By Chryseis Modderman and Amit Jhala
There are several ways seeds of Palmer amaranth can be introduced into your fields. Manure is one of them. Specifically, Palmer amaranth seeds that contaminate animal feed may survive digestion; and when that manure is spread onto cropland, those seeds may germinate. This article provides some answers on four topics:
- Overview of Palmer amaranth in Nebraska;
- Reducing Palmer amaranth seed in feed;
- Reducing Palmer amaranth seed in manure; and
- Field application of contaminated manure.
Herbicide-Resistant Palmer amaranth in Nebraska:
Palmer amaranth infestation is increasing in soybean and corn fields (Figure 1) in eastern Nebraska and several other crops such as dry bean and sugarbeet in the Nebraska Panhandle.
Figure 1. Glyphosate-resistant Palmer amaranth in soybean field (top) in south central Nebraska and atrazine/ALS inhibitors/ glyphosate-resistant Palmer amaranth in corn field (right) near Carleton, NE
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.), and common waterhemp (Amaranthus rudis Sauer) (Stubbendieck et al. 1994). They are usually found throughout Nebraska in dry prairies, cultivated and fallow fields, and roadside, industrial, and waste places (Stubbendieck et al. 1994). 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 (Chahal and Jhala 2018b).
Palmer amaranth has evolved resistant to several groups of herbicides in Nebraska, including glyphosate (Table 1). Additionally, some Palmer amaranth populations are resistant to multiple herbicides such as atrazine and HPPD-inhibitors. Therefore, growers should pay attention to management of herbicide-resistant Palmer amaranth as well as follow the best practices to reduce weed seed dissemination (Chahal and Jhala 2018).
Table 1. Herbicide-resistant Palmer amaranth biotypes in Nebraska (Sep 2020)
Type of resistance
Site of action
Occurrence in Nebraska
Acetolactate synthase (ALS)
Pursuit, Classic, Scepter
West central, south central
Callisto, Laudis, Armezon
West central, south central
Isolated field in west central
Reducing Palmer amaranth seed in feed
- Don’t assume animal digestion will kill all of the Palmer amaranth 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, 27% of amaranth seeds remained viable after digestion (Blackshaw and Rode 1991). 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 (Farmer et al. 2017).
- 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% (Simard and Lambert-Beaudet 2016). 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% (Blackshaw and Rode 1991).
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 (Larney and Blackshaw 2003).
- 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 weed seed survival; with varying levels of mortality escape based on operation and weed species. It was hypothesized that this mortality escape was due to cooler pockets that did 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 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 contribute to amaranth seed mortality (Allan et al. 2003). 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 (Eckford et al. 2012; Katovich and Becker 2004).
Field Application of Contaminated Manure
- Have manure that 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 (Mt. Pleasant and Schlather 1994). 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 seedbank 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 (Jhala et al. 2014).
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.
Figure 2. Palmer amaranth seedlings typically starts emerging in early May and continue emerging until end of August in Nebraska agronomic crops.
- 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 August (Figure 2), it is important to continually monitor fields.
Additional information and recommendations for Palmer amaranth control in Nebraska can be found in Chahal and Jhala, 2018 and Sarangi and Jhala, 2019). Minimizing the risks from animal manures is an important consideration in controlling this weed.Source : unl.edu/