Controlling weeds from the combine cab

By Jared Goplen, Extension Educators - Crops

October 2018

CombineIt is too late to control weeds this year, but the combine cab is a great opportunity to start managing weeds for next year. As you complete harvest this fall, think of which aspects of your weed management plan can be improved for next year, and think carefully about combining through weed patches. There are a few things you should pay special attention to.

1. Don't combine through weed patches

Combines make great weed seeders. Resist the temptation to combine through weed patches, which is a surefire way for patches to expand. Weed patches are much easier to control compared to whole fields, especially if weeds are herbicide-resistant. Herbicide-resistant weeds often start in patches and are spread throughout fields and across farms via equipment. Prevent this by combining around weed patches and waiting to harvest them until last, just before you clean out the combine. If you have whole fields that are weedy, consider harvesting those fields last to prevent spreading those weed seeds to other, less weedy fields. Keep in mind that an uncleaned combine can harbor over 150 pounds of dust, debris, and weed seeds. Simple cleanout techniques between fields can go a long way in preventing weed seed movement.

In Australia, where herbicide resistant weeds have become increasingly difficult to manage, harvest weed seed control strategies have been adopted to reduce movement and proliferation of weeds. Harvest weed seed control strategies include practices such as windrow burning, chaff carts, and the Harrington Seed Destructor, which is a cage mill that grinds up all of the chaff and weed seeds before spreading it back onto the field. Although these strategies cost time and money, they have been proven to be effective, and may be necessary if herbicide resistant weeds continue to spread in the Midwest.  

2. Take notes on weed escapes

Pay attention to which weeds escaped control this year. In recent years, waterhemp and ragweed have been some of the most common weed escapes in western Minnesota. Evaluate if weed escapes are throughout the field or in patches. If they are in patches, think back to whether or not there were issues with the herbicide application equipment such as plugged nozzles or faulty control valves. It is also possible that timing of the herbicide application was not ideal. Giant and common ragweed come up early in the growing season, before mid-June, while waterhemp is a very late-emerging weed that can emerge into August. If herbicide applications were made too early, you may have missed waterhemp, whereas if they were too late, the ragweed may have been too large for herbicides to be effective. Also think back to whether or not preemergence and residual herbicides were applied in a timely manner and activated by rainfall in time to be effective.

If herbicide applications were timely, it is also possible the escaped weeds are resistant to herbicides. Giant ragweed, common ragweed, and waterhemp have developed resistance to glyphosate (Roundup, etc.) and ALS inhibitors (FirstRate, Pursuit, etc.) in Minnesota, while waterhemp also has resistance to PPO inhibitors (Cobra, Flexstar, etc.). If resistance is suspected in your fields, a genetic test for some types of herbicide resistance is available from the University of Illinois. Consider sending a sample in as a cheap way to find out which herbicides will or will not be effective next year. 

3. Map the weed populations

One of the best ways to track weed control over time is to produce a weed map of your fields from year to year, noting which species are present and where (Figure 1). With precision ag technology, have the combine operator note where weed patches are and which species are present. To go low-tech, just keep a notebook and mark which areas of fields are affected. Having this record not only allows you to catch resistant populations sooner, but can also help save money on herbicide application costs if weed problems are confined to a specific area. If heavy weed infestations are present only in some areas, such as stressed areas of the field or headlands, perhaps a spot application on those areas should be considered for next year.

Make good use of your time in the combine this fall by combing around weed patches and thinking of how next year’s weed control can be improved. Even simple notes on weed control performance and distribution can help improve control in future years. Come next year you will be happy you took the time.

Weed map

Example of simple weed map. Maps can be produced using precision ag technology or just with a simple drawing in a field record notebook. Both types of weed maps are helpful in making future weed control decisions.


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