John Marshall, Cotton Seed Distributors Extension & Development agronomist - Dalby, outlines the plant population trials he has been conducting with both irrigated and dryland Bollgard II® varieties during the past six seasons.

John, what were your findings with the irrigated trials this year?

This is the sixth season we’ve conducted trials in irrigated, initially with INGARD® and then Bollgard II® varieties. This year the trend was fairly similar to what we’ve seen over the last couple of years.

The trial this year was conducted in a Bollgard field just outside Dalby, on a farm that we’ve conducted trials for six years now. The variety in the trial was Sicot 71BR, which yielded close to 4½ bales/acre, and across the population range there was very little yield difference at all.

As in previous years, the trial was conducted as a small plot trial, six replications of each treatment and picked with a trial picker, hand ginned and fibre quality done. The trend was very similar to what we’ve seen in previous years, with not much difference in yield across the range we looked at. The range we looked at was from 7 plants/m through to 17 plants/m.

You carried out segmented picking in this trial, across some of the different plant populations. Can you give us some idea on what sort of morphological differences where evident across of the range?

We looked at the two ends of the trial to some extent with the segmented picking.

We looked at 7 plant/m versus 15 plants/m, and we’d say the retention was down a little bit in the higher plant population.

It had slightly more bolls per metre but they were a little bit smaller in size, about an average of 10% lower in weight.

The boll numbers per plant varied a bit, in the lower population they were in the range of 11-22 bolls/plant, whereas there was a range from 2-18 bolls/plant in the higher population.

There was quite a noticeable difference in the fruit forming pattern between the two populations.

Obviously different looking plants, but were there any differences in fibre quality apparent across the plant population ranges?

No, that’s the interesting thing.

We’ve seen the same thing all years across the full range from 7 through to 17 - there has been basically no difference at all in fibre quality, in micronaire, length or strength.

The only thing there may have been a slight, trend towards was in micronaire - it may have been a little bit lower in the lower part of the plant in the denser plant stands.

However, there was no real difference there.

Based on this year’s work and obviously previous years’ works, what are your recommendations for planting rate with irrigated Bollgard II?

Combining this year’s data with previous years, we’d still stay with the same story we’ve put out for the last couple of years.

The aim should be to drop 14-15 seeds/m to achieve a stand of about 11 plants/m, - that’s with an average establishment of 75%, which is close to what most growers are achieving. In a situation where growers might be watering-up later in the season, this establishment percentage most likely would be higher and the seeding rate could come back a bit.

The idea is if things go against you and establishment drops right off and we’ve seen scenarios where establishment has dropped to 55%, you’re still achieving 8 plants/m. If you strike extremely good, conditions we’ve not seen any indication of a loss in yield or fibre quality by higher stands.

What has been the outcome to the plant population trial with the dryland this year and over the last couple of seasons?

We’ve carried out dryland plant population work for the last two seasons with Bollgard. As compared with the irrigated situation, from what we’ve seen from the last two years, our recommendation for plant stand wouldn’t be very much different from that for conventional. We’ve been looking at the range from 3-11 plants/m established, and across that we’ve not seen any significant difference in yield, although there’s probably been a slight trend towards a slightly improved yield towards the higher end. So based on what’ve seen in those two years, our recommendation would be aim to establish somewhere in the order of 6-8 plants/m in skip row configurations and maybe slightly lower in a solid plant. That trial work quiet surprisingly has shown no difference in fibre quality across that full range, right through from 3 to 11 plants/m. Length, strength and micronaire have been the same across the range in both years of trialling.

Further Information: John Marshall