Early maturing crops reduce wild oat densities
Early maturing crop rotations that included winter wheat had reduced wild oat populations compared to normal or late maturing rotations. This was likely because of increased crop competitiveness, rotational diversity and possibly harvest weed seed control.
The evolution of herbicide-resistant wild oats has made wild oat control increasingly difficult. The most recent survey in Manitoba found that 100% of wild oat samples were resistant to at least one herbicide mode of action, and in Saskatchewan 77% were also resistant to at least one mode of action. Additionally, multiple herbicide resistance to Group 1 and 2 herbicides are becoming more common. Recent research has focussed on integrated weed control to help manage weeds, and harvest weed seed control (HWSC) is seen as one of those tools.
A three-year research study was conducted at Lacombe and Beaverlodge, AB, Scott, SK, and Carman, MB from 2016 to 2018. The objective was to see if combinations of HWSC, earlier maturing crops, and swathing versus straight combining would affect wild oat populations. The premise was that HWSC combined with swathing of earlier maturing, more competitive crops would lead to the lowest in-crop wild oat populations.
The field trials were set up as a three-year rotational cropping system, and the results are the cumulative effects of the cropping system after three years.
The early maturing rotation was pea followed by winter wheat. The intermediate maturing rotation (normal) was wheat followed by canola, and the late maturing rotation was faba bean followed by flax. In the third year, barley was grown across all rotations to measure the treatment impacts on wild oat populations.
Crops were either swathed or straight combined. Because HWSC utilizing a weed seed destructor is not possible with plot combines, chaff collection was utilized to mimic the number of wild oat seeds collected and destroyed by a commercial weed seed cage mill.
In the first year, a glyphosate plus bromoxynil pre-seed burndown was applied, and subsequently, only broadleaf herbicide treatments were applied to the treatments – no wild oat herbicides were applied. Fungicides and insecticides were applied as required. Desiccation with saflufenacil (Heat LQ) was allowed as needed in straight-cut treatments.
Wild oat densities were lowest in early maturing rotations
Wild oat plant densities were measured each year just prior to in-crop broadleaf herbicide application. The early maturing rotation had a wild oat density that was 50% less than the normal and late maturing rotations after three years. The highest wild oat density was from the later maturing, straight cut treatment.
There was also an interaction between crop rotations and harvest management. The swathing treatment had lower wild oat densities in the normal and later maturing crop rotations. However, the results were quite variable across the sites. For the early maturing crop rotation, harvest treatment did not affect wild oat density.
The amount of wild oat seed collected in the chaff was variable between crop rotations, harvest type and sites. At each site, there was large variability and little consistency to the amount of wild oat seed collected in the chaff. This may have been due to differences in chaff collection devices that were fabricated differently at each site, differences in combine settings, and variable weather conditions. Unfortunately, this meant that the researchers couldn’t conclude that HWSC was a variable that contributed to lower wild oat densities in early maturing crop rotations.
Because wild oat herbicides were not used in the study, wild oat plant densities measured in the spring increased over time, although differently for crop rotation treatments with the early maturing rotation having significantly lower densities. This highlights the need for in-crop wild oat control along with the use of integrated weed management tools such as increasing seeding rates, crop silaging, and growing early maturing crops with HWSC.
Wild oat population densities over time in each of the six treatments included in the crop rotation maturity, harvest management and harvest weed seed control effect on wild oat populations study
Source: Tidemann et al. 2025
Wild oat densities in the soil seed bank were calculated in 2018 shortly after the final harvest. Twelve soil cores of 4 inches (10 cm) diameter were collected to a depth of 2 inches (5 cm) per plot. Wild oat seed was separated from the soil and counted.
Soil seed bank wild oat densities mirrored the results from plant densities measured in the spring. Crop maturity affected seedbank densities while swathed vs straight cut harvesting did not. The earliest maturing rotation had the lowest density at 750 seeds/ft2 (7,500 seeds/m2), which was slightly more than the 1400/ft2 (14,000/m2) in the late maturing rotation. The normal crop rotation was intermediate with seedbank densities around 1000/ft2 (10,000/m2).
Wild oat seedbank densities as affected by crop rotation maturity and harvest management, measured in 2018.
Source: Tidemann et al. 2025
Even though the use of chaff collection as a HWSC tool was inconclusive on its effect on wild oat management, the research does show that an early maturing crop can improve wild oat control because of its early time of emergence and increased crop competitiveness compared to spring annual crops.
Funding was provided by Alberta Canola.
Tidemann BD, Harker KN, Gulden RH, et al. Cropping system rotation in combination with harvest weed seed control for wild oat (Avena fatua) management. Weed Technology. Published online OPEN ACCESS 2025:1-32. https://doi.org/10.1017/wet.2025.14
Photo by Breanne Tidemann
