Seed & Chemical
Soybean response to fungicides
By Treena Hein
According to OMAFRA figures, the application of foliar fungicide has increased soybean yields for Ontario growers by about 2.1 b/ac since 2005. But fungicide efficacy with different soybean varieties is a question that needs answering, given the fact that some differences may exist for corn and cereal varieties, but no differences have been documented for other legume crops like dry beans.
Disease tolerance varies for each soybean variety, so each variety should respond differently to an application of Headline,” says Chris Gillard, an assistant professor in the area of field crop agronomy and pest management at the University of Guelph-Ridgetown College.
Over the last three years, Gillard has conducted trials to investigate this question, with Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) soybean specialist Horst Bohner and collaborators Albert Tenuta (OMAFRA field crops pathologist) and Dave Hooker (field crop agronomist and assistant professor at the University of Guelph-Ridgetown College). Some additional assistance was also provided by Don Depuydt at the Huron Research Station. Gillard and his team chose one site in Chatham and one in Exeter and planted three experimental trials at each site with the same 20 Roundup Ready varieties – four each from SeCan, Hyland, Pioneer, Monsanto and Syngenta, all selected to provide a range in disease susceptibility.
Headline, a well-established BASF fungicide that is one of the most popular fungicides used by soybean farmers for plant health, was applied at all sites once at R2-3, early pod development on lower nodes. Disease ratings were done at mid-season, and again late in the season, with yield and agronomic data (on maturity, seed quality and seed weight) collected at harvest. “Septoria brown spot was the primary fungal disease we observed,” Gillard notes. “Others such as white mould, SDS, frog eye leaf spot were present at a few sites, but at very low levels.” Disease levels differed from year to year, in response to rainfall and temperature at each site.
Overall, no interaction was found between fungicide and soybean variety in the study. “This means that collectively, the varieties acted the same as if the experiment was testing only one variety,” Gillard explains. While they did find a significant interaction between Headline application and soybean variety in terms of early disease control, this only occurred with a very small number of varieties. Gillard notes that the time at which they rated early fungal disease severity was very close to the application of Headline at most of the six sites, and concludes “I am not confident that Headline was the primary factor influencing differences between varieties at that point in crop development. The varieties we selected had a range in disease susceptibility, and that was likely the primary factor for the few differences we measured in early disease ratings.”
In five of six trials, the application of Headline delayed average plant maturity by 1.6 days (1.4 percent), compared to the untreated control. “This could be considered a plant health benefit, as a delay in maturity could result in increased yield,” Gillard says. “It was also observed that leaf defoliation of the soybean plants was 17.8 percent lower for Headline-treated plots, which is another indication of a delay in plant maturity.” As expected, in all six trials, the application of Headline increased the average seed weight (by 4.5 percent compared to the untreated control), and also increased the average yield (by 176 kg/ha, or 4 percent).
“Using a 50 bu/ac crop, this translates to a boost of two bushels per acre,” notes Gillard. “Assuming soybeans valued at $12/bu, this equals a gross economic benefit of $24.00 per acre. With Headline having a product cost of approximately $15 per acre, and an application cost of approximately $8 per acre, overall our study found that its use provides a net economic return of $1 per acre.” Growers should use their own average yield x four percent, to estimate Headline’s response on their farm. In addition, Gillard stresses that this calculation does not account for yield loss due to crop tramping from sprayer tracks during the fungicide application, or any additional cost of aerial application of the fungicide, to avoid tramping. “Tramping could easily result in a two percent yield loss, which would result in a net economic loss to the grower,” he says.
According to Gillard, several US studies have shown a larger yield response to Headline in soybeans, as well as a response to specific varieties. “This is the first replicated study conducted in Ontario, and growers will have to decide if they want a follow-up study,” he says, “or if this information is enough to decide how Headline fits in their crop.”