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Reduce Inputs With Whole-System Weed Control

Randy Anderson has studied weeds and their elusive ways since his beginning years as a researcher back in the early 1980s.

“I was looking then – as I am now – for alternative control methods for weeds that would let growers use fewer herbicides,” says Anderson, an agronomist at the USDA Agricultural Research Service North Central Agricultural Research Laboratory in Brookings, South Dakota. “That work led me to the study of population dynamics in weeds.”

From more than 30 years of watching weeds, Anderson has learned a critical lesson: Weed populations have the tenacity to evolve, shapeshifting to changing conditions imposed by humans. Weeds are most adept at adapting when varying conditions occur in predictable patterns.

“The most notable example I’ve seen over the years is the development of herbicide-resistant weeds,” says Anderson. “Despite use of herbicides, we haven’t achieved control. Weeds are still there all the time.”

Weeds are getting harder to control not only because of an increasing resistance to herbicides, but also because the hardiest species are survivalists. “We’ve eliminated the easy-to-control weeds, while our practices have served to select for the more difficult-to-control weeds like Palmer amaranth and jointed goatgrass, for example,” he says.

Yet, weeds have their limitations, and exploiting these in a synergistic, whole-system approach offers potential for significant control. “I believe we can get to the point in the future where we can control weeds using a system of herbicide-free no-till,” says Anderson.

Whole-system weed management is a multitactic approach comprising complex rotations, no-till, cover crops, synergism, and soil health.

“A single cultural tactic has minimal impact because the pest population is elastic and flexible. It can easily adjust to one tactic,” says Anderson. “If several tactics are used in different ways to suppress weed-population growth, for instance, weeds have trouble adapting. Thus, weed density declines over time.”

A decline in weed populations offers a savings in input costs because of reduced use of herbicide. “An economic assessment of farmers in northeastern Colorado showed that producers using a multitactic approach to weed control were spending half of what conventional winter wheat/fallow farmers were spending for weed control,” says Anderson.

Five Cornerstones

Whole-system weed management incorporates five cornerstones.

1. A broader perspective on managing weeds. “Consider that weeds are a natural part of your farming system  and focus on managing them rather than eliminating them,” he says. “For instance, weeds can add diversity to the plant community and favor beneficial insects. We tend to see weeds as an enemy, thus, emphasizing elimination rather than management. This elimination approach leads to resistant weeds.”

2. Improved soil health. When soil is healthy, it tends to produce crops that compete well with weeds. “Research shows that when you improve the health of the soil, the crop is more tolerant of weeds,” says Anderson. “Crops grown in healthy soil have healthier seedlings, which access more nutrients and moisture earlier. Healthier crop seedlings can outgrow weeds.”

Weeds are bound to come increasingly in check as you continue working to improve the health of your soils. “I’m noticing differences between past and present producers and how they view their soil,” he says. “When I started my career as a researcher, few farmers talked about soil health and what a weed could contribute.”

3. Systems complexity. Weed growth represents the first stage in the ongoing natural process of succession in plant life. “When the glaciers moved back, the first plants that moved in were weeds,” says Anderson.

As farmers shape production systems with increasingly diverse and dynamic components, they create the systems complexity that shifts the plant and soil-life communities further along the continuum of succession.

By contrast, says Anderson, “When we provide continuity of simple practices, we select for a certain plant community, and we give weed populations a chance to explode because they can adapt. The more complex the overall cropping system is, the easier it will be to control weeds and other pests.”

4. Crop diversity. “A systems approach would increase plant diversity in the rotation,” says Anderson. “All these crops would have different planting dates and differing growth periods, making it harder for weeds to adapt.”

One rotation design that is effective in fostering dynamic pest suppression is a rotation arranged in a cycle of four, with two cool-season crops followed by two warm-season crops.

“Diversifying crops with different planting dates within a life-cycle category, such as warm-season crops, for instance, accentuates the benefit gained with rotations comprised of two-year intervals of cool- and warm-season crops,” says Anderson.

A warm-season sequence comprising corn and sunflowers, for instance, can be more beneficial than a warm-season sequence of corn followed by corn. “The key is having crops with planting dates differing by at least three weeks,” he says. “For example, corn is usually planted in early May, while proso millet and sunflower are usually planted in early June.”

Adding cover crops and legumes increases the complexity needed to further confuse weeds.

5. No-till. Zero tillage reduces or eliminates the soil disturbance that encourages germination of weed seeds.

“Tillage buries weed seed in the soil, which increases long-term survival of the seeds,” says Anderson. “Weed seeds die rapidly if left on the soil surface, where they are exposed to extremes in temperature. On the surface, they also experience alternate periods of wetting and drying. This process starts splitting the seed coat, causing the seed to deteriorate. Lying on the surface, the seeds are exposed, too, to predation by insects, birds, and mammals.”

When no-till is used in conjunction with a diverse cropping system, weed control is improved. 

“Synergism between the two practices strengthens the cropping system,” he says.

As whole-system cropping and weed management continue to evolve, Anderson sees a future where weeds play a balanced role.

“If we took more of a systems approach, we wouldn’t spend so much time trying to eliminate that one aspect of nature that weeds represent,” he says. “Instead, we could invest more time and energy in simply growing more diverse crop communities.”

Learn More

Contact Randy Anderson at 605/693-5229 or randy.anderson@ars.usda.gov.

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