, rangeland production (lbs/acre) is likely to be below-normal (yellow or orange). Only if rainfall in June and July is above-normal can they hope to see near-normal (light green) or slightly above-normal (light blue) rangeland production.

Use Grass-Cast to Check Out Your County

In the future you’ll be able to go directly to the website to get this information. But, right now, the website is still under construction, waiting on additional funding that will allow the researchers to complete it. So I spoke with Dannele Peck, who has been working on this project and asked her to provide On Pasture readers with the two parts they need to find out what they can expect this grazing season. The first are the maps. You can download large versions of all three of the June 4 Grass-Cast maps here. The second is the Historical Productivity data from 1982 to 2015 for forage grown in your county. Download that list here, and then find your county.

Since forage production varies year-to-year, the Historical Productivity list includes the smallest amount of forage produced per acre during the 24-year time period (min ANPP) and the year that happened, along with the most forage produced (max ANPP) and the year that happened. They also include the average (mean ANPP), and the amount of forage that falls right in the middle of the dataset, halfway between the least and most (median).

You want to look at the mean (average), and then look at the maps to see the color covering your county. For this example we’ll use Baca County, Colorado, located in the very southeast corner of the state. Under the very best scenario, with above average rainfall in June and July, forage production will be 5 to 15% less than normal. So I multiply 689 x .15 and find out that I’ll have about 103.5 pounds per acre less than normal. That 585 pounds per acre is not as bad as things got in 2011 (285 pounds per acre). If I look at the worst case scenario, that drought will continue and precipitation will be below normal, I could have 15 to 30 percent less forage than average, or 689 x .30 = 206.7 pounds per acre less than normal or 482 pounds per acre.

From here I can consider if I can feed all the livestock I have, or if I need to think about destocking. I might also look at rangeland productivity in the broader region to determine where grazing resources might be more plentiful, and to find out if I could move my stock, or purchase a bit of emergency feed.

If you’d like to continue follow the forecast, you can find updates every 2 weeks here.

Predicting the Future is Hard

Grass-Cast is one tool you can use to help you make tough decisions based on the best information we have available. But you will need to combine the forecast information with your knowledge of local soils, plant communities, topography, past grazing pressure, and other conditions as part of the decision-making process.

Since Grass-Cast cannot tell the difference between desirable forage species 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. Producers should monitor these different vegetation types to see if one is responding to the weather better than the other and adjust Grass-Cast’s productivity estimates accordingly.

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

Would You Like Grass-Cast for Your Region?

Funding for this project came from USDA’s ARS and NRCS, and the National Drought Mitigation Center. As more funding is made available, they will continue to expand it to other regions and improve the website to make the information easily accessible. Please use the comments below to let us know how this tool might help you make decisions about your livestock, pastures and management, or share thoughts about what improvements or additional information would help you. Dannele says you should also feel free to email or call her directly with feedback—positive or critical—at, 970-744-9043. That way she can make improvements based on what you need.

Did you know? The Agricultural Research Service is the U.S. Department of Agriculture’s chief scientific in-house research agency. Daily, ARS focuses on solutions to agricultural problems affecting America. Each dollar invested in agricultural research results in $20 of economic impact.

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Kathy Voth
Kathy Voth
I am the founder, editor and publisher of On Pasture, now retired. My career spanned 40 years of finding creative solutions to problems, and sharing ideas with people that encouraged them to work together and try new things. From figuring out how to teach livestock to eat weeds, to teaching range management to high schoolers, outdoor ed graduation camping trips with fifty 6th graders at a time, building firebreaks with a 130-goat herd, developing the signs and interpretation for the Storm King Fourteen Memorial trail, receiving the Conservation Service Award for my work building the 150-mile mountain bike trail from Grand Junction, Colorado to Moab, Utah...well, the list is long so I'll stop with, I've had a great time and I'm very grateful.

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