Leopold Center - Researchers study nature's "little hammers" for weed control

Iowa State University researchers are studying how a small but plentiful mammal may play a big role in controlling weeds for Iowa crop farmers.

The weed management star is the prairie deer mouse, Peromyscus maniculatus.

Just over 6 inches in length (including its 3-inch tail), the prairie deer mouse has a dusky brown coat and white feet. Although suited to many habitats throughout North America, it prefers open spaces such as crop fields, which is how this little guy has found notoriety of sorts in a Leopold Center-funded study on weeds.

Like its cousin, the white-footed mouse that prefers field edges, the prairie deer mouse eats weed seeds. Also like the white-footed mouse, the prairie deer mouse does not hibernate, remaining active throughout the winter months.

Year-long activity
“These guys are working hard, every day around the clock,” said Brent Danielson, a mouse biologist from the ISU Department of Ecology, Evolution and Organismal Biology. “We found that they’re out there in significant numbers and that they’re active all the time.”

Danielson said that although they know mice are major consumers of weed seeds during the summer, the greatest period of vulnerability for weed seeds may be the period between harvest and spring planting. “We hope to learn more about how different crop types, tillage methods and rotations may affect winter mouse abundances and their diets,” he said.

The research is part of a larger project that began in November 2002 on the ISU Marsden Farm in eastern Boone County funded by a competitive grant from the Leopold Center Ecology Initiative. ISU agronomists Matt Liebman and Bob Hartzler set up 36 plots, each 60 x 275 ft., to study the effects of different crop management systems on velvetleaf and giant foxtail.

The study includes three rotations:

Each phase of each rotation occurs every year. During the past two cropping seasons, measurements have been taken to determine weed seed longevity in the soil, weed seedling emergence and survival, weed seed production, and weed seed loss to insect and animal predators.

Predation losses were determined by lightly gluing giant foxtail and velvetleaf weed seeds onto squares of sandpaper. The squares were placed throughout the fields for 48-hour periods between May and November each of the past two years. After each card was collected, researchers counted the number of seeds left on the card. Some of the cards were placed in cages designed to keep out the mice and other vertebrates.

Primary seed predators
“Our preliminary data suggest that mammals may strongly affect losses of weed seeds,” Danielson said. “When the rodents are allowed to forage for weed seeds, they can consume more than 40 percent of the seed in a single night.”

Averaged over 27 sampling periods in 2003 and 2004, about a third of the velvetleaf seeds and half of the giant foxtail seeds were lost to predators within two-day periods. In 2003, the loss of velvetleaf seeds to predators was greater in the four-year rotation than in the two-year rotation.
Andrew Heggenstaller, an agronomy graduate student who worked in the predation study, said predator activity depends on the rotation and time of year.

“Optimally, you want to provide a longer opportunity for seed removal during the season by having a diversity of crops that comes from a longer rotation,” he said. “In corn and soybean crops the demand for weed seeds by predators is highest only during the middle of the summer. The triticale and alfalfa provide a better season-long habitat for predators.”

Long-term effects
One concern about more diversified cropping systems is the influx of annual weeds in the three-and four-year rotations when red clover and alfalfa are grown with the small grain crop. Paula Westerman, a research associate in the ISU Agronomy Department, used data collected in the study to model long-term effects of management and seed predation on weed seedbanks.

“In the absence of seed predation, velvetleaf populations should go down. Giant foxtail, however, can increase if seeds are not consumed by rodents and insects,” Westerman said. “In these fast-growing weed populations, seed predators have tremendous impacts. A 25 percent loss of weed seeds due to predators may be enough to reduce population increases in giant foxtail populations. In other words, predation can reduce the risk of weeds.”

Liebman said he is pleased with the preliminary results, and compares the combinations of weed management tactics and ecological processes to “many little hammers.” He explained: “In sustainable farming systems, individual weed management techniques and ecological processes are usually inadequate for suppressing weed populations. However, combinations of tactics and processes like seed predation can get the job done. We call this approach “many little hammers” to contrast it with the usual “big hammer” approach: heavy reliance on herbicides.”

Leopold Center funds will support a third season of the Marsden Farm crop rotation project. Additional funds have been secured from the USDA-National Research Initiative competitive grants program and the ISU Agronomy Department Endowment.

ISU Extension field economist Craig Chase has calculated costs and returns for the three cropping systems averaged over two seasons.

Herbicide use in the four-year rotation was 78 percent lower than in the two-year rotation, which used herbicides (but not Roundup) at conventional rates. Reductions in herbicide use in the three- and four-year rotations were achieved by the use of banded sprays, rotary hoeing and interrow cultivation in corn and soybean years, and the elimination of herbicides in years when triticale, red clover, and alfalfa were grown.

The four-year rotation used 73 percent less synthetic nitrogen fertilizer than the two-year rotation. The longer rotation relies on nitrogen fixation by alfalfa and application of a low rate of cattle manure (seven tons per acre) prior to corn.

"Despite the management differences, our corn and soybean yields have been essentially the same in all systems," said Matt Liebman, one of the project leaders. Production costs in the three- and four-year rotations were lower by 27 percent and 31 percent, respectively, than the two-year rotation. Labor requirements were 52 percent and 69 percent higher in the three-year and four-year rotations.

Average returns to land, labor and management were greatest in the two-year system at $214 per acre, least in the three-year system at $187 per acre, and $205 per acre in the four-year system.