Luke M. Etheredge and James L. Griffin
In recent years purple nutsedge has become more problematic in Louisiana sugarcane fields. Purple nutsedge is considered among the world’s worst weeds because of its perennial nature and ability to produce abundant and viable underground tubers. The increase in purple nutsedge infestation in sugarcane is likely due to poor control with glyphosate herbicide applied during the summer fallow period before replanting in August and September. Limited herbicide options for nutsedge control in the crop also have contributed to the problem.
In Louisiana, when sugarcane is planted in August, the average soil temperature is around 85 degrees – ideal for nutsedge growth. When sugarcane begins to regrow in early March, the soil temperature averages around 60 degrees, which is not conducive to nutsedge growth. The competitiveness of purple nutsedge and its response to shade were evaluated under growing conditions corresponding to the time sugarcane is planted in Louisiana in August and September and when shading from the crop canopy could affect weed competition in May through July. Research also evaluated the ability of sugarcane varieties to compete with purple nutsedge at planting and nutsedge control options.
In the competition study, purple nutsedge tubers were planted along with one sugarcane seed piece of the variety CP 85-384 in a 7-gallon pot with a surface area of 1 square foot. The sugarcane seed pieces were planted in the pot’s center a half inch below the surface with the bud facing up. A weed-free control (sugarcane without nutsedge) was included for comparison. Purple nutsedge tubers were planted 2 inches deep and spaced evenly, using a grid to mark the locations.
In another study the sugarcane varieties LCP 85-384, L 97-128, Ho 95-988 and HoCP 96-540 were planted with either none or four purple nutsedge tubers per pot. Each study was terminated 64 days after planting (DAP).
An increase in purple nutsedge shoots was observed as initial tuber density increased from zero to 16 tubers per pot (Table 1). Dry weight of both purple nutsedge shoots and roots (including tubers) 64 DAP increased as initial tuber density increased, and root weight averaged 3.4 times that of shoot weight. Tuber production of 37.3 per pot occurred 64 DAP following an initial tuber density of one per pot. This is compared with 186.3 tubers per pot, where the initial tuber density was 16 per pot – a fivefold difference.
Sugarcane shoot dry weight averaged 24.9 grams per pot for the weedfree control and for one and two tubers per pot. This is compared with an average of 8.2 grams per pot for the four, eight and 16 tuber densities. In contrast, all tuber densities decreased sugarcane root dry weight compared with the weedfree control. With one and two tubers per pot, sugarcane root weight was reduced an average of 50 percent compared with the weed-free control. At the higher tuber densities, sugarcane root weight was reduced an average of 85 percent.
Purple nutsedge competition was more detrimental to sugarcane root growth than sugarcane shoot growth. Based on sugarcane shoot weight response to purple nutsedge competition, the critical weed density with LCP 85- 384 sugarcane was four tubers per square foot. Based on root weight, one tuber per square foot was the critical weed density.
Comparing sugarcane varieties in response to nutsedge competition, sugarcane shoot and root weight 64 DAP for L 97-128 averaged two times that of the other three varieties – LCP 85-384, HoCP 96-540 and Ho 95-988.
A purple nutsedge shade study was conducted in an abandoned sugarcane field with a heavy, natural infestation of purple nutsedge. Shade intensities were based on the light interception levels of black polypropylene fabric material. The experimental area was tilled to a depth of 4 inches and shaded. For the 30 percent shade treatment, 56 days after tillage, purple nutsedge shoot population was reduced 53 percent compared with the full sunlight. Purple nutsedge shoot dry weight was reduced 75 percent by 30 percent shade compared with full sunlight. Shoot population and shoot dry weight at 56 days for the 90 percent shade treatment were reduced an average of 92 percent.
In the control study, herbicides were applied in September around five weeks after planting when nutsedge was 4 to 6 inches and sugarcane was 10 inches. Nutsedge control with the Permit and Yukon treatments averaged 80 percent four weeks after treatment and 74 percent in April of the following year compared with an average of 74 and 44 percent, respectively, for the Envoke treatments (Table 2). The addition of 2,4-D low volatile ester did not increase nutsedge control.
As a follow-up study, Permit and Envoke were applied in March of the first production year when sugarcane and nutsedge had emerged from the winter dormant period. Even though nutsedge was controlled around 80 percent, an increase in sugarcane height and stalk population in July was not observed when compared with the nontreated control.
Since purple nutsedge produces underground tubers rapidly, the first line of defense should be implementation of control measures during the sugarcane fallow period. Glyphosate products will not provide control of purple nutsedge in fields with a heavy population. Permit, Yukon and Envoke will provide some control of nutsedge. Higher rates are needed when nutsedge is large and the population is dense.
For best results herbicide application should be made before nutsedge is 6 inches tall. If application is delayed until nutsedge forms a dense mat on the soil surface, a sizeable tuber population will have developed underground and control will be reduced. Permit and Envoke can be applied with glyphosate products without negatively affecting grass control. Additional surfactant is not needed if surfactant is already present in the glyphosate formulation.
If two applications of glyphosate are planned, Permit or Envoke should be applied with glyphosate in the first application. The follow-up application of glyphosate alone should be effective on nutsedge regrowth.
In Louisiana it is critical that sugarcane develop a strong root system to sustain viability of plants into the winter and to provide for aggressive emergence and growth of plants after the winter dormant period. This research shows that purple nutsedge is highly competitive with sugarcane, especially sugarcane root development, and that control measures should be implemented at planting to ensure adequate plant populations in the first production year.
Even though L 97-128 was more competitive with nutsedge than other varieties, an effective herbicide program will be needed to ensure good stand establishment in fields heavily infested with nutsedge. Permit, Yukon and Envoke can be applied after nutsedge emergence at the same rates as for the fallow application. Control around 80 percent can be expected four weeks after treatment. Sugarcane is very tolerant to overtop application of Permit and Yukon. Envoke can cause some yellowing and white banding on sugarcane leaves as well as slight stunting. No negative effect on sugarcane growth and emergence in the spring, however, has been observed. Envoke will also provide some residual control of winter weeds.
Because nutsedge emergence in the spring is affected more by cool soil temperature than is sugarcane, the earlier emergence of sugarcane enhances its competitiveness with nutsedge. Likewise, because nutsedge growth is affected by shading, the sugarcane crop should easily out-compete nutsedge at layby. Therefore, application of herbicides for nutsedge control in the spring or around layby may not provide an economic return.
Luke M. Etheredge, Jr., Former Research Associate, 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.)