The clock is ticking for postemergence weed control strategies in sugar beets

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.   

Recent rains and warm weather coupled with some of the earliest planted sugar beets in Michigan history has started the clock for postemergence weed control. Along with sugar beet emergence, weeds like common lambsquarters and velvetleaf have started to emerge in sugar beet fields. Starting now, timely postemergence herbicide applications will be critical for effective weed control programs, particularly for growers who will be using micro-rates. Micro-rate herbicide applications can be a cost effective strategy for weed control in sugar beets, but again timing is everything. The first micro-rate application should take place when weeds are less than 1/8-inch tall (cotyledon stage). If weeds get larger than 1/4-inch, micro-rate applications are less effective and standard herbicide rates should be used.

Timing of first POST herbicide applications

Whether a grower chooses to use micro-rates or standard-split herbicide applications, scouting will be an important step in determining when this first herbicide application should be made. For a micro-rate program, the first micro-rate herbicide application should be made when the weeds are less than 1/8-inch tall. This normally occurs between 225 and 275 growing degree days, base temperature 34°F (GDD) after the last weed control measure was made (tillage or herbicide application at or around the time of planting). In some cases if sugar beets were planted during the last week of March or the first week of April we have accumulated anywhere from 160 to 300 GDD with the majority of the accumulated GDD occurring during the last two weeks. In some areas where beets were planted early there were hard frosts and the GDD clock may actually start later, because the frost would have controlled some of the early emerging weeds. Under these conditions you may want to start accumulating GDD at the time of that frost and then start scouting at 200 GDD.

For standard-split herbicide applications, the first application should be made when the weeds are 1/2-inch tall which will generally occur between 350 and 400 GDD. This is the time when growers should start scouting for that first standard-split application.

What if a preemergence (PRE) herbicide was used?

If a PRE herbicide was used in the field this could also impact the timing on when the first postemergence herbicide application needs to be made. There are several factors that may influence this timing including: method of application (banded or broadcast), susceptibility of the weeds to the PRE product or products that were used, and whether the grower is planning on using cultivation for weed control. All of these factors need to be accounted for when making that first postemergence application. For example, if a PRE herbicide was banded and cultivation will be used for between the row weed control and there are no weeds emerging in the band, the POST herbicide application can be delayed.

Taking all of this information into consideration after the first micro-rate herbicide application is made, there are some general guidelines for timing micro-rate herbicide applications using GDD. These recommendations are listed below.

General recommendations for GDD micro-rate applications

After the first micro-rate application is made, MSU research has shown applying micro-rates on a 225 GDD schedule has shown the most consistent weed control across species while reducing the potential for sugar beet injury. However, in fields with sandy or darker soils (high organic matter) and high weed pressures, it is suggested to check fields at 150 GDD and time applications for 175 GDD. For growers who want to adjust the timing of micro-rates relative to weed emergence, it may be possible to lengthen the spray interval early in the season (April) to 275 GDD when common lambsquarters is the predominant weed. In May this interval should be shortened to 225 GDD until pigweeds (redroot pigweed and Powell amaranth) start to emerge, then micro-rate applications should be made every 175 to 200 GDD. If timing is missed, it is important to scout to determine the size of the weeds out in the field and reevaluate what the next weed management step should be.

Where do I get GDD calculations?

There are several good calculators out there to help calculate GDD for scouting and for micro-rate herbicide applications. Michigan Sugar Company through the use of BeetCAST ( and the Michigan Automated Weather Network or MAWN ( are two websites where there are excellent calculators to help determine GDD in the different sugar beet growing areas or you may choose to calculate your own.

GDD Formula = (High Temp + Low Temp)/2 - 34°F

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