Effect of Growth Stage and Water Stress on Barnyardgrass (Echinochloa crus-galli) Control and on Glyphosate Absorption and Translocation

Under greenhouse conditions, postemergence herbicide effectiveness was greater when barnyardgrass [ Echinochloa crus-galli (L.) Beauv.] plants were 5-or 10-cm tall as compared to 15-or 20-cm tall. Absorption of 14 C-glyphosate [ N -(phosphonomethyl)glycine] decreased from 58% of applied to 47% as plant height increased from 5 to 15 cm, and an additional reduction (to 32% of applied at 5 cm) occurred when terbutryn [2-(tert-butylamino)-4-(ethylamino)-6-(methylthio)-s-triazine] was added to glyphosate. Translocation of 14 C-glyphosate was reduced in older plants or when terbutryn was added to glyphosate. Water stress studies included soil moisture levels of 10, 20, 30, and 40% moisture (PW). Herbicide treatments were less effective at soil moisture levels below field capacity (30 PW). When soil moisture was 10 PW (-37 bar) absorption of 14 C-glyphosate was greatly reduced in plants treated at both 7.5-and 15-cm heights with or without terbutryn. At 10 PW only 15 to 20% of applied 14 C was absorbed whereas at 40 PW, absorption was over 45% and 62% with and without terbutryn, respectively. The addition of terbutryn to glyphosate resulted in decreased translocation for all soil moisture levels except the 20 PW treatment at the 15-cm height. Translocation of 14 C was greatly reduced for plants in soils at 10 PW for both herbicide treatments.