James L. Griffin, Blaine J. Viator, Patrick A. Clay, Jeffrey M. Ellis, Donnie K. Miller, Stacey A. Bruff, Andrew J. Lanie and Reed J. Lencse

Weeds are a major factor limiting production of sugarcane in Louisiana. The battle for water, light, nutrients and space between weeds and the crop can reduce sugarcane stalk population and yield. Sugarcane differs from other crops in that at least three harvests, and in some cases four to five harvests, are made from a single planting. Because the soil on the row top where the sugarcane grows is not appreciably disturbed during the multi-year crop cycle, weeds can become well established and difficult to control.

Although the herbicides labeled for use in sugarcane have not changed much in the past 12 years, LSU AgCenter research on herbicide use in combination with cultural practices and tillage has produced new recommendations for farmers. Several new herbicides with novel modes of action are being evaluated and should be available to producers within the next few years.

Summer: Preparing to Replant

After several years, weeds, diseases and insects take their toll on sugarcane stands, and it is no longer economical to keep stubble for another year. The stubble is destroyed, and the field prepared for replanting. This summer fallow period from May to September is costly to the farmer because there will be no crop to harvest until the following year, but inputs (tillage and herbicide) are necessary to prepare the land for replanting. Additionally, seedcane used to replant fallowed fields reduces the amount of sugarcane that would normally go to the mill for processing.

The summer fallow period is probably the most important phase of the sugarcane production system because this is when the grower has the opportunity to reduce weed infestations significantly. The most problematic weeds in sugarcane are the perennials, johnsongrass and bermudagrass, which survive from year to year by producing an underground network of fleshy stems, known as rhizomes. Since the soil on the row top where the sugarcane grows can remain virtually undisturbed for up to five years, these perennials can become well established and almost impossible to control without causing significant crop injury. Additionally, over time, annual weeds produce abundant seed that assures their continued presence.

The fallow period allows the grower opportunity to reduce weed seed and rhizomes in the soil to help get the newly planted crop off to a good start. Programs that can be used during the fallow period include frequent tillage (opening and closing of beds) to destroy emerged seedlings, use of soil-applied herbicides to prevent weed reinfestation and use of glyphosate products (Roundup Ultra, Roundup Ultra Max, Roundup Original, Glyfos, Glyfos-Xtra, Glyphomax Plus, Gly-Flo, Glyphosate Original, Touchdown and others). All can be effective, but in most cases where johnsongrass and bermudagrass are problems, glyphosate is necessary. Under limited rainfall conditions, frequent tillage can deplete moisture in the seedbed, resulting in delayed emergence and establishment of plant cane.

Another option during the summer fallow period is to plant early-maturing, glyphosate-resistant (Roundup Ready) soybeans on sugarcane beds and use glyphosate for weed control. Another article in this issue addresses the benefits of this alternative program.

Fall: Planting Period Critical

Following the summer fallow, sugarcane is planted in August and September using whole stalks or cut stalks (billets). This phase is critical because uncontrolled weeds have enough time before the first killing frost to reestablish and produce rhizomes or seed. If this occurs, efforts to reduce weed populations during the fallow period are nullified. Typically, soil-applied herbicides are used after planting but before cane emerges. Trifluralin products (Treflan, Trifluralin, Trilin, Tri-4 and Trific) require incorporation or mixing with soil covering the sugarcane seed to prevent herbicide loss. Other herbicides (Sencor, Prowl, Pendimax, Sinbar, Velpar plus Karmex/Direx, Aatrex/Atrazine or Karmex/Direx) can be applied to the soil surface and do not require mechanical incorporation. The combination of Command plus Karmex/Direx recently received a label for bermudagrass control when applied after planting and before bermudagrass is reestablished. With many of these herbicides, depending on the time of application and rate, control of some winter weeds can be expected.

Winter and Spring: Watch Weeds

In winter, when sugarcane is dormant, weeds in some fields can include the winter annuals, Italian ryegrass, timothy, rescue grass, annual bluegrass and an assortment of broadleafs including Carolina geranium, sowthistle, marsh parsley and others. Left uncontrolled, they will remain in the sugarcane crop until May or later when they mature and set seed. Winter weeds can interfere with early growth of sugarcane as it emerges from the dormant period. Broadleaf weeds are easily controlled with 2,4-D (sold under various trade names) or Weedmaster/Brash (2,4-D + dicamba mixtures). Control of winter grasses and certain broadleafs can be obtained with paraquat formulations such as Gramoxone Extra/Gramoxone Max/Boa. Advantages to removing winter weeds include earlier emergence and more rapid growth of sugarcane, increased drying of fields, more efficient tillage operations, and greater use of fertilizer by the crop. The Gramoxone Extra/Gramoxone Max labels allow for use on emerged cane up to the 4-leaf stage. Sugarcane injury with this treatment can be significant but is short-term and not detrimental to yield.

In March and April, the grower will use a soil residual herbicide to reduce competition of summer weeds. Weeds emerging with the crop in spring can reduce tillering of cane and hinder growth, often resulting in reduced yields at harvest. In most cases herbicides are applied to a narrow band on the top of the row, rather than broadcast, to reduce cost per acre. Various herbicides including trifluralin products that require incorporation along with Sencor, Prowl, Pendimax, Sinbar, Aatrex/Atrazine, Karmex/Direx and Sempra are labeled for use. Some growers may choose to combine paraquat or 2,4-D with the soil-applied treatments to eliminate a trip over the field.

