Herbicide Resistant Pigweed 2: Management in Conventional Row Crops

This video covers the best management practices for Palmer Amaranth and waterhemp in conventional row crops.
Herbicide Resistant Pigweed 2: Management in Conventional Row Crops - Videos

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- [Narrator] Palmer Amaranth and Waterhemp are a growing agronomic problem throughout Pennsylvania as well as many other states throughout the US and are continuing to spread.

They're resistant to mulitple herbicide modes of action including Glyphosate and ALS.

They're also a challenge to control because they germinate throughout the season, they grow quickly, and are highly drop tolerant.

Effective herbicides only manage seedlings under three inches tall and there is no herbicide rescue treatment for them.

So suppression of these weeds requires diligent scouting, prevention, and diversified management.

This Learn Now video will outline the best management practices for herbicide resistant Palmer and waterhemp in conventional row crops.

Before I go into the BMPs, there are a few key points to keep in mind in order to ensure successful management of these weeds.

First, it's key to prevent entry of new weed seeds because the easiest way to control Palmer is to never let it get in your field in the first place.

Apply a zero tolerance policy controlling every plant before it reaches three inches tall using residuals to stop emergence throughout the season and do not permit any plants to produce seed.

Palmer and waterhemp seeds only last about three years in the soil, so if you can stop all existing plants in the field then you can control the problem within a couple of years.

The first best management practice is to monitor seeds purchased, to make sure any seeds entering the farm are certified as clean.

Penn State seed laws are in place to prevent the contamination of crop seed.

Be cautious when bringing in specialty seed mixes such as wild life feed and native seed mixes as they could be potential carriers of pigweed seeds, as well.

For operations using or producing manure avoid spreading manure source from livestock that may have been in contact with Palmer or waterhemp.

Secondly, employ processes that help the crop be more competitive against the weeds.

Herbicide options for herbicide resistant pigweeds are more limited than for many other weeds and, those in Pennsylvania include resistance for both Glyphosate and ALS inhibitor herbicide.

So, using other practices alongside herbicides, will help ensure that zero tolerance policy that we're going for.

Decreasing crop row spacing can decrease weed germination and survival.

Performing nutrient tests helps ensure that the crop gets optimal nutrients for healthy growth early in the season and planting it to weed-free fields allows the crop to get off to a good start, as well.

Maintain this weed-free soil for the first several weeks of crop growth by using residual herbicides.

Planting a winter cover crop and terminating it a couple of weeks before planting the cash crop may also help the cash crop in its early growth by creating a heavy mulch to block out weeds.

If you'd like to learn more about how to do this cover crop management practice, or develop a plan contact your county extension agent to discuss site specific cover crop recommendations.

Three, manage infested fields with no-till, if possible.

No-till makes it so that any potential pigweed seeds are left on, or near the soil surface to be eaten by insects, or dried out by the sun.

Number four, use residual herbicides both pre and post throughout the season to prevent new flushes.

This step is important because Palmer and waterhemp seeds germinate throughout the season.

Number five, apply effective herbicides to small plants, no more than three inches tall.

Herbicide treatments are largely ineffective once these two weeds grow above about six inches tall.

Be sure to use the full recommended herbicide rate in order to optimize control and prevent the development of herbicide resistant populations.

Six, if any Palmer or waterhemp plants survive herbicide control, it's important to physically remove them by hand or with a hoe.

Since pulp plants can re-root carry them out to the field edge and then bury or burn them there.

Although this step is time consuming, it's worth it because it's necessary to prevent the plants from dropping millions of seeds that'll germinate the next year.

In infested fields, plant corn, or a perennial forage instead of beans.

Corn provides a wider range of effective herbicide options and perennial forage allows for mowing and mechanical control options.

Mowing can be a huge help in keeping pigweed populations at bay.

Because mowed plants often regrow it may be necessary to mow several times during the season such as an alfalfa.

Number eight, do not combine harvest areas of fields that are heavily infested with mature pigweeds.

Doing so would allow the transport of the weed seeds by the combine, and by mixing with crop seeds.

