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Soybean desiccation - diquat, ethephon and windrowing

M. Schulz

Department of Agriculture, Victoria, Institute for Sustainable Agriculture, Private Bag, Tatura VIC 3616

Yield and quality of soybeans, Glycine max, grown in Victoria can be reduced when rains delay harvest. To diminish this risk, early maturing varieties such as Stevens are recommended. Where an early maturing variety is not planted the chemical desiccant Diquat can be used as a pre-harvest aid (2). An experiment was conducted to assess the consequences of desiccation timing during a wet harvest period (1989).


Stevens and Bowyer soybeans were desiccated with Diquat when seed moisture contents were 72, 60, 50 or 25%. Moisture content, senescence, yield and quality were monitored. To investigate alternatives to Diquat, ethephon and windrowing treatments were implemented when the moisture content reached 60%.

Results and discussion

Diquat applied to Stevens soybeans when the average seed moisture content (M.) was 50%, brought forward harvest without reducing the final seed yield (3.5 t/ha). Three weeks after application the stem moisture content (M.) and the M. of desiccated plants were 13%, while non-desiccated plants had M. = 23% and M. = 66%. Desiccation at M. = 60 or 72% reduced yield to 2.3 and 1 t/ha respectively. Desiccation when M. > 50% inhibited seed dry weight accumulation, resulting seed weights being 7.5, 14.2, 18.3, 17.4 and 18.6 g/100 seeds for plants desiccated at M. = 72, 60, 50 and 25% or not desiccated respectively. Desiccation timing did not influence oil (20%) or protein (39%) content. Responses by cultivar Bowyer were similar except that yield was not reduced when Bowyer was desiccated at M. = 60% and desiccation at M. < 50% increased final seed yield 20% relative to untreated plots.

Ethephon treatment of Stevens at M. = 60% produced 90% leaf drop within one week but had little effect on M. and M,. In contrast, windrowing reduced leaf senescence and resulted in rapid plant dry-down, M. and M. reducing to 15% within two weeks of cutting. Stem dry-down after cutting was faster than after desiccation with Diquat. Ethephon did not affect final yield but windrowing reduced yield by 34%. This yield reduction was associated with smaller seeds but not with changes in oil or protein content.

Soybeans are physiologically mature when no longer gaining dry weight, which corresponds to M. = 54-62% (1). Plants desiccated after this should not experience yield losses. Caution is advised when applying desiccants as premature application substantially reduces yield. Windrowing could be substituted for desiccation if retention of necrotic leaves is acceptable and prone plants do not rot. The major advantage of desiccation is a rapid dry-down of stem material and subsequent ease of harvest. As ethephon application does not dry stem and seed material, it is unsuited as an aid to harvesting dry soybeans.


TeKrony, D.M. 1979. Agron. J. 71, 771-775.

Whigham, D.K. 1979. Agron. J. 71, 630-633.

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