It's been a wet spraying season in Iowa this year, and many farmers and custom applicators are running their sprayers through the fields faster than normal to try to make up for lost time. Sprayers around the state are poised for postemergence herbicide application as the soil dries in soybean fields and for mid-season applications of fungicide and/or insecticide.
Applicators using ground sprayers for herbicide may be using higher speeds to catch up with weeds and cover more acres in a narrow time window. If so, they should consider the travel speed they've used to calculate appropriate nozzle size, says Mark Hanna, an Iowa State University Extension ag engineer. If field speed increases by 25% (for example going from 12 to 15 miles per hour), spray pressure for a given nozzle must increase by 56% (from 40 psi to 63 psi) to maintain output. A better choice to reduce smaller droplets and minimize drift would be to increase nozzle size, he says.
What's the potential yield impact of wheel tracks?
A common concern with mid-summer applications when using ground-driven sprayers in soybeans is the potential impact of wheel tracks on bean yields. Research suggests that an adequate soybean stand (more than 100,000 plants per acre) planted in late April through mid-May can compensate for wheel tracks made when a field is sprayed at R1 growth stage of soybeans.
Yield loss can occur, however, when wheel tracks are made at the R1 or later growth stage in thin soybean stands (less than 100,000 plants per acre) or in late planted soybeans. "After the various hailstorms recently, we have a lot of fields of thin stands and of later planted beans, unfortunately," notes Clarke McGrath, an ISU Extension field agronomist.
Average yield loss for wheel tracks depends on sprayer boom width
Regardless of stand, plants could not compensate for wheel tracks made at R3 growth stage (early pod development) or R5 (early seed development). The average yield loss per acre is based on sprayer boom width (distance between wheel track passes). In ISU trials yield losses averaged 2.5%, 1.9% and 1.3% when sprayer boom widths measured 60, 90 and 120 foot, respectively. Multiple trips along the same wheel tracks did not increase yield loss over the first trip.
Comparison of ground vs. air applications in the Midwest is limited, says Hanna. An Iowa Soybean Association study on corn showed only a 0.2 bushel per acre difference between ground vs air. Both of these application methods can potentially be very effective on insect and disease pests in corn and soybean fields, but are dependent on operator/pilot skill and ensuring the right spray parameters for each type of application.