Monitoring fall CO2 efflux of dallisgrass and hybrid bermudagrass: Implications for nonselective weed control

Chemical control options for dallisgrass (Paspalum dilatatum Poir.) are limited, inconsistent, and damaging to desirable species. Dallisgrass and hybrid bermudagrass [Cynodon dactylon (L.) Pers. × Cynodon transvaalensis Burtt Davy] may enter dormancy at different times, allowing for safe, efficacious use of nonselective herbicides for dallisgrass control. Field experiments evaluated dallisgrass and hybrid bermudagrass dormancy by measuring plant respiration in fall and subjecting the carbon efflux data to a nonlinear exponential decay regression model fitted against cooling degree-days (CDDs). Total plant respiration differed between the species at trial initiation. Dallisgrass exhibited higher net respiration, indicating that each species proceeded into dormancy at different rates. A postemergence dallisgrass control trial was initiated at 5 to 125 CDD_22C to evaluate potential rates of quinclorac (1.5, 2.3, and 3.0 kg acid equivalent ha^–1), monosodium methanearsonate (6.1, 9.1, and 12.1 kg a.i. ha^–1), glyphosate (5, 7.6, and 10.1 kg a.i. ha^–1), and thiencarbazone + foramsulfuron + halosulfuron (0.14, 0.18, and 0.22 .kg a.i. ha^–1). Dallisgrass control 36 wk after the initial treatment was greatest with glyphosate (≥97%). Minimal bermudagrass survival was observed next spring in response to fall glyphosate. Total plant respiration per sample area differed between species at trial initiation. However, dallisgrass exhibited greater net respiration, indicating differences in behavior concerning the initiation and procession of dormancy between dallisgrass and bermudagrass. Microclimates such as shaded or low-lying areas may yield different results. Further research should determine the significance of this phenomenon to use nonselective herbicides to control dallisgrass in bermudagrass.