Investigating the impact of rain-fed cotton on grain production in northern farming systems

Take home messages

• Long fallow with good ground cover is paramount for preparing to establish a summer crop.
• November has the greatest probability of adequate planting conditions for summer crops in Northern grains regions.
• Chickpeas provided the best crop choice for double cropping in 2017 post the dryland cotton crop at Narrabri, due to high crop gross margins and its greater ability to extract soil moisture compared to wheat.
• Cultivating after dryland cotton crop did reduce cotton volunteers and ratoons by >100 plants/ha, but yield of the following chickpea crop was reduced by 42% in 2017.

Cotton’s fit in a dry-land farming system

Rain fed cotton production is an integral part of dryland farming systems in the northern grain regions of NSW, and southern Queensland. New cultivars with greater lint yield potential, high commodity prices and improved moisture management with the uptake of minimum-till farming have resulted in greater areas of farming land purposely kept for growing dryland cotton. As a result, questions are being raised about the sustainability of growers committing to growing a long-season summer crop in an unpredictable rainfall climate, and its impact on their farming system.

Issues for growing cotton in a dryland farming system include: How to sequence back into grain crops? What crop to grow after the cotton crop? Does cultivation of the cotton ratoons impact yield potential, and if so for how long? If cultivation does not occur, what is the impact of ratoon and volunteer cotton control?

Issues such as planting moisture opportunity, gross margins, rainfall efficiency and the impact on crop sequencing are investigated by the GRDC-funded farming systems projects. In collaboration with the Queensland Department of Agriculture and Fisheries (DAF), CSIRO and the NSW Department of Primary Industries (NSW DPI), the farming systems program is focused on developing systems to better use available rainfall to increase productivity and profitability. We present results from 2 sources here that investigate the options for transitioning from a cotton crop back to a grain crop and the legacy impacts on subsequent crops in a dryland farming system.

Summer planting opportunities for dryland cotton

One of the major decisions growers have when sequencing a cotton crop is the probability of receiving ideal planting conditions. APSIM modelling was used to predict the probability for ideal planting conditions (i.e. 30 mm of rain over 3 days and plant available water > 100 mm) for two northern NSW sites (Spring Ridge and Narrabri) (1a) and south east Qld (Pampas)(1b). Simulations outputs were taken from simulations of crop sequences involving a summer crop following either a winter cereal or chickpea in the previous year (i.e. Wheat – chickpea - wheat - long fallow, or wheat -chickpea – long fallow). All simulations assumed no-till, full stubble retention, with optimum fallow weed management.

All three regions follow a similar trend, although Pampas achieves planting probabilities earlier than both Narrabri and Spring Ridge. The probability of planting summer crop at Narrabri is not as strong during the month of October compared to the other two sites. The models indicate that in approximately 25-30% of years there is a probability of meeting these sowing conditions in October, which is the optimum planting month for cotton in northern NSW (Cotton Seed Distributers, 2013). Importantly this shows that at both Breeza and Narrabri there is only a 65% probability of summer planting conditions occurring by the 15^th December, meaning growers may miss out on ideal planting in 3 out of 10 years. On the eastern Darling Downs the simulations predict an extra 10% chance with 75% probability of planting before mid-December.

Figure 1. Summer planting opportunity for (a) northern NSW (Narrabri and Breeza) and (b) Pampas, Qld. Where conditions met >30 mm of rain over three days and >100 mm of PAW following a long-fallow after a winter cereal in the previous year (assuming no-till, full stubble retention and optimal fallow weed management).

Post-cotton crop management implications

A grain systems trial was established to evaluate selected farming system options post a cotton crop at the University of Sydney Narrabri research farm “Llara”. The study initiated by the NSW DPI northern cropping systems team, investigated various farming management treatments after growing dryland cotton, in particular grain production, soil nutrition, weed control, pathogen levels and system gross margins.

In total six treatments were developed consisting of three crop choices (wheat, chickpeas and a cover crop - barley), and three post cotton cultivation practices (full cultivation, plant line ripping and no till).

The tillage treatments post the cotton crop included:

• No till: No cultivation with following crops sown directly into cotton stubble with a no till planter. Only herbicides were used to control cotton regrowth or volunteers.
• Plant line ripping: Ripping tynes cultivated along the plant line of the cotton crop to a depth of 30cm. No cultivation occurred between the plant lines
• Full cultivation: Offset discs were used twice to ensure full disturbance.

Following a rain fed cotton crop grown in the 2016/17 summer, tillage events occurred approximately 1 month after cotton harvest and subsequent crops were planted on the 26^th June with approximately 40% of plant available water capacity (PAWC).

