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Weed management in soybeans

With high seed prices, farmers will be tempted to lower soybean seeding rates

Soybean farmers have been facing a growing threat to their bottom line: rising seed costs. Seeds for the very popular glyphosate resistant cultivars have increased in price by $47.75 an hectare (or 230 per cent) since they were first introduced in 1996. This trend is likely to continue given soon-to-be-introduced varieties with new herbicide resistant traits as well as increased soybean planting across the world.

The rising costs have, so far, been more than compensated for by soaring prices for soybeans in the past few years but that trend will not continue forever; eventually farmers will begin to feel the pinch and will increasingly want to find ways to boost yields while reducing their use of the glyphosate resistant seeds.

Lower seeding rates

A natural solution is to reduce seeding rates. Evidence from many recent studies shows this can be done without significantly reducing yields. For instance, soybeans have been shown to produce 20 per cent more branches per plant at lower seeding rates (under 250,000 seeds per hectare) than at higher rates (near 450,000 seeds per hectare). Simply put, each plant is able to develop more fully when it has more space in which to grow and faces less competition from its neighbours. That means a higher overall soybean yield.

Unfortunately that lower plant density can make the plants more vulnerable to weeds. Common Canadian Prairie weeds grow more aggressively and compete more ferociously for water and nutrients than soybeans. The main way soybeans compete is to form a thick canopy, which starves the competing weeds of the sunlight they need to drive their metabolism. Less dense planting means a less dense canopy, and a less dense canopy means a more welcoming environment for weeds. This in turn reduces soybean yields and cuts into the profits farmers would have enjoyed through savings on seed.

Dense soybeans canopies are a part of integrated weed management (IWM), an increasingly important practice as common weeds are becoming increasingly resistant to growing season applied glyphosate. This would seem to leave soybean farmers at an impasse.

Pre-season herbicides

Recent research by scientists at the University of Wisconsin shows a way to a relatively low cost solution, a way to seed less densely without worrying about weeds reducing your yield. The study is titled, “Can Soybean Seeding Rate Be Used As An Integrated Component Of Herbicide Resistance Management”, and was published in the October-December 2014 issue of Weed Science. In the article, these scientists, led by Ryan DeWerff, studied the effects of combining pre-season residual herbicides with lower seeding densities.

The researchers found that early season weeds are a substantial competitor with soybeans. These can take away vital water from the vulnerable growing plants, as well as reduce the availability of soil nutrients. If these weeds are also resistant to glyphosate (very possible in fields where glyphosate is regularly applied) and other common weed killers then they will become strongly established and significantly reduce yields later in the season. Denser soybean plantings were not very effective at suppressing these weeds.

In contrast, pre-growth herbicide treatments with metolachlor and fomesafen (sold under the brand name Prefix from Syngenta) were very effective at suppressing early season weeds and allowed the soybeans to enjoy the greater potential to develop in a less dense canopy. Fewer weeds in these plots also lowered the number of plants that could become resistant to herbicides, an important factor for farmers looking to achieve integrated weed management in their fields.

The scientists concluded that pre-season residual herbicides use produced maximum yields with fewer plants per hectare than late season applications. This is good news for soybean growers wondering how to control weeds this season while economizing on seeds. Though the research was done in Wisconsin, results should apply to soybean farmers everywhere.

About the author

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Michael Flood is a business writer and columnist. You can reach him at [email protected]

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