Sustaining productivity of a Vertosol at Warra, Queensland, with fertilisers, no-tillage or legumes. 9. Production and nitrogen benefits from mixed grass and legume pastures in rotation with wheat
W. M. Strong B , R. C. Dalal A E , E. J. Weston B C , K. J. Lehane B , J. E. Cooper B , A. J. King D and C. J. Holmes DA Department of Natural Resources and Mines, Indooroopilly, Qld 4068, Australia.
B Department of Primary Industries, Toowoomba, Qld 4350, Australia.
C Current address: 50 Broadwater Terrace, Redland Bay, Qld 4165, Australia.
D Department of Natural Resources and Mines, Toowoomba, Qld 4350, Australia.
E Corresponding author. Email: ram.dalal@nrm.qld.gov.au
Australian Journal of Experimental Agriculture 46(3) 375-385 https://doi.org/10.1071/EA05007
Submitted: 17 January 2005 Accepted: 11 August 2005 Published: 28 March 2006
Abstract
Reduced supplies of nitrogen (N) in many soils of southern Queensland that were cropped exhaustively with cereals over many decades have been the focus of much research to avoid declines in profitability and sustainability of farming systems. A 45-month period of mixed grass (purple pigeon grass, Setaria incrassata Stapf; Rhodes grass, Chloris gayana Kunth.) and legume (lucerne, Medicago sativa L.; annual medics, M. scutellata L. Mill. and M. truncatula Gaertn.) pasture was one of several options that were compared at a fertility-depleted Vertosol at Warra, southern Queensland, to improve grain yields or increase grain protein concentration of subsequent wheat crops. Objectives of the study were to measure the productivity of a mixed grass and legume pasture grown over 45 months (cut and removed over 36 months) and its effects on yield and protein concentrations of the following wheat crops.
Pasture production (DM t/ha) and aboveground plant N yield (kg/ha) for grass, legume (including a small amount of weeds) and total components of pasture responded linearly to total rainfall over the duration of each of 3 pastures sown in 1986, 1987 and 1988. Averaged over the 3 pastures, each 100 mm of rainfall resulted in 0.52 t/ha of grass, 0.44 t/ha of legume and 0.97 t/ha of total pasture DM, there being little variation between the 3 pastures. Aboveground plant N yield of the 3 pastures ranged from 17.2 to 20.5 kg/ha per 100 mm rainfall. Aboveground legume N in response to total rainfall was similar (10.6–13.2 kg/ha per 100 mm rainfall) across the 3 pastures in spite of very different populations of legumes and grasses at establishment. Aboveground grass N yield was 5.2–7.0 kg/ha per 100 mm rainfall.
In most wheat crops following pasture, wheat yields were similar to that of unfertilised wheat except in 1990 and 1994, when grain yields were significantly higher but similar to that for continuous wheat fertilised with 75 kg N/ha. In contrast, grain protein concentrations of most wheat crops following pasture responded positively, being substantially higher than unfertilised wheat but similar to that of wheat fertilised with 75 kg N/ha. Grain protein averaged over all years of assay was increased by 25–40% compared with that of unfertilised wheat.
Stored water supplies after pasture were <134 mm (<55% of plant available water capacity); for most assay crops water storages were 67–110 mm, an equivalent wet soil depth of only 0.3–0.45 m. Thus, the crop assays of pasture benefits were limited by low water supply to wheat crops. Moreover, the severity of common root rot in wheat crop was not reduced by pasture–wheat rotation.
Additional keywords: annual medic, lucerne, purple pigeon grass, Rhodes grass, soil nitrogen fertility, soil nitrate, soil water.
Acknowledgments
We thank Mr Peter Bock and Mr Tim Reid for providing land for the Warra Experiment, the Queensland Wheat Research Committee and Grains Research and Development Corporation for the financial support, and Dr Graham Wildermuth for data on common root rot of wheat in the trial.
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