JOHNSONGRASS

Abstracts:

• Johnsongrass (Sorghum halepense (L.) Pers.) Control in Corn. J.E. Rowehl and W.S. Curran, 1993. Proc. NEWSS 47:34.

Johnsongrass (Sorghum halepense (L.) Pers.) Control in Corn. J.E. Rowehl and W.S. Curran, 1993. Proc. NEWSS 47:34.

Johnsongrass (Sorghum halepense) is considered one of the major weed control challenges in corn in southern Pennsylvania. Nicosulfuron (Accent) and primisulfuron (Beacon) are two recently introduced post-emergence herbicide products available to control this weed. To learn more how these products perform under various conditions, two different field trials were conducted in 1991 and 1992 in York County, PA. These studies were intended to indicate what a farmer might expect to happen if a) the products are applied at different weed growth stages , b) split applications were used, c) lower than standard rates were used, or d) a combination of the products were used.

Treatments in 1991 compared the effects of using half versus the full label rate of product at both the recommended stage of weed growth (8-18") and at a later application (18-30") time, as well as the full rate split between both times. Crop oil concentrate was included in each treatment. The trial was done in a cooperating farmer's field that was chisel plowed and disked. In 1992 the same field was used however the planting was done with no tillage. Application of the full rate of each product was compared to two thirds the rate as well as a combined half rate of each product, all applied at the recommended weed growth stage (6-15"). Crop oil concentrate and UAN solution were included in each treatment.

Growing conditions in 1991 were poor. There was negligible rain after May 10 with above normal temperatures. Plants were under stress before, during, and after application. Results show that the performance of nicosulfuron and primisulfuron did not differ significantly when averaged over rates and timings, although ratings of nicosulfuron were generally higher, particularly later in the season. Application rate was more important with nicosulfuron than primisulfuron. Applications at the recommended growth stage gave better control than later applications at both rates. The split application of nicosulfuron gave the best control although the full rate at the first application was nearly as good.

In 1992 rainfall was below average but adequate. Temperature was below normal. Growth of the Johnsongrass from rhizomes was well ahead of the corn. Early season (28 days post application) control ratings showed no differences in control between all treatments. Later season ratings (54 days post application) showed the percent control provided by nicosulfuron was maintained and was significantly better than primisulfuron at all rates. Reduced rates provided comparable control to the full rates. Combining half rates of both products provided the greatest control but not significantly higher than nicosulfuron alone. Overall, control was better in 1992 than 1991.

Based on the results of these studies, application rates should not be reduced when growing conditions are hot and dry, particularly when applied beyond optimum growth stage. Reducing rates to two thirds may provide control comparable to the full rate when applied at the optimum time and under good growing conditions. Split applications provide slightly better control but probably not enough to justify a second trip across the field.

eason control in one of two years, but did not impact late season control of hemp dogbane. Spring chisel plowing increased hemp dogbane biomass in the absence of an effective herbicide in one of two years. Spring chisel plowing also reduced glyphosate performance in one year and primisulfuron plus dicamba performance in another. Spring chisel plowing plus primisulfuron and dicamba was the least effective treatment in one of two years, providing only 50% control. Glyphosate was more consistent than the early summer application across years and appeared to provide better control of perennial vegetative structures. The combination of fall glyphosate followed by primisulfuron plus dicamba in corn always provided greater than 90% control regardless of tillage system.