Statistical tests for hormesis and effective dosages in herbicide dose response

The log-logistic model is a popular candidate for modeling dose response in agronomy. One of its limitations is monotonicity: the response is either c ontinuously increasing or decreasing with changes in dosage. In herbicide d ose response investigations, authors have previously noted an increase in b iomass or grow:th for subinhibitory dosages. This hermetic effect, if ignor ed, can lead to substantial bias in estimates of effective dosages and can distort inferences about the differential effects of experimental treatment s. We present an extension of a model capable of modeling hermetic effects that allows inference about effective dosages. Furthermore, the model can b e used to compare effective dosages among different treatments if hermetic effects are present for some and absent for others and provides appropriate estimates of rates of change. In a designed experiment where velvetleaf (A butilon theophrasti Medikus) and barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] were treated with glyphosate [isopropylamine salt of N-(phospho nomethyl)glycine] + (NH4)(2)SO4 or glufosinate [2-amino-4- (hydroxymethylph osphinyl)butanoic acid] + (NH4)(2)SO4 at various rates, hermetic effects me re found for barnyardgrass treated with glyphosate but not with glufosinate . Procedures for a valid comparison of effective dosages between the two he rbicides are presented. Software code to perform the analyses is given in a n appendix.