Influence of Growth Stage and Soil Conditions on Bermudagrass Susceptibility to Glyphosate1

Abstract Bermudagrass (Cynodon dactylon L.) is a perennial grass weed species in many crop and industrial situations. Glyphosate (N‐(phosphonomethyl) glycine) has provided some control of this weed, but results have been variable. Field and laboratory studies were conducted to evaluate the conditions under which bermudagrass is most susceptible to glyphosate. Fall, spring, or summer glyphosate applications were made for bermudagrass control in 1974, 1975, and 1976. Bermudagrass at five growth stages was also treated with 1.7, 3.4, and 5.0 kg/ha of glyphosate. In the laboratory the influence of temperature, soil moisture, and N stresses before or after glyphosate treatments were evaluated for their influence on the degree of control achieved. The soil type was a Teller sandy loam soil (Udic arguistoll; fine loamy, mixed thermic). Glyphosate applications from July to early October, at a rate of 3.4 kg/ha or more, were the most effective for controlling bermudagrass. Spring glyphosate applications were ineffective even at 5.0 kg/ha. Substantial foliage (> 17 cm tall) had to be present to obtain the maximum control with August glyphosate applications. A 5‐day 20 C temperature pretreatment or cool temperatures during treatment did not decrease bermudagrass control with glyphosate. Dry soil conditions or wilted plants at treatment significantly reduced the effectiveness of glyphosate. Effective bermudagrass control with glyphosate appears to require actively growing plants, with the major portion of the photosynthates moving into the stems and roots. Plant stress prior to treatment reduces the control obtained.