Napiergrass has potential as a cellulosic biofuel crop because of its rapid growth habit in the southern United States. However, it is also listed as a potential invasive species by the Florida Exotic Pest Plant Council. For field renovation, information about napiergrass control in response to tillage and herbicides is required. Field studies were initiated to evaluate control of napiergrass established in fields for over 3 yr at Plains, GA, and Tifton, GA. For tillage and POST herbicides, imazapyr plus glyphosate consistently controlled napiergrass relative to diclosulam plus glyphosate, sulfentrazone plus glyphosate, or tillage in terms of visual injury, stem height and dry biomass reduction. One application of imazapyr plus glyphosate controlled napiergrass 74 and 94%, and reduced plant stem height to 6 and 15% of the nontreated control. When diclosulam plus glyphosate, sulfentrazone plus glyphosate, or tillage was used alone with no sequential herbicides, napiergrass control ranged from 12 to 33%; when these control tactics were followed by two sequential applications of either sethoxydim or glyphosate, napiergrass control varied from 45 to 99%. Reductions in plant heights were reflective of injury 47 d after final herbicide applications (May/June). Napiergrass yield in dry biomass production was reduced by imazapyr plus glyphosate ≥ 86% relative to the nontreated control (NTC). Diclosulam plus glyphosate, sulfentrazone plus glyphosate, or tillage alone was not effective in reducing napiergrass dry biomass yields ranging from 1 to 47% compared with the NTC; when these treatments were followed by sequential applications of sethoxydim or glyphosate, napiergrass dry biomass was reduced 46 to 91% compared with the NTC. Tillage plus two applications of sethoxydim or glyphosate exhibited control potential because they provided levels of napiergrass control similar to imazapyr-based treatments. Tillage plus multiple applications of sethoxydim or glyphosate offers flexibility to crop rotations as compared with the residual herbicide imazapyr, which has many crop rotation restrictions because of carryover concerns.

Nomenclature: Diclosulam; glyphosate; imazapyr; sethoxydim; sulfentrazone; napiergrass, Pennisetum purpureum Schumach.

Management Implications: Napiergrass is renowned as having the greatest biomass productivity among herbaceous plants, with estimates of 45 Mg ha⁻¹ yr⁻¹ for potential biofuel production. Although productive as a biofuel crop, elimination of this species from established populations for crop rotation, or controlling escapes that could be invasive, is a concern. Napiergrass can displace native species and alter community structure affecting ecological functions in many ecosystems. Herbicides or tillage or both are potential elimination methods of napiergrass populations from fields, roadsides, and natural areas. Our research indicated that napiergrass control could be achieved by combinations of multiple applications of the sethoxydim, glyphosate, or imazapyr, in combination with moldboard tillage of culms. To achieve complete control in Georgia, three herbicide applications were required, imazapyr was effective alone, tillage was only effective when used in combination with sethoxydim and glyphosate. These data indicate that napiergrass can be controlled. Because of the invasive nature of napiergrass, multiple control tactics will be required for eradication. In southern climates, such as Florida, where moderate winter temperatures are conducive to napiergrass survival, more-intensive management practices with herbicide and tillage may be required for eradication.