by QDAF researcher, Dr Paul Grundy:
There are a number of things to consider for nitrogen, first irrigation and pest management that will have an important bearing on crop development and yield potential in the CQ environment. The way in which these factors interact depends on the weather both leading up to and after operations take place. Critically these interactions vary significantly depending on whether a crop is sown early (August to early Sept) or late (late November and December).
In this edition we will focus on nitrogen and water interactions, leaving insect considerations for the next newsletter.
When we think about nitrogen, soil water and the weather the best place to start irrespective of planting date is to think about the plant and consider what it is trying to do and how external factors will influence growth. The important thing to remember about cotton is that it is eternal. The plant wants to live forever and therefore it grows with this pre-determined trait in place. If conditions do not suit the plant at a given point in time it can cease growth or shed squares only to resume when conditions improve down the track.
Alternatively if conditions are primed for growth, canopy expansion can occur at an explosive rate with the plant prioritising stem and leaf production at the expense of bolls with the plants underlying tactic being to provide an even bigger foundation for many more bolls down the track. The problem this presents for us as crop managers is that we need to manage the crop and grow an efficient canopy – which is of sufficient size to support a concentrated period of flowering to set bolls over 4-6 weeks that will mature 160-170 days from planting. Water, nitrogen and sunlight are key inputs that sustain growth and whilst temperature governs the rate at which growth development can occur.
For early sown crops cool overnight temperatures act as hand brake on the development and growth of the plant. The result is short internode lengths and reduced leaf area (smaller, denser leaves) and a plant that is very much kept in check. The limitation of this is that as conditions quickly warm up throughout late September, temperature becomes non-limiting and the plant is then self-constrained for a period due to its compact canopy’s ability to produce assimilates that are resourcing growth.
This in itself is not a problem except that Bollgard 3 varieties are likely to have high square retention going into to October and creating a potential imbalance between the production of assimilates from a reduced leaf area with the increasing relative demand from the fruiting sites. The result can be ongoing constraint of vegetative expansion leading to lower NAWF at the commencement of flowering and a crop that cuts out too soon. This becomes evident as plants cut out, failing to close the rows and achieve an optimum size that can support 120 or more bolls per metre row.
The tools available to crop managers to help overcome this typical trend for early sown crops are water and nitrogen. When soil water is constrained leaf expansion is reduced - a natural response as larger leaves require more water for transpiration. Therefore, a critical intervention for early sown crops is the timing of first irrigation to ensure that the crop has sufficient moisture to allow full leaf expansion. In practice for the trial plantings this has involved applying the first irrigation 4-6 weeks after planting around the growth stage of 6-8 nodes.
Typically this has entailed an irrigation on a deficit of 25-40mm depending on conditions. This might seem counter intuitive as many crop managers take the view that making the plant “work” during this early period will set it up with a deeper root system for the longer term. However, this “work” that the plant is forced to do comes at the expense of leaf area which then becomes further limiting in its own right as fruiting site demand soon follows. A better tactic is to increase irrigation deficits over the course of the second and third irrigations when the plant is experiencing warmer conditions and an expanding canopy is better able to support continued root and fruiting structure development in tandem.
Bringing forward nitrogen side dressing to co-incide with the first irrigation can also assist with early canopy expansion compared to an application just prior to flowering. This approach will ensure that nitrogen is incorporated and in plant available form in soil solution as the weather warms up which will also assist canopy expansion. The objective for the earlier application of irrigation and nitrogen is so that these resources are available in a non-limiting capacity as the weather warms up and that the plant is not constrained by these factors at that time.
Late sown crops experience the opposite conditions to early sown crops with hot temperatures often combined with humidity. These conditions predispose the plant to rapid growth and canopy expansion which left unchecked can result in a negative feedback loop whereby the rapidly expanding leaf area produces extra assimilates which in turn feeds more leaf growth as competition from fruiting site demand at this early stage is minimal. The result is rank plant that continues to prioritise assimilates towards stem and leaf as opposed to bolls which can be very difficult to manage if the crop then experiences bouts of cloudy or wet weather during flowering.
For these crops water and nitrogen can be used to curtail leaf and particularly internode expansion. The temptation with these crops is to put down a large basal application of nitrogen prior to planting and water more frequently during hot conditions. These two things in combination particularly if cotton it following a legume such as chickpeas can result in explosive growth with many late planted crops reaching nearly a metre in height at first flower in the 2016/17 season.
The opportunity to control growth in this instance is to hold back on the first irrigation and force the plant to work a little harder at these early stages particularly if the crop is growing rapidly. With minimal fruiting site demand and conditions that are conducive to vegetative expansion there is an opportunity at this stage to better balance the ratio of stem, leaf and square production. A more prudent approach for nitrogen management maybe to split a larger proportion of the crop budget to a side dressing that is timed to within 7-10 days of first flower.
Again this is about taking the edge off the crops growth. Rainfall at this time of year is a risk for nitrogen management so managers need to be mindful of weather outlooks and the ability to drive through crops to apply nitrogen. In some instances intermittent rainfall will make controlling growth with modification to nitrogen and water difficult and the use of Mepiquat Chloride will be necessary. This will be a topic for a later newsletter.
The key message for early and late crops is to think about where the plant is at, what it is likely to do in response to the weather and then work with the crop to either push it along or hold it back. Failure to work with the crop can quickly result in an early crop that cuts out prematurely or a late sown crop that grows rank and sheds fruit. Either of these outcomes can significantly reduce final yield potential.