Preharvest applications of glyphosate can be useful in controlling perennial weeds. Experiments were conducted from 1996 to 1999 to determine whether preharvest glyphosate applications are affected by differences in the amount of soybean canopy present at the time of application by measuring spray deposition and subsequently horsenettle or Canada thistle control. Soybean leaf interference levels were achieved by use of three soybean cultivars with different maturity groups to achieve no leaf interference, moderate leaf interference, and maximum leaf interference, and soybean was planted in three row spacings ranging from 19 to 76 cm. As soybean leaf interference increased, spray coverage of spray deposition cards decreased. There was a similar trend for relative spray volume, determined by intensity of the color change with water-sensitive cards. Row spacing did not influence spray coverage or relative spray volume. Percent change in horsenettle or Canada thistle stems from fall to spring counts was inconsistent. Differences detected in spray coverage did not influence weed control or weed stem density the following spring.
Nomenclature: Glyphosate; Canada thistle, Cirsium arvense (L.) Scop. #3 CIRAR; horsenettle, Solanum carolinense L. # SOLCA; soybean, Glycine max (L.) Merr.
Additional index words: CIRAR, Cirsium arvense, cultural practices, integrated weed management, perennial weed control, Solanum carolinense, SOLCA, spray deposition.
