Effective weed control is the goal of any herbicide application. Below are key questions to consider for successful weed control in a cereal crop.

What crop are you growing?

Cereal crops wheat, barley and oats vary in their tolerance to herbicides. Winter wheat is the most tolerant to herbicides followed by spring wheat then barley and oats are the least tolerant.1 Herbicide options decline as crop tolerance declines. There are no registered herbicides for grassy weed control in oats as these herbicides would cause unacceptable crop injury. Adjuvant options can also change by crop due to crop tolerance. When selecting a herbicide for weed control be sure to check which crops are registered and follow the label for tank-mixes and additives.

Consider past weed control.

Were there patches that looked like poor weed control? Herbicide resistance often looks like patches of poor weed control within a field. Risk of herbicide resistance is higher if the same herbicide mode of action is applied multiple years in a row and where a single effective mode of action is used against a weed species. If some of the patches look like they have consistently poor control, consider collecting and submitting weed seeds for herbicide resistance testing; acknowledging the presence of herbicide resistant weeds will help you select effective herbicides in the future.

Herbicide resistance often appears as irregularly shaped patches of uncontrolled weeds
Figure 1. Herbicide resistance often appears as irregularly shaped patches of uncontrolled weeds.

Typically, herbicide resistance is tested by growing out and spraying seeds of the plants that escaped the herbicide treatment with that same herbicide. The easiest way to collect seeds for testing is to wait until these plants mature in the field in late summer or fall. If you are planning to collect seeds to test, do not apply a pre harvest glyphosate as this could prevent the seeds from properly germinating and being tested. Seeds submitted should be mature and dry and clean of chaff. Sampling and testing in this way will not help change the results of the current growing season but can help inform future herbicide decisions. If you were unable to collect samples last season but would like to start planning for the next cereal crop, recall where weed escapes were and consider allowing some of these weeds to grow. The best method to achieve this is by tarping the area during herbicide application. This way seeds can be sampled and tested during a non-cereal year to help guide herbicide decisions in the next cereal crop. Keep in mind that resistance testing is limited by which herbicides are used to do the test. Weeds may be resistant to a herbicide within a group but may not be resistant to other herbicide families in the same group.4

Reach out to your local Bayer representative or retail agronomist for help with detecting herbicide resistance.

Which grassy weeds are present?

Group 1 herbicides are a good option for grassy weed control in wheat and barley crops except in fields where Group 1 herbicide resistance is a concern. In that case, group 2 herbicides can be an effective weed control option. Be sure to match the herbicide to the weeds present in the field. For example, Varro® herbicide can effectively control barnyard grass, canary seed, green foxtail, and wild oats with suppression on Persian darnel and yellow foxtail.2

Are perennial weeds present?

There may be an option to tank-mix or use a pre-mix to control perennial weeds with your grassy weed control. Perennial weeds can regrow from the root system; therefore, controlling the top growth with a contact herbicide does not kill these weeds.5 Systemic herbicides that are translocated to the root system are best for perennial weed control.

Re-cropping restrictions

Another consideration when selecting a herbicide for weed control are the re-cropping restrictions. Check the label. Some herbicides can control weeds without re-cropping restrictions. However, some herbicides have a residual aspect. This can help with prolonging the control of a weed but can sometimes affect a sensitive crop in the subsequent year. Be sure to keep good records of herbicides applied for reference the following year. Plan ahead. If sensitive crops are in the rotation, consider the past year’s moisture and be sure to allow the full re-cropping interval before planting to these crops.

Consider annual broadleaf control.

There are many options for annual broadleaf weed control in cereal crops. Some of these options must be applied prior to crop emergence (herbicides from Group 3, 9, 14, or 15). In spring, after crop emergence, herbicide options for broadleaf weed control are from Groups 2, 4, 5, 6, and 27. Tank-mixes or pre-mixes can combine multiple effective modes of action for enhanced weed control and resistance management. Controlling broadleaves in your cereal crop can help reduce the seed bank and future weed pressure.

Generally, herbicides are most effective on young, actively growing weeds. For tough-to-control weeds, or weeds that are nearly beyond the staging where they can be controlled, there may be an option to tank-mix with an additional mode of action. For example, Infinity® herbicide (bromoxynil and pyrasulfotole) can control small cleavers (one to three whorls) but by using Infinity® FX herbicide (which includes a third active, Group 4 fluroxypyr) cleavers can now be controlled up to the nine-whorl stage.2 This recommendation may not apply for herbicide-resistant biotypes. Be sure to use registered or supported tank-mixes, recommended water volumes and follow the label for application guidelines. Note that in high temperatures additional adjuvants may injure the crop.

It is important to keep in mind that antagonism can occur between herbicides in a tank-mix. One common result of antagonism is reduced weed control with the addition of an incompatible herbicide. If the two products being applied would cause antagonism when mixed together, the products can be applied three to four days after each other as an option to prevent antagonism to grassy weed control.6 Often, a more realistic approach is to use a registered pre-mix where antagonism is managed with the formulation. When tank-mixing, always follow the label to select the appropriate tank-mix partner and adjuvant requirements. Bayer provides a list of appropriate tank-mix partners for each Bayer herbicide and additional instructions for best results. The tank-mix list is located at https://www.cropscience.bayer.ca/en/Grower-Tools/Tank-Mix.

What does AMS do when added to Bayer herbicides in cereal crops?

Ammonium sulphate 40% solution can be added to Bayer herbicides at a rate of 0.5 litres per acre in order to provide improved and more consistent weed control, as can be seen in cooler growing conditions. If using AMS with a different concentration, adjust the rate accordingly.

In Varro® herbicide and Velocity m3 herbicide, AMS can increase control of wild oat infestations or when managing Japanese brome and yellow foxtail. A non-ionic surfactant can be used for the same benefits in durum wheat where AMS is not recommended. AMS can also help mitigate a loss of control if there are cool temp or overnight low temperatures.

In Tundra® herbicide and Infinity® herbicide, AMS can help control larger cleavers (four- to six-whorl growth stage), Canada fleabane, volunteer soybean and round-leaved mallow as well as provide suppression of giant ragweed and spreading atriplex. In Infinity® FX herbicide AMS can help control Canada fleabane, larger soybean (up to the ninth trifoliate) as well as suppression of giant ragweed and spreading atriplex.2