The use of herbicides in crops has led to theevolution of herbicide resistant weeds. Worldwide, there are 487 unique cases of weeds resistant to differentmodes of action with 253 (147 dicots and 106 monocots) different species havingresistance to at least one mode of action (Heap, 2017).
Group 2 herbicides account for a largepercentage of herbicide resistance. There are approximately 159 different weed species resistant toacetolactate synthase (ALS) inhibitors (Group 2) (Heap, 2017). The popularityand repeated use and reliance on imidazolinone chemistry in lentil since theintroduction of the herbicide tolerant varieties has led to the development ofherbicide resistant weeds (Beckie et al., 2013). Imidazolinone based herbicides arecharacterized by their inhibition of the acetolactosynthase inhibitors (ALS ).Herbicides in the group the ALS/AHAS enzymes and stop the production of severalamino acids (Whitcome, 1999; Tranel and Wright, 2002). These herbicides have high efficacy andcontrol a broad range of grassy and broadleaf weeds (Whitcomb, 1999; Shaner andHornford, 2005). This class of herbicides is popular due to the low activeingredient required for weed control, reduced environmental impact, cropselectivity, soil persistence for residual weed control, and low mammaliantoxicity (Devine and Shukla, 2000; Tan et al.
2005). In Clearfield® lentil ALS resistance is a seriousconcern. Weeds that escape or emergeafter the post emergent application can cause harvest complications as well asmature and deposit more seeds into the seed bank. If these weeds escaped due toherbicide resistance than they offspring being deposited into the seed bank mayalso carry the same herbicide tolerance.With no new herbicide mode-of-action being introducedin the last 20 years there has been increasing reliance on existing herbicides.The increasing rise of herbicide resistant weeds (Heap 2016) indicates thatcurrent agronomic practices are beginning to fail as the number of weed speciesnot controlled by herbicides rises.
Otherforms or methods of weed management may become more important if moretraditional herbicide chemistries start failing to control weeds in crops. Integrated weed management (IWM) is a collection ofsystems designed to minimize the growth and reproduction of weeds including butnot limited to reduced tillage, crop rotations, agronomic practices such asseeding rates and cover crops, mechanical means such as inter row tillage (Blackshawet al., 2018; Stanley, 2016). These approaches focus on preventing weeds frommaturing but to not address how best to manage weeds once they have emergedin-crop and the risk of crop damage is too great as variable managementoptions. Other none herbicide weed seed management approachesinclude mechanical controls such as swathing, combining, in-field seeddestruction, chaff collection, windrow burning (Walsh and Newman, 2007; Walshand Powels, 2007; Walsh et al.
, 2012; Walsh and Powles, 2014). These approachesall have different measures of success controlling weeds. They also havedifferent limitations or drawbacks limit the potential, depending on the weedspecies, control time, and soil health and environmental consequences.Utilizing just one of these methods may not provide the correct degree ofcontrol to reduce the weed seed bank or prevent or stop the spread of herbicideresistant weeds. In lentil, and other crops, the use of a desiccant can providemore than only crop dry down, late in the growing season, prior to harvest.
The above-mentioned approaches can be used as controloptions to help reduce weed seed dispersion, population increases, andherbicide resistance spread. Often the decision to use any weed management toolis based off an economic threshold (ET) approach and decisions are not timebased but only focused on weed control at the current decision making monumentand not the long term implications of the spread weeds, particularly herbicideresistant weeds (Bauer and Mortensen, 1992; Norris 1999). Another approach in combating the rise of herbicide resistanceis the no-seed threshold or zero-tolerance threshold introduced by Norris aswell as Bagavathiannan and Norsworthy (Norris, 1999; Bagavathiannan and Norsworthy, 2012). They found that the economic models usedfor weed management decisions only consider the cost of the control measureversus the loss of yield if the weeds are not controlled (Zimdahl, 2004; Bagavathiannan andNorsworthy, 2012).
The zero-tolerance approach to weed management considers the contribution oflate season weeds contributing offspring to replenish the seed bank andgenerating further issues into the future (Walker and Oliver, 2008; Bagavathiannan andNorsworthy, 2012). Currently. the critical period of weedcontrol looks to limit crop loss due to weeds and does not consider futureimplications of later emerging weeds producing offspring (Gallandt, 2006). According to Davis, the size of the seed bank andthe seed production every growing season are critical to the long-term survivalor success of weed populations (Davis et al., 2003).
Understanding theimportant of considering a long term view on weed management is importantbecause weed control programs that do not consider the future growing seasonswill not help to manage overall weed population growth or to help reduce or delaythe emergence of herbicide resistance developing (Gallandt, 2006). This approach is particularly important toconsider when contemplating the rise of herbicide tolerant weeds. In situations where late weed escapes are nota product of herbicide resistance they still produce offspring that are addedto the seed bank, but those offspring should then be controlled by the nextherbicide application they are sensitive to.
With herbicide resistant weeds that escape control of a PRE or POSTherbicide application, they may not be controlled by the next mode of actionand continue to grow and deposit seeds increasing in number and causing yieldlosses. If POST applications fail tocontrol herbicide resistant weeds then the next herbicide timing that can preventthe weeds from producing offspring are pre-harvest application of herbicides (Bagavathiannanand Norsworthy, 2012).