The impact of delayed post-emergence herbicide in corn

Date:09-21-2022

By Dr. Peter Sikkema

Ontario farmers grow most of the corn produced in Canada, with 2.2 million acres produced in 2020, a value of more than $2 billion. However, the crop is sensitive to early weed interference. The Weed Science Society of America (WSSA) reported an average corn yield loss of 50 per cent if weeds are left uncontrolled.

Wild buckwheat was one species found at the Exeter site researching the effects of delayed herbicide applications. Photo: Mike Cowbrough, OMAFRA

The optimal time to control weeds is field-specific and influenced by weed species composition, weed density, competitive indices of each species, the relative time of weed and crop emergence, tillage practices, row spacing, nutrient availability and environmental factors.

A study of nine field experiments was conducted from 2017 to 2019 at Exeter and Ridgetown in Ontario. Seed preparation at all sites consisted of fall moldboard plowing followed by two passes with a field cultivator in spring. Experiments were arranged in a random block design with four replications and included a weedy control, a weed-free control and six post-emergence treatments. The first herbicide application was made when weeds were 5, 10, 15, 20, 30, or 50 cm in height.

Growers are advised to keep corn weed-free from emergence until the V6 stage to minimize yield loss. photo: Mike Cowbrough, OMAFRA

Weed species composition at Exeter included green foxtail, wild mustard, lamb’s quarters, flower-of-an-hour, redroot pigweed, smartweed, wild buckwheat, lady’s thumb and barnyard grass. At Ridgetown, weed species composition consisted of green foxtail, barnyard grass, lamb’s quarters, velvetleaf, common ragweed and pigweed.

An obvious difference between the two groups of environments was the overall weed density: two environments had lower weed densities ranging from 12 to 82 weeds per square metre at individual application timings, averaging 57 weeds per square metre for the season. Seven environments had higher densities, ranging from 132 to 411 weeds per square metre at individual application timings and averaged 148 to 353 weeds per square metre.

Results

Relative corn yield was calculated as a function of four different parameters: weed size at the time of herbicide application, days after crop emergence (DAE), crop heat units (CHUs) accumulated from planting and crop stage. The delay in herbicide application that caused a one, 2.5, five, 10, 25, and 50 per cent corn yield loss was determined.

Table 1: Corn yield loss due to delayed herbicide application based on weed size at time of herbicide application (bu/ac). photo: Source: Dr. Peter Sikkema, University of Guelph, Ridgetown Campus

Overall, research indicated that corn is very sensitive to weed interference. Relative yield decreased with increased weed size, DAE, CHU from planting and corn growth stage.

The results of this study highlight the importance of a planned two-pass weed control program. Growers are advised to keep corn weed-free from emergence until the V6 stage to minimize any yield loss from weed interference.

Funding for this project was provided in part by the Grain Farmers of Ontario and the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA).