Abstract

In a mixed weed population, an optimum herbicide strategy maximizes turf canopy coverage and health.  The objective was to study the effect of postemergence herbicides alone and in mixtures on populations of green kyllinga (Kyllinga brevifolia Rottb.), tropical signalgrass (Urochloa subquadripara (Trin.) R.D. Webster), and 'Tifgreen' bermudagrass turf (Cynodon transvaalensis Burtt-Davy x C. dactylon (L.) Pers.)  Initial coverage was green kyllinga, 43%; tropical signalgrass, 29%; and bermudagrass, 28%.  There were 12 herbicide treatments, including a water control, in five replications. Treatments were bentazon, 1.12 kg ha-1; halosulfuron, 0.07 kg ha-1; imazaquin, 0.56 kg ha-1; and MSMA, 2.52 kg ha-1 applied alone and in all binary combinations, plus MSMA + metribuzin at 0.28 kg ha-1.  Halosulfuron and imazaquin rates were reduced in half in mixtures.  There were two sets of applications separated by 11 weeks, each with two applications split by 10-11 days.  Treatments with halosulfuron or imazaquin were applied only once per set of applications.  Only treatments with halosulfuron, imazaquin, or MSMA + metribuzin injured (P < 0.05) green kyllinga.  Only treatments with MSMA injured tropical signalgrass.  Only MSMA + metribuzin and MSMA + halosulfuron injured green kyllinga and tropical signalgrass, but did not maximize bermudagrass coverage.  The optimum treatments, bentazon + MSMA and MSMA alone, increased bermudagrass coverage to 97% and 94%, respectively, compared with 43% for the water control.  Although imazaquin did not injure bermudagrass, it reduced bermudagrass coverage to 13%.  In removing kyllinga, imazaquin had opened a niche that was colonized by tropical signalgrass at the expense of bermudagrass.  The optimum herbicide strategy must consider not only injury to weeds but competition among weeds.