U. Minn. Offers guidance for pre-emergent herbicide application


Post-emergence herbicide supply issues creating challenges for 2022 and possibly into 2023, the use of a residual activity pre-emergent herbicide as part of a weed management program herbs is more important than ever. Pre-emergence herbicides can help control troublesome weeds such as hemp waterhemp and giant ragweed. They can also result in lower weed densities and more uniform weed height at the time of postemergence herbicide application, aid in the management of herbicide resistant weeds, and ultimately help to protect yield potential and crop profitability. What if you want to plant a cover crop later in the season? Will a residual herbicide that controls weeds also affect your cover crop?

Determine your cover crop goals

If you are planting a cover crop for food or forage (for example, to graze), you must follow any restrictions listed on the herbicide label because the cover crop will enter the human food chain and animal. Most herbicide labels do not indicate the waiting time between application and when a cover crop can be grazed or harvested for forage. In these cases, you must follow the rotation restriction indicated on the label. This restriction is the amount of time you must wait after applying the herbicide before you can seed the cover crop. If the cover crop you wish to plant is not listed, you should follow the most restrictive rotation interval. Some products have extremely long rotation restrictions for certain cover crops (eg, 30 months), so these products may not be suitable in fields where you want to plant a cover crop for food or forage.

On the other hand, if you are planting a cover crop for soil health reasons and the cover crop will not enter the food or feed chain, you have more flexibility. In this case, you can plant the cover crop whenever you want, but you assume all risk of injury or cover crop failure.

Considerations When Using a Cover Crop for Soil Health

1. Cover crop species

Research shows that in general, cereal rye, wheat and oats tend to tolerate many of the herbicides commonly used in a corn/soybean system. If you are planting a cover crop mix, keep in mind that broadleaf herbicides tend to pose more risk to broadleaf cover crops, and similarly to herbicides against grasses and grass crops. grass cover.

2. Time between herbicide application and cover crop seeding

In general, the longer the time between herbicide application and seeding of the cover crop, the less chance there is of damage to the cover crop. For example, a cover crop planted in the fall is less likely to be affected by a herbicide applied in the spring than a cover crop intercropped with the cash crop shortly after application.

3. Characteristics of the herbicide

Although a long residual herbicide may be attractive from a weed control perspective, this characteristic may present a higher risk for a cover crop seeded after application. A herbicide with a long half-life – the time it takes for a pesticide to break down to 50% of the original amount – may pose a greater risk to a cover crop sown after application, but only whether the product has soil activity (ie the herbicide can be taken up by plants after application).

4. Soil conditions

Depending on the herbicide, extremes in soil pH or soil organic matter can influence how long a herbicide remains active in the soil, and therefore the potential to affect a planted cover crop after application. For example, atrazine and chlorimuron (ai in Classic) do not break down easily in soils above pH 6.8, while imazethapyr (ai in Pursuit) is more likely to cause damage to a sensitive crop in soils with a pH below 6.5. .

5. Weather conditions

Microbial activity is a determining factor in the degradation of most herbicides. Accordingly, conditions that favor microbial activity, such as warm conditions and adequate humidity, tend to favor herbicide degradation. On the other hand, extreme conditions (eg drought, cold/freezing temperatures) inhibit microbial activity. These conditions can increase a product’s longevity in the soil and the potential to injure a susceptible cover crop planted after application.

MN Search Results

Research in several states has shown that herbicides can impact the establishment and growth of cover crops. To help address this concern in Minnesota, the University of Minnesota conducted research on the trade-offs in weed control when cover crops were integrated into a corn/soybean system at 3 sites (Waseca, Rochester and Rosemount) from 2019 to 2021.

Since water hemp is one of the main weed problems for many farmers in the state, herbicides with residual activity on water hemp were selected. Resicore (3 qts/ac), Verdict (18 oz/ac), Outlook (21 oz/ac) and Outlook (16 oz/ac) fb Outlook (8 oz/ac) 30 days later, were applied in the spring, and compared to a control with no residual herbicide. The cover crops evaluated (red clover @ 12 lb/ac, camelina @ 10 lb/ac and cereal rye @ 60 lb/ac) were seeded by drill in the fall following corn silage harvest. Red clover establishment was poor in all treatments in both years, so results are only shown for camelina and cereal rye.

In these trials, the herbicides evaluated had no impact on the biomass of cereal rye and camelina in the spring (Figures 1 and 2). This is encouraging because it shows the potential for using effective residual herbicides on water hemp, even when planning to seed a cover crop later in the season. It is important to note, however, that these results are based on only 6 site-years of data. We were unable to assess all possible scenarios of weather and soil conditions and there is a risk of poor or no establishment if herbicide label warnings or restrictions are not followed. Individual results should vary based on the factors listed above.

Figure 1. Average spring biomass of a fall-seeded camelina cover crop at three locations in southern Minnesota over two years (2020-2021), impacted by the pre-emergence program used in the spring prior to seeding. No difference was detected between treatments (p = 0.05).



Figure 2. Average spring biomass of a fall-seeded winter rye cover crop at three locations in southern Minnesota over two years (2020-2021), as affected by the pre-emergence program used in the spring before sowing. No difference was detected between treatments (p = 0.05).

Weigh the risks and trade-offs

Planting a more tolerant cover crop such as cereal rye in the fall poses less of a risk to establishment when a residual herbicide is used in the spring, compared to interplanting a cover crop soon after the fall. application in spring or at planting of a mixture containing potentially susceptible species.

Keep in mind that using a reduced rate of a herbicide is NOT recommended. Reduced rates can result in reduced weed control while increasing the selection of herbicide resistant weeds, and you could still damage a susceptible cover crop.

Cover crops can provide many soil health benefits and be a source of feed for livestock. Using a robust weed management program that tackles your biggest weed problems, even when planting a cover crop, will contribute to the long-term success of your cropping system. .

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