Field sandbur (Cenchrus spinifex) control and bermudagrass (Cynodon dactylon) response to herbicide and nitrogen fertilizer treatments

Field experiments were conducted in 2009 at Chickasha (CHK-1) and Mulhall (MHL-1), OK, and in 2010 at Chickasha (CHK-2) and Hennessey (HEN-1, HEN-2, and HEN-3), OK, to measure the effects of herbicide and N fertilizer treatment combinations on field sandbur control and bermudagrass response. Densities of field sandbur ranged from 0 (weed-free locations) to 23 plants m -2. Field sandbur control 6 WAT in 2009 was not evaluated due to drought conditions at MHL-1. At HEN-2 in 2010, no difference among herbicide treatments occurred when evaluating field sandbur control 6 WAT, with control ranging from 92 to 96%. At HEN-3 in 2010, an interaction of herbicide and N fertilizer main effects occurred at the field sandbur control 6 WAT evaluation. Pendimethalin applied alone controlled field sandbur 57% at 0 kg N ha-1. As the N fertilizer rate increased, field sandbur control increased to 90%. Nicosulfuron plus metsulfuron-methyl applied alone controlled field sandbur from 80% at 0 kg N ha-1 to 93% at 340 kg N ha-1. All other herbicide treatments exhibited 83 to 100% field sandbur control regardless of N fertilizer. Field sandbur control 9 WAT in 2009 at MHL-1 and 2010 at HEN-2 and HEN-3 were similar. Pendimethalin applied alone controlled field sandbur 80% in 2009 and 96% in 2010. Nicosulfuron plus metsulfuron-methyl treatments controlled field sandbur 88 to 90% in 2009, and 97 to 99% in 2010. Imazapic plus glyphosate controlled field sandbur 87% in 2009, and 100% in 2010. No difference in field sandbur control occurred as the N fertilizer rate increased in 2009 and 2010. In 2009, field sandbur control 9 WAT ranged from 87 to 90%, and in 2010, control ranged from 96 to 99%. Bermudagrass injury 3 WAT in 2009 and 2010 were similar. In both 2009 and 2010, pendimethalin applied alone had minimal bermudagrass injury. Nicosulfuron plus metsulfuron-methyl treatments exhibited 9 to 18% bermudagrass injury 3 WAT where imazapic plus glyphosate exhibited 32 to 50% bermudagrass injury 3WAT. There was no difference in bermudagrass injury 3 WAT as the N rate increased, with injury ranging from 12 to 19%. At CHK-1 in 2009, pendimethalin alone did not cause any yield reduction, nicosulfuron plus metsulfuron-methyl treatments reduced yield 14 to 17% and imazapic plus glyphosate reduced yield 33%, when compared to the untreated. When evaluating the N fertilizer main effect, bermudagrass yield increased among the herbicide treatments as the N fertilizer rate increased. At CHK-2 in 2010, all herbicide treatments reduced bermudagrass yield except pendimethalin applied alone. Nicosulfuron plus metsulfuron-methyl treatments reduced yield 30 to 38%, whereas imazapic plus glyphosate reduced bermudagrass yields 55%, compared to the untreated. When evaluating the N fertilizer main effect, bermudagrass yield increased among the herbicide treatments as the N fertilizer rate increased. At HEN-1, there was no difference in bermudagrass yield due to wet conditions prior to harvest; however, when evaluating the N fertilizer main effect, bermudagrass yield increased among the herbicide treatments as the N fertilizer rate increased. Pendimethalin followed by (fb) nicosulfuron plus metsulfuron-methyl and nicosulfuron plus metsulfuron-methyl provided excellent field sandbur control with minimal bermudagrass injury and yield reductions. The addition of N fertilizer increased bermudagrass yield and results may suggest that N fertilizer may increase field sandbur control.