Pre-harvest herbicides are typicallyused in lentil after the crop is matured and when the seed colour is changing.Typically, herbicides are used as desiccants to reduce seed moister, improvequality, and increase harvest efficiency by controlling weeds that caninterfere with harvest (Yenish and Young, 2000). Desiccation is particularlyimportant in lentil as its indeterminate growth pattern can reduce seed qualityby introducing a higher percentage of immature green seed into the harvestyield as the plant continues to flower and produce seed until conditions forcethe into senescence. (Saskatchewan Ministry of Agriculture, 2016). Differentherbicides need to be applied at different stages of maturity, based on seedcolour change each herbicide (Saskatchewan Ministry of Agriculture, 2016).
These herbicides are used to reduce the timeuntil harvest with herbicides desiccationrequiring lesstime to dry down a cropcompared to other methods (Tanget al., 1992). Pre-harvest herbicides can beclassified as either true desiccants or harvest aids as well as by theirmodes-of action (contact or systemic.
) True desiccants are used to rapidly dry down plants. These desiccants are often contact based,though not all contact based herbicides are true desiccants, and need adequatecoverage to maximize the amount of plant surface area in contact with theherbicide. (Ware and Whitacre, 2004; Schemenauer,2011). Contact herbicides have little or no systemicactivity. Harvest aids consist ofherbicides with systemic qualities or contact herbicides with slowermodes-of-action and some limited systemic properties. Systemic herbicides aretakin in by the plant and translocated to other parts of the plant (Baumann,2008).
Severaldesiccants and harvest aids are actively used in Canada for a variety ofcrops. Some of these herbicides includediquat, glufosinate, saflufenacil, pyraflufen, flumioxazin, andglyphosate. In western Canada,diquat (group 22) is registered as atrue desiccant for lentil as Reglone® (Saskatchewan Ministry ofAgriculture, 2015). As a true non-selective herbicideand desiccant, diquat isveryeffective as a crop dry down herbicide (Cobb andReade, 2010; Zagonel 2005; Saskatchewan Ministry ofAgriculture, 2016). Diquat does not affect seed germination in other crops as its rapidbreakdown of tissue impedes it limited systemic properties (Zagonel 2005; Saskatchewan Ministry ofAgriculture, 2016). Diquat has no follow croprestrictions as is has strong soil binding properties with the negative charged particles (Cobb and Reade,2010; Saskatchewan Ministry of Agriculture, 2016).
As a contact herbicide, it is importantto ensure good coverage for maximum efficacy (Zagonel, 2005; Saskatchewan Ministryof Agriculture, 2016). Diquat’s mode ofaction affects photosynthesis in Photosystem I by disrupting the cell membranesand blocking protein and lipids (Cobb and Reade, 2010). This blocking of proteins andlipids produces peroxide radicles, creating a quick dry down and plant death (Blackand Meyers, 1966). The pre-harvest interval for lentil and Reglone is 4 to 7days after application (Saskatchewan Ministry of Agriculture, 2016). Glufosinateis registered in western Canada as a desiccant under the trade name GoodHarvest® (Fleury, 2015; Saskatchewan Ministry of Agriculture, 2016).Glufosinate is non-selective herbicide with limited translocating properties.
In western Canada, it is more commonly known as the post-emergent herbicide Liberty®for in-crop use in Invigor canola varieties. As a Group 10 mode-of-action, Glufosinate works by inhibiting glutaminesynthesis and reducing the conversion of glutamate and ammonium into glutaminewhich reduces photosynthesis effectiveness (Cobbe and Reade, 2010). Likediquat, glufosinate has no residual properties with rapid breakdown in the soiland water volume is also important to insure adequate coverage across the plant(Saskatchewan Ministry of Agriculture, 2016) For lentil desiccation,glufosinate (as Good Harvest®) application is when 40 to 60% of pods turn yellow or brown (Saskatchewan Ministry of Agriculture, 2017)Saflufenacil is now registered in Western Canada as aharvest aid for red lentil varieties under the trade name Heat LQ® (SaskatchewanMinistry of Agriculture, 2017). Saflufenacil is a Group 14 herbicide thatinhibits the protoporphyrinogen IX oxidase (PPO) enzyme, which converts protoporphyriogen IXto protoporophyrin IX (Grossman et al., 2010; Soltani et al.
,2010). This prevents the biosynthesis of chlorophyll and causes the cellmembranes to die (Dayan et al., 2010; Grossman et al., 2010). The pre-harvestinterval is 3 days and the application timing for saflufenacil is approximatelywhen 15% of the bottom pods are brown and rattle when shaken (SaskatchewanMinistry of Agriculture, 2016). Saflufenacil has both contact and systemic properties which makes it aversatile herbicide. Like all contact herbicideswater coverage is important.
It isinteresting to note that saflufenacil is unique in that it has some mobility inboth the xylem and phloem unlike other PPO inhibiting herbicides with systemicactivity only through the xylem (Ashigh and Hall, 2010, Soltani et al., 2010).Flumioxazinand pyraflufen-ethyl are also Group 14 PPOinhibiting herbicides used in othercrops throughout Canada (Saskatchewan Ministry of Agriculture, 2016).
Both products are used to dry down certaincrops. Flumioxazin is registered inManitoba under thecommercial nameValtera®and is only registered for applicationprior to soybeanseedingand dry bean desiccation (Valent Canada, Inc., 2009; Soltaniet al, 2013). Pyraflufen-ethyl isalso used as a desiccant for cotton and potatoes (Nichino Europe Co. Limited,2012). Neither flumioxazin or pyraflufenare registered in lentil.Glyphosate (oftenreferred to as Roundup) is registered in lentil as a harvest aid (SaskatchewanMinistry of Agriculture, 2016) As a non-selective, systemic herbicide,glyphosate can translocate throughout the plant phloem and xylem and slowly inhibitsplant growth (Cobb and Reade, 2010; Saskatchewan Ministry of Agriculture, 2016). Glyphosate has no contact properties and doesnot have any residual soil properties that would affect the germination orgrowth of following crops (Saskatchewan Ministry of Agriculture, 2016).
Glyphosates systemic properties mean thatapplication timing is important. Harvestaid applications made too early can have a negative effect on seed quality withchemical residue in the seed which is why glyphosate is not registered as aharvest aid for any crop grown for seed (WilsonandSmith 2002; Saskatchewan Ministry of Agriculture, 2016). Glyphosatewas first developed my Monsanto in 1971 byDr. John E.
Franz and first marketed in 1974. (Ashighand Hall, 2010; Franz et al., 1997). Glyphosate is now an important globalherbicide and has changed the type of cropping system farmers now use todayfrom conventional to minimal or no tillage (Franz et al., 1997). The uniquemode-of-action of glyphosate is the inhibition of enolpryruvyl-shikimatephosphate (EPSP) synthase which is an enzyme used to produce amino acids in theshikimate pathway.
(Ashigh and Hall, 2010; Cobb and Reade, 2010). Final plantdeath occurs due to the inhibition of photosynthesis, as the plant cannotcreate proteins stemming from the buildup shikimate-3-phosphate (Franz et al.,1997; Duke and Powles, 2008). Glyphosate is typically applied to lentil whenthe lower 35% of the pods have turned brown (Schemenauer, 2011; SaskatchewanMinistry of Agriculture, 2014).