The application of herbicides prior to seeding cereal, oilseed or pulse row crops, otherwise known as ‘pre-seed burnoff’, is a common crop production practice in Western Canada. Conventional practice is to spray entire fields with select herbicides using a blanket or full spray application, i.e., herbicide is applied at the same rate across the entire area to be seeded. This can result in the wasteful application of herbicides to random areas or pockets dispersed throughout a field where there are no weeds present. This can pose a number of negative impacts to the environment, including soil health; water quality, following runoff of chemicals into surface water bodies like creeks; infiltration into groundwater sources; air quality, with the possibility of subsequent ecological damage to trees or other vegetation from spray drift.

Precision spray application of herbicides provides the opportunity to apply herbicides exactly where the treatment is required, i.e., directly on weeds. Although it is meant to curtail environmental risks, pre-seed burnoff precision sprayers still have some of their own limitations. For instance, the inability of the accompanying camera technology to detect the presence of tiny weeds can result in unsprayed weeds that thrive, only to compete with the crop for essential resources – soil, water and light – needed to sustain growth.

This research project sought to investigate the performance and effectiveness of a precision sprayer technology, WEED-IT, when used for pre-seed burnoff weed management in Western Canada. WEED-IT has three spray modes – a full spray (blanket) mode; a spot spray (precision spray) mode; and a bias spray mode (i.e., a combination of full spray at a reduced application rate, boosted by spot spray on detected weeds).

In 2020, a range of functionality and operational performance tests were conducted on the precision sprayer. This resulted in some enhancements or corrective modifications to the sprayer technology by the manufacturer.

In 2021, the first field trial was completed on strip plots. The effects of four pre-seed burnoff spray treatments on weed density and the amount of canola harvested were studied. Three of the four spray treatments were the three WEED-IT spray modes – full, bias and spot – while the fourth treatment was a control, i.e., there was no pre-seed burnoff herbicide application. In keeping with conventional crop management, fertilizer was applied to the strip plots prior to the application of the spray treatments; a herbicide-tolerant canola variety was seeded; and two herbicide applications were applied at different growth stages of the canola.

Compared to the full spray application, between 68% and 97% less herbicide was applied in spot spray mode. However, this also corresponded to a lower reduction in weed density, ranging between 55% and 81% under the spot spray treatment, versus 79% to 96% under the full spray mode. Under the bias spray mode, the reduction in weed density ranged between 82% and 88%. About 19% to 26% less herbicide was applied in bias spray mode versus full spray mode.

There were no significant differences in the amount of canola harvested between the sprayed strip plots and the control strip plots, regardless of spray mode. In other words, the two, post emergence, herbicide applications at the different growth stages of the canola had optimum effect on weed management in this trial. This was evident from the amount of canola harvested from plots without the pre-seed burnoff herbicide application. This strongly suggested that the full spray, bias spray and spot spray, pre-seed burnoff herbicide applications were unnecessary.

In 2022, the field trial was repeated with a different crop, using the same strip plots. A herbicide-tolerant barley variety was seeded following the pre-seed burnoff herbicide applications. Under the spot spray application, between 89% and 98% less herbicide was applied compared to the full spray application. Similar to 2021, the reduced herbicide use under the spot spray application resulted in less reduction in weed density (83% to 88%) compared to the full spray application. Conversely, about 13% more herbicide was applied after the bias pre-seed burnoff spray application versus the full spray application.

A similar outcome was observed at the end of the 2022 field trial in relation to the amount of barley harvested from the strip plots under the different herbicide spray treatments. Again, there were no significant differences in the amount of harvested barley between all treatments, including the control plots that did not receive a pre-seed burnoff spray application.

In both trial years, all pre-seed burnoff spray applications did not result in a greater harvest compared to the non-application of pre-seed burnoff herbicides. Hence, the practice of pre-seed burnoff spray application does not seem to be essential, i.e., in cases where post-emergence herbicide applications are planned to aid weed control. However, if it is deemed an essential weed management practice, then the use of the WEED-IT precision sprayer spot spray mode for pre-seed burnoff weed management will more than likely reduce the amount of herbicide used in most, if not all pre-seed burnoff applications, of course, depending on sizes and number of weeds present.