To determine the distribution and abundance of GR or auxinic-resistant kochia and other targeted weeds, including Russian thistle, waterhemp, and ragweed species, in Manitoba in 2018, Saskatchewan in 2019, and Alberta in 2021 through a survey of 300 sites in each province.
Kochia is a problem weed in Prairie field crops which reduces crop yield and impedes harvest. Kochia is mainly managed with herbicides and has become resistant to several. Herbicide-resistant kochia will survive and reproduce when sprayed with a herbicide which normally kills it. The study objective was to survey glyphosate-resistant and dicamba-resistant kochia within Manitoba in 2018, Saskatchewan in 2019, and Alberta in 2021. Kochia seed was collected at approximately 300 sites in each province. Samples were threshed and seeded in the greenhouse then sprayed with labeled doses of glyphosate or dicamba then evaluated after 3 or 4 weeks, respectively.
The occurrence of glyphosate-resistant kochia has increased from previous surveys, found in 58% of Manitoba sites, 87% of Saskatchewan sites, and 78% of Alberta sites. Dicamba-resistant kochia was found at 1% of Manitoba sites, 45% of Saskatchewan sites, and 28% of Alberta sites. This documents the first instance of dicamba-resistant kochia in Manitoba. Previous surveys have shown that all kochia is resistant to ALS-inhibitors (Group 2) so triple-resistant kochia was present in 40% of Saskatchewan samples, <1% of Manitoba sites, and 10% of Alberta sites. Additional screening showed 44% of Alberta sites were fluroxypyr-resistant and 25% were triple-resistant.
Herbicide-resistant kochia has become widely distributed on the Prairies. Best management practices (BMPs) for controlling herbicide-resistant kochia are grouped into four categories: A) the premise BMP, B) knowledge BMPs, C) control BMPs, and D) space-time BMPs. The premise BMP is to reduce the soil seedbank.
The knowledge BMPs include scouting routinely and understanding weed biology. Scouting will be critical; a GPS can be used to map patch size over time to evaluate how effective management strategies were.
The control BMPs include: 1) use a diversified approach, 2) use full labeled herbicide doses to the proper stage, 3) rotate modes of action, 4) use cultural practices to achieve canopy closure rapidly, and 5) use mechanical control strategies when possible. A diversified approach incorporates many management strategies including cultural, chemical, and mechanical control. The aim is to reduce how many weeds a herbicide must kill at any time by incorporating non-chemical control strategies into management programs.
Cultural methods should aim to achieve canopy closure of the crop. The main principle is to have the crop use the available space to intercept sunlight (among other resources) so it is unavailable to kochia. Cultural strategies include: using crop rotation, competitive cultivars, increased seeding rates, and narrow row spacing.
The space-time BMPs include: 1) managing weeds prior to seeding, 2) managing weeds at or after harvest, 3) preventing within and between field movement of kochia seed and tumbleweeds, and 4) managing field borders. Pre-seed herbicide burn-down and spot tillage may be used prior to seeding to manage emerged kochia. Seed destructor mills for combines are available on the Prairies, have shown to destroy kochia seed, but longer term control requires further study. Depositing chaff on marginal kochia patches may help limit emergence and move weed seed found in chaff out of the field. Sanitation of seeders and combines is critical to prevent introducing herbicide-resistant kochia seed across and between fields.