From Cotton Seed Distributors
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Don Biffen, Research & Development specialist - Bayer Cropscience, explains how defoliation is initiated.

Don, could you explain simply how defoliation or leaf abscission is initiated?

Yes certainly, the products that we have within our portfolio are all growth regulators and as such they rely on the constant metabolism of the plant when they’re applied during the leaf abscission, boll opening process, right up until harvest. We have a number of products within our portfolio, some of those lead purely to leaf abscission, mainly products like Dropp® Liquid and Dropp® Ultra and there are other products like Prep and Finish® that actually cause leaf maturation, removal of new leaf growth. Leaf abscission is caused by the build up of ethylene absissic acid within the plant. You get the leaf abscission layer, which is there when the leaf actually, or shortly after the plant actually germinates and any leaves that form. By putting these products on the cells within the abscission layer increase in size, they elongate, the cell wall ruptures and then the leaf falls away from the plant.

So what sorts of factors influence defoliation or that process?

The factors leading to defoliation or successful defoliation at the end of the season are many and varied and are influenced by things that we have control over. Even planting dates, seed density, plant uniformity throughout the crop right through to a whole range of environmental conditions, be they temperature, water stress, heat stress but certainly a crop that’s been grown very well without any excessive stress through the season is certainly much easier to defoliate at the end.

You’ve got a lot of different factors obviously there that do influence defoliation and a lot of things can influence it. Another important thing is application of the product. Do you have any tips on aerial applications and ground rig applications and getting the best out of defoliants?

Yes certainly. The growth regulators or those products that we used as harvest aids at the moment only act within a leaf, they don’t translocate from one leaf to another so coverage on any particular leaf within the canopy is extremely important. To get the best coverage depends on the situation in the canopy, density etc, so it’s important to get good even coverage right down through the canopy or certainly as far as you can. Aerial applications are normally conducted around about 30-40 l/ha and that’s appears to be the optimal range, whereas ground applications are normally put on in the range of around about 90-100 l/ha. But certainly the most important thing is getting good even coverage right as far as you can down through the canopy. Certainly on irrigated crops we wouldn’t expect to get the full coverage with the first pass.

So you’re looking at multiple passes in most cases?

In most cases yes, certainly. In some dryland crops it’s feasible to get good even coverage down through the canopy and then achieve a commercial result with one pass.

A lot of the defoliants have the addition of oils or wetters. Any comments on how important they are for a good job?

Getting back to the point that these products aren’t translocated; they have to be applied to any particular leaf to drop it off the plant or any boll to effect full boll opening, the addition of oils is essential and our early trial work with the Dropp® based products showed us that the addition of an oil gave us about an extra 20% in performance over and above not using an oil. But also the other point with oils is they’re very important particularly in drought stressed crops or crops that towards the end of the season that have run out of water and certainly it’s important to put on the label a label recommended rate of oil to get good coverage over leaves and get the best out of the harvest aid that’s applied.

Concentrating on some of the more difficult situations to defoliate, you mentioned drought stresses crops and I guess most dryland crops fall into that category. So what are the best ways of handling those crops that have had a pretty tough time? 

It’s certainly within drought stressed crops or alternatively crops that have ran out of water are probably not such an issue this year but certainly last season and the season before we had crops in a number of valleys that weren’t able to receive their last couple of waterings. The important things were to firstly use Dropp® liquid if you are able to unless it was getting really cold where people had to go to Dropp® Ultra and the other important point was to use the correct amount of oil as we’ve just mentioned. By increasing rates we weren’t seeing any increase in performance so you’d stick to the normal rates that would be recommended for the temperature regime at that time and the actual canopy load within the crop but certainly you didn’t need to increase the amount of products. So I guess the two most important aspects are if you can, use Dropp® Liquid and make sure you use the full-recommended rate of oil.

Would more oil help?

No, we haven’t seen any benefit from increasing the oil so whatever the oil you use, use it at that recommended rate.

The other situation that we often come up with is very rank leafy crops, crops that for one reason or the other have got a little bit out of control. Any special tips for defoliating those?

I guess rank cotton it’s important to have either a tank mix that includes either Prepp or particularly Finish®. Both of those products are very good at stopping that new growth that’s on top; pulling it up and taking it off the plant. So where you have got rank cotton it’s certainly important to use either Prep or Finish® in those tank mixes.

Another thing that comes up every year is a lot of growers like to add Roundup® to defoliants or try and do things in conjunction with defoliants where they’ve got weeds or even in some cases I guess non Roundup Ready® cotton to get some suppression of regrowth, particularly if it keeps raining and crops are likely to keep on growing. Any guidelines on that or limitations or when you shouldn’t do that?

Yes certainly, when you are using it on conventional cotton we don’t recommend the use of Roundup® or Glysophate based mixtures in with harvest aids simply because the two products have an antagonistic mode of action. Roundup® for example or glysophate tends to want to shut the plant down, whereas we want to keep the plant metabolised and keep it growing so we get good distribution of the growth regulators or harvest stages more commonly known within the plant structure. If you want to put on a herbicide such as Glysophate to clean up any weeds, it’s best that that’s put on at least three weeks before the first application of the harvest aid, simply to get the best out of the product. We’ve seen consistently a degree of leaf freezing where that gap between the Glyphosate application and the first application of harvest aid is shortened or alternatively added in with the tank mix.

You mentioned Roundup Ready®, obviously there are a lot of crops now that are Roundup Ready®. Would you expect to see the same thing there?

Well it’s a good point and we haven’t fully investigated that and that’s something that we’re going to have to take into consideration particularly this year. We don’t envisage any issues with that but that’s something that we would like to look at this season and follow up.

Going onto that issue of some of the reasons why Roundup® used is to control regrowth but are there any other options for regrowth if it occurs?

Thidiazuron, which is the active ingredient within Dropp® and Dropp® Ultra is very good at suppressing regrowth that is if you put the product on before you get a rainfall event or whatever it’s very good at inhibiting that regrowth. However, once you’ve got regrowth on a plant, products such as Finish® and Prep in the tank mix it is probably the most successful way of controlling it.

Going on now to boll opening, where people are trying to speed up harvest. What factors are likely to influence how well boll openers work and any tips on using boll openers to get the best advantage?

With boll opening, the temperature considerations, growing considerations are very similar for boll openings as they are for just straight defoliants. But obviously if you’ve got large amounts of bolls to open, ethephon based products that is Prep or Finish® are the most successful products to use in that situation. And certainly we’ve formulated a plan within our own products whereby we recommend Prep in situations where you just need boll opening but use Finish® as either a spike rate to kick that off at the beginning of the season or to give you good boll opening and defoliation as a second pass in some of your heavily defoliated irrigated crops.

Further Information:  Robert Eveleigh, John Marshall, or Craig McDonald