Summer fallow and in-crop weed management programs in sugarcane (Saccharum spp. hybrids): control of perennial weeds and purple nutsedge (Cyperus rotundus L.) interference

In fallowed sugarcane fields, perennial weeds and sugarcane regrowth must be controlled to maximize productivity of sugarcane in the first production year. Isopropylamine salt of glyphosate applied in April at 1.68 kg ai/ha to 15 cm sugarcane provided 85% control 28 d after treatment (DAT). Control of sugarcane 56 DAT with isopropylamine and potassium salt formulations of glyphosate averaged 83% for 1.12 kg/ha. Bermudagrass control with glyphosate at 1.12 kg/ha was 86% 40 DAT and increased to 98% with a sequential application at 1.12 kg/ha. In comparing various combinations of tillage and glyphosate in summer fallow programs, bermudagrass ground cover was 37% in November for tillage alone compared to no more than 7% ground cover for the tillage/glyphosate and no-tillage/glyphosate programs. Perennial weed control was greater when glyphosate replaced a tillage operation. Purple nutsedge tubers were planted in 26.5 L pots with a surface area of 0.093m2 at densities of 0, 1, 2, 4, 8, and 16 tubers/pot along with a single node cutting of 'LCP 85-384' sugarcane to evaluate interference. At 64 days after planting, both purple nutsedge shoot and root (including tubers) dry weight increased as initial tuber density increased. Based on sugarcane shoot dry weight, critical weed density was four nutsedge tubers/pot. Using root dry weight, critical weed density was one nutsedge tuber/pot. The sugarcane variety 'L 97-128' was more competitive with purple nutsedge than LCP 85-384, 'Ho 95-988', and 'HoCP 96-540'. In a shade response study, 30% shade reduced nutsedge shoot dry weight 75% compared with full sunlight. Field studies were conducted to evaluate purple and yellow nutsedge control in sugarcane with herbicides applied postemergence (POST) in September around 5 weeks after planting. Halosulfuron at 53.0 or 70.6 g/ha averaged 80% 4 weeks after treatment (WAT) and 77% 6 WAT. Control with the trifloxysulfuron at 15.7 g ai/ha was no more than 71% 6 WAT. Injury to sugarcane was not observed 6 WAT for either herbicide. In April of the following year, nutsedge control with the halosulfuron treatments averaged 74% compared with 44% for the trifloxysulfuron treatments, but sugarcane shoot population did not differ.