With Cotton in the Tamarang and Caroona region this summer, it is pertinent to provide some information about hormone damage, the relative amounts required to cause damage and the extent of damage that can be caused by hormones.  This is an article from 'Fallow Solutions' a regular newsletter from Dow AgroSciences. 

Extensive publicity has been given to the risks associated with the application of phenoxy herbicides to fallow fields near cotton fields. Inevitably, most focus falls on 2,4-D because of the extreme sensitivity of cotton to doses 1% or less than those typically applied in the field. It is just as important to consider the risks posed by the application by other phenoxy herbicides (MCPA), other herbicides with an auxin like mode of action including picloram e.g. Grazon™ Extra, triclopyr e.g. Garlon™ 600, fluroxypyr e.g. Starane™ Advanced and dicamba as well as to other translocated herbicides including glyphosate and metsulfuron.

In a 2004 US study investigating the effects of simulated drift of 7 “hormone” herbicides on 6 to 8 leaf cotton (AlKhatib et al. 2004), researchers at Kansas State university found that susceptibility to visual symptoms decreased in the order: 2,4-D, picloram, dicamba, fluroxypyr, triclopyr and clopyralid. All rates of 2,4-D and the highest rate of picloram tested caused greater than 60% floral abortion. When fibre yield was evaluated, only 2,4-D and the highest rates of picloram caused significant reductions. However, local (Upper Namoi Valley) studies have shown that fluroxypyr and triclopyr can cause significant cotton injury when directly applied to cotton plants. Metsulfuron has been shown to cause serious injury to 9-11 node cotton at rates as low as 2 g/ha of 600 g/kg product while it is estimated that glyphosate applied at rates in the range 23-45 gac/ha can cause significant effects on growth of young non-Roundup® Ready cotton.

Herbicides containing fluroxypyr, triclopyr and picloram should not be applied directly upwind of cotton or under still or near-calm conditions where the possibility of temperature inversion or unpredictable off-target airflow could transport un-deposited particles onto a nearby susceptible crop. The possibility of drift can be mitigated by the use of coarse droplet producing nozzles but minimising boom height can have a more profound effect by ensuring that the greater proportion of emitted output from the sprayer reaches the target before droplets evaporate to a driftable size. Avoidance of application under hot (leading to high evaporation and convectional air movement) and low humidity conditions should be avoided.

Other summer crops exhibiting particular sensitivities to auxin like herbicides include soybeans (particularly to dicamba), sunflowers (2,4-D) and mungbeans.

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