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Palmer amaranth seeds in manure — What can you do?

Palmer seeds eaten by livestock can survive digestion to later germinate in manure spread on fields.

November 3, 2020

6 Min Read
Palmer amaranth infestation
INCREASING PRESENCE: Palmer amaranth infestation is increasing in fields of soybeans and corn in eastern Nebraska, as well as several other crops like dry edible beans and sugarbeet in the Nebraska Panhandle.Tyler Harris

The seeds of Palmer amaranth can be introduced into your fields in several ways. Manure is one of them. Specifically, Palmer seeds that contaminate animal feed may survive digestion, and when that manure is spread onto cropland, those seeds may germinate.

Palmer amaranth infestation is increasing in fields of soybeans and corn in eastern Nebraska and several other crops such as dry bean and sugarbeet in the Nebraska Panhandle.

Palmer amaranth, a member of the pigweed (Amaranthaceae) family, is native to the southwestern United States and northern Mexico. It is a small-seeded broad leaf weed and is a relatively new weed in Nebraska.

Historically, common weeds from the pigweed family reported to occur in Nebraska are:

• tumble pigweed (Amaranthus albus L.)
• prostrate pigweed (Amaranthus graecizans L.)
• redroot pigweed (Amaranthus retroflexus L.)
• common waterhemp (Amaranthus rudis Sauer)

They are usually found throughout Nebraska in dry prairies, cultivated and fallow fields, and roadside, industrial and waste places. Palmer amaranth has been identified in the last few years in several north-central states, including Wisconsin, Michigan, Ohio, Minnesota and Illinois, which has raised concerns among weed scientists and growers about the spread of this species into areas not previously reported.

Because of its rapid growth, ability for prolific seed production, and ability to evolve herbicide-resistance, Palmer amaranth can be hard to control in agronomic crop fields.

Herbicide-resistant Palmer amaranth biotypes in Nebraska, Sept. 2020

Reducing weed seed

Palmer amaranth has evolved resistant to several groups of herbicides in Nebraska, including glyphosate (Table 1). Also, some Palmer populations are resistant to multiple herbicides such as atrazine and HPPD-inhibitors. Therefore, growers should pay attention to management of herbicide-resistant Palmer amaranth, and follow these best practices to reduce weed seed dissemination:

Don’t assume animal digestion will kill all Palmer seeds. Though it will reduce seed viability, simply feeding the contaminated material to livestock will not eliminate all Palmer amaranth seeds. Grass and soft-coated broadleaf seeds (such as clover and pennycress) are more easily destroyed in digestion than hard-coated seeds, such as Palmer amaranth.

In rumen animals, such as cattle, research shows 27% of amaranth seeds can remain 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, according to recent research.

Ensile the feed (if appropriate for the feed type). The fermentation and heat generated during ensiling is quite effective for killing weed seeds. Research in Canada in 2016 found that 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. A 1991 study in Brazil found that 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%.

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, one study in 1998 found that sustaining the compost at 140 degrees F for three days will nearly 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 160 degrees for four days with 50% moisture is recommended. Another study in 2003 found that it took between 21 and 50 days of composting with proper management to eliminate amaranth seed.

However, both of these studies reached zero viable weed seeds under the best compost management practices possible in a very controlled environment. In contrast, a 1992 California study surveyed actual on-farm composting sites. It found that while composting did reduce weed seed viability 90% to 98% over six to eight weeks, there was still potential for weed seed survival, with varying levels of mortality escape based on operation and weed species.

It has been hypothesized that this mortality escape was due to cooler pockets that did not sustain high temperatures for long enough. Therefore, just because manure has been composted does not necessarily mean it is free of weed seed.

Obviously, liquid manures cannot be piled for composting, and research shows 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 seed germination beyond the benefits of animal digestion alone.

Transport manure to fields that can be frequently scouted. 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 August, it is important to continually monitor fields.

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 zero to 400,000 seeds. A 2% survival of 75,000 seeds would leave 1,500 viable seeds per ton remaining. Applied at 8 tons per acre, that would increase the weed seed bank by 12,000 seeds per acre.

This “numbers game” is especially precarious in the case of Palmer amaranth, a prolific seed-producing weed species. A single female plant can produce somewhere between 100,000 to 500,000 seeds depending on competition with crops, other weeds and management practices.

Apply the highest rates of manure to the fewest number of fields as possible to minimize the spread of Palmer amaranth seeds. If these fields can be planted to more competitive crops such as alfalfa, grass pasture or small grains that could also help to suppress Palmer amaranth growth and reduce seed production.

Minimizing the risks from animal manures is an important consideration in controlling this weed.

Modderman is a University of Minnesota Extension crops educator. Jhala is a Nebraska Extension weed management specialist.

Source: UNL Water, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.


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