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Post-Drought Agronomic Challenges: Herbicide Carryover, Weed Management, Soil Fertility

Post-Drought Agronomic Challenges: Herbicide Carryover, Weed Management, Soil Fertility


As if last year's drought didn't cause enough problems in 2012, now there are some agronomic factors that could impact the 2013 crop if producers don't manage them appropriately. Those factors include potential herbicide carryover and heavy weed pressure in existing crop stands.

Herbicide carryover

While some areas were fortunate to receive some much-needed rain last fall, soil profiles in other areas continue to desperately lack moisture. Therefore, one concern facing crop production during and following a drought is herbicide carryover. Rotational restrictions listed on herbicide labels aren't as accurate under hot and dry conditions, as herbicides typically dissipate much slower. As a result, an application made last year could impact this year's crop if a field hasn't received enough precipitation.

In a Purdue University article, Professor Bill Johnson and Program Specialist Travis Legleiter say that producers who lacked fall rainfall and miss out on significant snow totals this winter should "be wary of atrazine and HPPD inhibitor (Callisto, Laudis, Corvus and Impact herbicides) carryover into soybean, especially in high-pH and/or high-clay-content soils." (Soils with a 7 pH level or more are considered to be high, as are soils that have been limed since last fall.) They add that growers should also be cautious of imidazolinone chemistry in products like Scepter and Pursuit herbicides carrying over into this year's corn crop.

Iowa State Professors of Agronomy Bob Hartzler and Mike Owen list seven factors that help determine the risk of crop herbicide carryover: chemical half-life, rate of application, application date, soil characteristics (texture, organic matter, pH), rainfall (total amount and distribution throughout year), sensitivity of rotational crop and growing conditions following planting this spring.

Since the only factor that can be controlled at this point is which type of crop to plant this spring, Winfield Regional Agronomist Bob Beck suggests growers have a lab analyze soil for herbicide residues or conduct a bioassay if they are concerned about carryover and the potential risk of injury to susceptible rotation crops. A bioassay is when susceptible seeds are grown in samples of soil from the field that was treated and compared against seeds grown in untreated soil samples.

"Samples should be taken from multiple points throughout a field, and either type of test should be done in early spring so that the soil is most representative of the potential injury at planting," Beck explains. "When conducting a bioassay, place your collected soil ½-1 in. deep and plant 50 seeds of the crop that you intend to plant in that field this spring. Then moisten the soil and observe plants for two weeks or more as they germinate and grow. If test results dictate, growers should look at selecting a different crop."


Weed growth in existing crops

When crops suffered last summer, many growers stopped spending time and resources on weed control. As a result, more weed seed is sitting out in the fields, waiting to rob crops of early moisture this spring. While burndown and pre-emergent applications can be very effective clearing a field of weeds before planting, growers need to take a different approach when controlling weeds in pastures and crops that overwinter, like alfalfa, wheat and canola.

"To give grasses and winter crops the best chance to recover this spring, growers need to time their spring herbicide applications to hit that first wave of weeds when they are about 3-4 in. tall and follow up with a second application this summer if necessary," Beck advises. "If weeds are allowed enough time to form a canopy, crop recovery will be severely impacted and getting a handle on the weeds will be that much more difficult."


Drought effects on soil fertility

Soils react differently to weather changes, but some nutrients that are vital for plant growth, such as nitrogen (N), phosphorus (P) and sulfur (S), have a tendency to be at slightly higher levels in soils after drought than before. While these higher levels could assist production in the short term, Beck says growers should continue to perform soil tests prior to planting and have tissue samples analyzed over the course of the growing season to make sure they're supplying their crops with necessary nutrients. In areas where pre-drought levels were distinctly undersupplied, nutrient deficiencies will still deter growth.

"Nitrates and sulfates will remain in soil until tile lines run and take those nutrients with it," Beck says. "If the tile lines don't run, then sample soils between the surface and 1 ft. and again at a depth between 1 and 2 ft. to identify how much N and S is available for the 2013 crop."

An area where the drought is unlikely to cause any significant long-term effects is soil pH.


Read more from AnswerPlot, WinField.

TAGS: Fertilizer
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