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.”

Crop fruit retention

Crop fruit retention is another factor that impacts when to apply the final water. Martin says pushing a crop with additional water to gain more than 45 percent fruit retention can be a waste of water and money.

“Research indicates 45 percent or higher fruit retention on the first two fruit sites on each fruiting branch in an extended season will likely result in lower yield gain,” Martin said. “With 45 percent fruit retention, much of the plant’s energy is going to the fruit. Growers will likely not get much more fruit.”

Martin shared a PowerPoint plant graphic with 20 potential fruiting sites; two per fruiting branch. The actual flower-boll set was 12. The total fruit retention was about 60 percent; 12 bolls on 20 branches. This particular plant, Martin noted, would not be a good candidate for pushing water late in the season.

To grow or not grow a top cotton crop with additional water is another consideration. Martin says full-season varieties have the best chance for a second (top) crop which can significantly increase yields. Mid- to short-season varieties generally have problems pushing a top crop.

Hot, dry weather is required to successfully grow a top crop. If the decision is made to grow a top crop in desert production, 600 heat units are required from flower to hard boll. Another 400 heat units are required to open the boll.

“That’s 1,000 additional heat units needed, or about five weeks worth of heat, in the late summer in the Parker Valley,” Martin said. “At least three weeks of that requires irrigation. Once we get to the hard boll, we can dry the crops down a little to help open the boll.”

When it comes to turning off the irrigation faucet, Martin offers these guidelines:                 

1. Pick the last flower for harvest and irrigate normally for an additional four to five weeks;

2. Don’t get greedy. Flowers which show up at the top of the plant are not worth much. Most of the boll load, including the lint, is at the bottom of the plant.

The scheduled defoliation date is another factor which impacts the last irrigation date. Martin suggests forgoing two irrigation cycles before applying defoliant.

“If you are irrigating every 10 days, apply the defoliant about 20 days after the last irrigation assuming no rain has occurred,” Martin said. “This helps ensure the effectiveness of the defoliant. You want a plant in stress, but not too much so the defoliant can do its job.”

Final spray for lygus control

Making a good decision on when to stop insecticide use for late-season lygus control in cotton is also important, says Peter Ellsworth, UA integrated pest management specialist, also based at the MAC.

Thresholds for lygus bugs in cotton have long been established in Arizona. Insect sprays are recommended and usually cost-efficient when the minimum threshold density reaches 15 lygus including four nymphs (15/4) per 100 sweeps any time during the peak flowering period.

As crop growth slows and the blooming rate declines, Ellsworth says the amount of salvageable yield from lygus sprays is also reduced.

Factors which impact the best spray termination decision mirror Martin’s irrigation points: the variety maturity class (early to mid- to full-season); the planting date — April is optimal in central Arizona; production goals tied to irrigation termination timing linked to the primary fruiting cycle; and cotton development measured by NAWF counts.

Draft lygus control termination guidelines are available on the Arizona Crop Information Web site at http://ag.arizona.edu/crops/presentations/DRAFT_LT_guide2-pg.pdf.

“Generally, considering all types of production scenarios, I’d recommend stopping lygus sprays anywhere from two weeks before the initiation of cut out (i.e., before NAWF = 5) to one week after the initiation of cut out,” Ellsworth said. “Growers and pest control advisers should make the best decision based on their particular situation.”

The lygus bug, Lygus hesperus, reduces yield and quality. Adult lygus feed on the plant, but plant-bound nymphs create the bulk of the preventable damage. The insect changes the fruit’s spatial pattern and delays fruiting by feeding on and eliminating fruiting sites. This also changes how the plant allocates carbon and nitrogen.

“The energy captured from the sun enters the stems and leafy growth instead of the bolls,” Ellsworth said. “The results are taller plants that are more difficult to defoliate, plus more leaf trash and poorer lint quality.”

Ellsworth has conducted numerous cotton field trials including insecticide use on lygus. He recommends the use of Carbine as the first spray for lygus control.

“Carbine is the best game in town right now for lygus control; it’s very effective, selective and safe on beneficial insects,” Ellsworth said. “The product effectively targets aphids, a potential threat in cotton fields located along the Colorado River.”

He also urges growers to rotate Carbine with other products and chemistries including Orthene, Vydate, and Belay to reduce the chance of resistance over time.

“Belay might be the best rotational alternative to Carbine for lygus control, because the other options are very broad spectrum and less safe for beneficials,” Ellsworth said.

Surveys have revealed about 81 percent of central Arizona cotton growers used Carbine to control lygus in 2009, up from 66 percent in 2008.

Ellsworth’s studies suggest the insecticide Sulfoxaflor, which is several years from registration, has one of the most promising new chemistries for lygus control.

Insecticide use has decreased substantially in Arizona cotton, says Ellsworth; down to 1.5 sprays per season for all pests over the last four seasons.

Ellsworth’s work is supported by grants from the U.S. Department of Agriculture’s Risk Avoidance and Mitigation Program, Arizona Cotton Growers Association, and agrochemical companies.

cblake@farmpress.com