Tracey Farrell (photo), Department of Primary Industries and the Cotton Catchment Communities CRC Extension Team, outlines a controlling Roundup Ready® volunteer trial conducted this season.

Tracey, we have just had a look at a trial you have done comparing or controlling Roundup Ready® volunteers, can you tell me why you actually did the trial?

At the end of the last cotton season I did a phone survey with cotton growers in the Lower Namoi Valley, and the feedback from them in terms of how they had gone about controlling volunteers last season, was that when they had chosen to use herbicides and trying to target small volunteers, there was actually about in 90% of situations where unsuccessful results were achieved. So that highlighted to me the need to do some investigation in how robust some of the herbicides are for use in volunteer control. And also to further investigate the issue of timing in using herbicides to control volunteers.

Just going into some of the different herbicides used, what were the particular treatments that you looked at in the trial?

The trial was split into two timings. We were looking at Spray.Seed® and Hammer®, these were the two herbicides of interest. The reason being that they actually have ‘control of volunteer cotton’ on their labels. The growth stage recommendation for Hammer® is from 2 – 6 leaf and for Spray.Seed® it is up to 9 leaf. We used 6 leaf as the arbitrary growth stage cut-off that we were really looking at and then did a late treatment to see what is the robustness of those two growth stage ranges. We targeted cotton volunteers at 6 leaf or at 9 leaf with a low rate of Spray.Seed® 2.4L according to the label. A high rate of Spray.Seed® that’s 3.2L and the glyphosate – Hammer® mix was based partly on what other weeds were in the paddock at the time we did the treatments.

When the field was flushed, the bladder ketmia grew away quicker than the volunteer cotton. This photo was taken on the day the first treatments were applied. The volunteer cotton was ~ 6 leaf.

We should say something about the other weeds in there because what you did was plant cotton into the side of a field and then watered that field. Any comments just on those other weeds and what impact they may have had on the control?

Yes, it was a little bit artificial in that we needed to hand plant some Roundup Ready® seed in order to guarantee we would got some Roundup Ready® volunteers to look at and what we found was that when the paddock was flushed, (it had irrigated sorghum in the rest of the paddock) that the bladder ketmia and the peach vine in the paddock actually germinated and grew at a much quicker rate than the volunteers. We actually had quite a large mass of other weed species by the time our cotton got to the 6 leaf stage that we were looking for.

What impact do you think that may have had on some of the treatments?

From the perspective of the glyphosate treatment, the bladder ketmia in particular had past the recommended growth stage on the label, so it was always going to be a challenging situation for that treatment to get a good look at the volunteers. From the perspective of Spray.Seed® we are looking at a contact mode of action which then is a function of coverage so the more leaf area you have got the more water you need the more likely you are to need a higher water rate. And there is the opportunity then that slower growing weeds such as the cotton volunteers may actually have been shaded by that canopy and that may have influenced the ability for that product to control those weeds.

The photo above was taken 7 days after the 6 leaf timing. This plot is Spray.Seed® @ 3.6 L/ha (high rate). 100% control of cotton volunteers was achieved. The green that can be seen in the back ground is peach vine that wasn’t completely controlled – also seen in the photo below.

Can you make some comments then about the performance of how those different herbicides appear to control the volunteers?

I was actually surprised by how little rate response there was between the 2.4L and 3.2L of Spray.Seed®. Particularly at the early timing both rates were highly reliable. There was probably a little bit of difference with regard to the total brownout on the bladder ketmia but it still, I thought, was effective in that it totally prevented fruit production, the setting of seed. At least in a real life commercial situation it increases that window of time you have got before a cultivation or something that may actually take the plant out altogether without actually adding to you weed seed bank.

And it completely killed off the volunteer cotton. With regards to the glyphosate – Hammer® treatment I think both timings suggested to us that the reliable growth stage range is probably quite narrow for that product, in that it really  struggled at the 6 leaf growth stage. And yet we have seen from other trial work done by other weeds researchers like Ian Taylor (NSW DPI & Australian Cotton CRC) that it can be very effective but at younger growth stages.

You made some comments about what would be the ideal sizes and the maximum sizes that you try and target with those two products.

Aiming for that 4 leaf stage is probably a pretty good idea just because then it gives you that little bit of margin for error if you do get that shower of rain and it does set you back to that 6 week mark but if you do get delays and can’t go with ideal timing then  we expect that there is more reliability with a product like Spray.Seed® then relying on a spike type rate of Hammer® with a glyphosate for fallow control. The other thing being not only from the volunteer cotton perspective but also with regard to the bladder ketmia when it was getting larger as well the Spray.Seed® was more reliable.

The ‘spraying mantis’ used to apply the treatments.

They are both contact type products or particularly the Hammer® component in there and the Spray.Seed®. Any comments on getting the best out of those in terms of controlling the volunteers?

To give you some background on the footage on our trial, it was actually, the product applied using an experimental style ‘spraying-mantis’ and it’s set up with 110º-01 flat fans. It is a very fine spectrum of droplets and may not be ideal in a commercial situation but it did allow us to use 100L of water very effectively. To actually go into the Spray.Seed® label in a bit more detail, the canopy of weeds like that (we were spraying) probably is close to needing 150L and additional wetter. So you just really need to aim for coverage, realize you are aiming for coverage and match your water volume to the nozzle setup that you have got so that you can compensate one with the other where needed to get the best overall situation.

Finally, you have looked at herbicides there and any comments with regard to when you would move towards cultivation rather than herbicide for trying to control those volunteers?

Yes the cultivations are a really handy tool for volunteer control and in the phone survey that I alluded to earlier it was actually quite a successful means of controlling smaller volunteers. The issues with cultivation come with if you have got large weed canopyies of other weed species that that could actually slow down the efficiency of a cultivation activity. But definitely it’s got a fit in our system where volunteers may get large or be too numerous to use either herbicide or chipping.

Survey of 24 Lower Namoi Cotton Growers, May 2004. The graph shows the proportion of times when poor efficacy was achieved with each volunteer control method (and follow-up control was required). Cultivation of small (up to 6-leaf) volunteers was the most reliable method of volunteer control. In no situation (0%) was follow-up control needed.

Tracey, have you got any comments with regard to coverage on the particular weeds, we have got some samples here of different plants from the same treatment, same plot so would you make some comments?

What we are looking at here is some plants that I have pulled up out of the 2.4L per hectare of Spray.Seed® treatment that was applied when the cotton was about 6 leaf stage. And while the cotton was quite consistent across the plot totally annihilated, we did have varying levels of efficacy on the bladder ketmia and that was a part of the fact that it was such a dense canopy, there was such a high plant population of the bladder ketmia. So you are seeing that where some plants were a little bit more exposed there was complete kill while others had a bit of shading affect have been stopped in their tracks but they are not dead and highly likely that with the next shower of rain we would actually see re-growth in those plants. But the point I want to make is that they are not producing fruit (seed) and it does actually buy you time til when a cultivation activity for bed forming or something else may have been required in the paddock anyway to try not to, to be able to ‘kill two birds with one stone’ as much as possible and prevent that seed getting back into the seed bank.

Where that is particularly helpful, even if the weeds and plants are getting a little bit big, if it is wet, too wet to cultivate, at least you have got something that’s holding.

Yes that is right, you are holding the system in place.

Further Information: 
Dr Stephen Allen, Robert Eveleigh, John Marshall,  Craig McDonald, David Kelly or James Quinn