Grow a summer cover crop to improve fallow rainfall capture

Storm rains this past summer and autumn saw massive water runoff, considerable soil erosion, and also commonly very poor water storage for the next crop.

This poor fallow water capture contributed to the winter-spring drought with poorer crop performance.

Research, presented at the recent Australian Agronomy conference, found that growing a summer "cover crop", such as millet, is worth considering to add ground cover that will improve fallow rainfall capture.

While the cover crop uses water for its growth, provided there is later summer and autumn rainfall after it is desiccated, the net benefit can be greater summer/autumn water capture and better water extraction in the following crop.

The research was carried out at Bungunya, approximately 70 kilometres WNW of Goondiwindi by Andrew Erbacher and David Lawrence of the Queensland Department of Agriculture and Fisheries.

The study was established in a long-fallow paddock following skip-row sorghum harvested in February 2017.

It had deep phosphorus applied in August 2017 and was 'Kelly-chained' in September, leaving it with little cover.

Cover crops were sown on October 11, 2017.

Three cover crop termination dates were used with millet; early at first node (dry-matter 1.5 t/ha); mid at flag leaf emergence; and late at anthesis (4.7 t/ha).

Sorghum, lablab and a mixed-species treatment and were sprayed out at the same time as the mid-terminated millet.

Wheat was sown across all treatments on May 1, 2018.

Key rainfall totals included 86 millimetres between planting of cover crops to mid-termination, 205mm (11 events) between mid-termination to planting of wheat crop and 41mm between wheat sowing to maturity (three events).

All cover crop treatments, except lablab, had recovered the water they used prior to termination, with some accumulating more plant available water (PAW) than the long fallow control.

Apart from added soil water storage, additional ground cover from cover crops improved wheat establishment compared to bare long fallow in this drier than average season.

Delaying millet termination provided more ground cover, with it persisting longer.

Late-spray-out millet and sorghum collapsed across the inter-row after spray-out, increasing ground cover later in the fallow period.

Between mid-termination and February 28, 2018, 175mm of rain fell in 10 events, and millet treatments all recovered PAW used by them. They had similar soil water to the bare fallow.

Late-terminated and rolled millet, had 19mm more PAW than the control.

When wheat was planted on May 1, the mid-terminated millet had 13mm more PAW than the long fallow control.

Late terminated millet had 15mm more, and late millet rolled had 31mm more.

Water extraction by the wheat was greater from all millet cover crop treatments.

Wheat following millet and sorghum cover crops yielded $200 to $400/ha more than bare long fallow.

Wheat on long fallow yielded 1.45 t/ha, but after early removed millet, wheat yielded 2.2 t/ha.

After medium stage millet desiccation, wheat yielded 2.4 t/ha and after late removal 2.9 t/ha.

Soil water differences at end of fallow explain some of the yield differences.

Cover crops had more water in the 0-10cm layer, largely contributing to improved wheat establishment.

Expected yield increases from the higher fallow water storage alone would typically be about 15kg/ha/mm stored soil water and account for yield increases of 0.2-0.54 t/ha grain.

However, the measured yield gains for these treatments were 0.95-1.46 t/ha.

These results confirm that a crop can be grown and sprayed out to improve ground cover in a long fallow, without having a net negative effect on PAW, with yield benefits in the following crop in excess of what can be explained by increased soil water.

Proceedings of the 2019 Agronomy Australia Conference are available at www.agronomyaustralia.org/conference

Next week: Clay granules, infused with species-specific rhizobia bacteria; the way forward to add, and where necessary, upgrade legume nitrogen fixing ability.