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Agronomy Update
Agronomy update: Integrated weed management to control herbicide-resistant weeds

March 8, 2024  By Bruce Barker


Herbicides remain the foundation of weed control, but additional tools are needed to provide alternative weed control strategies, which can also help to slow the development of herbicide-resistant weeds. Over the years, integrated weed management strategies (IWM), such as increased seeding rates and more diversified crop rotations, have been researched to assess their impact on weed control.

 The objective of a recent study led by weed scientists at Agriculture and Agri-Food Canada at Lacombe, Alta., was to further IWM strategies that had previously shown potential on some weed species to see if they could be used to help control the entire weed community. Additionally, the impact of harvest weed seed control (HWSC) was assessed as an addition to IWM. 

Research was conducted at six locations from 2016 to 2020: Beaverlodge, Lacombe and Lethbridge, Alta.; Scott and Saskatoon, Sask.; and Carman, Man. Fourteen treatments using IWM strategies, such as rotational crop diversity including winter annuals and perennials, increased seeding rates, crop silaging and chaff collection were tested — with or without in-crop herbicides. 

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Each treatment was initiated in 2016 with all plots seeded to wheat at a seeding rate of 2× the normal rate, and no herbicide or HWSC was used in 2016. During the next three years, the IWM treatments were applied. In total, there were 14 different treatments. 

Each year, the treatments were described by the crop grown, the seeding rate of the crop, whether or not herbicides were used, and whether or not harvest weed seed control was incorporated through the use of chaff collection. 

One IWM treatment compared was crop seeding rate. Spring and winter cereals were seeded at 1× and 2× the typical seeding rates, and canola, field pea and faba bean at 1× and 1.5× the typical seeding rates. Alfalfa was seeded at 1× rate. 

The HWSC strategy used in the trial was chaff collection and removal. This simulated the use of a physical impact mill that is of interest in Western Canada but not available for plot combines.

 The standard crop rotation was wheat-canola with herbicides and no HWSC. Various diversified crop rotations included alfalfa, faba bean, pea, winter wheat, winter triticale, barley for grain and fall rye. For example, a diversified rotation utilizing spring crops included spring wheat/faba bean/barley/canola/spring wheat, and this rotation included treatments with and without herbicides, standard and increased seeding rates and with or without HWSC.

 Another diversified crop rotation was spring wheat/pea/winter wheat/canola/spring wheat. Again, this rotation included treatments with and without herbicides, standard and increased seeding rates, and with or without HWSC.

 An IWM treatment in some rotations was early-cut barley silage harvested one week after head emergence (Zadoks 65) to control weeds prior to seed shed. An example of this crop rotation was spring wheat/silage barley/fall rye/canola/spring wheat. This rotation utilized the high seeding rate for each crop, with and without herbicides and HWSC in non-silage years. The cumulative effects of the treatments were measured in the final year of the study in 2020. In this year, wheat was seeded at 2× rate with no herbicides or HWSC. This eliminated the confounding effects of different crop types and the reduction of weeds due to herbicides or HWSC.

 Overall, the data analysis found success in managing some species of weeds, while other weeds were not successfully managed with these IWM strategies. Wild oat weed densities were lower with increased seeding rates, two years of early-cut silage barley and a rotation with competitive winter cereals, even when no in-crop herbicides were applied.

Conversely, IWM strategies that did not result in improved wild oat control were the use of winter cereals that had poor overwinter survival, increased seeding rate as a stand-alone IWM strategy and diversified spring annual crops. Similar results were found with green and yellow foxtail species.

 The impact of IWM treatments on broadleaf weeds was more variable. Treatments with IWM had improved weed control of lamb’s quarters, cleavers, kochia, wild mustard and redroot pigweed.

However, IWM did not have an impact on roundleaf mallow, hemp-nettle, henbit and narrowleaf hawksbeard. Chaff collection was not a replacement for herbicides, but it did result in incremental control of weeds when incorporated into a weed control strategy.

 The density of weeds was also an important factor in the success of IWM. Where weed densities were high, IWM was less successful in improving weed control and reducing the weed seed bank. This indicates the importance of implementing IWM sooner rather than later to help better manage weeds.

 Overall, the research showed IWM strategies can be effective in reducing the reliance on herbicides and improving weed control for some weed species. It also illustrated the need for further research to understand how IWM can fit into farming practices in Western Canada and what specific strategies are best for which species. 


Bruce Barker divides his time between CanadianAgronomist.ca and as Western Field Editor for Top Crop Manager. CanadianAgronomist.ca translates research into agronomic knowledge that agronomists and farmers can use to grow better crops. Read the full Research Insight at CanadianAgronomist.ca.

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