Key Bollgard® II nutritional needs

Cotton Seed Distributors - Web on Wednesday

Dr. Chris Dowling, Research & Development Agronomist with Incitec Pivot, discusses key issues in relation to addressing nutritional needs of high yielding Bollgard® II cotton crops.

Chris could you just outline some of the improvements in soil sampling procedures both positional and profile that you’re extending these days?

In the past we’ve had a very randomised “walk all over the field” approach to encompass all of the variability in the field in one sample and to a degree that’s served our purpose in growing cotton in the recent past, but we need a little bit more precision with higher yields.

What we’re trying to do is, given the tools we’ve got with precision ag, yield maps, CMPDH surveys, cut and fill maps, is to try and do a better job on locating where we sample and doing it fairly precisely for obvious reasons, like picking vigorous parts of fields from yield maps and seeing what impact that’s having on the soil test values compared to other parts of the field.

In that way we can probably predict what’s going to happen to the rest of the field should we start to produce that sort of yield all over the field. Secondly, traditionally we’ve looked at zero to thirty cm as being the layer of the soil we’ve concentrated on.

That was fine in the days of full water and very few rotations in cotton but with rotations and again with higher yield I think we’ve got to look below 30cm to see what’s going on down there.

There’s plenty of research going on in the grain industry, even in cotton on sub soil limitations and the sub soil for cotton is really that layer from 30 to 80cm. Problems with chloride toxicity, dispersive soils all impact on high yield cotton. We’ve got to know what they are, where they are and then manage them.

With Bollgard® II, many growers now have the potential to produce 12 bales/Ha fairly regularly. What are some of the restrictions for meeting nutrient requirements for these crops mid season?

The biggest change is the realization that for the major nutrients, and I’ll concentrate on N P and K, is that you’re looking at a 25 to 40% increase in peak demand in boll fill.

In other words the crop is producing that extra yield in the same time we used to grow the lower yield, so the demand on the plant is a lot higher. Those conditions are taking place under a set of situations where if that’s happening the roots and the leaves that used to support the plant take up those nutrients at lower yields are now not getting that nutrient and are probably less effective.

The leaves are unable to recycle nutrient because they have a boll attached to every one of them with higher retention and the root systems are getting a lot more competition because of the higher retention, for carbohydrate and nutrients, so we’ve got this higher demand but less sort of resilience or rebound potential in the crop.

I know in the past and probably quite correctly a lot of side dressing research showed no difference between an application at sowing or putting it all up front versus doing side dressing. Now we need to really review that, particularly with N and K and maybe with P in the light of these increases in instantaneous demand during peak fill.

Plant tissue analysis has been around for many years and you yourself have been involved with a lot of the work with that. What are some of the improvements in procedures in recent times, which should give the cotton industry more confidence in using this technique?

I am probably one of the people that have caused the lack of confidence. I probably put my hand up there in supporting the sampling protocols that we used in the past. Again it was an approach to try and encompass the variability that occurred across the fields to try and produce an average.

Unfortunately in doing that, when you go back and sample the second time, you encompass a different amount of variability and we end up with these saw tooth type run down graphs, which you never know from one sampling to the next whether you’re going to be going up or down and that’s just random variability that we’re inducing in the sampling.

It struck me a couple of years ago that researches are able to produce nearly perfect nutrient decline graphs from petioles and the way they do that is usually in small plots or in glasshouse conditions under very strictly controlled conditions It occurred to me that rather than incorporating variability across the field, that we should reduce the variability by monitoring single points in the field that represent either high or low vigor parts of the field, sampling at the same soil moisture, use the same leaf, do everything possible to reduce the variability between samplings.

By doing that last season we produced, on a field basis, the same things that you see in research. It is possible - it takes a lot of mental effort and a very strict reliance on doing it properly. If you’re not going to do it to a very strict protocol, it’s probably not worth starting to begin with.

Just a few comments from you from some of the tools for what you’d class as “managing the crop forwards”, such as 10-day forecasts, pre-emptive use of foliars, etc?

It occurs to you when you think about what causes cotton to grow in the middle of the season that it’s usually around a couple of things and I’m talking about the rate of growth here – they are the availability of water, the availability of nutrients, light and heat. We measure water, we can measure nutrients and manage it through tissue analysis and we’ve just discussed what opportunity there is.

We can manage water pretty closely with Neutron Probes and that sort of thing, but the things we’ve never really looked at and integrated into that management are light and heat. Light for most seasons is not a problem although I will say that I believe that cloudy weather in mid January, should we get it, is going to be a real challenge to manage in these high yielding crops because of the carbohydrate drain on the plant under those conditions with high yield potential and growing bolls, especially need carbohydrate.

But the other one that does vary and we can get good maps of, is temperature. We have available now temperature forecasts up to 10-days out and I think when you make your decision around plant tissue analysis on where the plant is nutritionally, what the water is doing and if we know 10 days forward whether it’s going to be hotter or colder than the last 10 days then it’s just another tool to integrate in there and predict whether we need to apply something now and get a response or we can wait a little bit longer to make that decision.

And just going on with that, the use of foliars with preventing water logging?

Foliars are an interesting thing, and another is plant tissue analysis it’s probably not got a pretty good image. I think tactical use of foliars for specific reasons, not just as a general spray because things don’t seem to be about right has a place.

Certainly the nitrogen research that Arthur Hodgson did in the 80’s clearly shows that tactical use to reduce waterlogging effect after flood irrigation has real benefits and I think that’ll be just increased by the high yield situation. We’ve also got to look at the situation around problems with early senescence.

Foliars have a place - as we said the plant with this high yield potential has a lot less recycling of nutrients in the plant because it’s running at a higher level of demand so to fill in those stress periods I think foliars have got a place - but it’s got to be a tactical use and well planned.

And finally, just the importance of phosphorus and potassium nutrition in high yielding crops. What do you see as some of the immediate research for these two?

I think probably some of the major research needed is not whether we need it or not because there’s no doubt that if we don’t need it now with the removal rates occurring with these high yields, then it won’t be long before we do need it.

The question mark is about the efficiency of application, how do we put it on to get the best effect, what’s the timing and where we need to place it. With these high yield crops taking out 20, 30, 40kg of P and then probably 60 or 80kg of K annually that it’s only a matter of time. The cracks are showing already and I would hate to think we ran it down to the situation where we’re losing a bale or two bales before we decide to do something about it.