Micro-rate herbicide applications need to be timely 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 temperatures have contributed to good sugar beet emergence in many areas of the state. However, along with the sugar beets has come excellent emergence of several weeds. Growers who are planning to use micro-rate herbicide applications for weed control, should have made their first micro-rate application for many of the beets planted on or before April 21 in most areas of the state. Rain and wet conditions have caused many of these applications to be delayed. Missed timings early in the season can cause major weed problems as the season progresses. So, what are some of the options that can be done to control these weeds if a micro-rate application timing is missed? Below are some general guidelines that should be considered in choosing alternative weed control strategies if a micro-rate application timing is missed before the weeds become too large and require hand labor.

Factors to consider

Know the size of the weeds in the field. Your decision on what to do next for weed control is primarily going to be based on weed and sugar beet size. Weed species like common lambsquarters and velvetleaf are going to be the primary driver weeds this time of year. Five different weed control strategies are listed in this article that are dependent on weed size. See these different options below for each weed size (Option A, 1/8 inch tall weeds; Option B, Option C, or Option D, 1/4 inch tall weeds; and Option E, 1/2 inch tall weeds).

Was a pre-emergence (PRE) herbicide used on this field?
If a PRE herbicide was used, this could impact the susceptibility of the weeds, as well as the potential susceptibility of sugar beets to increased injury. This factor should be considered when deciding on what weed control strategy to use. With cool-wet conditions, both weeds and sugar beets will be more susceptible to POST herbicides, particularly if there was a PRE herbicide applied. Keep this in mind when applying higher Betamix rates or when switching to a standard split herbicide program. Also, if a PRE herbicide was banded, check to see if there are any weeds emerging in the band. If weed control is good in the band, cultivation may be all that is needed.


Option A: Weeds are 1/8 inch tall.Even if the maximum number of GDDs has passed for your micro-rate application schedule (generally around 225 GDD34), and weeds are still less than an 1/8 inch (0.125 inch) tall, continue using a standard micro-rate herbicide program. This may be the case with some of the slower growing early summer annual weeds.

Option B: Weeds are 1/4 inch tall.
One option when weeds exceed the maximum size for a standard micro-rate herbicide application is to increase the rate of Betamix to 12 fl oz/A or 16 fl oz/A in the micro-rate application. According to the label, the rate of Betamix at 16 fl oz/A should only be increased if the smallest beets in the field are in the 4-true leaf stage. Increasing the Betamix rate may help control slightly larger weeds. Applications can be made later in the day to help reduce sugar beet injury. Also use caution when using higher Betamix rates if sugar beets are in the early 2-leaf stage. Sugar beets are more sensitive to stresses at this stage of growth.

Option C: Weeds are 1/4 inch tall.
Another option is to make the standard micro-rate herbicide application when conditions allow and then shorten time period before the next application. For example, if the second micro-rate application timing is delayed, you may want to decrease the amount of time between the second and the third application. Larger weeds that are injured may be more susceptible to the third micro-rate application and won’t have completely recovered. Remember, sugar beets will also be more susceptible and that could potentially lead to an increase in sugar beet injury or stunting, especially if conditions are cool and wet. According to research we conducted last year, we were able to achieve better common lambsquarters control with this strategy compared with simply increasing the Betamix rate. In our research, we shortened the interval to approximately 75 GDD, which was approximately 5 days between the applications early in the season.

Option D: Weeds are 1/4 inch tall.
Yet another option when the micro-rate herbicide application timing has been missed, is to switch to a standard split low rate herbicide application. If you choose to switch to a standard split herbicide application, do not add MSO or surfactant to this application. Applications should also be made after 4:00 PM. to reduce sugar beet injury. If weed control is adequate after the standard split application, growers can switch back to a micro-rate herbicide program (start GDD accumulation after the standard split application) or continue with a another standard split rate later in the season. This strategy was also more effective than simply increasing the Betamix rate for common lambsquarters control.

Option E: Weeds are 1/2 inch tall.
If weeds reach 1/2 inch tall, it is best to plan on a standard split rate herbicide program. Apply the first split rate when sugar beets are less than the 4-true leaf stage. Weeds that are not completely controlled by this application will be controlled by the second application. The second application should be made 7 to 14 days after the first application. In most cases this is generally between 350 and 400 GDD. Applications of standard split rates should be made after 4:00 PM to reduce sugar beet injury.

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