Dr Paul Grundy of QDAF and CottonInfo talks first irrigation in a water challenged 2019-20 season for Central QLD growers:

Last season highlighted a diversity of opinions and consternation around first irrigation decisions, as well as whole-of-season irrigation strategies, with the spectre of a dry season ahead and limited water.

With another limited water season ahead of us, this newsletter aims to help you identify the range of things that are now in play which will hopefully enable you to make effective decisions that suit your farm, water supply and planted area.

Let’s start by considering the purpose of the first irrigation and why it is critical for the yield potential and harvest weather risk management of early planted crops. In essence, the first irrigation enables the plant to develop a timely, proportioned and healthy canopy (and root system) in the lead up to first flower.

Time is important because irrespective of all else, the crop is marching towards first flower with the accumulation of day degrees. The condition in which your crop will arrive at this key juncture in time is up to you the crop manager. In other words the crop will arrive at first flower at a similar time whether or not the first irrigation is brought forward or delayed but the quality of the canopy by that stage will have a lot to do with when you apply that first irrigation.

This leads to the proportion part of the equation. Bollgard 3 plants are likely to have high fruit retention by first flower. If leaf area is insufficient to support the rapid increase in carbon demand that occurs with the commencement of boll development your crop will be at risk of premature cut-out. This problem is difficult to manage if you reach first flower with a small canopy and low nodes above white flower (NAWF). Secondly if you find yourself in this situation it often even takes more water and nitrogen to reverse the problem with the result being delayed crop maturity (potentially not a great outcome if the decision to plant early was to avoid picking in February).

Crop health is the important third factor. I use the term health in a generic sense but the point is that a crop at first flower should be healthy and not under stress. The measure for this is a crop with a healthy and expanding canopy and root system, with greater than 7 NAWF and with a ratio of leaf area vs retention that will enable plenty of assimilate "in the tank” to ensure continuing rapid expansion - increasing yield potential in the lead up to cut out.

First irrigation is a balance between applying water either too early and too late.

The timing of the first irrigation is critical for getting this balance right. Stretching an irrigation at this stage to save water may be a false economy for a range of reasons that we will explore.

The risk associated with irrigating too early can be waterlogging and a general waste of water, while the risk with being too late is restricted canopy development and root exploration. Irrigation enables leaf area expansion and provides a soil environment that is conducive to root exploration and extraction of nutrients that need to be in solution for uptake.

Stretching the first irrigation to “encourage” root exploration can be a counter-productive idea. Whilst water logging will certainly limit root development, roots sitting in dry soil also stop growing and there will quickly come a point where running a larger deficit will not encourage “lazy” plants to put down deeper roots (as is often assumed to be the case).

This is where the best tool in your decision making kit is a shovel. Many early planted crops this season have had a rough start. The last part of August was cold followed by warm windy weather. These conditions will have slowed plant growth, however with the sunny, dry and windy conditions, the soil profile will have continued to dry down and it is likely that crops are now “sitting”, having become largely stranded due to the earlier stressful conditions and now dry soil. Take a shovel and dig down beside the crop row to ascertain where good soil moisture begins and how this relates to your crops root zone. Chances are the majority of the crops roots are sitting inactive in dry topsoil and this will now be contributing to slow growth. Delaying irrigation on a crop in this situation is just likely to exacerbate the problem.

This video, filmed in Emerald with Michael Bange, will show you what to look for. It is also worth noting that the crop in this video was filmed was overdue for an irrigation (just as a point of reference look at the limited lateral root expansion in the dry hill towards the end of the video).

Irrigating a crop that is partly stranded in dry soil will do a couple of things. Firstly the dry part of the root system will be reactivated to grow and continue exploring the profile, increasing the size of the bucket and providing access to the most fertile part of your soil profile – the top 30 cm. This activity and access to nutrient allows faster growth and enables better root exploration. Trying to save water by delaying the first irrigation at this stage is likely to represent a false economy when you consider the CQ environment and future picking risks. Focus your efforts on making your first irrigation as efficient as possible.

Get the water on and off your field quickly. The lower soil profile should still be relatively full so this irrigation if applied effectively should not require the volumes of water that a later irrigation would require. Another tactic that might suit your circumstances could be to avoid inter-row cultivating prior to watering to again aid the rapid movement of water through the field. The idea here is to provide a quick drink for your crop to get it moving.

Keep in mind that root development continues for a long time after first flower and the opportunity to stretch irrigations and make subtle water savings is more likely during the later parts of the season. This will also encourage timely maturity after securing adequate yield potential. The likelihood of having some help from in crop rainfall will increase during November and December. Making the most of boll setting during November and early December also saves water as the crop is growing under optimal conditions at this time and transpiration demand is less. Basically your crop will travel further between irrigations at this time of year without ill effect. This is why the early sown treatments at the Orana experiment utilised approximately 0.9ML less water over the crop cycle compared to the mid-September sown treatment.

When thinking about how to get the best bang for your buck with the water that you have consider how you are going to maximise the number of bales per megalitre of water available to you. This might mean irrigating a smaller area and maximising yield potential rather than spreading water out more thinly over a larger area of lower yielding cotton. The latter scenario could prove to be profitable if there is spring rainfall but current forecasts are suggesting otherwise. The former scenario might require the hard decision to abandon some fields now and not apply any more water.

Over the longer term the wide planting window does provide some unique opportunities for growers depending on the source of irrigation water. For those on the channel system and not subject to on-farm storage losses, a valid strategy could be to plant late using a single skip configuration as a way of growing a guaranteed high yielding “dryland” style crop that capitalises on in-crop summer rainfall. For those with water in ring tanks that are rapidly evaporating away, the best approach could be to plant early and convert water to fruit as efficiently as possible. This of course does not take into account the vagaries of water allocation rules and regulatory time frames that can dictate when carry over water is used but with all of the issues at hand and the potential for water to be limited next season as well these factors become all the more significant when deciding when and how much area to plant.

For more information, contact Paul Grundy.