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The impact of undersown medic on wheat yield in a marginal cropping area

T.W.G. Graham, C.M. Schefe and R.J. Jettner

Queensland Department of Primary Industries, Roma QLD 4455
Queensland Wheat Research Institute, Toowoomba QLD 4350

Many areas of the Maranoa, on the western margin of the wheat cropping lands of Queensland, have been cropped for more than 50 years. Grain yields and grain protein levels are low from wheat crops grown on these old paddocks. Undersowing the last wheat crop in a sequence with annual medic appears to be a pragmatic approach to establishing a medic ley to build soil nitrogen levels and restore grain protein and yield. While this has been a successful practice in other areas, local farmers are concerned about possible yield loss from medics in the wheat. Trials were conducted over three years to determine the adverse effects of medic competition on wheat grain yield in drier (200 mm, average winter rainfall) marginal cropping areas.

Methods

In 1988 wheat was undersown with annual medic and the effects of regenerating snail, Medicago scutellata, barrel, M. truncatula, and burr, M. polymorpha, medics in subsequent wheat crops (1989 and 1990) were monitored. The yields of wheat from plots of varying medic density were compared with plots kept free of medic by spraying with post-emergence herbicides chlorsulfuron (Glean') and metsulfuron (Ally`). Regression analysis was used to determine the relationship between wheat grain yield and medic dry matter production.

Results and discussion

In the second year (1989), increasing growth of medic significantly reduced the grain yield of the companion wheat crop (Fig. 1). Wheat grain yield was reduced by 10% in the presence of 15 to 20 medic plants/m2 producing approximately 1000 kg dry matter/ha. At higher medic populations, wheat grain production declined rapidly to only 200 kg/ha of grain where undersown medic yields reached 2000 kg dry matter/ha. In 1990, where fallow medic growth was managed by spraying once with 2,4-D (Amicide 5001 at 1 L/ha prior to planting wheat, there was no effect of medic growth on wheat grain yields of 2350 s.e. 26 kg/ha. Post-planting rains germinated medic under the wheat crop but medic yields were too low to affect the wheat.

Figure 1. The effect of medic growth on the grain yield of wheat at Roma in 1989.

The beneficial effects of medics on soil fertility are known, but autumn medic populations need to be managed to minimise grain yield loss. This can be done by strategic use of pre-plant herbicides or perhaps higher wheat-planting rates. Further research on medic-wheat competition is needed to develop appropriate recommendations on planting strategies.

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