Two separate experiments were conducted in 2015 and 2016 in Citra, FL to investigate the effects of preplant application timing of 2,4-D and dicamba on sesame stand and yield. Nonlinear regression analysis was performed to determine the application timing that caused 10% stand or yield reduction (GR₁₀) compared to the nontreated control (NTC) and expressed as d before planting (DBP; longer intervals indicate more injury). Likewise, regression analysis was used to determine sesame stand that resulted in 10% yield reduction (YR₁₀) expressed as plants m^-1 row. Stand measured 3 wk after planting (WAP) revealed 2,4-D applied at 0.53 kg ae ha^-1 to be the least injurious treatment to sesame stand (GR₁₀ = 6.4 DBP). Conversely, dicamba at 1.12 kg ha^-1 produced a GR₁₀ of 15.7 DBP for sesame stand at 3 WAP. 2,4-D applied at 0.53 and 1.06 kg ha^-1 and dicamba applied at 0.56 kg ha^-1 had the lowest GR₁₀ for yield of 2, 3.7, and 3 DBP, respectively. Dicamba applied at 1.12 kg ha^-1 proved to be the most injurious treatment to yield, which produced a GR₁₀ value of 10.3 DBP. To simulate possible stand losses associated with dicamba or 2,4-D and the subsequent effect on yield, a separate experiment was conducted in which sesame was thinned to various plant densities and yield was recorded to determine the relationship between plant stand and seed yield. The regression analysis of these data was then compared to that of the experiment treated with 2,4-D and dicamba to separate any physiological effects of the herbicides that would lead to yield reduction from yield effects due to stand loss only. Rate constants were compared and no statistical differences were detected between herbicide and non-herbicide treatments, suggesting that yield reductions that occur from preplant applications of 2,4-D and dicamba were purely due to stand reductions.

Nomenclature: 2,4-D; dicamba; sesame, Sesamum indicum L.