Bill Williams and Steven T. Kelly
Perennial weeds such as alligatorweed and redvine are becoming more difficult to control as producers adopt conservation tillage practices. These weeds begin spring growth before crops are planted and are difficult to manage even with multiple herbicide applications. Fall applications of the herbicide glyphosate for johnsongrass and dicamba (Banvel, Clarity) for redvine have long been recommended for managing these weeds. A standard recommendation for alligatorweed has not been established.
Many producers complain of inconsistent results from fall herbicide applications and the high cost of dicamba for redvine control. Most fall applications have been made in late October to early November. One of the main reasons for this is because those are the earliest dates possible following many cotton crops. Another important reason is that it is commonly believed that the best time for fall applications is within a couple of weeks of the first frost.
In recent years the best johnsongrass control following Fall corn harvest has been observed in September. The increased use of a cotton-corn rotation provides an opportunity for producers to make earlier fall applications. Glyphosate prices have dropped dramatically in recent years, which may make high rates of glyphosate more economical. The objective of this research was to evaluate fall herbicide programs for managing alligatorweed and redvine. Research Approach
Studies were established in the fall of 2003 near Monroe, La., to evaluate fall applications of herbicides for alligatorweed control. The field was planted to rice in 2002 and 2004 and fallowed in 2003 and 2005. During the fallow years the field was disked 2 to 3 times and leveled. The field was not tilled during the cropping years. In the first study, several phenoxy herbicides (Tordon, Banvel, 2,4-D and Grandstand) and glyphosate were applied on Sept. 15, 2003. Alligatorweed control was evaluated monthly from March to October in 2004 and in April and September of 2005. A second study evaluated the effect of application timing on alligatorweed control with glyphosate, glyphosate plus 2,4-D and glyphosate plus triclopyr (Grandstand).
In the fall of 2002, a study was established near Crowville, La., to evaluate fall applications of herbicides for redvine control. The field was planted to cotton in 2001 and 2003 and planted to corn in 2002 and 2004. The field was prepared for planting in the fall by clipping stalks and re-hipping (or re-shaping the planting beds) within two weeks of harvest. Cotton was harvested early in 2003. As a result, the field was prepared for planting by early September, presenting an opportunity to investigate sequential fall programs. The plots from 2002 (12, 40-inch rows) were reduced to 4-row plots and treated with dicamba (Banvel) or glyphosate in late September, 2003, or not treated at all. A second study was established in the fall of 2004 to evaluate the effect of glyphosate application timing and rate on redvine control. Two replications were near Crowville, La., and two replications were near Saint Joseph, La. Results
Picloram (Tordon) at 0.5 pound per acre and glyphosate a 1.0 pound per acre resulted in the best alligatorweed control. Dicamba, 2,4-D, triclopyr and picloram at 0.25 pound per acre resulted in 70 percent or lower alligatorweed control. Synergistic responses were not identified when glyphosate was tank mixed with dicamba, 2,4-D, triclopyr or picloram. Glyphosate applied alone resulted in excellent alligatorweed control for as much as two years after application (Figure 1). Control was best from mid-September to early-October and was considerably lower with mid-October applications. Tank mixing glyphosate with either 2,4-D or triclopyr did not improve alligatorweed control. In many cases, especially with a mid-October application, alligatorweed control was reduced when glyphosate was mixed with 2,4-D or triclopyr compared to glyphosate alone.
After six months, redvine control was similar for glyphosate at 2 and 4 pounds per acre and for dicamba at 2 pounds per acre (Figure 2). Triclopyr resulted in the lowest redvine control. One year after application, glyphosate at 4 pounds per acre controlled redvine 95 percent, dicamba at 2 pounds per acre controlled redvine 85 percent and glyphosate at 2 pounds per acre plus dicamba at 1 pound per acre controlled redvine 80 percent. The remaining treatments resulted in 50 percent or lower redvine control.
Glyphosate at 2 pounds per acre applied in late September of 2002 and 2003 resulted in redvine control in 2004 equal to or better than single or multiple applications of 2 pounds per acre dicamba or 4 pounds per acre glyphosate.
Glyphosate applied at 2, 3 and 4 pounds per acre on Sept. 15, 2004, resulted in similar levels of redvine control in observations made in April 2005. The best control from 2 pounds of glyphosate per acre was observed from mid-September to mid-October. Timing had little effect on redvine control with glyphosate at 4 pounds per acre. By September, treatments began to differentiate, demonstrating the need for at least 3 pounds of glyphosate per acre. These results show that glyphosate at higher rates controls redvine as well as dicamba. Glyphosate at 4 pounds per acre resulted in the most-consistent redvine control from single applications. However, 2 pounds of glyphosate per acre applied two years in a row appears to be the best approach.
Overall, fall applications of glyphosate can help manage alligatorweed and redvine. This research indicates that glyphosate applications at 1 pound per acre for alligatorweed or 2 to 3 pounds per acre for redvine should be made between Sept. 15 and Oct. 15 when conditions are favorable for rapid weed growth.
(This article was published in the summer 2006 issue of Louisiana Agriculture.)