Imagine — being able to accurately predict which weeds will hit your fields and when. It may sound like pie in the sky, but that's the goal researchers had in developing WeedCast, a windows-based software program.

Users input weather, soil type, tillage and crop rotations. “The model then predicts the weeds that will come up,” says Frank Forcella at the USDA-ARS Soils Lab in Morris, MN. Forcella designed the software along with university and other ARS agronomists in Iowa, Michigan, Missouri, Colorado, South Dakota and Ohio.

“It all started by growers calling and saying, ‘Tell me when the weed emerges, then I'll know what to do,’” Forcella recalls.

The model predicts when weeds emerge and how fast each species grows. The software can help you plan tillage or post herbicide applications.

Organic growers use it to predict how fast early season weeds grow. “Typically, organic growers wait to plant and they tend not to know when weeds are fully emerged. The software helps growers time rotary hoeing, harrowing or cultivating between rows,” Forcella says.

Organic grower Carmen Fernholz used soil temperature and weather to time generic weed control before the software was released. “The program fine-tunes what specific weeds and what percentage of weeds emerge,” says the Madison, MN farmer. Fernholz, who grows 100 acres each of corn and soybeans, inputs the previous day's maximum and minimum temperatures in WeedCast. By looking at the previous day's weed emergence, he plans his day. For example, if green or yellow foxtail reaches 15% emergence, he'll rotary hoe or harrow.

“Even if you're not an organic farmer, knowing the weed emergence can raise the efficacy of herbicides,” Fernholz points out.

Forcella finds growers use the information to time post herbicide applications. “They know that, when 10% of the weeds have emerged, it doesn't make sense to apply the herbicides. But when 80% of the weeds have, it does.”

The software can pinpoint the time weeds will emerge; it can't forecast the exact number of weeds or what to do. Although the software has been available for three years, the agronomists now have concrete examples of how to use it.

For example, agronomists use the model's emergence information to predict green and yellow foxtail control. Forcella recommends using a rotary hoe or harrow at 15% emergence and again at 30% emergence. Then at 60% emergence, he recommends cultivating between rows.

“There's never excellent control, but pretty good and consistent control from one year to the next,” Forcella says.

The software uses graphs and tables to illustrate the weed emergence percentages. Control recommendations are made outside the program.

“After we made the recommendations for foxtail, then farmers asked, ‘What about wild mustard, velvetleaf and others?’” The software has emergence potential for 17 annuals, including foxtail, pigweed, smartweed, velvetleaf, lambsquarter and ragweed.

Agronomists use the program to predict problem weeds and tailor control to their climate. In Iowa, giant ragweed has a short season. But in Ohio it germinates all season long, says Ohio State University extension weed specialist Mark Loux.

Timing is everything in a total post program. Applying herbicides when giant ragweed is the ideal 6” is nearly impossible. That's because it grows so fast and new ragweeds are emerging.

In general, most applications work best on 6” weeds. But if applications are made to foxtail less than 3” tall, new weeds can cause yield lags. With a pre herbicide, growers have some control and make the post program more manageable, Loux says.

WeedCast software can be downloaded from www.mrsars.usda.gov by cliking on Computer Software Products, then Weed Ecology Management.