Irrigation termination of Cotton

Ayman Mostafa and Ed Martin

Last irrigation is a very crucial decision for cotton grower to make. There are some very applicable procedures to determine when to stop irrigating your cotton crop in Arizona. Three primary considerations are involved in determining the terminal irrigation:  agronomic stage of the crop, weather conditions and insect pest pressures. Putting these considerations in mind, the following steps can help you make your decision on irrigation termination:

1. To know where you are in the fruiting cycle.  Are you at cutout? (In Arizona cutout is the point at which the crop has an average of 5 or less nodes above the white flower (NAWF) during the primary fruiting cycle.)
2. Once you have determined where you are (at cutout – 2 weeks out) you need to start deciding whether or not to go for a top crop. The decision on whether to go for a top crop primarily lies with how well your crop has set bolls and what the overall boll load was after the first primary fruiting cycle.
   1. Research has shown that if you have 45% or more fruit retention (first two fruit sites on each fruiting branch) the probability that extending the season will result in significant yield gain is low. This probability is reduced even further as fruit retention goes up.
   2. Full season varieties have the best chance for a top crop with significant yield increase.
3. In order to realize any real significant increase from a second crop, you need hot and dry weather. Remember you need about 600 HU in order to go from flower to hard boll, and then another 400 HU to get to open boll.
4. Keep in mind that the last bloom you want to harvest will typically take an additional 3-5 weeks of growth. You will need to irrigate and manage pests for that extended length of time. 
5. If you want, you can pick the last boll you want to harvest.  This typically only takes 2-3 weeks of additional management.

6. Don’t get greedy, be reasonable and practical – cotton will keep on producing flowers and before you know it, you’ll be irrigating in October.
7. A good rule of thumb is to defoliate at about twice your normal irrigation interval. 
   1. So if you are irrigating every 10 days, you want to apply defoliant about 20 days after your last irrigation (assuming no rain has occurred). 
   2. You need to do this to assure the effectiveness of the defoliant.
   3. You want a plant in stress – but not too much so the defoliant can’t do its job

Additional references can be found at the following links:

• Evaluation of Irrigation Termination Management on Yield of Upland Cotton https://repository.arizona.edu/bitstream/handle/10150/210969/370108-218-222.pdf?sequence=1&isAllowed=y
• Effect of Irrigation Termination Date on Defoliation and Yield of Upland Cotton https://repository.arizona.edu/bitstream/handle/10150/204821/370077-061-064.pdf?sequence=1
• Management Considerations for Short Season Cotton in Arizona https://extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1244.pdf
• It’s All About Timing https://cals.arizona.edu/crops/cotton/files/DefoliationTimingvFc.pdf