ISSUE #469

469 - Using GDD for Herbicide Timing

4-29-05

Timing of herbicide applications to maximize weed control and limit costs is always a challenge. Uncontrolled weeds dramatically reduce yields and on-farm profit. Weeds are perennially listed as growers most serious production problems. Feedback from growers, agriculturists and consultants in 2004 indicated using Growing Degree Days (GDD) as a tool to time herbicide applications was quite successful.

How Are GDD's Used?

Research at NDSU, the U of MN, and Michigan State University established the relationship of temperature to sugarbeet growth stage. Each American Crystal Sugar Company agriculturist tracked sugarbeet growth stage versus GDD in a cooperative project with Alan Dexter in 2004. This field data strongly supported the relationship between GDD and sugarbeet growth stage developed by Dr. Dexter, Extension Sugarbeet Weed Specialist and Dr. Carlyle Holen, U of MN Extension Agronomist at Crookston in their 1990's research.

Postemergence herbicide combinations for weed control in sugarbeet generally have been applied three to five times at a seven-day interval. A seven-day interval works well in "average" conditions but an interval longer than seven days may be more cost effective in cool weather when plant growth is slow and an interval shorter than seven days may provide better weed control in a warm environment when plant growth is rapid. GDD can be used to adjust the intervals between POST herbicide applications to improve herbicide efficiency.

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Guidelines for Utilizing GDD for Timing Post Sugarbeet Herbicides

The first POST herbicide treatment should be applied when weeds are in the cotyledon to early two-leaf growth stage. The number of GDD between applications can be adjusted for various situations.

Examples of recommended GDD intervals:

175 GDD

  • Utilize between applications of the micro-rate with dry soil conditions and/or high weed densities.
  • ALS - resistant kochia control with the micro-rate may be less than adequate even when the 175 GDD interval is used.
  • Use intervals greater than 175 GDD for POST¬†conventional¬†rates (except for a bad kochia problem) and for fields treated with a soil-applied herbicide.

225 GDD

  • Utilize between applications of conventional rate treatments with average soil moisture.
  • Use 225 GDD between micro-rate applications with good to excellent soil moisture or micro-rate applications over a soil-applied herbicide treatment for an average weed population.

275 GDD

  • Utilize between applications for conventional rate treatments with good to excellent soil moisture.
  • Use 275 GDD for the micro-rate over a soil-applied herbicide with weeds that are not difficult to control.
  • Do not use 275 GDD for kochia control.

Avoid applying POST sugarbeet herbicides the day before a frost. Herbicides applied 12 to 24 hours after the frost will cause less sugarbeet injury than herbicides applied 12 to 24 hours before the frost. Sugarbeet injury from POST conventional herbicides can be reduced by starting application at 4:00 pm or later. Severe sugarbeet injury is most likely immediately after a weather change from cool and cloudy to hot and sunny. Conventional rates do cause more sugarbeet injury than micro-rates and fields treated with a soil-applied herbicide may have more injury from POST herbicides.

GDD Information on the Internet

Dan Bernhardson and Al Cattanach from American Crystal met with Alan Dexter and John Enz from NDSU to discuss improvements to the NDAWN site for GDD information and applying herbicides. The new GDD application will allow grower users to input spray dates for up to 10 fields and get their accumulated GDD since the last application. The site will store planting dates or spray dates for each individual grower and display them the next time he logs on to the site. The grower can refer to "GDD Guidelines for Timing Herbicide Applications," to plan his herbicide program at anytime.

Contact your agriculturist, crop consultant or university specialist for answers to weed control problems.

Thank You

A special thanks to Alan Dexter, John Enz and Dan Bernhardson for contributing assistance to this Ag Notes.