Mike Bange, Senior Principal Research Scientist, CSIRO and James Quinn, Marketing & Extension Lead, CSD are often asked questions regarding limited water decisions. Here are their answers to commonly asked questions:

Which configurations yield highest?

Fully irrigated solid planted cotton will out-yield wider row configurations on a per hectare basis NOT necessarily on a per ML basis. (Figure 1).

As water becomes limited skip row configurations can become an option as they function by increasing the volume of soil that plants have to explore, providing a bigger reservoir of available moisture and allowing the plants to hold on for longer during dry periods.

Figure 1: Comparison of yield between solid and skip row configurations

How do solid and skip row configurations compare?

CSD have data from 18 sites where solid and single skip row configurations have compared (Table 1). While the solid plant has a higher yield, the single skip can have gains in water use efficiency by better utilising later rains (January/February), which can boost their yield.

Table 1: Summary of 18 case study comparisons where solid and single skip row configurations were present.

Which configuration is best? 

No simple answer as every situation is going to be a bit different.  Different climate, different rainfall, different soil type and different attitudes toward risk. For more detail on how these factors affect choice of row configuration refer to CSD’s publication Getting the Most out of Skip Row Irrigated Cotton.

Is there a decision tool to help make a choice on row configuration?

Yes, CSD’s Semi irrigated row configuration decision matrix (Figure 2).  We need to get at least 800mm water through the cotton plant (crop evapotranspiration (ETc)) to reach its full yield potential.  There are three water sources that can make up this 800mm:

  1. Irrigation water available (what you can apply to the crop)
  2. Soil water (Plant available water holding capacity, PAWC).
  3. Effective rainfall, rainfall that ends up in the soil profile and is available for the plant to extract, not runoff water.

The choice of row configuration is then based on combining the various sources to achieve in excess of 800 mm over the life of the crop.

Figure 2: CSD’s Semi Irrigated Row Configuration Decision Matrix.

If you've got above greater than 900mm, then solid is going to be the option.  If you are below 800mm, then you have to and improve the bucket.  The lower the available water, the wider the skip configuration.

Example 1: (e.g. Darling Downs, Breeza Plain)

  • PAWC = 300mm
  • 3 Irrigations = 300mm
  • In-crop Rainfall = 400mm
  • Avail. Moisture = 1000mm

In example 1 there is enough available moisture to potentially plant in a solid configuration to achieve a high yield potential - or single-skip if the rain forecast is not looking promising.

Example 2a:

  • PAWC = 200mm
  • 3 Irrigations = 300mm
  • In-crop Rainfall = 200mm
  • Avail. Moisture = 700mm (100-150mm short)

Example 2b (Single Skip):

  • PAWC: 300mm
  • 3 Irrigations = 300mm
  • In-crop Rainfall = 200mm
  • Avail. Moisture = 800mm

In example 2a, there is a deficit of 100-150 mm of available moisture and the crop will not be able to reach its full potential if planted on a solid row configuration. The one thing the grower can alter is to go out to a single skip row configuration as in example 2b which will effectively increase the amount of PAWC available to that crop.  This would give the crop enough to get through. If the growing season ends up being drier than expected or the forecast for rainfall is not high, then double skip planting may be an option for risk management.

How do single plant lines (60”, 80” solid) and paired lines (single or double skip) compare?

At the lower yield level, below 4 bales/ha there is not a great deal of difference between the two.  It is only when we start to get into the higher yield ranges that the single plant lines will out yields the paired lines.

CSIRO have found is that the equidistant spacing of rows is a really good option in terms of light use efficiency of the crop also in terms of water use as equidistant row spaces actually forces plants to go looking for water deeper.

The decision on moving to a single plant line opposed to a double plant line should be made in full consideration of planting equipment width, tractor wheel spacing and also picking machinery configuration and availability.

Has water use pattern changed over time?

Yes, Bollgard cotton has an increase in water use earlier in the season which is attributed the higher and earlier fruit load, which has important implications on timing of irrigations.

Figure 3: Comparison of seasonal crop water use between Non-Bollgard and Bollgard Cotton Varieties.

Should we be delaying the planting date?

We may have no choice if we are planting on rainfall. However, ideally the later in the planting window the better (within reason we need to make sure we finish that crop off). Planting later in the window enables us to take advantage of the summer storms in January/February period but it’s also taking that critical boll fill and flowering period out of that heat of that December January. However, establishing these crops on rainfall is where we make some big savings in terms of water.

Do we have to increase the plant population in semi-irrigated cotton?

No, the same principles apply for a fully irrigated crop as a semi-irrigated crop. As long as you have a fairly uniform, even plant stand (it can range between six to thirteen plants per meter depending on your area) with no gaps. Large gaps are exacerbated by the skip area and the plant has trouble trying to do compensate for those gaps.

