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Water management for irrigated pasture and salinity control in south-west western Australia

P.R. Scott and R.A. Sudmeyer

Department of Agriculture, Baron-Hay Court, South Perth WA 6151

Average water applications of over 100 mm have resulted in ground water recharge and salinity in the south-west irrigation area of WA. Poor pasture growth has raised questions about optimum frequency and amount of irrigation.

Methods

The effect of water stress on the growth, dynamics and water use of an irrigated perennial ryegrass, Lolium perenne var. Ellet, and New Zealand white clover, Trifolium repens, was examined in 1989 at the Wokalup Research Station. The soil was sandy loam with sandy clay at about 50 cm depth. The pasture was grazed and mown to maintain above-ground biomass between 1 and 2 t/ha, and sprinkler irrigated to maintain available soil moisture above 75%. Two plots of 324 m2 were compared after irrigation ceased on one of them.

Soil moisture was measured by neutron moisture meter in six access tubes to a depth of 1.8 m in each plot. Readings were taken every Tuesday and Thursday, with additional readings immediately before and after each irrigation. Pasture growth was measured on random samples by tagging individual plant leaves, and recording their rate of elongation. Above-ground biomass was cut, separated into species components, live and senesced, then dried and weighed. The quality of the live material was assessed by measuring metabolizable energy, digestibility and crude protein. Meteorological variables were recorded at the site.

Results and discussion

The rate of senescence of pasture under water stress increased markedly (from 17 kg/ha/day to 70 kg/ha/day about 10 days after watering ceased on the non-irrigated plot. This was accompanied by a reduction in accumulation of living biomass from 80 kg/ha/day to 23 kg/ha/ day and the ratio of senescence to growth increased from about 0.2 under well watered conditions to 3.0 under moisture stress (after day 10). Quality of the remaining living material increased marginally as soluble carbohydrates were translocated from senescing leaves. Total feed quality was likely to have been reduced, however, because of the increased senescence.

Significant differences in leaf elongation rate between the treatments occurred after 37-51 mm of soil moisture depletion (40-85 mm of Class A pan evaporation) or 6 to 12 days from the cessation of irrigation. Ryegrass appeared to be more sensitive to soil moisture deficit than white clover.

To maintain maximum pasture growth rates in the south-west of Western Australian, farmers should water ryegrass-clover pastures at about 10-day intervals, or better, after about 60-80 mm of pan evaporation. This is more frequent than currently practised and similar to recommendations for northern Victoria (1). Water per irrigation should be matched to soil moisture depletion to minimise groundwater accessions. Good water control is essential to avoid waterlogging and wastage.

Acknowledgments

Assistance was receil, ed from the Western Australian Water Authority, Wokalup Research Station, Harvey and Bunbusy offices of the Department of Agriculture.

Reference

Blaikie, S.J., Martin, F.M., Mason, W.K. and Connor, D.J. 1988. Aust. J. Exp. Agric. 28, 315-319.

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