By Amy Duke
For farmers, weed management is an ongoing battle, especially as the number of herbicide-resistant weeds seems to grow each year. And when weeds invade crops, everyone loses, from the grower, who loses money on damaged crops and herbicides that don't work, to the consumer, who might have to pay higher prices at the grocery store as a result.
Fortunately, growers have important allies in their war on weeds — researchers in Penn State's College of Agricultural Sciences, who are developing strategies to combat weeds while reducing reliance on herbicides.
"At one time, herbicides were considered the end-all in weed control," said William Curran, professor of weed science in the Department of Plant Science. "But weeds can quickly adapt and become more tolerant or resistant to herbicides."
Herbicide resistance isn't the only drawback to their use — herbicides can contribute to environmental pollution, especially to surface water sources. There also is a significant cost associated with their use — farmers spend anywhere from $30 to $100 an acre for herbicides used in growing corn and soybeans, and the herbicides don't always work as well as they should.
"The bottom line — the less we rely on herbicides and the more we can diversify weed control, the better," Curran said.
Weeds have been developing resistance to chemical herbicides — commonly known as weed killers — as far back as 1970, and the problem continues today. More than 350 confirmed instances of herbicide resistance have been reported in 197 weed species globally, according to the International Survey of Herbicide Resistant Weeds, a worldwide group of weed scientists. It's estimated that there are more than 150 types of herbicide-resistant weeds in the U.S. alone.
Curran said some of the most damaging and fast-growing weeds in the Northeast are Palmer amaranth and waterhemp, two species of pigweed. Like most weeds, they compete with crops for nutrients, water and sunlight, can reproduce rapidly, and have few natural controls to keep them at bay, such as herbivores and disease. And after weeds take over, a counterattack sometimes can be futile and expensive.
Along with other faculty and graduate students, Curran has been experimenting with various weed management methods that reduce herbicide use. Two completed studies took place at the Agronomy Farm at Penn State's Russell E. Larson Agricultural Research Center at Rock Springs, focusing on corn and soybean crops. Experiments examined the use of cultural tactics (changing the environment) and mechanical methods (kill or suppress weeds through physical disruption) in a no-till production system.
As for cultural methods, the team planted cover crops of cereal rye before soybean, and hairy vetch or red clover before corn. These cover crops grow fast and blanket the ground, inhibiting the growth of weeds by competing for limited resources such as sunlight. They also protect the soil through the fall and winter and can be used as a weed-suppressive mulch after planting the cash crop, Curran noted.
At planting time, researchers employed mechanical tactics by using a roller crimper to help manage the cover crops and increase weed suppression. The team then used a technique called "zone herbicide application," in which herbicide is applied at the time of planting in a "band" just over the crop row. The scientists then used a high-residue, inter-row cultivator designed for no-till systems to control the weeds between the crop rows.
"This technique differs from a broadcast herbicide application, which basically covers an entire field," Curran said. "Banding or zone herbicide application typically can reduce residual herbicide use by 50 to 75 percent. The high-residue, inter-row cultivator controls the weeds between the crop rows by slicing or cutting the weeds just below the soil surface. The slicing sweeps on the cultivator are designed to travel just below the soil surface to limit the amount of soil disturbance."
Over several planting seasons, the team compared results of cultural and mechanical controls and banded herbicide use to the more standard practice of relying solely on herbicides. Although the results were sometimes mixed, the banded program reduced corn herbicide use by almost 50 percent and soybean herbicide use by 12 percent. This reduction can be smaller or larger depending on the herbicides selected. The results from the first three years of this research were published last year in Weed Science, a peer-reviewed journal.
The experiment is ongoing, with researchers looking at weed control that involves both banding and cultivating, banding and post-emergence herbicide application, and comparing 15-inch- vs. 30-inch-wide soybean rows.
An additional study helped the team determine the optimal timing and frequency of high-residue cultivation in reduced tillage soybean and corn, using cover crops and banded herbicides. This research confirmed the need for two cultivator passes to optimize weed control and prevent yield loss, and it confirmed that banded herbicide application together with high-residue cultivation can achieve results similar to herbicide-only programs. A paper detailing this research was published in the peer-reviewed Agronomy Journal.
While the study outcomes are positive, Curran said these methods require investment in equipment and labor, and that can be an obstacle for some growers.
"To quote a colleague: 'It's hard to sell complexity when simplicity is craved,'" Curran said. "But we must reduce reliance on herbicides. Our studies demonstrate this is achievable, and it's our charge to provide growers with the information and resources they need to make it happen. And when these methods become the norm, benefits will be reaped by growers, consumers and the environment."
Detailed information on weed management — including videos, fact sheets and research reports — can be accessed online at Penn State Extension's website, http://extension.psu.edu/pests/weeds.
Joining Curran in this research were Elina Snyder and Clair Keene, graduate students; Heather Karsten, associate professor of crop production/ecology; Sjoerd Duiker, associate professor of soil management and applied soil physics; Jeffrey Hyde, associate director of programs, Penn State Extension; and Glenna Malcolm, lecturer in the Eberly College of Science.
Funding was provided by the U.S. Department of Agriculture's Northeast Sustainable Agriculture Research and Education program, and Penn State's College of Agricultural Sciences.