Grain crop yields

After the 2016/17 dryland cotton crop, there was low residual soil moisture in the profile (77 mm of plant available moisture to a depth of 120 cm). The implementation of the cultivation treatments further reduced the plant available water in both the full cultivation and plant line cultivation treatments (Table 1). Along with below average in-crop rainfall during the winter of 2017, these factors combined resulted in low grain yields for both wheat and chickpea. The no till systems resulted in greater grain yield for both crops. The wheat no till treatment yielded 0.28 t/ha higher than the wheat plant line cultivated treatment, while the chickpea no till yielded 0.275 t/ha higher than the chickpea plant line cultivated treatment. This equated to a yield difference of 38% for wheat, and 42% for chickpeas. Crop choice also impacted final grain yield with the wheat no till treatment yielding 34% higher than the chickpea no till treatment (0.97 t/ha and 0.64 t/ha respectively).

Table 1. Plant available water (PAW) after cultivation implementation and subsequent grain yield and crop biomass of wheat or chickpea crops following cotton at Narrabri, NSW (2017)

Economic returns of crops

An important aspect of the study based at Narrabri was to evaluate the economics of implementing the various management treatments. Due to the low yields, only two treatments were profitable after the 2017 winter harvest – no till chickpeas and no till planted wheat (Figure 2). Although both treatments did receive an extra herbicide than the cultivated treatments, the yield advantage resulted in higher gross margins. While crop choice did impact gross margin, as no till chickpeas resulted in a higher income than the no till wheat ($132/ha and $44/ha respectively). The results show that both cultivation and crop choice had an impact on the gross margin for the grain crop following cotton. For growers considering the value of planting a strategic cover crop after a dryland cotton crop, the farming system’s 2017 cover crop (barley) resulted in a cost of $100/ha. The cost includes planting cost, seed purchase and herbicide applications and fallow maintenance up to December 2017.

Figure 2. Crop gross margins post a dryland cotton crop from Narrabri, 2017. Grain values used for gross margin analysis are 10 year median prices at port, minus transport costs. Prices used at Narrabri were: wheat - $269/t and chickpea - $504/t

Crop water use efficiency (WUE)

After the 2016-17 rain fed cotton crop, there was 77 mm of plant available water (PAW) at the Narrabri farming systems site (equal to 42% of plant available water capacity (PAWC)). As expected, soil disturbance due to cultivation treatments led to a loss in soil water. The full cultivation reduced plant available water by 21 mm, while the plant line cultivation reduced plant available water by 12 mm. As a result the no till treatments had higher plant available water at planting. Subsequently, the no till chickpeas had the greatest crop water use for all the treatments planted in 2017 (189 mm, p<0.05). The impact of cultivation was highlighted by the three wheat treatments, with no till wheat using more moisture than plant line cultivated wheat, which in turn had higher crop water use than the fully cultivated wheat (178, 161 and 144 mm respectively).

While wheat had higher water use efficiency (kg grain/mm crop water use) than chickpea (4.6 and 3.4 respectively), a no till sowing operation resulted in higher water use efficiency than plant line cultivation for both crops (Table 2) (p<0.001).

Interestingly crop choice had an impact on the plant available water left in the profile after the 2017 winter crop harvest. There was an average of 57.8 mm plant available water for the wheat no-till and plant line cultivation treatments at harvest, while the chickpea averaged (for the same cultivation treatments) a lower amount of 42.5 mm. This result supports the theory that chickpeas can access more soil water than wheat. They were able to produce grain later in the season, while the wheat treatments were observed to have matured earlier due to moisture stress (Figure 3).

Figure 3. Plant available water, Narrabri 2017-18



Table 2. System water use efficiency, Narrabri 2017

Cotton regrowth and volunteer control

A major concern for growing rain fed cotton is the number of ratoon and volunteer cotton plants that occur after cotton harvest. Controlling ratoon and volunteers can be expensive and become hosts for pests and diseases. Weeds counts conducted 184 and 300 days after the harvest of the cotton crop show the longevity of the volunteers and ratoon plants. The application of the two cultivation treatments did reduce the number of cotton ratoons and volunteers, with the plant line cultivation having the greatest effect. While both the cultivation activities did incur an extra cost for the management systems, the higher number of ratoons and cotton volunteers resulted in extra herbicide applications for the no-till treatments.

It should be noted that there are no registered or consistently reliable herbicide options available for the control of cotton ratoon.

