Weed And Seed

How does spring flooding impact weed seed movement and dispersal? The flooding that is occurring from spring snow melt may cause weed seeds that are on the soil surface or eroded soil to move, and possibly long distances. 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. Fall Flame Weeding: Targeting weed seeds before they enter the seedbank In this project we aim to test the hypothesis that fall flaming can efficiently and effectively kill weed seeds on the soil

Effects of Spring Flooding on Weed Seed Movement

How does spring flooding impact weed seed movement and dispersal? This question may be at the bottom of most producer’s problematic list as spring rolls around. The importance of it lies in the inevitable spring that will come with the possible elevated diverse weed seed emergence in some fields due to flooding. Producers that farm areas next to rivers see this more than others. For example, those who farm next to the James River may see Canada thistle seed movement down the river as it floods and then recedes.

The flooding that is occurring from spring snow melt may cause weed seeds that are on the soil surface or eroded soil to move, and possibly long distances. The current flooding situation this spring may increase the number of problematic weeds in those flooded areas. Weed seeds such as kochia, waterhemp, thistle, ragweed and many other commonly found weeds are easily moved by water.

The first step to take after the water recedes and plant growth starts, is to scout your fields. After the field has been accessed, a weed control plan needs to be found and initiated. Field scouting for weeds is always a good practice that will pay dividends later. Scouting fields for weeds helps to determine what weed control measures need to take place before the cash crop is planted or drilled, and what may need to be done later in the growing season. The areas where flooding is worst may need to be extensively scouted for new weeds that where introduced into your property, and possibly higher density of some common weeds.

It is hard to think, but there may be benefits to weed control in a flooding situation. If winter annual weeds such as marestail exist on your property, they may be killed by the flood waters. This is caused by the excess water forcing the oxygen out of the soil, which causes the plant roots to die or drown due to the lack of oxygen.

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.

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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.

See also  Grass Weed Seeds

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.

Additional Reading

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

Fall Flame Weeding: Targeting weed seeds before they enter the seedbank

In this project we aim to test the hypothesis that fall flaming can efficiently and effectively kill weed seeds on the soil surface. While flaming is widely used to kill small weed seedlings, and field stubble burning is known to kill weed seeds, we are not aware of any research in which flaming has been used to target seeds. This idea comes from the experience of Rob Johanson, a diversified organic farmer in Maine who, while flaming potato vines in particularly weedy fields observed smoldering weed residues and reasoned that he was likely killing weed seeds. He has further observed that flamed plots generally have lower weed pressure the following year. We aim to confirm this observation with a series of on-farm experiments evaluating three flame doses and their effect on the following season’s germinable weed seedbank and early-season weed seedling densities. Laboratory studies conducted with condiment mustard (Sinapis alba), indicated that a 900C propane flame required 3 sec. or more exposure to reach mortality levels of 80%; mortality was 100% at 5 sec. of exposure (Figures 1 and 2). Field studies will include higher temperatures to reduce the required exposure time, and will evaluate the combined stresses of flaming and overwinter mortality. Results will be shared with other growers during a Field Day and a session on “Managing the Seedbank for Improved Weed Management” at the 2012 Maine Organic Farmers and Gardeners Association Farmer to Farmer Conference. We will also produce a short video on fall flaming which will be posted to our YouTube Channel (zeroseedrain), and the national eXtension website.

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Project objectives from proposal:

Objective: To measure weed seed mortality in response to flaming in the field.

This project will include field experiments conducted at one farm in Maine, and another in Vermont. The on-farm studies will be conducted on a uniformly cropped weedy field following cash crop harvest. At each farm, in the fall of 2011, weeds will be flail mowed and four replicated strips will be subject to the following treatments: no flaming; flaming at 40 kg propane per ha; flaming at 80 kg per ha; and flaming at 160 kg per ha (note: a 20-40 kg rate would kill sensitive weed seedlings). In the spring of 2012, soil sampling and subsequent greenhouse germination assays will measure the weed seedbank in each treatment. Ten soil cores (6.5 cm diam by 10 cm deep) will be randomly chosen from each plot, bulked, and spread in greenhouse flats, and watered to encourage weeds to germinate. Seedlings will be removed monthly, followed by drying and crumbling of soil and re-watering (four cycles in total). To measure the contribution of fall flaming on weed management in the subsequent crop, in June of 2012 we will record weed density after the final cultivation event at each farm. Specifically, eight 0.5 sq. m quadrats will be censused in each replicate of each treatment. Weeds will be identified by species and counted.

Propane usage will be recorded by weight using portable field scales, and will be used to estimate per ha costs for the flaming treatments. Thermocouples and a portable temperature recorder will be installed in each treatment to record the thermal dose.

Performance Target: We expect that the germinable weed seedbank density will be inversely related to the propane flaming rate. Further, the spring seedling census data will proportionally reflect the effects of flaming on the germinable seedbank, with the lowest surviving weed density in the 160 kg per ha propane treatment, and the greatest weed density in the control treatment. Adoption of this promising management practice will depend on the expected tradeoff between faming cost and weed control benefits measured the following year.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.

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