If you are planting into fairly rough conditions and you know things aren't ideal, adjust your planting rate up a little bit just to make sure you get that uniform plant stand.

There is often only one opportunity to get this crop in and established, so you want to do it right. Don’t want to have to go back and flush the field as that’s a waste of valuable water. 

Pix management of skip row configurations

If things go well during the season we can get some large plants in wider configurations.  It's very important that you don't grow more than 4 nodes of vegetative growth. We don't want to be getting an overly rank crop we want to make sure that we get a crop that's still around 25 nodes (if it comes to that) and still makes it just over a metre in height. 

A large bulky plant is inefficient and can be difficult to pick.

The same rules apply for skip as solid planted in terms of pix management. It’s important to remember that in some cases these skip row crops will have more soil area more nutrients and more water available to them so that the rate is going to be more difficult to determine.  If we are getting the end of season and we know that we haven't got any rainfall or water available to keep it going a good cutout rate of pix before the last irrigation might just help stop new growth.

How do you time your irrigations in a semi-irrigated system?

The timing of irrigation is going to change depending on how much water you have available and how the crops are developing, but the principles are the same as for a solid plant. Water stress during flowering, particularly at peak flowering through to late flowering will result in the greatest yield loss (Table 2), so this is where you want to be targeting your irrigations. Important to use both soil moisture probes and continue to closely monitor crop development eg Nodes above white flower (NAWF).

Table 2: Yield loss (%) per day of water stress (extraction of >60% plant available water content). (Source Yeates et al. 2010#; Hearn and Constable 1984*).

Timing of the first irrigation?

Timing of the first irrigation is critical as it sets up the crop. If you stretch this irrigation, it will adversely affect the crop architecture so it will not be as efficient at generating yield is rainfall or water is received later in the season.

One irrigation available: It might be the one that establishes the crop and you grow a dryland crop after that.

Two irrigations available: The first irrigation would be at a similar time as a fully irrigated crop.  The 2nd irrigation is really about managing that nodes above white flower.  Delay irrigating for long as possible into flowering without letting the crop go into serious stress or fully cut out – maybe 5-6 NAWF. This will limit yield potential should further irrigation water become available later on but will give the best opportunity for good fibre quality for the fruit that is already set.

Two irrigations available, established on rainfall: Target that first irrigation not before flowering but in the early flowering period and then the second one around cut out time to make sure you fill those last set bolls. Crop monitoring around this second irrigation is essential as growth regulator may be required to prevent re-growth and target resources into filling bolls.

Three irrigations available: Here you are trying to extend that flowering period. These 3 waters would be applied as the crop NAWF declines from around about 8 NAWF nodes down to 4 NAWF (cutout).

Does row configuration affect fibre quality?

Yes it can. There is an important interaction between row configuration, yield potential and fibre quality. As the row configuration widens, inherently there is the potential for less yield in better seasons, and this difference can be quite large. However, wider configurations yield similar to tighter configurations in harder years, while maintaining base grade fibre quality. Therefore, varying row configuration is a method by which yield can be secured and quality discounts minimised.

The relationship between yield and fibre quality in dryland cotton is shown in figure 2. This is the summary of 12 dryland row configuration trials conducted by the CSD Extension and Development team since 2006. The upper points of each set of individual data are a result of a wider configuration. Please note that, within many cases in these trials, the final yield was not significantly affected by the choice of row configuration.

While the data presented here is for dryland crops, the principals remain the same for semi irrigated cotton on a similar row configuration.

Figure 4: Relationship between dryland cotton yield and fibre length

Is there a tool help decisions regarding Dryland row configuration? 

Yes, the CSD Dryland Row Configuration Tool. Selecting the right planting row configuration is a critical factor in the decision to plant dryland cotton and, depending on seasonal factors, will impact on the final yield and fibre quality, and the profitability of your crop.

There are several considerations which may influence the selection of a row configuration, including soil Plant Available Water Holding Capacity (PAWC), planting date and the seasonal forecast or risk profile. Select the optimal configuration by considering and overlaying these factors, but remember it has to fit into existing tractor wheel bases and picking technologies available.

Figure 5 demonstrates combining these three variables into a matrix to visually describe the row configuration decision making process. If your situation falls within the red shaded area then a wider row configuration such as super single may be a consideration.The orange, yellow and green sections, for which the majority of dryland cotton planting occurs would highlight considerations for either double or single skip planting options. The blue area would get the best result using a solid planted configuration. The decision is ultimately based on an individual’s attitude to risk.

See the CSD publication Considerations for Dryland for more information.

Figure 5.