Johnsongrass interference in corn and control with sulfonylurea herbicides

Field studies were established at Jackson and Grand Junction, Tennessee in 1988 to investigate the effects of johnsongrass [Sorghum halepense (L.) Pers.] on corn and to determine the optimum time and method of removal of johnsongrass interference in corn. Experiments were established in areas naturally infested with rhizome johnsongrass. Experimental design was a randomized complete block. Treatments were replicated four times. DPX-V9360 (35 g/ha) and CCA-136872 (30 g/ha) applied postemergence at four weekly intervals beginning at three weeks after planting (WAP) were compared to six dates of mechanical removal at weekly intervals beginning at three WAP. A weedfree check and weedy check were also included. At Jackson, johnsongrass control for all chemical treatments was lower than that for the weedfree check with the exception of DPX-V9360 applied at five or six WAP. Corn injury was not significant for any treatments except for DPX-V9360 and CGA-136872 applied at six WAP. Yield reductions were observed in the treatments hand hoed at eight WAP, DPX-V9360 applied at three WAP, CGA-136872 applied at three or six WAP, and the weedy check. At Grand Junction, DPX-V9360 applied at three, four, or five WAP provided good to excellent johnsongrass control and did not differ from the weedfree check. All other chemical treatments provided less johnsongrass control than the weedfree check. Regrowth of johnsongrass was greater with CGA-136872 than with DPX-V9360. Corn stunting was observed in treatments hand hoed at seven or eight WAP, for treatments with CGA-136872 applied at five or six WAP, for treatments with DPX-V9360 applied at six WAP, and in the weedy check. Stunting was due to the combined effects of chemical injury and johnsongrass interference, but late-season stunting was due predominantly to johnsongrass interference. Corn yields were lower than the weedfree check in all treatments with the exception of those hand hoed at three or four WAP, and DPX-V9360 treatments applied at three, four or five WAP. The optimum time for mechanical removal of johnsongrass was at three to four WAP, while the optimum time for chemical removal of johnsongrass was at four to five WAP. Field studies were established at Jackson and Grand Junction, Tennessee in 1988 to determine the effects of various johnsongrass densities on corn yield. The experiments were established in areas naturally infested with rhizome johnsongrass. Johnsongrass was thinned to the desired populations. The experimental design was a randomized complete block. Treatments were replicated four times. Johnsongrass densities of one plant per 1.5 m of row did not reduce yields, while johnsongrass densities of two or more plants per 1.5 m of row yielded lower than the weedfree check. Field studies were conducted in 1988 at Jackson and Milan, Tennessee to determine the effects of DPX-V9360 and CGA-136872 on corn growth and yield when applied to weedfree corn at various growth stages under differing environmental conditions. The experimental design was a randomized complete block with a split-split plot factorial arrangement of treatments. Main plots were planting dates; sub-plots were herbicide application dates; and sub-sub-plots were herbicides. Treatments were replicated three times. Corn was planted at four weekly intervals. For each planting date, each of the two herbicides was applied at five weekly intervals beginning at two weeks and continuing through six weeks after planting. Corn planted at planting dates two and three was injured more severely than corn planted at planting dates one and four. CGA-136872 injured corn more than did DPX-V9360 across all planting dates and herbicide application dates. Corn injury levels were greatest when the herbicides were applied to two week-old corn. Corn injury levels decreased for herbicide applications made to successively older corn. Barren plant percentage was related to corn injury. Barren plant percentage increased as the level of prior corn injury increased. At Jackson corn yields increased for corn planted through the third planting date. Yields were reduced for corn planted at the final planting date. Corn yields were reduced when herbicides were applied to two week-old corn. Corn yields for all planting dates were reduced when herbicides were applied on May 26. This application was made during an extended period of cloudy weather, and may have reduced the corn\u27s ability to metabolize the herbicides. Yields were not different at Milan