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David Kelly, CSD agronomist Goodiwindi, talks about the "Nodes Above Cracked Boll" (NACB) technique for determining when to apply the last incrop irrigation.

Timing the last in-crop irrigation is one of the more difficult management decisions for growers. What are some of the issues that need to be considered?

The objective of the final irrigation is to complete all of the maturation of every boll on the plant without any water stress so that all these bolls complete their fibre development with full water and the other objective is at the end of the boll maturation that your soils are that of refill point or fairly empty.

Counting nodes above last cracked boll is an assessment technique for timing of last irrigation it’s gained a lot of popularity, particularly with un-tipped crops that we see characteristic of Bollgard® II. What are the basics of this technique?

Well there are two basic assumptions that nodes above cracked boll are based on. The first one is that four nodes above the cracked boll on the plant is fully mature, so on this particular crop four nodes up that boll has completed all of its fibre development and all its got to do now is open up.

The second basic assumption is that the bolls will mature up the plant at intervals of 42 day degrees so in most situations that’s about 3 or 4 days. So if today that bolls cracked, three days time the next node up will be cracked. There are the two basic assumptions which the nodes above cracked boll is based on.

What are some of the practical considerations that need to be taken into account when doing NACB counts in the field?

The first thing is to make sure you have got a good sample. If you have got a field that’s fairly uniform it makes it easier but if you have got a field where there is variability in plant size you probably want a few entry points in the field and you want a fairly good sample size so you are getting good representation of the plants in that field. You may want to do three or four entry points in the field and at each place do 10 – 20 plants.

The second important thing is to start off with the cracked boll, which is the boll that’s just starting to crack. You don’t count the open boll you start from the cracked boll. The next important thing is, is when you are counting up the nodes of the plant you also include the nodes that don’t have a boll on them and the last thing is when you are doing nodes to above cracked boll you only count as high as your last harvestable boll so in this particular plant it is this boll here. You don’t count those nodes at the top of the plant where there’s no boll.

How do you actually use the technique to determine the date of final irrigation?

When you are doing date of final irrigation you use three important pieces of information. Firstly obviously is nodes above cracked boll which is working our how mature your crop is at the particular time. So on this particular plant here, this is a cracked boll here. We have 8 nodes above cracked boll. 1,2, 3, 4, 5, 6, 7, 8. We will assume that 1, 2, 3, 4 the boll that was on here would have been mature so all of these four nodes here we have got to supply adequate water for them to mature. Now if we assume that it is about 3 ½ days for each node to become mature, well that’s about 14 days until this boll will be fully mature.

The second important piece of information is daily crop water use, trying to work out how much water in millimetres per day that your crop is using and to determine that you could use your soil capacitance probes and neutron probes or any of those things. An example is if we decide that this particular crop here was using 6 mm a day at this time of the year, we know that we have got 14 days that we need full water for. So 6 x 14 is 84mm to complete this crop. So we need 84mm to mature that top part of the plant.

The third piece of information we need to know is what sort of irrigation deficit you are using or what your soil water holding capacity is. So if you are using an irrigation deficit of 60mm we need 84mm to finish off the crop so that’s one full irrigation of 60 mls plus 24 mls. In that sort of situation what you have often got to do is bring that second to last irrigation back a bit so we have that objective of having the soil profile fairly empty when this last boll is mature.

What happens if this timing of this last irrigation is out?

There are two situations obviously there. If your last irrigation is too late so that when this last boll is mature the soil profile is still wet. What you can do is a crop that is fully opened up doesn’t use a lot of water so its not going to dry that soil out so there is a risk that when you go through and do your picking or any trafficking on that field you are going to cause compaction. Secondly obviously is the crop could produce vegetative growth, putting on nodes on the top that you are not going to get anything with. Which you could run into risk of being a haven for insects whether its heliothis in conventional or aphids and white fly in any sort of cotton and also regrowth can increase potential for stain from green leaf when you are picking.

What about the dry end of things?

In the dry end of things if you have pulled the last irrigations so that there is not adequate water to fill some of these last bolls, firstly the crop will go into stress which means that it will be difficult to defoliate because the crop needs to be growing actively to take in some of those defoliants. 

The second thing is you could get a reduction in yield because those bolls haven’t fully developed so there is just going to be less lint per plant going into the plant that you are picking and thirdly is micronaire. The micronaire or the fibre thickening process happens in the second part of fibre development so if you pull water and these bolls are maturing when the crops stressed well you could get decreased micronaire.

In summary, what are the key things about choosing nodes above cracked boll for irrigation decision making?

The assessment should take place as soon as you start getting cracked bolls but you should do it regularly and very often because obviously rainfall during that period is going to mean that you need to re-evaluate your decisions and also as we said before the rate of increase of the bolls opening will vary with temperature, with day degrees, so in hot conditions its going to open up quicker, in cooler conditions its going to open up slower. As your daily water uses will vary with the conditions so measure nodes above cracked boll, keep a good idea of your soil moisture status and do it fairly regularly and keep re-evaluating it.

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