Pre-harvest is changing. Typically, herbicides are used as desiccants

Pre-harvest herbicides are typically
used in lentil after the crop is matured and when the seed colour is changing.
Typically, herbicides are used as desiccants to reduce seed moister, improve
quality, and increase harvest efficiency by controlling weeds that can
interfere with harvest (Yenish and Young, 2000). Desiccation is particularly
important in lentil as its indeterminate growth pattern can reduce seed quality
by introducing a higher percentage of immature green seed into the harvest
yield as the plant continues to flower and produce seed until conditions force
the into senescence. (Saskatchewan Ministry of Agriculture, 2016). Different
herbicides need to be applied at different stages of maturity, based on seed
colour change each herbicide (Saskatchewan Ministry of Agriculture, 2016).  These herbicides are used to reduce the time
until harvest with herbicides desiccation
requiring less
time to dry down a crop
compared to other methods (Tang
et al., 1992). 

Pre-harvest herbicides can be
classified as either true desiccants or harvest aids as well as by their
modes-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 adequate
coverage to maximize the amount of plant surface area in contact with the
herbicide. (Ware and Whitacre, 2004; Schemenauer,
2011).  Contact herbicides have little or no systemic
activity.  Harvest aids consist of
herbicides with systemic qualities or contact herbicides with slower
modes-of-action and some limited systemic properties. Systemic herbicides are
takin in by the plant and translocated to other parts of the plant (Baumann,
2008).

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Several
desiccants and harvest aids are actively used in Canada for a variety of
crops.  Some of these herbicides include
diquat, glufosinate, saflufenacil, pyraflufen, flumioxazin, and
glyphosate. 

In western Canada,
diquat (group 22) is registered as a
true desiccant for lentil as Reglone® (Saskatchewan Ministry of
Agriculture, 2015). As a true non-selective herbicide
and desiccant, diquat is
very
effective as a crop dry down herbicide (Cobb and
Reade, 2010; Zagonel 2005; Saskatchewan Ministry of
Agriculture, 2016). 
Diquat does not affect seed germination in other crops as its rapid
breakdown of tissue impedes it limited systemic properties (Zagonel 2005; Saskatchewan Ministry of
Agriculture, 2016). Diquat has no follow crop
restrictions 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 important
to ensure good coverage for maximum efficacy (Zagonel, 2005; Saskatchewan Ministry
of Agriculture, 2016).  Diquat’s mode of
action affects photosynthesis in Photosystem I by disrupting the cell membranes
and blocking protein and lipids (Cobb and Reade, 2010). This blocking of proteins and
lipids produces peroxide radicles, creating a quick dry down and plant death (Black
and Meyers, 1966). The pre-harvest interval for lentil and Reglone is 4 to 7
days after application (Saskatchewan Ministry of Agriculture, 2016).

Glufosinate
is registered in western Canada as a desiccant under the trade name Good
Harvest® (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 glutamine
synthesis and reducing the conversion of glutamate and ammonium into glutamine
which reduces photosynthesis effectiveness (Cobbe and Reade, 2010). Like
diquat, glufosinate has no residual properties with rapid breakdown in the soil
and 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 a
harvest aid for red lentil varieties under the trade name Heat LQ® (Saskatchewan
Ministry of Agriculture, 2017).   Saflufenacil is a Group 14 herbicide that
inhibits the protoporphyrinogen IX oxidase (PPO) enzyme, which converts protoporphyriogen IX
to protoporophyrin IX (Grossman et al., 2010; Soltani et al.,
2010). This prevents the biosynthesis of chlorophyll and causes the cell
membranes to die (Dayan et al., 2010; Grossman et al., 2010). The pre-harvest
interval is 3 days and the application timing for saflufenacil is approximately
when 15% of the bottom pods are brown and rattle when shaken (Saskatchewan
Ministry of Agriculture, 2016). 
Saflufenacil has both contact and systemic properties which makes it a
versatile herbicide.  Like all contact herbicides
water coverage is important.  It is
interesting to note that saflufenacil is unique in that it has some mobility in
both the xylem and phloem unlike other PPO inhibiting herbicides with systemic
activity only through the xylem (Ashigh and Hall, 2010, Soltani et al., 2010).

Flumioxazin
and pyraflufen-ethyl are also Group 14 PPO
inhibiting herbicides used in other
crops throughout Canada (Saskatchewan Ministry of Agriculture, 2016).  Both products are used to dry down certain
crops.  Flumioxazin is registered in
Manitoba under the
commercial name
Valtera®
and is only registered for application
prior to soybean
seeding
and dry bean desiccation (Valent Canada, Inc., 2009; Soltani
et al, 2013). Pyraflufen-ethyl is
also used as a desiccant for cotton and potatoes (Nichino Europe Co. Limited,
2012).  Neither flumioxazin or pyraflufen
are registered in lentil.

Glyphosate (often
referred to as Roundup) is registered in lentil as a harvest aid (Saskatchewan
Ministry of Agriculture, 2016) As a non-selective, systemic herbicide,
glyphosate can translocate throughout the plant phloem and xylem and slowly inhibits
plant growth (Cobb and Reade, 2010; Saskatchewan Ministry of Agriculture, 2016).  Glyphosate has no contact properties and does
not have any residual soil properties that would affect the germination or
growth of following crops (Saskatchewan Ministry of Agriculture, 2016).  Glyphosates systemic properties mean that
application timing is important.  Harvest
aid applications made too early can have a negative effect on seed quality with
chemical residue in the seed which is why glyphosate is not registered as a
harvest aid for any crop grown for seed (Wilson
and
Smith 2002; Saskatchewan Ministry of Agriculture, 2016). Glyphosate
was first developed my Monsanto in 1971 by
Dr. John E. Franz and first marketed in 1974. (Ashigh
and Hall, 2010; Franz et al., 1997).  Glyphosate is now an important global
herbicide and has changed the type of cropping system farmers now use today
from conventional to minimal or no tillage (Franz et al., 1997). The unique
mode-of-action of glyphosate is the inhibition of enolpryruvyl-shikimate
phosphate (EPSP) synthase which is an enzyme used to produce amino acids in the
shikimate pathway. (Ashigh and Hall, 2010; Cobb and Reade, 2010). Final plant
death occurs due to the inhibition of photosynthesis, as the plant cannot
create proteins stemming from the buildup shikimate-3-phosphate (Franz et al.,
1997; Duke and Powles, 2008). Glyphosate is typically applied to lentil when
the lower 35% of the pods have turned brown (Schemenauer, 2011; Saskatchewan
Ministry of Agriculture, 2014).