Weed Control in Sunflower (Helianthus annuus L.) with Soil-applied Herbicides Affected by a Prolonged and Limited rainfall

The field trials were carried out in the Bitola and Titov Veles regions during two sunflower growing seasons (2018 and 2019) to estimate a weed control in sunflower with the soil‒applied herbicides, influenced by a prolonged and limited rainfall. Polygonum aviculare L., Solanum nigrum L., Chenopodium album L., Amaranthus retroflexus L., Portulaca oleracea L., and Echinochloa crus-galli (L.) P. Beauv. were the most dominant weeds in both regions. The efficacy of PRE-em herbicides varied among the weed species, treatments, periods of efficacy estimation, regions, and years. The overall performances of the PRE-em herbicides were correlated with weather and soil conditions. The inconsistent weather patterns between the two years of the study likely influenced the weed control. All weeds in 2018 in the Bitola region were poorly controlled (<77% and <62%, 28 and 56 days after application, respectively) due to a higher amount of rainfall (57 mm) during the 10 days of the 1st decade after herbicide application. The herbicide efficacy has only produced a marginal control of weeds in 2019 in the Titov Veles region as well (<68% and <59%, 28 and 56 days after application, respectively), due to the drought conditions observed in this region in early spring and in mid-spring. A PREem application followed by a heavy rainfall resulted in a sunflower injury in the Bitola region in 2018, ranging from 9 to 28% across the PRE-em treatments seven days after emergence. The injuries by oxyfluorfen and dimethenamid-P were more serious (24 and 28%, respectively). The sunflower achene yields for each treatment in both regions generally reflected an overall weed control and crop injury.