Early Summer: Grass Threat

When johnsongrass, itchgrass and annual grasses are not controlled with the soil-applied spring treatments, growers can make a postemergence application of Asulox/Asulam. These herbicides can be applied broadcast, banded or as a spot treatment over the top of sugarcane. Johnsongrass control with these herbicides can be erratic since environmental conditions greatly impact herbicide uptake into the plant. A characteristic yellow discoloration and stunting occur with johnsongrass. Weed control is slow, often requiring as long as six weeks, before weed death occurs.

For best results, actively growing johnsongrass should be 12 to 18 inches tall and other grasses no more than 8 inches tall. With some variation caused by weather conditions, johnsongrass will be at the recommended treatment size in most years during April. Johnsongrass control can be reduced if the root system is disturbed by cultivation or fertilization seven days before or seven days after Asulox/Asulam application. A 20-hour, rain-free period following herbicide application may be needed to maximize johnsongrass control.

A second application to johnsongrass regrowth, usually about eight weeks after the first application, can enhance control but may not increase yields more than a single application. The variety LCP 85-384 has shown sensitivity to Asulox/Asulam (yellowing and stunting) when temperatures are high and when applications are made after May 15.

Layby: Last Run-through

Layby is the term generally used to describe the farmer’s last crop run-through. At layby in late April and May, sugarcane is cultivated for the last time, and a soil-applied herbicide is used to keep fields weed-free until harvest. Various trifluralin products that require incorporation and Sencor, Prowl, Pendimax, Sinbar, Aatrex/Atrazine, Karmex/Direx and Sempra are labeled. Herbicides are applied broadcast and directed underneath the crop canopy to avoid contact with the young leaves in the top of the plant. Weeds that escape the layby herbicide treatment generally are not yield-limiting but can reestablish and set seed, causing problems the following year.

The morningglory group of weeds, referred to as tie-vines by growers, can be a particular problem after layby, especially red morningglory. These weeds, capable of climbing and forming a dense mat over the crop canopy, create difficulty during harvest, especially for combine harvesters. Red morninglory can be controlled preemergence at layby with Spartan, a new herbicide recently approved.

Late Season: Late-season Treatments

Weeds that escape the layby treatment may, in some cases, require control measures for efficient harvest of the crop. Late-season herbicide applications are most often made by air in late June through August and involve use of 2,4-D products. 2,4-D is effective on morningglories, if the rate is matched to the weed size. Even though a pint per acre is effective on 2- to 3-leaf plants, 1.5 quarts per acre is needed if vines are climbing sugarcane plants. Encroachment of residential areas and legal restrictions in some parishes prohibit use of 2,4-D for late-season control.

Use caution when applying 2,4-D to sugarcane for seed. For some of the older sugarcane varieties, 2,4-D applied within four weeks of cane harvest for seed resulted in significant reductions in planted stands both in the fall and the next spring. LSU AgCenter researchers are investigating the effect of 2,4-D application timing on LCP 85-384 planted using both whole stalks and billets.

To control bemudagrass reinfesting row middles, Gramoxone Extra/Gramoxone Max/Boa can be applied as a directed spray in late June and early July. To avoid significant injury to young cane, herbicide should be applied with a high clearance sprayer with spray solution directed to the stalk bases to cover weeds. This desiccates bermudagrass and, combined with shading from the crop canopy, can prevent or reduce bermudagrass reinfestation and transport in sugarcane used for planting.

Fall: Fall Treatment

In sugarcane planted in August and September and in sugarcane harvested for seed or harvested in September for early delivery to the mill, bermudagrass can reestablish before the winter dormant period. One excellent option is to apply Roundup or other glyphosate products to the row middles using a hooded or shielded sprayer in mid-October. This treatment has significantly reduced bermudagrass infestation the following year. Growers should use caution, though, because severe sugarcane injury can occur if glyphosate comes in contact with green cane foliage.

Asulox/Asulam may be applied around mid-October to early planted or early harvested sugarcane to control emerged johnsongrass. This has been an effective treatment even when herbicide rates are reduced compared with those used in April and May. Significant reductions in johnsongrass have been observed the next spring.

The reason for the effectiveness of glyphosate products and Asulox/Asulam is that in the fall, when days become shorter and nights cooler, perennial plants like bermudagrass and johnsongrass begin to move food reserves to the rhizomes for winter survival. Herbicide applied at this time moves readily within the plant along with the food reserves to the rhizomes where it inhibits growth.

Planning Herbicide Programs

Knowledge of specific weed problems and cultural practices is critical to planning cost-effective weed control programs. The registration status of herbicides is rapidly changing. Weed control recommendations in sugarcane are published annually in the LSU AgCenter publication titled “Louisiana Suggested Chemical Weed Control Guide” and are based on research conducted by weed scientists in the LSU AgCenter and with USDA. This publication is available online at the LSU AgCenter’s Web site and from any parish extension office.

Acknowledgment The American Sugar Cane League provided partial funds to support this research, which over a 12-year period contributed to thesis projects for several graduate students working in cooperation with Ed Richard, U.S. Department of Agriculture, Houma, La.

This article appeared in the fall 2001 issue of Louisiana Agriculture.