Cogongrass is a noxious perennial grass that has invaded many countries in the tropical and subtropical regions of the world. Its management has been a significant challenge because of large rhizome and bud reserves in the soil. The emergence pattern of this weed under field conditions has received little attention. Field trials were conducted in 2002 and 2003 in the humid forest zone of southeastern Nigeria to model shoot emergence. The experiment had four treatments: (1) count and tag crop-free cogongrass shoots, (2) count and suppress crop-free cogongrass shoots with paraquat, (3) count and cut crop-free cogongrass shoots, and (4) count and cut cogongrass shoots in cultivated corn. The rationale for these treatments was to determine the effect of different monitoring techniques on shoot emergence of cogongrass. The development of the model was based on hydrothermal time, which was calculated from soil moisture and soil temperature at a 2-cm depth. A Weibull function was fitted to cumulative percent shoot emergence values of Treatment 4 and hydrothermal time. The model closely fit the observed pattern of cogongrass shoot emergence (r² = 0.95, n = 36). It also predicted shoot emergence satisfactorily in six treatments (r² > 0.85, P < 0.001, n = 7 in each treatment) that simulated farmers' practices in southwestern Nigeria. This is the first model developed for cogongrass shoot emergence based on hydrothermal time under field observations. The model should facilitate further analyses of cogongrass emergence patterns and the timing of its management.

Nomenclature: Paraquat; cogongrass, Imperata cylindrica (L.) Beauv. IMPCY; corn (maize), Zea mays L.