2010 has been a white-knuckle year for many Western cotton farmers who eked through the cool and wet spring planting season and now face important decisions as fall harvest looms.

Two common questions that pique growers’ interest at this point are when to terminate irrigation and when to apply final insecticides.

Ed Martin, University of Arizona associate director for Extension programs and state irrigation specialist, says to follow three good rules to determine when to terminate irrigation in cotton. Oddly, the rules have nothing to do with water.

Final irrigation decisions, Martin says, should be based on agronomic, weather and entomological information.

“First, know where you are in the plant’s fruiting cycle,” Martin says. “Determine the plant status, boll load and overall plant health. Take out the crystal ball and see what the weather forecast is for the next month. If you prolong the season through more water to gain the next flower, remember late-season heat units decrease and insect pressure can increase.”

Martin, based at the Maricopa Agricultural Center (MAC), Maricopa, Ariz., doled out feedback during a late-season crop, pest and weed management workshop in Parker, Ariz., in mid-August.

“Knowing the status of the fruiting cycle helps determine plant cut out (the final stage before boll opening),” Martin said.

“In Arizona, cut out is defined as when the crop has an average of five or less nodes above the white flower (NAWF) during the primary fruiting cycle.”

Total Heat Units After Planting (HUAP) in part determine when cut out occurs. Varieties also play a major role. Generally, cut out occurs in desert cotton at 2,000 HUAP to 2,700 HUAP for short-season varieties; 2,300 HUAP to 3,000 HUAP for mid-season varieties; and 2,500 HUAP to 3,000 HUAP for full-season cotton.

Martin says about 1,200 HUAP are required on average to achieve first bloom in Arizona; about 2,000 HUAP for peak bloom; and about 2,500 HUAP for cut out.

The Parker area, located along the Colorado River in La Paz County, was about 19 days behind in total HUAP in late August, according to Arizona Meteorological Network data. Growers who planted March 15 were about 15 days behind (2,300 HUAP total). Plantings around May 15 were six days behind schedule in HUAPs.

“In the spring we had a hard time getting our cotton up,” Martin said. “The hot weather this summer provided much needed higher heat units.”