Take a Proactive Approach to Managing Palmer Amaranth in Minnesota Crop Production Fields

By Jeff Gunsolus

Early detection and eradication of Palmer amaranth will pay you dividends in reduced management costs. Differentiating Palmer amaranth from the other common amaranth species is challenging. However, because Palmer amaranth and tall waterhemp are biologically similar, you can approach this challenge with the weed management tactics that you would use for effective tall waterhemp control.

Steps to successful management of Palmer amaranth and tall waterhemp:

1. Identify and report Palmer amaranth to ensure proper management.
2. Determine if they are herbicide-resistant.
3. Prevent seed production.
4. Use residual herbicides in both corn and soybean for effective control. This is essential.

Identification and reporting process for Palmer amaranth

Prevention of Palmer amaranth infestations is the goal of the partnership between the U of MN Extension, Minnesota Department of Agriculture and the farmer and crop consultant relationship. Early detection and eradication of Palmer amaranth will reduce farmer management costs, help to determine primary means of seed movement and prevent the rapid spread of this challenging weed.

Are the Palmer amaranth plants found in Minnesota expressing herbicide resistance?

In the six counties where Palmer amaranth has been detected only plants from Jackson and Redwood counties tested positive for glyphosate resistance (Group 9). None of the populations tested positive for PPO resistance (Group 14 – e.g. Flexstar and Cobra).

Tall waterhemp populations are more likely to express resistance to multiple herbicides. Figure 1 shows the results of PPO and glyphosate resistance testing for 86 plants from west central and southern Minnesota. The largest portion of tall waterhemp plants was resistant to both PPO and glyphosate herbicides (43 percent), reinforcing the idea that this is a serious and prevalent issue.

In addition, the majority of tall waterhemp populations are historically resistant to group 2 herbicides (ALS – e.g. Pursuit).

Do not let these plants go to seed

Plants can adapt quickly to herbicides

Palmer amaranth and tall waterhemp resistance to various groups of herbicides (e.g. glyphosate –group 9, Pursuit – group 2 and Flexstar – group 14) brings uncertainty about which herbicides will be most effective in any given field. Molecular tests for group 9 and 14 herbicides are readily available through the University of Illinois Plant Clinic for $50 per sample.

Pollen can transfer herbicide resistance traits

Pollen from these weeds has been shown to transfer herbicide resistant traits to nearby plants of the same species. Therefore your weed management goals should not only focus on reducing weed impact on crop yield but just as importantly, to reduce weed seed production that increases the amount of seed in the soil seed bank.

Plants produce large amounts of seed

Palmer amaranth and tall waterhemp are capable of producing hundreds of thousands of small seeds that can be easily transported via field equipment, water, wildlife and manure. Fortunately if Palmer amaranth and tall waterhemp seed production is eliminated, seed already present in the soil can be reduced to much more manageable levels in just four to five years. The bottom line is, annual herbicide costs can easily double and multiple passes through a field will be necessary once either of these weed species becomes established. Setting a goal of zero weed seed production for the next four to five years can reap long term dividends. Prevention of establishment is still the best option.