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.”