The timing of a pre-seed burndown herbicide in direct seeding systems is dependent on the weed species present
November 27, 2007 By Top Crop Manager
The timing of a pre-seed burndown herbicide in direct seeding systems is dependent
on the weed species present, their size, populations and the tillage system.
Based on research and farmer experience, the generally recommended timing is
just prior to seeding.
"If there are weeds present, then a pre-seed burndown as close to seeding
as possible is the best practice," says Eric Oliver, soil conservation
agrologist with Saskatchewan Soil Conservation Association at Swift Current.
"But in this area, a lot of guys are seeding as early as possible, so the
timing is difficult because not all of the weeds have emerged."
Research at Agriculture and Agri-Food Canada at Melfort and Scott showed that
regardless of the type of openers used, a glyphosate application just prior
to seeding resulted in the greatest yield response in wheat and barley, while
minimizing weed competition and injury to the crop.
In those studies, glyphosate was applied at two to three weeks before seeding,
one day before seeding, and three to four days after seeding in a narrow hoe
and simulated sweep system. At Scott, wheat yield was greatest when glyphosate
was applied one day before seeding with a narrow hoe opener. Yields were reduced
by more than 25 percent when glyphosate was applied two to three weeks before
seeding, and 15 percent when applied three to four days after seeding.
With the sweep system at Scott, pre-seed application timing did not have an
impact on yield, likely because the sweeps controlled the weeds at seeding.
But yields were reduced by 15 percent when glyphosate was applied three to four
days after seeding. It is believed that the late application of glyphosate did
not cause crop damage, but it allowed for large winter annual weeds to compete
for moisture and nutrients.
At Melfort, while the timing of the pre-seed burndown had no significant effect
on barley yield, the trend was towards higher yields when glyphosate was applied
one day prior to seeding.
|Pre-seed burndown timing and yield response
(Melfort and Scott, Saskatchewan 1997 and 1998).
|Glyphosate timing||Narrow hoe direct||Sweep direct|
|Grain yield (bu/ac)||Grain yield (bu/ac)|
|Two to three weeks prior||26.4||30.1|
|One day prior||36.7||30.5|
|Three to four days after||30.1||26.0|
|Two to three weeks prior||27.3||20.0|
|One day prior||28.0||25.0|
|Three to four days after||27.4||20.3|
|Source: E.N. Johnson et al. 2001.|
Other research conducted by Ken Sapsford of the University of Saskatchewan
and Eric Johnson at AAFC Scott also showed that delaying a pre-seed glyphosate
application after ground crack can result in crop stand and yield reductions
in wheat and barley.
Currently, pre-seed burndown is used on between 40 and 45 percent of the seeded
acres in western Canada. The trend has been on the rise over the past five years,
generally mirroring the rise in no-till and min-till. Not surprisingly, no-till
acres get the largest amount of pre-seed treatments, at around 80 percent of
no-till acres in 2005. No-till relies on pre-seed burndown to control weeds
prior to seeding, while min-till and conventional systems can rely on some mechanical
weed control while seeding. In 2005, 47 percent of min-till and 28 percent of
conventional acres received a pre-seed burndown.
Weed species also dictate the timing. For example, when temperatures are warm
in the spring, winter annuals and biennials will grow more quickly than under
cooler temperatures, perhaps requiring earlier control. A warm spring can make
timing of the pre-seed applications difficult because of the rapid growth of
weeds. And even high-disturbance seeding systems can benefit from pre-seed burndown
if winter annuals and early germinating weeds are present several weeks before