Winning the Weed War

It’s not just herbicides. For example, a big factor in the battle between crop and weed is light – getting that crop canopy to close as quickly as possible

By Gary Moran

How do you get the most return from your herbicide investment? Pay more attention to agronomy and its role in weed control rather than rely totally on herbicides, says K. Neil Harker, weed scientist at the Agriculture and Agri-Food Canada Research Center at Lacombe, Alberta.

Harker says there are effective weed management techniques that can augment herbicide performance and reduce the need for herbicide applications. One of the major keys is crop health, with robust roots and shoots that will compete with weeds to the full extent of their genetic potential.

A big factor in crop versus weed competition is light – it’s important that the crop canopy closes as quickly as possible to produce an environment for weeds that is as inhospitable as possible. Agronomic practices that favor rapid canopy closure will lower herbicide requirements and favor better herbicide performance.

Careful seeding to produce a uniform stand is one important factor. Gaps or skips in the crop stand create weed “nurseries” that allow unnecessary weed seed production. Putting too much nitrogen in the seed row can also compromise crop health and favor weed proliferation. Seeding methods and rates that rapidly provide dense crop canopies as quickly as possible are crucial.

The best biological control for weeds is more crop seed, Harker says. Weed biomass goes down with a higher seeding rates, but seed costs may be high, so this technique is most feasible in crops with relatively low seed costs.

Combining agronomic practices can dramatically reduce weed infestations, he says. Studies at the Lacombe center are investigating cultivar traits, seeding rate, herbicide rate and crop rotation effects on crop productivity and weed management. Each of these factors had considerable effect on controlling wild oat, but when the practices were combined the results were dramatic, says Harker.

For example, in a study involving a quarter of the recommended herbicide rate, a higher seeding rate for three consecutive years decreased wild oat biomass and seed production by four times, while a tall barley cultivar for three consecutive years decreased wild oat approximately 10-fold compared to a short cultivar. Combining higher seeding rates with the taller cultivar for three consecutive years decreased wild oat biomass and seed production by approximately 40-fold when compared to three consecutive years of short barley at low seeding rates. 

Low herbicide rates are very consistent on their own, but years of good agronomy can set up the potential for growers to successfully use low herbicide rates, Harker says.

“Confusing” weeds
Crop diversity and rotation is another important factor in reducing weed infestations. Harker says that diversity “confuses” weeds. Rotations involving only summer-annual crops with little change in seeding and harvest dates allow build-up of particular weed species. Including a winter annual crop such as winter rye in the rotation will give the crop a head start over wild oats. In many cases wild oat herbicides may not even be needed in winter cereal crops. Perennial forages are especially tough on wild oats.

Early seeding to allow crop emergence ahead of weed emergence can reduce weed problems and minimize crop yield losses. Rotating varieties with different maturity dates can provide the operational diversity needed to “confuse” weeds. In addition, crop variety rotation can also reduce crop disease if the varieties have different disease resistance backgrounds.

Widespread use of herbicide-resistant crops has increased weed control to unprecedented levels, but these levels of control may lead to unintended consequences, Harker cautions. These levels of weed control may lead to weed shifts and weed resistance. Any repeated agricultural practice will favor some weed species over others, but repeated herbicide use also leads to weed resistance to herbicides.

For example, in U.S. soybean growing regions where large amounts of glyphosate are used, glyphosate resistance has been confirmed in horseweed and common ragweed. It is possible that glyphosate-resistant weed species may continue to be selected because of the repeated use of this herbicide, he says.