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Desiccate or wait? – it depends

Desiccating peas can mean a better quality crop in the bin sooner, but products are not interchangeable.

November 19, 2007  By Helen McMenamin

Desiccating peas shortens the time from physiological maturity to combining.
It is worthwhile if time management is critical or if you can improve the quality
of the peas going in the bin by a grade or two.

"A grade or two can bring an extra dollar a bushel, or even two,"
says Ken Lopetinsky. "That sort of price boost easily covers the cost of
the desiccant and application, if the crop has a reasonable yield potential.
But, the application must be at the right time or there's no quality benefit.
You can also lose yield through poor timing."

The Alberta Agriculture pulse research agronomist set up trials in the Edmonton
region and sprayed both Reglone and Roundup on green and yellow peas at three
different dates relative to physiological maturity. That is the point at which
seed filling is complete and the crop just needs to dry to be ready to combine.


Yield and quality dropped when the drydown chemical was applied too early,
the trials concluded.

Applying Reglone or glyphosate 10 days too soon, before upper pods developed
the orange peel texture of mature pods, reduced seed size of both green and
yellow peas and as a result yields were also lower. Yields dropped by as much
as 21 percent. Seed size was reduced by three to 11 percent in 2002 in all applications.

Spraying four days before maturity, when upper pods were just getting their
leathery 'orange peel' texture with soft, wet seeds that dented easily, caused
a slight yield loss that was not significant.

At one site in one year of Lopetinsky's study, July temperatures were very
high and caused flowers to abort. In this situation, there was very little difference
in the maturity of the lowest and highest pods, and desiccation offered no significant
advantage in terms of yield or quality.

A green pea variety, Nitouche, had fewer bleached seeds and higher quality
after Reglone application, whether it was applied early or at maturity. The
best quality Swing yellow peas were harvested after Reglone application at maturity.
"There's a narrow window for application for desiccant," says Lopetinsky.
"Read and follow the label directions closely to get your best return out
of harvest management chemicals.

"Buyers of peas for human consumption are strict in their demands. Peas
have to grade #1 or #2 and they won't accept any green peas with more than three
percent bleached or four percent shrivelled seeds. For yellow peas, the maximum
is two percent green and five percent shrivelled. If you miss these standards,
your peas are feed, worth $50 or $100 less per acre."

The decision whether to use a desiccant depends on your need to manage your
harvest workload and the risk of the crop losing quality if you leave it to
ripen naturally.

"It's a matter of balancing the value of premium quality against the cost
of chemical and application," says Lopetinsky. "There's little risk
of losing quality as the crop dries naturally when the lowest and top pods have
similar maturity. The risk is greatest when flowering has been prolonged by
a good growing season. The lowest pods can be mature, with orange peel textured
pods and seeds detached from the pod, while top pods are still smooth with soft
seeds that don't split in half when squeezed."

Although Lopetinsky's research has shown quality and yield benefits from pre-harvest
glyphosate, it is not a true desiccant. On the other hand, diquat, the active
ingredient in Reglone, is a true desiccant that dries the crop on contact. Its
herbicide effect is secondary to its drying effect on plants. Generally, you
can spray and plan on combining five to seven days later. But, if there is a
couple of weeks of wet weather, the crop may turn green again.

Plants yellow and start to dry almost immediately on contact with diquat. Leaves
can crisp within hours, but heavier plant parts such as stems take longer to
dry. This can cause harvest problems, so spraying in the evening is sometimes
advised to slow the process.

Glyphosate is a systemic herbicide with the secondary effect of desiccating
the crop. Because the chemical must be absorbed and translocated to growing
points to kill plants, it takes longer to dry the crop, usually 14 to 21 days.

Pre-harvest glyphosate is the best option if you have perennial weeds poking
above the pea crop. Glyphosate has its best activity against Canada thistle
and perennial sowthistle when they are in the bud to mid-flower stage. Translocation
of food to the roots is at its peak then, and the herbicide active ingredient
piggybacks on this translocation. Dandelions are also more vulnerable at this
time. If the crop is infested with wild buckwheat, Reglone is a better herbicide
choice because glyphosate has only weak activity on this annual.

A final warning is that glyphosate is definitely not for use on seed crops.
It can cause emergence problems even if a fall germination test is good. A second
test in spring is needed to ensure high germination rate and normal shoots.



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