Weeds are vulnerable to more than chemicals. They are also susceptible to natural pests and that creates opportunities for farmers to control them using the natural environment.
Weed seeds with cracks and cuts in their seed coats are more prone to damage from natural pests.
And because the seeds survive for up to 25 years in the soil, a good time to attack them is while they are lying dormant, waiting for their time to grow, said a researcher with the United States Department of Agriculture.
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“Soil seed banks are comprised of both dormant and non-dormant seeds persisting at varying depths in the surface soil profile,” said Adam Davis, an Urbana, Illinois, researcher with USDA.
“If all the seeds in the seed bank were to germinate within a given year, the weed management task would be simple. Leave the field fallow for a couple of years, spray or till.
“They don’t, so it’s not (simple),” he said.
Davis’s research is looking to better understand why weeds don’t germinate at similar times, even with rain and under conditions that should support them.
“Most viable weed seeds are found below the surface or in the top 30 centimetres of the soil,” he said.
“But some, they fall into cracks or get tilled deeper and become dormant for longer periods.”
He said generally weeds that spend their energy making large numbers of seeds tend to have short lives in the soil.
At 50,000 seeds per plant, kochia falls into this category, with a viable life in the soil of about six weeks.
Lambsquarters on the other end of the scale, tends to produce far fewer seeds, but their waxy seed coat defences make them viable for 25 years.
Weeds with moderate persistence tend to invest in a combination of chemical and physical defences.
But no matter which system of defence or seed yield population the weed uses, mechanical damage successfully controls them.
“If you can cause damage to the seed’s outer defences, you can limit its ability to survive. Then the environment becomes your herbicides,” Davies said.
Different methods of damaging the weed seeds resulted in different levels of mortality in the studies carried out by Davis’s team at Michigan State University.
The weed seeds were pierced, sliced and ground to see which methods could be used to damage seeds before allowing them to spread in the field.
“With the exception of the piercing treatment, seed chemical defences did not mitigate seed decay due to mechanical damage treatments.
Presumably, the slicing and grinding treatments removed a large enough portion of the seed that opportunistic fungi or bacteria were able to overwhelm seed chemical defences.”
In the piercing treatment, however, seed mortality declined with increasing concentrations of chemical compounds.
The seed piercing, commonly caused by insects, is thought to increase weed seed decay due to fungal attack.
“Therefore, although weed seed phenols don’t appear to serve as the primary defence against decay, nor drive seed bank persistence, they might provide a secondary defence against superficial damage to the seed coat by granivores (insects),” he said.
Davis said Canadian researchers including University of Manitoba scientist Martin Entz and University of Saskatchewan researcher Steve Shirtliffe, join a long line of scientists who have pointed out the vulnerability of weed seeds to natural elements.
Shirtliffe said his research indicated that while up to 60 percent of weed seeds fall to the ground ahead of the combine, opportunities exist to reduce spread by chaff spreaders.
“Spreaders are getting bigger and the spread of weed patches, especially resistant weeds can be contained if you collect or damage weed seeds before they hit the spreader,” he said.
Davis said research into weed seed spread is not new.
“There has been a lot of work around collecting weeds seeds and burning them, but that won’t work in many locations where burning isn’t allowed. There is another strategy,” he said.
“We can damage the seeds while still at the combine. They don’t need to pulverize the seed, only wound it and let Mother Nature take care of the rest.”
Davis’s work has attracted farm equipment maker John Deere and he is working with engineers at Deere to develop a device that would damage weed seeds while in the chaff stream of the combine.
“If you reduce the weed population, you reduce selection for tolerance to herbicides and you will lower weed control costs overall,” he said.
