Pre-harvest desiccation: effects on the physiological quality of soybean seed

Temperature fluctuations associated with high rainfall and the oscillating relative humidity of the air during the pre-harvest stages of seed maturation may result in losses in physical and physiological quality and also in seed health. Pre-harvest desiccant application helps to reduces seed exposure to unfavourable climate conditions and reduce seed vigor and germination. The object of this research was to determine the effects of the application period of desiccants and of early harvesting on soybean seed quality. The experiments were conducted in Alto Garças, MT, with the cultivars MG/BR-46 Conquista and FMT Tucunaré in the 2005/2006 growing season, and with BRS MT Pintado and FMT Tucunaré in 2006/2007. The experimental design was a randomized block in a factorial arrangement 6x2x2x2 [6 treatments (5 desiccants and 1 control), 2 application times, 2 harvest times and 2 storage periods]. Five desiccant treatments were evaluated: paraquat, diquat, paraquat + diquat, paraquat + diuron and glyphosate, applied at the R6.5 and R7 growing stages, which were compared to a control treatment. Seeds were harvested at two stages: first, when moisture content reached 17% to 20% (early harvest), and secondly, 12% to 13% (normal harvest). Seeds from early harvesting were dried to 12% to 12.5% moisture content in a forced air chamber at temperatures ranging from 38 ºC to 40 ºC. Seed quality was evaluated by the following tests: germination, seedling vigor classification, tetrazolium, field seedling emergence and seedling growth. Paraquat, diquat, paraquat + diquat and paraquat + diuron as pre-harvest desiccants do not affect seed physiological quality but glyphosate causes phytotoxic damage to seedling roots. With the exception of glyphosate, pre-harvest desiccation at the R6.5 and R7 growth stages results in the production of seeds having a similar quality. Early harvest results in the production of higher quality seeds compared to those from a normal harvest.

Glycine max; physiological maturity; glyphosate; germination; vigor

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