A University of Missouri weed science Powerpoint presentation posted on the web brings forward a lot of interesting thoughts about fall herbicide applications to crop fields, especially for Missouri and the Southern Corn Belt.

It is noted that winter annual weed prevalence in the Southern Corn Belt appears to be greater these days because there was such a long-time of Roundup Ready soybean glyphosate dominated production, fewer residual herbicide applications and relatively mild winters, according to the researchers who compiled the presentation.

Some common claims about fall herbicide applications include:

  • Good time to control winter annuals and perennials.
  • Larger winter annuals need higher rates of burndown herbicides in spring and could interfere with planting.
  • Clean fields are less likely to attract black cutworm moths and other insects.
  • Winter annual weeds take nutrients away and can compete with the emerging crop.
  • Weeds keep the ground wet, which slows planting.
  • Warmer soils lead to quicker crop planting.

University research has clarified some claims. The presence of winter annual weeds caused significant reductions in soil moisture in all corn and soybean experiments. Winter annual weed removal increased soil temperatures when above 50°F in corn experiments and 68°F in soybean experiments.

Winter annual weeds apparently act as alternative hosts for corn flea beetle and some lepidopteran insects, according to researchers.

“We did not observe black cutworm damage in any corn trial, but the black cutworm-winter annual weed interaction is a known relationship that should be considered when thinking about fall herbicide applications,” it is noted in the Powerpoint presentation.

Removal of winter annual weeds decreased total insect and negro bug populations after planting in soybean experiments.

The impact of fall herbicide applications on other factors within the “agroecosystem” should not be overlooked, and growers have to determine the value to them of all the agronomic factors.

“Our data indicate that early spring applications of residual herbicides provide better control of emerging summer annual weed seedlings than fall herbicide applications. But ... how many years will we be able to get on the land and do that?,” is one question posed since spring weather dictates some field operations every year.

When looking at what might have to be done to control weeds in the spring, the researchers question making a fall application as perhaps not being needed every year but complaining that when it isn’t needed is an unknown because of weather. “Ultimately, are we adding a cost to the weed management system or eliminating one?”