Luke M. Etheredge Jr., James L. Griffin and Michael E. Salassi
In Louisiana, four to six harvests are made from a single planting of sugarcane. After that, the sugarcane must be replanted because of disease and weed pressure. In a typical program, the sugarcane stubble from the last harvest is destroyed in the spring or early summer, and fields are prepared for replanting in August and September. During this fallow period, producers are able to control perennial weeds, which have established over the crop cycle, with postemergence application of glyphosate and timely tillage operations. The fallow period is critical for perennial weed management because once sugarcane is planted, the row top will not be mechanically disturbed for the remainder of the multi-year crop cycle. Therefore, management of perennial weeds in fallow is essential to maximize yields for the next few years.
Historically, producers have relied heavily on frequent tillage operations during the fallow period for weed control. As sugarcane farms become larger, however, use of conventional tillage programs in fallowed fields is cost-prohibitive. With fuel and labor costs rising and the decrease in cost of glyphosate products, a no-tillage or reduced tillage system may be more cost-effective. This research addressed the potential use of reduced tillage programs in fallowed sugarcane fields with heavy perennial weed pressure. The economics of various reduced tillage, no-tillage and conventional programs were compared.
Field experiments were conducted using third stubble (fourth production year) LCP 85-384 sugarcane. Roundup UltraMax, an isopropylamine salt of glyphosate plus surfactant, was applied at four application timings and four rates in April and May. The first application was made when average sugarcane canopy height was 6 inches. Three subsequent applications were made at two-week intervals when average canopy height was 10, 16 and 18 inches.
At each timing, Roundup UltraMax was applied at 25.6, 38.4, 51 and 64 oz/A, which corresponds to 1, 1.5, 2 and 2.5 lb ai/A glyphosate. When Roundup UltraMax was applied at 38.4 oz/A or higher to 6-inch sugarcane, control 28 days after treatment (DAT) was at least 92 percent. When application was delayed until sugarcane reached 16 inches, control 28 DAT was maximized at 78 percent for 38.4 oz/A. At 18 inches, control 28 DAT was maximized at 76 percent for 51 oz/A. At 42 DAT sugarcane was controlled at least 90 percent when Roundup UltraMax was applied at all rates to 6-inch sugarcane. This level of control was not observed, however, when applied to 18-inch sugarcane.
In another study third stubble LCP 85-384 sugarcane was treated when average sugarcane canopy height was 10 inches in late April to compare isoproylamine salt formulations – Roundup UltraMax, Mirage and Honcho Plus – and potassium salt formulations – Roundup WeatherMax and Roundup Original- Max. Each glyphosate formulation was applied at the same rate of active ingredient per acre – 1, 2 and 3 pounds.
At 56 DAT when regrowth of stubble could be evaluated, control increased as rate of glyphosate increased, and differences were not observed among the formulations. Sugarcane control with glyphosate at 1 pound per acre averaged 83 percent, which would be acceptable if a follow-up application of glyphosate is made. Sugarcane control with glyphosate at 3 pounds per acre averaged 95 percent.
These studies show that low rates of glyphosate can be effective, especially if sugarcane is small and a follow-up application is planned. Of utmost importance is that an application is made in April to allow for complete control of sugarcane so stubble does not interfere with planting. Bermudagrass control with glyphosate
Single and sequential applications of glyphosate were evaluated for bermudagrass control in fallowed sugarcane fields. Roundup UltraMax was applied at 25.6, 38.4, 51 and 64 oz/A to actively growing bermudagrass in late May to mid-June when stolons (runners) were 6 to 10 inches long. Following each initial glyphosate application, sequential applications at 25.6, 38.4, 51 and 64 oz/A were made in July to bermudagrass regrowth when stolons were 2 to 6 inches long. At 40 days after the initial application (the same day the sequential application was made), bermudagrass control was at least 86 percent for all rates of Roundup UltraMax. At 18 days after the sequential application, bermudagrass control was at least 98 percent, regardless of rate. Economics of weed control
For this study three locations with silty clay loam and silt loam soils were selected. These locations had natural infestations of bermudagrass and johnsongrass that had developed over four and five years. Experiments consisted of tillage/weed control programs imposed during the fallow period (April-August). See the table for specific treatments and costs. At the end of the fallow period in August, all plots were planted with LCP 85-384.
Bermudagrass ground cover in planted sugarcane was 37 percent in November where only tillage was used during the fallow period compared to no more than 7 percent ground cover for programs that included Roundup UltraMax applications. There was no difference in bermudagrass control between the tillage plus glyphosate and no-tillage plus glyphosate fallow programs. Johnsongrass was controlled at least 83 percent for the tillage or no-tillage plus glyphosate programs, but control was 73 percent where only tillage was used. Although weed control was less for the tillage alone fallow program, sugarcane was able to compensate. Sugarcane and sugar yield for the plant cane crop was not reduced. Sugarcane yield averaged 33.3 tons per acre and sugar yield was 8,960 pounds per acre.
Because differences in sugar yield were not significant, gross return for each fallow program could not be calculated. A total cost for each fallow program, however, was calculated using tillage and herbicide costs. Tillage cost was based on the number of passes across each plot and implement used. Herbicide cost varied depending on the number of glyphosate applications and whether or not a preemergence herbicide was used. The costs of the fallow program treatments ranged from $45 per acre to $76.30 per acre. The most expensive program was where a preemergence herbicide was used along with frequent tillage. Conclusions
Even though fallow programs will always be a cost to the grower, the investment is necessary to control perennial weeds and to maximize sugarcane production over the multi-year cropping system. From the standpoint of weed control, fallow programs using glyphosate to replace a tillage operation were more effective than a conventional tillage program. Although economically the tillage-alone program was by far the least expensive, the attractiveness of this program would be directly related to fuel and labor costs.
In this research herbicide cost was based on the Roundup UltraMax formulation, and less expensive glyphosate formulations were shown to be equally effective when applied at the equivalent rate of active ingredient per acre. Additionally, the glyphosate rate effective for control of both bermudagrass and sugarcane is half that used in the fallow program study, which would further reduce cost. A fallow program in sugarcane that includes one or two timely applications of glyphosate in combination with reduced tillage or a no-tillage program can be effective in controlling both perennial weeds and sugarcane stubble. A no-tillage fallow program would reduce soil loss from fields and subsequent costs associated with clean-out of drainage ditches and would also help conserve soil moisture critical to stand establishment. These factors, however, are difficult to quantify economically but should be considered in making the decision to implement a no-tillage or reduced tillage fallow program in sugarcane.
Luke M. Etheredge, Jr., Former Research Associate and James L. Griffin, Lee Mason LSU Alumni Association Professor, School of Plant, Environmental, and Soil Sciences, and Michael E. Salassi, Department of Agricultural Economics and Agribusiness, LSU AgCenter, Baton Rouge, La.(This article was published in the spring 2008 issue of Louisiana Agriculture.)