Palmer amaranth in manure: what can you do?

By Chryseis Modderman, Extension Educator

September 2019

Palmer amaranthPalmer amaranth and other weed seeds can be spread through contaminated feed as manure. If it’s possible your livestock feed is contaminated, follow these guidelines to minimize the risk of introducing this problematic weed onto crop or pasture land through manure application.

Animal digestion won't kill all amaranth seeds

Research studies found:

  • 27% remained viable in cattle manure
  • 3.5% remained viable in poultry litter

Ensiling may help reduce Palmer amaranth seed in feed

Research studies found:

  • 1 month of silage storage reduced amaranth seed viabilty 41% in alfalfa and 60% in corn
  • 2 months killed 87% of amaranth seed

Reducing Palmer amaranth seed in Manure

Composting is your best option

The internal heat generated by proper composting will kill up to 98% of viable Palmer amaranth seed. Proper composting requires active management and must be monitored and aerated for correct weed-killing conditions to develop. For best results:

  • Internal temperature of 160⁰F should be maintained for 4 days or more
  • Moisture content must be above 35% and below 80%, though 50% is recommended
  • Total composting time should not be less than 21 days, and over 50 days is optimal

Even well-managed compost might not entirely eliminate weed seeds, and 2-10% of total seeds can remain viable.

Liquid manure options are limited

Obviously, liquid manures cannot be piled for composting, and pit storage – including the anaerobic conditions in deep pits – does not significantly help kill amaranth seeds. Barring expensive heat treatment of the manure, the best option here is application to nearby fields that can be scouted easily and frequently.

Anaerobic digesters are not effective at reducing Palmer amaranth viability

Anaerobic digestion has not been found to significantly reduce amaranth seed viability beyond what is provided by animal digestion alone.

Field application of contaminated manure

When you have manure that you suspect or know is contaminated with palmer amaranth, transport it to nearby fields that can be easily and frequently scouted. Even if the feed was ensiled and the manure was composted before spreading, it’s still possible for weed seeds to remain viable. 2% survival of 1,000 seeds still leaves 20 viable seeds remaining. Apply the highest rates of manure (according to MN Pollution Control Agency guidelines) to the fewest number of fields as possible to minimize the spread of the seed. If these fields can be planted to more competitive crops such as alfalfa, grass pasture, or small grains that could also help by smothering the weed.

Scouting fields after application is crucial

Monitor fields early and often since this weed has an extended emergence period ranging from May through July.

What to do if you find a Palmer amaranth plant

Don't transport it

Palmer amaranth is an Eradicate Prohibited Noxious weed, meaning it cannot be legally transported even if it’s just to town to get an expert’s opinion. That goes for all stages of growth, whether it has produced seeds or not. Instead, take photos or bring the expert to the suspected plant.

Contact MDA or UMN Extension

To report a suspected Palmer amaranth plant, call the MDA Arrest the Pest line at 1-888-545-6684 or email arrest.the.pest@state.mn.us and provide:

  • Your location
  • Description of where the plant was found
  • Your contact information
  • Feed or seed source (if known)
  • 3-6 photos targeting 3 portions of the plant:
  1. Entire plant
  2. Petiole (where leaf connects to stem)
  3. Inflorescence (seed head)

After reporting, destroy the plant

If the plant has produced seeds, cover it with a bag before disturbing it. Palmer amaranth is a prolific seed-producer with around 500,000 seeds per plant. Digging up or disturbing this weed without bagging the inflorescence can cause the seeds to disperse, making the problem worse. Burning is the best method of destruction.

Additional Resources

UMN Extension information on Palmer amaranth

MDA press release

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