Controlling weeds in organic systems is a bit like balancing your chequebook, except that the goal is to get the (seed) bank account as low as possible.
“When managing annual weeds, the important thing is the seed bank,” said Eric Gallandt, a weed ecologist in the latest webinar of this spring’s Western Canadian Organic Webinar series.
Gallandt, who hails from the heavily forested northeastern state of Maine, said that aside from some large potato and dairy operations, most of the farms in the state are diversified vegetable growers, of which 380 are certified organic.
Driving the account down to lower levels requires “many tiny hammers” aimed at minimizing “credits” to the bank that arrive via weed seed “rain.” That could include timely weed control tillage, short-season cover crops to pre-empt weed growth, or using winter cereals to increase crop competition with summer annual weeds.
Other tools in the tool box capitalize on “debits” to the seed bank through accelerated weed seed decay, germination, burial or predation by rodents, birds or bugs.
Relying solely on tillage is problematic, because the results are highly variable, said Gallandt.
Tests in 70 locations in a field using condiment mustard as a “surrogate weed” found that a four-row, S-tine harrow killed anywhere from zero to 100 per cent of the weeds.
“In 25 per cent of the locations, the efficacy was below 50 per cent,” he said, adding that variance might be due to soil conditions.
Also, the research showed that the kill rate average of 69 per cent per pass was the same no matter if the weed density was low or high. That means areas with a greater weed problem tend to produce more survivors, and increasing the intensity of tillage doesn’t always result in better weed management over time.
Samples taken from his region found that the germinable seed bank ranged from zero to 30,000 seeds per square metre. The mid-range was 3,000-10,000, but many farms were much higher.
“If you are in this situation, it means that you are going to have to cultivate more and kill 70 per cent with 30 per cent surviving. You just keep doing that until you get an acceptable density of weeds,” he said.
Seeding rate and size
To speed that process, Gallandt has looked at various “multiple-stress” approaches that look beyond just killing weed seedlings by tillage.
Enhancing crop competition via heavier cereal seeding rates is one strategy. When attempting this, it’s important to remember that seeding rates based on seed mass must take into account the varying size and weight of individual seeds. For example, Harvard cultivar wheat seeds average 45 mg per seed, while Red Fife is much smaller, at 26 mg/seed.
To hit a target of 45 seeds/sq. ft., you need almost 200 pounds per acre of Harvard, compared to just 112 lbs./acre for the smaller-seeded Red Fife.
Most seeders are designed to place seeds close together in long, narrow rows that leave room for weed growth in between.
But to boost crop competition, a team of equipment engineers led by Jacob Weiner at Denmark’s Royal Veterinary and Agricultural University developed a precision seeder that planted seeds in a wide uniform grid pattern with the aim of minimizing the empty spaces that could be taken over by weeds.
From the Grainews website: Herbicide carryover may not be in the past
Comparison trial results were promising with a significant reduction in weed pressure, but the equipment is still experimental, said Gallandt.
Some farmers have duplicated the seeding arrangement by seeding in a criss-cross pattern, or using a broadcast unit on the front of the tractor that dispersed one-third of the seed, and the remaining two-thirds via the drill behind it.
Both pre- and post-emergent tine harrowing to control early flushes of weeds in cereals is a popular choice, but although it appears to have 95 per cent efficiency at the white thread stage and 75 per cent for those in the cotyledon stage, being too aggressive risks injuring the young crop even at increased seeding rates.
“This white thread stage is important, and if you can nail it, it’s awesome,” said Gallandt.
“But there are occasions when there doesn’t appear to be a net benefit to crop yield. You’ve got to balance the negative effect of damaging crops with the positive effect of reducing weed density.”
Soil disturbance that occurs when seeding fall cover crops risks burying and protecting surface weed seeds from critters that might eat them, he noted.
However, studies of the effect of predators on weed seeds have shown mixed results. Test plots that left crop and weed residue on the surface in fall instead of seeding a cover crop or doing tillage in the hope that mice, birds or bugs might deplete the seed bank, showed impressive results only one year out of three.
Gallandt spoke in the second organic webinar scheduled for the noon hour this spring. Next up on Feb. 11 is a one-hour talk entitled “Growing hemp on the Prairies,” followed by, “The plant’s role in soil fertility management” on Feb. 18. To sign up, contact [email protected]