Weed seed in dairy manure depends on collection site
Dairy manure collected for 2 years from various sites in seven Central California dairies was found to contain viable weed seed. Weed seed Contamination Was most Severe when manure was taken from dry COW pens and liquid manure sedimentation handling facilities. Composting did not eliminate all viable weed seed.
Dairy manure has been an important soil amendment, particularly to less productive soils. It also has, however, a poor reputation among growers. Its use on farms has sometimes been correlated with weedier fields compared with farms not using it. The reasons for higher weed populations could be due to weeds in the manure, lack of effective composting, or contamination of the manure by windblown seeds from adjacent areas.
Tulare County has one of the largest dairy industries in the United States, and the manure produced as a by-product represents a major revenue source. It is spread on crop land within the region, but because of its reputation as a weed seed source, poultry manure or commercial fertilizers are often preferred as sources of nitrogen and phosphate.
Poultry manure is nearly weedfree, depending on how much soil and litter becomes mixed with it. Weeds are broken down by the grinding action of sand in the gizzard. In dairy animals, many seeds pass through the digestive system and remain viable. The digestive system may, in fact, loosen the hard seed coats of weeds, thus enhancing germination.
Fig. 1. Weed seed species found in dairy manure.
The objectives of our study of weed seeds in dairy manure were to learn which sites on the dairy contribute the most weed seed, what weed species are most prevalent and whether composting as practiced in commercial dairies effectively reduces weed seed viability.
Manure samples were taken from six dairies in Tulare County and one in Kings County. Dairies selected were typical of those within the southern San Joaquin Valley. The dairies ranged in size from 350 to 1,500 cows and were categorized as traditional open corral with shades.
Seven sets of samples were taken from each dairy in 1988 and 1989. Samples were taken seasonally in April, July, October and December of 1988 and in April, July and December of 1989. Sites of sampling at each dairy included: dry cow (nonlactating) pens, milk-producing cow pens, compost piles, solid waste separators and sedimentation ponds. Each sample was a composite of five subsamples representative of the site and was 2.2 pounds in weight. Compost samples were taken from five of the dairies having manure piles 6 to 8 weeks old and were sampled from within the piles at least 2 feet deep.
To compare the relative content of viable weed seed in manure at the various sites of collection, we adhered to the following procedure. Manure samples were air dried and then gently pulverized. The manure was spread out over the surface of two 18- x 18-inch flats containing UC No. 2 potting mix (vermiculite, peat and lightly sprinkled two to three times a day thereafter, depending on moisture demand, to maintain surface moisture. Weed seed germination was accomplished in a greenhouse held at 20° to 25°C to assure germination of both cool- and warm-season weeds.
At 10 days, the first weed seedling counts were taken. The germinated weeds were removed, the flats were air dried for 1 week, and the surface was remixed and watered again for another 10 days, after which a second weed count was taken. Weeds were identified by species. Those that were difficult to identify were transplanted and grown to maturity for identification. It was recognized that only a fraction of the weeds present in the manure could be germinated and counted by this method due to dormancy, but relative comparisons could be made among the collection sites.
The weed species most prevalent in manure samples from all of the dairies over the seven sampling times were grasses and broadleaf species (fig. 1). Broadleaf species included: pigweed (Amaranthus spp.), lambsquarter (Chenopodium album), sowthistle (Sonchus oleraceus), chickweed (Stellaria media) and such mustard species as shepherds purse (Capsella bursa-pastoris) and London rocket (Sisymbrium irio). Grassy weeds included yellow foxtail (Setaria glauca), bermudagrass (Cynodon dactylon), annual bluegrass (Poa annua) and barnyardgrass (Echinochloa crus-galli).
When different sites of manure collection were compared, samples differed in viable weed seed content. Manure taken from producing cow pens had notably fewer viable weed seeds than that collected from dry cow pens. Presumably, this was due to the lower quality of feed (weedier feed) fed the dry cows (fig. 2). Sediments from liquid manure-handling facilities (solid separator and sedimentation ponds) also contained a high viable weed count.
Fig. 2. Viable weed seed by site of collection, a composite of seven dairies.
Nebraska: Palmer Amaranth Seeds in Manure – What Can You Do?
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.
1. 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 (Photos by Amit Jhala). Click Image to Enlarge
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).
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.
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.
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
2. 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. 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%.
3. 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., Larney and Blackshaw reached 0% viable weed seeds under the best compost management practices possible in a very controlled environment. In contrast, Cudney et al. 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. 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.
4. 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. 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.
Figure 2. Palmer amaranth seedlings typically starts emerging in early May and continue emerging until end of August in Nebraska agronomic crops. Click Image to Enlarge
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.
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.
Common Spikeweed Standards
GENERAL STANDARDS — The standards on this sheet are in part condensed and apply to Common Spikeweed. For greater detail and additional provisions, see the Pre-Variety Germplasm Standards .
APPLICATION — Applications should be submitted electronically on CCIA’s website ( Application to grow and certify seed ) as soon as possible and no later than three (3) weeks after planting. New applicants should contact the CCIA office for instructions on obtaining access to the online application system.
FIELD HISTORY — Land must not have grown or been seeded to any Centromadia species during previous five years to be eligible to produce G1 or G2 seed. Land must not have grown or been seeded to these Centromadia species during the previous two years to produce G3-G10 seed unless the previous crop was of the same released material and of a class equal or higher to that of the crop seed. Common Spikeweed must be planted in distinct rows. Exceptions must be approved by the Seed Certification Office prior to planting.
ISOLATION — Fields or portions of fields producing seed must be isolated the following distances:
G3 to G10
Isolation requirement (any size field)
FIELD INSPECTION — Include a seedling and a seed crop inspection.
Off-Types — Every field should be rogued to remove any plants of another crop or variety, including volunteers. Pre-variety germplasm selections cannot always be differentiated at field inspection. When differences can be distinguished, the maximum mixture of other varieties or definite off-types permitted is as follows:
G3 to G10
Weeds — Fields must be free of any prohibited noxious weeds. Restricted noxious weeds and common weeds difficult to separate must be controlled. Prohibited and Restricted noxious
weeds are listed in the California Seed Law/CA Code of Regulations/Sections 3854 and 3855. See California Seed Law – Prohibited and Restricted Noxious Weed List .
Fields may be refused certification due to unsatisfactory appearance caused by weeds, poor growth, poor stand, disease, insect damage, and any other condition which prevents accurate inspection or creates doubt as to identity of the variety.