Small but mighty, weed seeds in manure can be problematic when they result in overgrown, weedy fields after manure application. Some manures can be a source of these troublesome weed seeds. But, luckily, there are some measures that can be taken to reduce the viability of those weed seeds. Weeds are a major threat to biodiversity including in areas of high conservation value. Unfortunately, people may be unintentionally introducing and dispersing weed seeds on their clothing when they visit these areas. To inform the management of these areas, we conducted a systematic quantitative li …
Managing Weed Seeds in Manure
Small but mighty, weed seeds in manure can be problematic when they result in overgrown, weedy fields after manure application. A survey found that fresh manure on dairy farms had an average of 75,000 seeds per ton. But, luckily, there are some measures that can be taken to reduce the viability of those weed seeds.
First of all, don’t assume that animal digestion will take care of the problem. Though it will reduce weed seed viability, simply feeding the material to livestock will not eliminate all seeds. Grass and soft-coated broadleaf weed seeds are more easily destroyed in digestion than hard-coated seeds. In a study conducted on rumen animals, such as cattle, 27% of hard-coated seeds remained viable after digestion. The gizzard digestive system of poultry is highly effective at destroying weed seeds, and only 3.5% of hard-coated seeds fed to ducks were recovered and found viable in a similar study.
So what can you do to reduce weed seed viability beyond the gut? In general, heat is the enemy of weed seed survival. The benchmark for good seed mortality is 140⁰F (60⁰C) sustained for three days. Hot temperatures that fall below that mark or a shorter duration will still kill some weed seeds, but not as thoroughly. How you subject the weed seeds to heat is up to you, but below are a few suggestions.
Minimize weed seeds in feed and forage by ensiling
What goes in, must come out; so killing seeds before they get to the animal is a good strategy. One way to do that is to ensile the feed (if appropriate for the feed type). The fermentation and heat generated during ensiling is quite effective for killing weed seeds. One study found that just one month after seed-contaminated alfalfa haylage was stored, viability of the toughest seeds 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 was shown to kill up to 87% of viable seeds; and when feed went through both ensiling and rumen digestion, the seed mortality increased to 89%.
Minimize weed seeds in manure by composting
What if ensiling isn’t feasible? What if your manure is already contaminated with weed seeds? In those cases, composting is a very effective method for killing weed seeds – more effective than ensiling.
Internal heat generated by properly composting manure will kill most weed seeds – even the hard-seeded weeds. The key word here is “properly.” Aged manure is not composted manure. I’ll say it again: 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. Studies have shown that sustaining the compost at that benchmark of 140⁰F for three days can reduce weed seed viability 90-98%, so long as a minimum of 35% moisture is maintained. Another study found that overall duration was important and that it took between 21 and 50 days of composting for best results.
Even under the most diligent composting program, there can be seeds that survive. It is theorized that since manure is not a uniform product, this mortality escape is due to cooler pockets that do not sustain high temperatures for long enough. Therefore, just because manure has been composted does not necessarily mean it is weed seed free.
Field application of contaminated manure
Remember, 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 2% survival of 75,000 seeds 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! Therefore, it is crucial to scout fields that receive manure to head off any severe weed infestation.
Watch for a second article on “Palmer amaranth Seeds in Manure – What Can You Do?” later in October.
Cudney, D., Wright, S., Schultz, T., and Reints, J. 1992. Weed seed in dairy manure depends on collection site. California Agric. 46:31-32. http://calag.ucanr.edu/Archive/?article=ca.v046n03p31
Larney, F. and Blackshaw, R. 2003. Weed seed viability in composted beef cattle feedlot manure. J. Environ. Qual. 32:1105-1113. https://www.researchgate.net/publication/10706540_Weed_Seed_Viability_in_Composted_Beef_Cattle_Feedlot_Manure
This article was reviewed by Amit Jhala, University of Nebraska-Lincoln Extension Specialist and Ron Seymour, Nebraska Extension Educator
Weed seeds on clothing: a global review
Weeds are a major threat to biodiversity including in areas of high conservation value. Unfortunately, people may be unintentionally introducing and dispersing weed seeds on their clothing when they visit these areas. To inform the management of these areas, we conducted a systematic quantitative literature review to determine the diversity and characteristics of species with seeds that can attach and be dispersed from clothing. Across 21 studies identified from systematic literature searches on this topic, seeds from 449 species have been recorded on clothing, more than double the diversity found in a previous review. Nearly all of them, 391 species, are listed weeds in one or more countries, with 58 classified as internationally-recognised environmental weeds. When our database was compared with weed lists from different countries and continents we found that clothing can carry the seeds of important regional weeds. A total of 287 of the species are listed as aliens in one or more countries in Europe, 156 are invasive species/noxious weeds in North America, 211 are naturalized alien plants in Australia, 97 are alien species in India, 33 are invasive species in China and 5 are declared weeds/invaders in South Africa. Seeds on the clothing of hikers can be carried to an average distance of 13 km, and where people travel in cars, trains, planes and boats, the seeds on their clothing can be carried much further. Factors that affect this type of seed dispersal include the type of clothing, the type of material the clothing is made from, the number and location of the seeds on plants, and seed traits such as adhesive and attachment structures. With increasing use of protected areas by tourists, including in remote regions, popular protected areas may be at great risk of biological invasions by weeds with seeds carried on clothing.
Keywords: Biological invasion; Human-mediated dispersal; Invasive species; Long distance dispersal; Tourism impacts.
Copyright © 2014 Elsevier Ltd. All rights reserved.
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