Defoliation: What do the consultants say?

Defoliation:

Last season's weather conditions resulted in problems for some growers in getting leaves to drop. Leaf levels were higher than usual, in some case resulting in leaf discounts. 

As a result, CottonInfo's team of Regional Extension Officers recently surveyed key consultants in their regions to gain insight into how they make the decision to defoliate, to maximise yield and fibre quality. Below is a snapshot of the commentary provided.

Top tips for the best results?

  • Ensure the crop and the field are ready to defoliate. Ensure moisture and N have been run down in crop as it makes the job easier.
  • Check your crop regularly. Watch weather forecasts closely, particularly temperature.
  • Try to defoliate when it's warm. Use weather to your advantage (more so than crop maturity).
  • Don’t spray defoliants if rain is coming. A window of 12-14 hours is preferred.
  • Considering using side sprayers to get better coverage on 60-inch cotton.
  • If you are unsure: call someone. There are always people willing to help and provide advice. 

What harvest aid products do you use and rates? What are the considerations you take into account when choosing harvest aids?

  • If conditions are good: Thidiazuron 100-200ml/ha depending on weather, with Ethephon 500ml first pass, same rate of Thidiazuron and 2.4L/ha Ethephon each pass after that + 1L DC Tron Oil each pass. If conditions are cold: go to Etee 80ml/ha + 2.4L/ha Ethephon +1L DC Tron each pass.
  • Aiming for two passes. First pass, Thidiazuron 150 ml + 750 ml of Ethephon. Second pass Thidiazuron 125 ml/ha + 2 L/ha Ethephon.
  • 50-150mL/ha Thidiazuron and 1.5-2L/ha Ethephon first pass with oil. Secon pass 7-10 days later same rate of Thidiazuron but 2-2.5L Ethephon with oil, and again, and again (hopefully not but possibly again!). Rates depend on temperatures and stage of crop, canopy size and nitrogen levels.
  • We use Thidiazuron when overnight temp is greater than 12 degrees (C). In a range of 150-200 ml/ha depending on temperatures. Usually also mix with 300-500 ml/ha of Ethephon depending on maturity. If pushing a crop that is not fully mature, often leave Ethephon out. On second pass often use 100-150ml of Thidiazuron with 2-3L of Ethephon, and same rates on third pass if and when required. When overnight temp is below 12 degrees, I switch to Thidiazuron + diuron using rates from 0.3-0.4 L/ha. We never mix with Ethephon on first pass as the incidence of leaf freeze is too high.
  • I think we’re all pretty well resigned to the fact that with how we manage the current varieties with nitrogen and water, three passes is normal and generally Thidiazuron and Ethephon do a pretty good job most of the time.

What factors are taken into account for the timing of defoliation? What method/s do you use to assess timing of defoliation?

  • Maturity of the bolls, temperatures and the forecasted weather. Picker availability is also a consideration. 
  • Timing wise, I use cutting bolls as the best judge. Other considerations:
    • picker availability, and whether we can afford to wait for a few late bolls to fill while we wait for the pickers or need to get straight in.
    • whether there are any early bolls cracking that need to be chased by a defoliation or whether they can sit and wait. With these late bolls handy things to note are: when your last water was; how dry the profile is; if they are actually likely to fill regardless.
    • whether there is a possibility of residual N, or honeydew on the leaf.  
    • weather is a big consideration in defoliation timing.
  • We will go fractionally early on first pass if the current temperatures are good, and if weather forecasts have temperatures dropping away.

How do you address regrowth?

  • Try to pick on time. If the picker is delayed, then delay defoliation.
  • Manage soil moisture and N status. Left overs of either result in late growth. If this occurs, diquat can be applied by air. 
  • Regrowth can be a funny thing: sometimes difficult, sometimes manageable. Most of the regrowth I’ve ever dealt with has been addressed by using herbicides by ground or diquat by air. When using herbicides you have to be careful that there isn’t too much leaf still on the plant as it becomes very crumbly very quickly and whole leaves can drop into open bolls, break apart and stick to the lint, resulting in discounts. The other concern is regrowth coming back. Herbicides only burn off the leaf, so if your pickers are longer than 5-10 days away, your leaf will come back. I’ve heard of people desiccating regrowth multiple times. Water rate is the biggest thing to consider with Group L herbicides and the addition of an oil and ammonium sulphate is essential. 

Ground vs air: which is better for defoliation?

  • I generally like to do all the solid configuration by air, and any other configuration that’s not going to be bashed around by a ground rig; by ground. Ground rigs do a better job especially with droppers and angled nozzle bodies that surround the plant line you can get a far superior job, but you would never put them into a solid crop for defoliation unless they’d been in there all season training the cotton. Even then I probably wouldn’t put a ground rig in after row closure.
  • Ground: better coverage but knocks more cotton on the ground; Aerial – better for timeliness and no cotton knocked on the ground.
  • Aerial application is the preference due to timeliness. When temperatures are suitable able to get over more ground.
  • Strongly believe that we are not using ground rigs enough, especially for the first pass. When using ground must have boom height high enough for nozzles to overlap to prevent concentration of product on narrow bands of the plant.