Minnesota: Palmer Pigweed in Manure – What Can You Do?

Palmer amaranth pigweed. Photo: Aaron Hager, University of Illinois

Palmer amaranth pigweed. Photo: Aaron Hager, University of Illinois

The Minnesota Department of Agriculture has identified manure as a pathway of introduction for Palmer amaranth. Specifically, Palmer amaranth seeds that contaminated sunflower screenings were fed to cattle. Some of those seeds survived digestion, and when that manure was spread onto cropland, those seeds germinated.



As a newly-identified problem in this state, it is worth taking a moment to examine seed viability in manure and what this might mean for Minnesota livestock and crop producers. Note that this article’s focus is manure and it will not delve into the specifics of Palmer amaranth and the challenges it presents.

In short, this is an Eradicate Prohibited Noxious Weed, meaning the goal is to entirely eradicate this weed; and it is illegal for this seed to be transported or sold. UMN Extension has more information, and MDA has put out a press release about this problematic weed.

Reducing Palmer amaranth seed in feed

Don’t assume animal digestion will kill all of the seeds

Though it will reduce weed seed viability, simply feeding the contaminated material to livestock will not eliminate all Palmer amaranth seed. Grass and soft-coated broadleaf seeds are more easily destroyed in digestion than hard-coated seeds – such as Palmer amaranth. In rumen animals, such as cattle, 27% of amaranth seed remained viable after digestion. The gizzard digestive system of poultry is highly effective at destroying weed seeds, and only 3.5% of Palmer amaranth seeds fed to ducks were recovered and found viable.

Ensile the feed (if appropriate for the feed type)

The fermentation and heat generated during ensiling is quite effective for killing weed seeds. Just one month after contaminated alfalfa haylage was stored, amaranth seed viability dropped by 41%; and in corn silage, the drop was even greater at 60%. Logically, seed viability continues to decrease as silage storage time increases. Eight weeks of ensiling killed up to 87% of viable amaranth seed; and when feed went through both ensiling and rumen digestion, the seed mortality increased to 89%.

Reducing Palmer amaranth seed in manure

Compost solid manure

Internal heat generated by properly composting manure will kill most weed seeds – even the hard-seeded Palmer amaranth. The key word here is “properly.” Aged manure is not composted manure. Proper composting requires active management and must be monitored and aerated for correct weed-killing conditions to develop.



Temperature and moisture are the two most crucial elements for seed mortality in compost. For Palmer amaranth, Wiese et al. (1998) found that sustaining the compost at 140⁰F for three days will virtually eliminate seed viability, so long as a minimum of 35% moisture is maintained. To account for temperature and moisture uniformity issues that are prevalent in composting, exceeding these minimums and composting at 160⁰F for four days with 50% moisture is recommended. Another study found that it took between 21 and 50 days of composting with proper management to eliminate amaranth seed.

However, research by Wiese et al. (1998), Larney and Blackshaw (2003) reached 0% viable weed seeds under the best compost management practices possible in a very controlled environment. In contrast, Cudney et al. (1992) surveyed actual on-farm composting sites and found that while composting did reduce weed seed viability 90-98% over six to eight weeks, there was still potential for seed survival; with varying levels of mortality escape based on operation and weed species. It is theorized that this mortality escape is due to cooler pockets that do not sustain high temperatures for long enough (Grundy et al. 1998). Therefore, just because manure has been composted does not necessarily mean it is weed seed free.

Liquid options are limited

Obviously, liquid manures cannot be piled for composting, and pit storage – including the anaerobic conditions in deep pits – does not significantly contribute to amaranth seed mortality. Barring expensive heat treatment of the manure, the best option here is application followed by diligent and frequent scouting.

Don’t rely on anaerobic digestion

Though anaerobic digestion of manure may reduce seed viability of some weeds, it has not been found to affect amaranth germination beyond the benefits of animal digestion alone.

Field application of contaminated manure

Have manure you think is contaminated?

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. A 98% reduction in viability seems sufficient, but even low seed survival rates can be problematic. A survey of fresh dairy manure in New York found an average of 75,000 viable seeds per ton and a range of 0 to 400,000 seeds.

2% survival of 75,000 would leave 1,500 viable seeds remaining per ton. Applied at 8 tons per acre, that would increase the weed seedbank by 12,000 seeds per acre. This “numbers game” is especially precarious in the case of Palmer amaranth since Minnesota hopes to entirely eradicate it, as it would be fairly easy for just one or two seeds to slip through to the field.

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.

Scout fields after application

It is crucial to scout early and often for Palmer amaranth in fields that have received possibly-contaminated manure. Since this weed has an extended emergence period ranging from May through July, it is important to continually monitor fields.

If you find a suspected Palmer amaranth plant

Don’t transport it

Palmer amaranth is considered 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

Palmer amaranth can be tricky to identify, especially since it closely resembles other common amaranth species such as redroot pigweed and waterhemp. MDA and UMN Extension have experts that can help identify this weed and tell you how to proceed. 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 the following:

  • Your location
  • Description of where plant was found
  • Contact information
  • Seed source (if known)
  • Three to six photos targeting three portions of the plant:
    • Entire plant
    • Petiole (where the leaf connects to the stem)
    • Inflorescence (seed head)

After photographing and reporting, destroy the plant

If the plant has produced seeds, cover it with a bag before disturbing it to control seed loss and shatter so it can be destroyed. Your instinct might be to immediately cut down the suspected plant, but that might be a bad idea. 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.

For a fact sheet on this topic, see Palmer amaranth in manure (736 K PDF)

Source URL: https://blog-crop-news.extension.umn.edu/2019/06/palmer-amaranth-in-manure-what-can-you.html