Weeds Compete with Crops for Water

July 22, 2013

Summer heat has finally arrived.  In Wisconsin, our wet May and June (2013) caused an extended and delayed crop planting and hay harvest season.  As a result, it is not uncommon to see a wide range of crop maturities within the same neighborhood, and on individual farms.  The current heat has advanced crop growth, yet already brings a challenge to the soil moisture content of sandy fields and steeper landscapes where earlier water was more prone to running off instead if infiltrating.

An example of the wet, and now drying soil conditions can be seen near Cashton, WI, where it has rained approximately 15.5 inches since April 25; 8 inches through May + 7 inches through June + 0.5 inches now into July.  As of Mid July, the last appreciable rainfall was 0.72 inches on June 25th.  Our recent week of 90 degree days combined with 3 weeks of limited rain has some fields showing exactly where their pockets of low moisture holding capacity soil are.

Again, near Cashton, corn maturity ranges from the V4 to V10 + stage (5 inches tall – ready to tassel next week).  The younger corn fields did not get planted until late June due to wetness.  Other fields of younger corn have been no-tilled into sod after first crop hay harvest; and others have had winter rye forage taken off, then tilled and planted.

The first scenario of young corn can have weedy areas because even after squeezing a planting between rain storms on soil that was already wet, it often rained again before herbicide application, thus delaying weed control too.  The second scenario of young corn shows that even in a wet year, the first crop in a double crop system uses more soil moisture than you think.  And, weed control in these fields can be problematic too as custom herbicide applicators continue catching up.

Photo: Red Root Pigweed

Red Root Pigweed as seen in a WI field on July 17, 2013.

Corn requires about 20 to 24 inches per year of water to yield 150 to 200 bushels per acre. Even with all the precipitation this spring, corn will still need an average of one inch of rain each week in late summer.

The wet, extended and delayed planting season has many corn, soybean, and new seeding alfalfa fields showing moderate to heavy weed populations.  In fields that still have young growth stage crops – be prepared to scout for weeds.  Consult your agronomist as needed for post emerge weed control strategies and herbicide options.

The list below shows the importance of adequate weed control to minimize competition for soil water between corn and 3 common broadleaf weeds.  In early corn growth stages – when any of these broadleaf weeds become “large and in charge”, the corn and weed plant dry matter accumulation is similar, creating a water competition between the two.

Plant                                     Water Requirement

                               (grams water / 1 gram plant dry wt.)

Corn                                     350

Redroot Pigweed            300

Lambsquarter                 650

Common ragweed          910


Written by Kevan, Outreach Specialist  kevan.klingberg@ces.uwex.edu

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