Use of Glyphosate and Imazapyr for Cogongrass (Imperata cylindrica) Management in Southern Pine Forests

Cogongrass (Imperata cylindrica [L.] P. Beauv. var. major [Nees] C.E. Hubb) is one of the most invasive perennial grasses worldwide and has progressively infested managed and natural habitats in the mid-South over the past 100 years. To extend past research toward the goal of eradication on forested sites, we tested the most effective herbicides (glyphosate and imazapyr) over a 3-year study period in a series of low to very high rates at two application timings, all followed by one retreatment a year later. A commonly used combination of glyphosate and imazapyr was also tested at three spray volumes. Factorial studies were established at two sites in the Coastal Plain of Alabama involving new or old infestations. Herbicide efficacy, measured 24 months after treatment and retreatment, increased linearly with increasing rates of glyphosate and imazapyr, but eradication was not achieved. On average, September applications provided greater control than October applications at both sites (61 versus 50%, respectively). The best levels of control 2 years after retreatment occurred with repeated September applications using 2.2 kg acid equivalent/ha imazapyr, providing 88 and 90% control in the new and old infestations, respectively. Control with the glyphosate and imazapyr combination did not differ with spray volumes, but the combination gave greater control than similar rates of the single herbicides on the new infestation.