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Cotton irrigation scheduling in the South Burnett

P.N. Vance

Queensland Dept. Primary Industries, P.O. Box 23, Kingaroy Q 4610

Trials to determine the yield response of cotton to irrigation on Byee soils and to promote improved irrigation scheduling techniques to growers are part of a Queensland project funded by the Cotton Research Council.

Trial procedures

Unreplicated irrigation scheduling treatments were imposed on six-row strips of Siokra in commercial blocks on black earth alluvial soils (Ug 5.15) at Byee. Treatments were scheduled based on potential crop water use estimated by a crop factor Class A pan evaporation model using data from the weather station at Byee.

Results and conclusions

Peak yields in both seasons in excess of 8 bales per hectare were achieved with the 90 mm deficit and the farmer's commercial practice (around 120 mm deficit) in the 1987/88 season (Table 1). These irrigations occurred during flowering-boll development.

Table 1. Yield response to irrigation scheduling treatments.

Very frequent waterings were unnecessary and depressed yields slightly. The dryland yields reflect the marked contrast between seasons. The 1986/87 soil moisture for the dryland treatment was close to field capacity at flowering in early January and did not reach wilting point until after bolls had started to open. By contrast in 1987/88 little rain fell after the start of December with soil moisture dropping 120 mm below field capacity soon after flowering and passing wilting point in the top metre of soil by mid January.

High yields of cotton (8 bales/ha) can be produced by scheduling irrigations so that the soil moisture does not drop more than 120 mm below field capacity on Byee alluvial soils. This should allow greater flexibility in irrigation management than in other Queensland cotton districts where deficits of 75 to 90 mm are suggested as optimum. The likely yield depression at deficits greater than 120 mm will be studied in the coming seasons trials.

Growers used trial data to calculate crop water requirements when applying for allocations from a new dam. During the season they will be supported in the use of "WATERSCHED", a decision support package to order water and irrigate on time.

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