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. Western Integrated Pest Management Center For the latest IPM news and funding announcements, subscribe to our monthly newsletter. Targeting Weed Seeds at Harvest As herbicide-resistant
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
Western Integrated Pest Management Center
For the latest IPM news and funding announcements, subscribe to our monthly newsletter.
Targeting Weed Seeds at Harvest
As herbicide-resistant weeds become more common across the country, researchers and growers are looking for other ways to control weeds.
In Colorado, they’re looking to techniques and technology developed in Australia, which has significant issues with herbicide-resistant weeds.
Known as harvest weed-seed control, these IPM-friendly methods are designed to destroy or remove weed seeds during harvest to prevent them from raining down onto the soil and replenishing the weed seed bank. In Colorado wheat, weed species of concern are winter annual grasses that share the grain’s growing cycle, like jointed goatgrass, feral rye and downy brome.
“In harvest weed-seed control, the objective is to prevent those seed-bank increases,” explained Colorado State University doctoral candidate Neeta Soni. “There are a number of ways to do it, and we’re investigating to see if they could be adopted in Colorado.”
One way to destroy the weeds seeds is by directing chaff during harvest into a cage mill – imagine a giant coffee grinder – and pulverizing the chaff and weed seeds into powder. That’s the idea behind an Australian innovation known as the Harrington Seed Destructor (and a new competitor called the Seed Terminator).
Another option is to use a piece of equipment called a chaff deck to gather chaff into mounded strips behind the harvester, capturing the weed seed in those mounds of chaff. In some places those chaff strips can be burned, and in others they’re left alone to allow the weed seeds to decay without entering the soil.
A third option is to use chaff carts and collect all the chaff and captured weed seeds for off-site destruction.
“Our research is focused on finding out if there is potential to use these methods in Colorado,” explained Soni, a graduate student of assistant professor Todd Gaines. “So what we needed to know is whether, at harvest, the majority of the winter annual grass seeds are retained in the upper wheat canopy, where they would be vulnerable to the seed destructor or other methods.”
If the weed seeds have already shattered and fallen to the soil, or if the weed seeds are below the cutting height of the combine, the methods would not be as effective.
So the Weed Research Lab team measured and counted a lot of weeds.
“What we found is that the majority of seeds are still retained at harvest,” Soni said. “Downy brome is the same height as wheat, rye is taller and jointed goatgrass a little shorter, but growers could adjust their cut height to manage it.”
Soni then counted out 1,000 seeds of each weed species into a specified amount of chaff and drove to the University of Arkansas where they have a seed destructor set up on a test platform. She ran each bundle through the destructor. The pulverized material was dusted across beds of soil to see if any weed seeds germinated. Virtually none did.
“The seed destructor was 98 percent effective on downy brome and jointed goatgrass, and 99 percent effective on feral rye,” Soni said.
The Gaines lab hopes to conduct field trials with the equipment. They will also study if the strips of mounded chaff are effective in Colorado, or if the state’s dry and windy conditions enable weed seeds to survive and spread.
The seed destructor isn’t commercially available in the United States yet, but a number of researchers are testing versions in different regions and in different crops. The initial model was a tow-behind trailer, but both Australian manufacturers now offer the technology integrated into a combine harvester that retails between $120,000 and $160,000 Australian dollars.
Not every grower would need to buy one.
“It is very common that growers here have their harvesting done by a contractor,” Soni said, “so this could be an extra service they provide.”
But not at every harvest. Because whatever specific iteration of harvest weed-seed control Colorado growers may eventually adopt, it should be just one element of an integrated management strategy, Soni cautioned.
“Repeated use could lead to the selection of earlier-shattering weed seeds, or shorter weeds,” she said. “It has to be used in rotation with other integrated measures, including herbicides and crop rotation.”
In short, it should be part of an IPM program.