If it's still necessary to harvest to heavily infested field, harvesting it last will prevent the spread of pigweed seeds to additional fields via the combine.

Number nine, clean harvest equipment, mowers, and tillage equipment before leaving infested fields to prevent the spread of these seeds to other fields.

For more details on equipment cleaning for weed control can be found in Part Five of this Pigweed Learn Now Series titled Prevention and Proper Disposal.

Number 10, be sure that imported equipment and feed is not contaminated with weed seeds.

Avoid purchasing products from heavily infested regions.

And if purchases are necessary, carefully inspect the seed and clean any equipment thoroughly before use.

Number 11, scout field edges, ditches, and fencerows for Palmer and waterhemp plants especially in the late summer through harvest.

Infestations can sometimes start near barns and barn yards, as well.

Number 12, since each plant can produce up to one million seeds, it's very important to not allow any Palmer or waterhemp plants to mature and drop seed.

We strongly recommend a zero tolerance policy when it comes to these weeds.

When using herbicides, diversifying herbicide modes of action is important for controlling Palmer and waterhemp.

Both weeds have multiple herbicide resistance and have the potential to develop additional resistance relatively quickly.

Hitting the weeds with multiple herbicides modes of action helps ensure total control and reduces the chance of encountering new resistance.

Be sure to include effective modes of action in the burndown in order to plant into weed-free soil.

Include residuals both pre and post to maintain the critical weed-free period of crop growth and to control pigweed seeds that would sprout throughout the season.

Herbicide recommendations for soybean include an effective burndown mix and residuals are key to include as close as possible to the planting date.

Always apply to plants under three inches tall.

If applying liberty in the burndowns, spray under full sunlight with good spray coverage.

Avoid relying on Glyphosate and ALS inhibitor herbicides as many Palmer and waterhemp populations are resistant to both.

In the pre-treatment, residuals are key.

In the post application, Fomesafen safely containing herbicides will residual control or combine residuals with your post to control later emergency seedlings.

Feel free to pause this video here, if you need more time.

For corn, again, plant into a weed-free field using tillage, and/or an effective burndown mix and apply pre-emergence seven to 10 days before planting to maximize residual control.

A range of pre-emergence options for corn that includes both a Group 15 herbicide and atrazine can provide effective control.

Some examples are listed here.

Additionally, herbicide premixes that include HPPD inhibitors, can be successful but do not rely on HPPDs for both pre and post because some Palmer and Amaranth resistance to HDDPs does exist.

Feel free to pause this video here, if you need more time on this slide.

While many producers hope that new herbicides will be developed, that are effective against Palmer and waterhemp, we don't know of any new modes of action currently being developed.

It's been over 20 years since a new and unique herbicide mode of action has been discovered and many resources now go into seed technology not chemicals.

Additionally, if a new mode of action were discovered today, it would take at least 10 years to get it to market.

And Palmer and waterhemp have the ability to develop new resistance fairly quickly.

So this highlights our need to use the herbicides we have judiciously, and to integrate alternative tactics alongside chemical ones to preserve the effectiveness of the herbicides we have now.

To recap, here are a few main take home messages for managing Palmer and waterhemp.

Use caution with all inputs to the farm to prevent new entry of these seeds.

Control them with effective herbicide modes of action before they're three inches tall.

And use residuals because plants emerge throughout the season.

Integrate other tactics, as necessary, such as hand pulling and cultivation to ensure a zero tolerance policy.

And don't permit any plants to produce seed.

Since each plant produces up to a million seeds leaving even one plant in the field could lead to a costly infestation next year.

Palmer Amaranth and waterhemp might be in a field near you.

So developing a proactive management plan now using the recommendations in this video will save time and money later.

Talk to your county extension educator.

They could help you find out if these weeds are in your area.

And help identify a suspect weed or offer further recommendations.

For additional resources on herbicide resistant pigweeds visit our website.

While you're here, be sure to watch our other Learn Now Videos on Herbicide Resistant Pigweed including Weed ID, Proper Prevention and Disposal Practices, and Management in Horticultural and Organic Systems.

Thanks for watching.


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