Interactions between nitrogen and velvetleaf (Abutilon theophrasti) densities on glufosinate and glyphosate efficacy
Soil nitrogen (N) levels and weed populations are known to vary within agricultural fields. This variability in soil N may contribute to weed species escaping control with herbicides such as glufosinate and glyphosate. Weeds occur in variable patches across agricultural landscapes and despite continued control with herbicides, weeds continue to persist in agricultural fields. In order to explore how herbicide efficacy can be influenced by N rate and weed seedling density, growth room and field studies were conducted. In growth room experiments glufosinate efficacy was affected by velvetleaf density while glyphosate efficacy was affected by both velvetleaf density and N rate. Also, under growth room conditions, addition of ammonium sulfate (AMS) and to a lesser degree urea ammonia nitrate (UAN), were found to increase glyphosate efficacy at 225 g ae ha-1 on velvetleaf plants grown under high N but especially low N. However, when glyphosate was applied at 900 g ae ha-1 there was no improvement in velvetleaf control with the addition of UAN or AMS. Under field conditions, glufosinate efficacy was affected by velvetleaf density. For example, under low N, application of 50 g ai ha-1 of glufosinate, increased velvetleaf biomass to 137% while biomass was reduced to 77% at low density when compared to untreated. Alternatively, N rate affected glyphosate efficacy such that biomass of velvetleaf seedlings grown at high density and high N was 45% of the untreated control compared to 75% for plants grown at the same density under low N, following treatment with 900 g ae ha -1 of glyphosate. In addition, velvetleaf seedlings treated with either glufosinate or glyphosate continued to grow and produce flowers and seeds. This research provided the first evidence from the field of reduced glyphosate activity caused by velvetleaf plants grown under low soil N and high density. Knowledge of soil N by seedling density interaction is important in terms of understanding how weed species can escape herbicide treatment while not having the genetic mutations associated with herbicide resistance. As well, the ability of velvetleaf to survive and set seed following treatment with either herbicide, helps to ensure persistence of the population for years to come.