Field experiments were established in 1998 and 1999 to evaluate the effect of giant smutgrass competition and hexazinone application on bahiagrass forage yield. The experimental design was a split-plot, with low (< 20% groundcover), medium (20 to 70% groundcover), and high (> 70% groundcover) giant smutgrass density as the main plot factors and hexazinone application or no hexazinone application as the subplot factors. In 1998, without hexazinone, bahiagrass biomass accumulation was 1,164 kg ha⁻¹ mo⁻¹ under low giant smutgrass infestation but 590 and 154 kg ha⁻¹ mo⁻¹ under medium and high giant smutgrass densities, respectively. From harvests occurring 1 yr after hexazinone application, bahiagrass yield in the weed-free area was similar to that growing under low giant smutgrass density. However, as giant smutgrass density increased to moderate or high levels, bahiagrass yield was reduced relative to the weed free. Giant smutgrass biomass accumulation was also measured over time. Giant smutgrass biomass, in both years, increased dramatically in the late summer months at the medium and high densities but not at the low density. It was concluded that bahiagrass was competing with the giant smutgrass at low density and depressed late season growth but was not capable of doing so at higher infestation levels. A rapid increase in late-season giant smutgrass growth was partially explained by the fact that bahiagrass is a short day plant that begins to flower in mid- to late summer, and aboveground biomass production decreases in late summer. This shift in carbon allocation in bahiagrass would exert less competition on giant smutgrass and thus partially be responsible for the late season increase in giant smutgrass growth. Economic analysis performed on these data illustrated that a net loss of $15.20 per stocking unit (cow–calf pairs) would be realized if hexazinone were used to control low densities of giant smutgrass. However, a net gain of $29.28 and $55.75 per stocking unit was observed if hexazinone was used to control giant smutgrass that had reached medium or high levels of infestation, respectively. It was concluded from these data that giant smutgrass should not be controlled until densities reach approximately 35% infestation.

Nomenclature: Hexazinone; giant smutgrass, Sporobolus indicus (L.) R. Br. var. pyramidalis SPZIN; bahiagrass, Paspalum notatum Fluegge PASNO.