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Move Irrigation To Top Of “TO DO” List
Author: Tom Barber, Extension Weed Scientist

This is one of the earliest planted cotton crops in Arkansas history with 80 percent of the crop planted by May 6th and all of it planted by May 15th.  Cotton growth ranges from cotyledons to 10 nodes with 4 squares.  Fields planted the last week of March and first couple weeks of April look good for the most part.  The cotton that seems to be struggling the most was planted around the week of April 22 during the only cool/rain period we have experienced.  Arkansas is currently about 300 heat units above our normal accumulation for the months of April and May.  Several days in May have reached the 90 degree mark and moisture is very limited in most locations.  Thrips have been terrible this year on cotton in all stages, from emmergence through 5 leaf.  The dry weather has compounded this issue, while areas that have recieved scattered showers are starting to turn the corner and look really good.

Herbicide damage from preemerge applications have taken a toll on cotton stands in many areas, but most can be attributed to the 2-4 inch rain that was recieved in April.  In many cases the Reflex that was applied did not recieve a rain prior to cotton planting.  Cotton emmerged and the heavy rain splashed Reflex up on the plant causing severe stand loss and re-plant situations.  Numerous stands were also affected with Diuron, Cotoran and Caparol applications during that time.

Irrigation preparation should be at the top of the “To Do”  list as we move into weeks of 90+ degree weather and little rain in the forecast.  Over the last four years in our irrigation work, we have been able to gain an extra node at flowering by watering one week earlier.  The cotton was approximately 9-10 nodes when we initiated the early irrigation treatment, and at flowering, the early irrigated cotton was 10 in taller and bloomed with 9 NAWF (nodes above white flower), while the cotton where irrigation was delayed for one week, bloomed with only 7 NAWF.  The moral of the story is to be prepared to water before cotton growth slows.  By watering on time, we were able to prevent drought stress that ultimately delayed the cotton further.  Keep in mind that this crop may have a shallow root system from early season stresses.  In the fields where the tap roots are permanently damaged from cool soil temperatures and seedling disease, timely irrigation will be critical.

Cotton water demand prior to squaring is generally less than 0.10 inch per day.  However, once the plants begin to square, the demand goes up from 0.10 to 0.25 in/day as the plant biomass and square number increases.  Because it is so dry this season, it is important to keep this seedling cotton growing through the heavy thrips populations.  With current temperatures, we are accumulating enough heat units to grow a new node every 2.5 to 3 days.  Monitor your fields closely, and if the seedling cotton is not growing a new node at this pace, supplemental irrigation is needed.

Irrigation will also be needed in many fields to activate residual herbicides and move nitrogen into the soil solution.  Nitrogen applications into dry dirt or dust will not be available for plant uptake.  If you recall 2010, we received no rain in June and almost all furrow irrigated fields were streaked because nitrogen was not activated in every row.  Unfortunately, if no rain is received on furrow irrigated fields, every row will need to be watered the first time to activate the applied nitrogen more efficiently.

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