Benjamin L. Legendre, Kenneth A. Gravois, Keith P. Bischoff and James L. Griffin
In Louisiana, a sugarcane crop cycle usually begins with a fall-planted crop (called plantcane), which is harvested about 16 months after planting. That field produces two or more stubble (ratoon) crops in subsequent years and is then plowed in the spring for planting the following fall. South Louisiana has a seven-month to nine-month growing season that extends from late February or early March to late November or until the first freeze of the winter season stops plant growth. The date of the 50 percent probability (average date) for the first fall freeze in South Louisiana is from November 25 to December 15. Harvest generally extends from late September through early January.
Although the Louisiana sugarcane variety development program has selected and released varieties that produce high sugar (sucrose) content early in the harvest season, the level of recoverable sugar per ton of cane at the beginning of harvest is still relatively low. Sugar yield generally increases as the harvest season advances, depending upon the variety and weather conditions during the year. Natural ripening is caused by a combination of factors, including high-incident sunlight, cool nights and drying soil before and during harvest. These conditions generally slow vegetative growth and promote sugar accumulation in the plant.
Artificial ripening of sugarcane as a complement to natural maturity is possible because of the development of glyphosate, a plant growth regulator, for use as a chemical ripener that hastens sugarcane maturation and increases sugar yield per ton of cane and per acre. Glyphosate is the active ingredient in a number of herbicides, including several formulations of Roundup, a broad-spectrum burndown herbicide from Monsanto Company, and Touchdown from Syngenta Crop Protection. Roundup is used with Roundup Ready crops, in fallow-field weed control programs and in residential areas.
Glyphosate is also one of the most effective chemical ripeners used in the world. It apparently influences the way dry matter is partitioned in a plant, increasing the ratio of sugar to fiber and enhancing the level of sugar in the juice and cane. Glyphosate treatment, however, usually decreases cane yield after treatment by slowing cane growth and reducing stalk weight. In Louisiana, the effectiveness of glyphosate for ripening sugarcane is highly dependent upon the sugarcane variety, the rate of glyphosate applied, the treatment-to-harvest interval and the growing season.
Glyphosate was first approved by the U.S. Environmental Protection Agency to be labeled and marketed as the chemical ripener Polado in 1980 and later as Polado L for use as a management tool to increase sugar yield. Polado (or Polado L) was the only glyphosate formulation labeled for commercial use until 2003, when Touchdown IQ from Syngenta Crop Protection was approved by EPA. These formulations were also labeled for sugar enhancement in Florida, Hawaii, Texas and Puerto Rico, although sugarcane is no longer grown commercially in Puerto Rico. Their use is restricted to the stubble crops only.
The Polado L label stipulates a use rate of 4 to 14 ounces per acre of the formulated product depending upon sugarcane variety and crop condition. In 2007, three glyphosate formulations were available for use as chemical ripeners for sugarcane in Louisiana – Touchdown Total, which replaced Touchdown IQ, Roundup WeatherMAX and a limited supply of Polado L. Syngenta decided not to market Touchdown Hi-Tech in 2007 although it was labeled for use as a ripener.
These products all contain glyphosate as the active ingredient and act with the same mode of action. When they are applied at the equivalent rate of Polado L, users can anticipate similar results.
Slow stand development or shoot emergence in spring following the use of glyphosate is commonly observed in sugarcane treated with glyphosate. Research has shown that annual treatments with Polado L within the same crop cycle will usually increase mean annual sugar yield. But depending upon the sensitivity of the sugarcane variety and the treatment- to-harvest interval, those treatments can negatively affect the yield of sugar per acre in subsequent stubble crops. Additional research has shown that regrowth of cane treated with glyphosate can be further affected by leaving plant residue in the field following harvest.
The effect on regrowth is varietydependent, and some varieties are more sensitive to repeated use of glyphosate within the crop cycle. Polado L and Touchdown Hi-Tech are formulated without added surfactant – a chemical that changes the surface tension of the liquid and causes it to spread out rather than bead up on the plant surface. Research has demonstrated, however, that a quality non-ionic surfactant can improve the efficacy of these products. The remaining three products – Roundup WeatherMAX, Touchdown IQ and Touchdown Total – are formulated with a surfactant, and no additional surfactant is recommended.
Currently, glyphosate is used on approximately 250,000 to 300,000 sugarcane acres in Louisiana each crop year, netting the state’s sugarcane growers, processors and landlords an estimated $100 per acre in increased revenue. The average increase in recoverable sugar per ton of cane is approximately 20 pounds, in a range of 5 pounds to 30 pounds, depending upon variety, crop condition at and following glyphosate application, and weather conditions between treatment and harvest. The increase in recoverable sugar per ton of cane is accompanied by a corresponding decrease in cane tonnage of approximately 2 tons per acre in a range of 1 ton to 4 tons, depending upon variety and date of glyphosate application.
For early harvest beginning in late September, glyphosate must be applied when the cane is still actively growing in August; therefore, a treatment-to-harvest interval of 28 days is generally recommended. As the harvest season progresses into late October through November when vegetative growth has decreased, the treatment-to-harvest interval can be increased to 49 days for maximum response in recoverable sugar per acre.
The anticipated increase in sugar per acre can range from 350 pounds early in the harvest season to more than 600 pounds from mid-October through mid- November. This increase in sugar per acre adds approximately $30 million in increased gross revenues each year for the state’s sugarcane industry. With material and application costs of approximately $4 million annually, the use of glyphosate has a benefit-to-use ratio of 7.5 to 1. This cost is typically paid for partly or entirely by the sugar factories because of the increased recovery of sugar per ton of cane.
For the past three years, LSU AgCenter research has been conducted on the use of trinexapac-ethyl (marketed as Palisade) from Syngenta as an alternative to the use of glyphosate. Palisade is a plant growth regulator labeled for use on perennial ryegrass. It slows the growth of grass stems when the product is applied at the manufacturer’s suggested rate. LSU AgCenter research has shown that Palisade can increase the yield of recoverable sugar per ton without a dramatic decrease in cane tonnage or effect on the subsequent stubble crop as seen with glyphosate.
No current glyphosate formulations are labeled for use on the plantcane crop because of the potential for these products to cause significant yield reduction in the subsequent stubble crop, especially when used at higher rates. Therefore, additional research is ongoing to find alternative ripeners like trinexapac-ethyl that can be used on the plantcane crop without subsequent-year yield loss and without deleterious effect on the current yield.
Benjamin L. Legendre, Denver T. Loupe/American Society of Sugarcane Technologists Sugar Heritage Professor and Interim Head, Audubon Sugar Institute, St. Gabriel, La.; Kenneth A. Gravois, Graugnard Brothers Professor and Resident Coordinator, and Keith P. Bischoff, Andrew P. Gay Professor, Sugar Research Station, St. Gabriel, La., and James L. Griffin, Lee Mason LSU Alumni Association Professor, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.
(This article was published in the spring 2008 issue of Louisiana Agriculture.)