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Managing weed control timing

Balance the costs of your herbicide choices and spray early.

November 14, 2007  By Bryan Cook

Management decisions in cropping are routinely based on the necessity to plant
corn early. The emphasis on timely planting dilutes the management appeal of
timely spraying. As growers continue to consolidate, Roundup Ready (RR) corn
may appear as a management tool to ease the burden on timely spraying.

Extending the spray window
Glyphosate technology has allowed crop managers to extend the spray window.
Spraying weed infested corn 40 days after planting with good control and no
visual crop injury has created the illusion of a successful spray program. Because
entire fields are treated equally, yield loss from early weed competition is
never considered or verified.

As a result of the 'Ultim epidemic' in the early 1990s, many research institutions
have studied the impact of early weed competition for more than 15 years. Results
are not absolute, but generally indicate a critical weed-free period commencing
as early as three, and continuing until eight leaf corn. Weed pressure during
this period creates irreversible yield loss. The critical period can also be
defined by using days after planting or weed height. Refer to the charts for
research alluding to yield loss from delayed weed control.


Research clearly indicates that using RR corn to extend the spray season into
mid June is going to cost growers money. Crop managers should plan to have all
acres sprayed within 25 days of planting. Custom applicators, additional staff
or additional sprayers may all be required to make timely spraying a reality.
Use previous weed scouting records to prioritize fields with heavy weed pressure.

Cost savings offered by RR corn
Choosing RR corn to save on weed control may prove disappointing. Beyond some
obvious advantages in no-till, glyphosate technology is merely price-competitive.
Herbicide expense may appear lower as the cost is split between seed and herbicide.
The Technology Use Agreement (TUA) in RR corn adds a premium to the seed. A
standard TUA fee does not exist between, or even within companies, so the cost
seems to fluctuate between varieties and traits selected. Single, double and
triple stacked varieties all may have different TUA prices.

When comparing herbicide cost, do not use the price of one litre of glyphosate.
Glyphosate is unique in that growers often exceed label rates. Also, be cautious
about using glyphosate alone. If using the technology properly with respect
to resistance management, a registered tank-mix partner is required. A minimum
cost for a glyphosate based herbicide program must be $20 per acre, whereas
a base guideline for a good reliable conventional herbicide program rarely drops
below $35 per acre.

After all the dollars are on the ground, the savings offered by RR corn are
minimal. Economics is not the selling feature of the most successful herbicide
on the planet. Minimal weather restrictions, good weed control, no visual crop
damage and obscure crop stage limits are the addictive features. The simplicity
of the technology can breed complacency and serve as a catalyst to delay spraying.

Practical considerations
For growers who manage acreage with a livestock base, growing RR corn can be
attractive. Perennial weed pressure can be greater on livestock operations as
a result of forages in the rotation. RR crops provide an additional opportunity
to control tough perennial weeds such as quackgrass or plantain. Do not substitute
spring applications of glyphosate for late fall burndown type situations. Controlling
perennial weeds is more successful with fall applications due to crop staging.

If RR corn is being considered on a small acreage, do not choose a random field.
Growing RR corn adjacent or near conventional corn creates a potential drift
problem and invites disaster from spraying the wrong field. The hectic planting
and spraying season routinely compromises the best intentions. Small acreage
demands that all of it be RR corn, or none.

Be responsible
Regardless of herbicide choice, management must set goals. Be aware of critical
weed-free periods and reconsider the definition of acceptable weed control.
Realize the value of glyphosate to future farm managers. Avoid abusing the technology
with repeated solitary use. Consider herbicide rotation, tank-mixing and any
tillage options. Prioritize decisions based on early planting and early spraying.

Bryan Cook is a CCA with Cropland Consulting based in Prescott,
Certified crop advisers across Ontario assist growers
with crop management decisions. Since 1996 individuals from most growing
regions in Ontario have entered this intensive program. They maintain their
designation with continuing education credits by attending courses and workshops.
Look for the CCA emblem!


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