Irrigation by cotton variety demonstration

Jenelle Hare, Department of Primary Industries and Fisheries/Cotton Catchment Communites CRC and Simon White, National Centre for Engineering in Agriculture

A field scale cotton variety by irrigation demonstration was established on the Darling Downs during the 2006/07 summer following discussions with growers and consultants. They had identified a need to assess the local yield performance and water use productivity of Bollgard varieties grown under a range of irrigation deficits.

Trial Overview

The 25 ha demonstration site was established on a farm in the Nandi area near Dalby as part of a Department of Natural Resources and Water funded Rural Water User Efficiency Three project. Commercial-sized plots of 32 rows were sown to each variety in each irrigation strategy. Eight rows were sown between each irrigation strategy to negate lateral water movement effects. A further 40 rows of single skip, at the growers’ instigation, were sown beside the trial area to compare the performance of solid with single skip.

The varieties assessed were Sicot 71B, Sicot 80B and Sicot 43B (Sicot 80B was sown in the single skip). The three irrigation treatments applied were:

* Early (Strategy A) – 80mm deficit
* Commercial (Strategy B) – 100mm deficit
* Late (Strategy C) – 120mm deficit

The deficits were measured using a Diviner capacitance probe (as per commercial practice). A calibrated neutron probe was also used to measure the true deficit. All irrigations were measured and evaluated using Irrimate ™ technologies.

In September 2006, 260kg/ha of urea, 100kg/ha of Starter Z and 40kg/ha of muriate of potash were applied based on soil tests.

Pre-water

Pre-water started 16 October 2006 and took 36 hours. Starflow meters in the head ditch and sump showed an application of 2.17 ML/ha and tailwater of 0.16 ML/ha (a net application of 2.01 ML/ha or 201 mm). Given the field was a failed sorghum crop from 2005-06 and had little fallow rainfall, this was not surprising.

The irrigation advance was very even – evidence of the large deficit and extensive cracking which was present in the field.

Sowing

The site was sown on the 31 October 2006 at 13.2 seeds/m for Sicot 71B, 12.9 seeds/m for Sicot 80B and 13 seeds/m for Sicot 43B. Starter Z fertiliser at 20kg/ha was applied. Emerging seedlings experienced every adverse condition possible – from hot windy conditions directly after sowing to cool, cold conditions thereafter. This affected final plant populations (see Table 1).

Table 1: Final plant populations (plants/m row)

The single skip configuration had a final plant population of 6.9 plants/m.

Soil moisture monitoring

Soil moisture was monitored by:

• calibrated neutron probe (3 tubes in each plot) installed at the top and bottom of the field
• a Diviner (2 tubes in each plot)
• an EnviroSCAN in each plot (to demonstrate plant extraction rate from continuous soil moisture monitoring).

In addition, neutron probe access tubes, diviner tubes and enviroscans were installed across the single skip area to investigate soil moisture extraction of the crop into the skip. The tube placements were between the rows, on the planted row, in the furrow and on the top of the skip.

In-crop

Cultural operations and insect management of the site was as per normal commercial practice. Two mirid sprays were applied to all plots and a Pix® spray to control vegetative growth was applied to the Sicot 80 B plots. Rainfall recorded at the site for the season was 146.5mm.

There was some useful in-crop rain in December and early January (see Table 2). Total effective rainfall for the season was only 45 mm, denoting the numerous small showers that occurred throughout the season.

Table 2: Monthly rainfall (mm) at site

Throughout the demonstration there was evidence of Sicot 80B´s more indeterminate growth habit and its suitability for adverse (dryland) soil moisture conditions compared to the other varieties. Sicot 80B consistently maintained a more desirable plant water status prior to irrigation and was the only variety requiring an application of growth retardant in mid January.

In later growth stages Sicot 71B appeared to adapt to all conditions imposed extremely well. Its compact growth habit was very supportive of high fruit retention and large bolls and very little shedding occurred during boll fill.

Strategy C and the single skip were defoliated on 30 March 2007. Strategy A and B were defoliated on 2 April 2007. The second defoliation was applied to the whole site in mid April 2007.

Irrigations

The irrigation strategies were implemented after the first in-crop irrigation. All irrigations were measured and evaluated using Irrimate™. The irrigations were not optimised as the aim was to evaluate current commercial practice. The final irrigation was strategically implemented to achieve desired soil moisture conditions at defoliation. Five irrigations were applied to strategy A, four to strategy B and strategy C, and three to single-skip. Forty-six Irrimate™ evaluations were conducted during the season.

Irrigation water applied throughout the season (including pre-water) is shown in the table below. On average, an additional 0.35ML/ha of irrigation water was applied to the early strategy compared to the commercial strategy.

Table 3: Water applied (ML/ha) to each treatment

Yield Results

The site was picked on 26 and 27 April 2007. The yield results are shown in Table 4.

Table 4: Yield (bales/ha) for each treatment

Water use indices

Gross Production Water Use Index (GPWUI = Yield ÷ Total Applied Water) for each variety is shown below in Table 5. Total applied water includes irrigation plus estimated effective rainfall plus soil moisture reserve (starting soil moisture minus final soil moisture at harvest).

Table 5: GPWUI (bales/ML) for each treatment

Outcomes

• Across the range of irrigation treatments the same varietal trend existed. Sicot 71B had the highest yield for each irrigation strategy, followed by Sicot 43B.
• Improvement in GPWUI was achieved by irrigation earlier at a smaller deficit than current commercial practice. No benefit was found in delaying irrigations past commercial practice for any variety.

Acknowledgements

We thank all growers and consultants who were involved in preliminary discussions for their valuable participation in directing the RWUE3 field activities. We acknowledge the contribution of David Hall (ex Landmark) for his valuable input during the early stages of the trial and Will Woolcock, Landmark, for his contribution and co-operation. We also thank John Marshall and Cotton Seed Distributors (CSD) for supplying the seed for this trial and importantly his input on variety selection. Thank must go to our growers Russell, Sue and Justin Commens, for without their support and co-operation all this work would not have been possible and demonstration trials such as this would not happen.

Watch for the Darling Downs Trial Booklet which will include a comprehensive report on all results from the demonstration site. Anyone wishing to obtain a copy should contact the Darling Downs Regional Cotton Extension Officer.