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The effects of lupin management on wheat yields in lupin-wheat rotation

Ann Petch

CSIRO Division of Land Resources Management, Floreat Park, W.A. 6014

Grain legume crops such as lupins may be able to supply some of the nitrogen requirements of a succeeding wheat crop. An experiment initiated by Mr. E.R. Watson is being completed to determine the effect of lupin crop utilisation and stubble management on subsequent wheat yields.

Five lupin crops (L. angustifolius cv. Uniharvest) were grown in replicated plots in a 600 mm rainfall area of W.A. At flowering one crop was mown off as if for silage. One month later one crop was mown off as if for hay. The residues from each were discarded. The other three crops were harvested for grain at maturity. In mid-summer the stubbles of two crops were spread evenly across the plots and one lot was burnt, while the other was left. The third stubble was raked off the plots and discarded. In autumn all the plots were ploughed. turning in the remaining stubble, and wheat (T. aestivum cv. Gamenya) was sown on all five areas, as well as on a sixth area which had been cultivated and fertilised but not sown in the lupin year ('Fallow'). Yield and nitrogen content of the wheat was measured. The experiment has run for a total of ten cycles on three sites. Response to additional nitrogen has been measured in some years. Some results from one of the sites are presented.

The 'Silage', 'Hay' and 'Burn' treatments were usually followed by the best-yielding wheat crops. However, no one treatment was always superior. Wheat grain yields for three consecutive cycles of lupin-wheat are shown in the figure.

Fig. 1. Wheat yields, means of 18 reps, after five lupin treatments or fallow the previous yr. Different symbols show significantly different (P<0.05) values within yrs.

Although grain nitrogen content varied from year to year it was not affected by treatments, so nitrogen uptake reflected wheat yields, and ranged from 12.9 to 19.9 kg N ha-1 in 1973 and from 22.9 to 29.5 kg N ha-1 in 1975. The effect of nitrogen application to the wheat crop was tested in 1977. Grain yield responded significantly to an application of 50 kg N ha-1 on all treatments except 'Turn'.

The responsiveness to added nitrogen suggests that other nutrients are not limiting, but the ploughing-in of lupin stubble may tie up large amounts of mineralised and applied nitrogen in decomposition.

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