Top Crop Manager

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The cost of trampling your crop

Spraying later in wheat results in lost yield.

November 19, 2007  By Top Crop Manager

Every bushel counts when it comes to putting wheat in the bin, so when you
cut down on field losses, the benefit goes right to the bottom line. Research
in wheat shows that crop trampling while spraying can result in up to a four
percent yield loss if spraying is delayed. While not huge, that is over 1.5
bushels of wheat on a 40 bushel crop. And that four percent yield loss does
not take into account the loss due to delayed weed control.

"We were doing a lot of sprayer testing in the 1990s, and with the wide
range of tires, tank sizes and wheel configurations, we were getting a lot of
questions on wheel track damage," says Brian Storozynsky, project technologist
with the Agricultural Technology Centre (ATC) at Lethbridge, Alberta. As a result,
the ATC conducted a three-part trial to see how various combinations of tractor,
sprayer tires and sprayer tank sizes, at different crop stages, affected yield.

The first study looked at the effect of single versus tandem sprayer tires,
sprayed at the two to three leaf stage, five to six leaf stage and tillering
stage. Four hundred and 800 gallon water loads were also compared. Various combinations
of wheel assemblies were compared, including single turf sprayer tires, tandem
walking beam sprayer tires, tractor tires pulling a sprayer tank with single
turf tires, and tractor tires pulling a sprayer tank with a tandem walking beam
transport system.


The results showed a clear trend of reduced yield at larger crop stages due
to trampling by the wheels, where spraying at the tillering stage resulted in
two to six times more yield loss as spraying at the two to three leaf stage.
It also highlights the need to run the sprayer tires in the wheel tracks caused
by the tractor tires.

"We definitely were seeing some yield effects of wheel tracks as the crop
was larger," says Storozynsky. "The earlier you get in the crop the

Table 1. Impact of wild oats emergence
and density on wheat yield.
Wild oats time of emergence Wild oats density – number
per square metre
1 2 4 6 8 10 12 14 20 30
Wild oats are one leaf stage ahead of the wheat 1 2 4 6 8 10 12 14 19 26
Wild oats are at the same stage as the wheat 1 1 2 4 5 6 7 8 11 16
Wild oats are one leaf stage behind the wheat 0 1 1 2 3 3 4 5 7 10
Source: O'Donovan; Alberta Environmental
Centre (Vegreville, Alberta).

The second part of the study compared single versus dual wheels on the tractor.
Storozynsky says that many farmers run duals in the hopes that the wheel pressure
will be less. The results showed that there was not any difference in crop damage
between single and dual tires, except when dual tires are used at the tillering
stage, which showed significantly lower yield than the single tractor tires.
Again, this would have been because of crop trampling. And, once again, under
all tractor tire loads, whether single or dual, the yield loss was higher when
the crop was sprayed later.

The final study compared single, tandem and lugged sprayer tires at two growth
stages; two to three leaf stage and tillering. In this trial, no trends were
apparent between tire types, indicating all would be suitable for crop spraying.

The overall results indicated that sprayer tires used today are suitable for
crop spraying. Both single and dual tractor tires are suitable, reducing the
need to drop the dual tires when spraying. However, with all tires and combinations
of configurations, the later the spraying operation took place, the greater
the amount of crop damage. The other recommendation coming out of the study
is to ensure that the sprayer tires follow the tractor tires.

"The damage can really show up if spraying later, like what happened in
2005. Farmers couldn't get on the land to spray because of the wet weather in
June. When the crop gets up to seven inches tall, some of the narrow tires on
high clearance sprayers leave tire tracks that are still visible at harvest,
so there is likely some loss there," explains Storozynsky.

Early weed removal also important
While crop trampling losses at later spraying stages can be significant, they
can be overshadowed by crop losses due to weed competition. In wheat, generally,
the earlier the weed is removed from the crop, the greater the chances the crop
yield will be unaffected. Studies from Agriculture and Agri-Food Canada and
the Alberta Research Council have found that delaying weed control will result
in higher yield losses, especially if the wild oats are left to compete past
the five to six leaf stage, or past 20 days. Additionally, if wild oats emerge
before the crop, yield losses are higher as well.

Table 2. Affect of wheel configuration
and crop stage on yield.
Wheel configuration Crop stage (percentage yield
Two to three leaf stage Five to six leaf stage Tillering
Single sprayer tires in tractor tracks 1.170 1.428 2.177
Tandem sprayer tires in tractor tracks 0.577 1.089 2.294
Single sprayer tires plus tractor tracks* 1.906 1.644 3.857
Tandem sprayer tires plus tractor tracks* 0.651 2.687 4.216
*Sprayer tires do not follow tractor
wheel tracks. Source: Ag Tech Centre.

Patrick Haikal, a researcher with Arysta LifeScience, agrees that many independent
studies show that early weed removal often results in the highest yields. Arysta
is also conducting time of weed removal trials, looking at common grassy and
broadleaf weed herbicides. He says the trends in the research support other
findings that early weed removal is best.

"Where we are seeing some differences is with a product like Everest,
which has both leaf and soil activity. It can be applied early to control existing
weeds and then the soil activity will control later flushes," Haikal says.
"That is where we are seeing the difference, with Everest applied early
versus other post-emergence products. The other products can suffer from either
being applied too early, resulting in later flushes, or applied later in an
effort to target more weeds, resulting in yield losses due to early weed competition."

Naturally, growing conditions in any given year can impact weed removal timing
and crop trampling damage. However, most research trials indicate that controlling
weeds as early as possible is the best way to set wheat crops up for their full
yield potential. -30-



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