Tolerance of forage bermudagrass [Cynodon dactylon (L.)Pers] hybrids to imazapic plus 2,4-D

Abstract

Research was conducted to evaluate the tolerance of 'Coastal', 'Jiggs', and 'Tifton 85' bermudagrasses [Cynodon dactylon (L.) Pers.] to Oasis® which is a l:2 premix of imazapic plus 2,4-D containing 0.91 and 1.82 kg ai L⁻¹, respectively. The effects of herbicide rates, application timings, and the influence of water or urea ammonium nitrate (UAN; 32% nitrogen) as herbicide carriers were examined. The herbicide rates were 0, 0.11, 0.21, 0.32, and 0.42 kg ai ha⁻¹. The herbicide was applied in either water or UAN to bermudagrass stubble (5 to 10 cm tall), and regrowth (15 to 20 cm tall). Weed free experiments were conducted with each well-established variety at different locations. Visual crop injury, yield, and quality [crude protein (CP) and acid-detergent fiber (ADF)] of all varieties were measured throughout the growing seasons of 2001 and 2002 after herbicide applications. All evaluations were combined for years, since year was assigned as a random variable in the data analysis procedure. At the first harvest date, all rates of imazapic plus 2,4-D caused yield reductions regardless of application timing, carrier, or variety. By the second harvest, all three varieties had fully recovered from the initial injury and no yield reductions were observed. In the Coastal and Jiggs varieties, the initial herbicide injury in the first harvest, regardless of application timing or carrier, caused significant seasonal yield reduction. In the Tifton 85 variety, seasonal yield reductions were observed when 0.315 and 0.42 kg/ha of imazapic plus 2,4-D were applied at the stubble timing, or when 0.21, 0.315, and 0.42 kg/ha of imazapic plus 2,4-D were applied at the regrowth timing. Application timing did not have an effect on seasonal yields with any variety. In some instances, water carrier treatments yielded significantly more forage than UAN carrier treatments. However, these differences were less than 12%, and are likely due to the difference in fertilizer formulation that accompanied the herbicide carrier. In the first harvest, no application parameter examined resulted in detrimental effects on bermudagrass CP or ADF content, and some instances of improvement were observed. However, by the second harvest, CP and ADF content of all varieties were unaffected.