Table 3. Residual ratoon and volunteer cotton plant numbers (plants/ha) at Narrabri, at 184 and 300 days after cotton harvest

Crop yields following cotton compared to other crop sequences

Farming system trials at Narrabri and Pampas have provided opportunities to compare the yield of crops grown as a double crop after a summer crop, with the yield of crops grown after different previous crops in the cropping sequence. At both Narrabri and Pampas, cotton and sorghum crops were followed by a double crop of wheat. Here the yield of these wheat crops were compared with the yield of wheat crops grown after chickpeas followed by a summer fallow. As shown Table 4, a chickpea- fallow-wheat sequence clearly resulted in a higher yield at both the Pampas and Narrabri trial sites when compared to wheat yields following cotton or sorghum. The yield of the wheat crop following directly after cotton was 65% lower at Narrabri and 47% lower at Pampas compared to following chickpea. It must be noted that both Pampas and Narrabri received below average rainfall during the 2017 winter growing season, but the results show the large impact cotton has on the following crop’s yield. At the Pampas site, it should also be noted the impact of a long season summer crop (cotton) compared to a shorter growing summer crop (sorghum). Wheat yield when double-cropped following sorghum yielded significantly higher than following cotton (1.75 and 1.06 t/ha respectively – Pampas 2017).

Table 4. Wheat yield at farming systems research sites Narrabri and Pampas in 2017 following cotton compared to other previous crop sequences.

The Pampas experiment also compared the impact of different summer crops (maize and cotton) on the pre-plant soil water and yield for subsequent summer crops (sorghum or mungbean) (Table 5). When cotton was the previous crop compared to maize, starting plant available water for the next summer crop was approximately 20 mm lower and yields of sorghum were reduced by 0.4 t/ha and yields of mungbean were reduced by 0.3 t/ha.

Table 5. Comparison of soil water pre-plant and subsequent grain yields of sorghum or mungbean crops following either cotton or maize the previous summer, Pampas 2017.

^A cv. Taurus, sown 3 Nov 17, soil N 150-180 kg/ha, 65 000 plants/ha
^B cv. Jade, sown 8 Dec 17, 360 000 plants/ha

Conclusion

There are many challenges sequencing cotton in a dry land farming system. Firstly, growers need to evaluate the impact and risk of growing a long season summer crop in a variable climate with unreliable summer rainfall. Northern NSW and south-east Queensland do have high probability of adequate spring – summer planting conditions especially after a long fallow with good ground cover; however, the planting conditions may occur later than the ideal planting date for full lint yield potential.

The opportunity to plant a double crop after cotton in optimum conditions is limited; therefore, if growers do plant, the crop will benefit from capacity to tolerate moisture stress. At Narrabri, chickpeas stood out as the ideal second crop in a double cropping sequence, as they were able to extract a greater amount of soil moisture in a low moisture environment and also resulted in the greatest gross margin. Wheat and the cover crop (barley) did have greater biomass accumulation and did result in greater residual stubble cover, which may have a beneficial impact on future grain crops. While cultivating did have benefits such as reducing the cotton ratoons and volunteer numbers, the cost of the implementation on soil moisture caused significant yield reduction. If growers are able to defoliate their cotton within the regulated date, the ideal treatment is to leave the field in a no till situation. It is noted that there are no registered or reliable options for control of ratoon cotton with herbicides.

We have also found that the greater moisture extraction of cotton compared to other summer crop options can have legacy impacts that last > 12 months, resulting in lower grain yields compared to growing crops after other summer crop options. These negative impacts should be considered when evaluating the profitability of dryland cotton compared to other summer grain crop options (e.g. sorghum, maize).

References

Bell, L. et al (2015). “Improving northern farming systems performance”. GRDC Update paper

Keating, B. A. et al (2003). "An overview of APSIM, a model designed for farming systems simulation". Europ. J. Agronomy 18: 21.

Cotton Seed Distributers. “A guide to dryland cotton”. csd.net.au

Acknowledgements

The research undertaken as part of this project (DAQ00190 and CSA00050) is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support. We must also acknowledge the assistance provided by staff at the Pampas experimental site (Jack Mairs, Duncan Weir, John Lawrence) and co-operators and hosts at ‘Anchorfield’ and ‘Llara’ who assist us implement this experiment in a variety of ways (too many to mention).

Contact details

Jon Baird
NSW Department of Primary Industries
21888 Kamilaroi Highway, Narrabri 2390
Ph: 02 6799 1520
Email: jon.baird@dpi.nsw.gov.au

Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994