The effect of air moisture content (relative humidity (RH)/vapour pressure deficit (VPD)) on the absorption and efficacy (measured as dry weight reduction) of the herbicide glufosinate ammonium on wild oat ('Avena fatua') was investigated. Wild oat is susceptible to glufosinate ammonium, but dry conditions (RH < 45%, VPD > 1.2 kPa) at the time of spraying can significantly reduce its efficacy for weed control. Results from preliminary dose-response experiments indicated that humidity/VPD had the greatest effect on efficacy within several hours of spraying. In subsequent experiments on wild oat plants grown continuously at low humidity, it was found that less than 1 hour of exposure to high humidity (RH > 95%, VPD < 0.1 kPa) before and after spraying resulted in efficacy similar to that observed in plants kept continuously at high humidity before and after treatment. Uptake experiments using 14C glufosinate ammonium applied as a spray revealed that plants grown continuously at low humidity except for short exposure to high humidity before and after spraying, had significantly greater uptake compared to plants kept continuously at low humidity. However, this effect was not observed when 14C glufosinate ammonium was applied as 1 [mu]l droplets. In experiments designed to examine the effect of droplet drying on glufosinate ammonium efficacy it was found that at low humidity, addition of short chain non-ionic humectants (i.e. glycerol) to the spray solution slowed droplet drying thereby significantly increasing efficacy. Methods were developed to isolate sections of wild oat cuticle/epidermis to examine the effect of humidity on cuticular absorption of 14C glufosinate ammonium immediately after spraying. At low humidity, cuticular uptake of 14C glufosinate ammonium ceased within 1 hour of application, while at high humidity cuticular uptake continued for several hours. Cuticular absorption of 14C glufosinate ammonium at low humidity, from solutions containing glycerol, was approximately two times greater than that from solutions with no humectant. In conclusion, uptake of glufosinate ammonium formulated as Ignite was dependent on the rate of droplet drying, which in turn, was dictated by the amount of moisture in the air. The results of lab and growth chamber studies suggested that field efficacy of Ignite at low humidity can be improved by adding the humectant glycerol to slow droplet drying.