James L. Griffin, Curtis A. Jones and Jonathan D. Siebert

Red morningglory is one of the more common and problematic weed pests found in Louisiana sugarcane fields. The traditional way to control it is to apply the herbicide atrazine in April or May when sugarcane is cultivated for the last time. The intent of this application is to eliminate weed competition until the crop is harvested beginning in September.

Atrazine, however, provides weed control for only about eight weeks after application, and at least 16 weeks can occur between the application and harvest. During this period, rainfall and warm soil can cause rapid herbicide degradation, often resulting in late-season red morningglory infestations.

When control fails, morningglory, known by growers as tie-vine, climbs and entwines sugarcane stalks, causing the plants to fall over. This reduces both the number of harvestable stalks removed from the field and the efficiency of mechanical harvesters. Producers are often forced to apply the herbicide 2,4-D in late season to facilitate harvest.

Even though highly effective on red morningglory, 2,4-D is restricted in some areas of Louisiana because of problems with off-target movement and injury to sensitive crops, particularly cotton. During 2001-2002, LSU AgCenter scientists conducted research to evaluate red morningglory emergence across the growing season and the effect of the sugarcane canopy on the weed’s growth. Knowledge gained in these areas will help develop effective management strategies. Additional research has evaluated alternative herbicides for atrazine and 2,4-D.

Tillage and Red Morningglory

The purpose of this study was to determine the effect of tillage on red morningglory seed germination and seedling emergence. Soil samples were collected to determine red morningglory seed populations in April before the treatments began and in October when the study was terminated. Treatments included tillage to a 4-inch depth in May, June, July and August and no tillage. Weed emergence 20 to 41 days after each tillage operation was determined.

Soil samples before the study began contained between 100 and 450 red morningglory seeds per square foot.

On the July sampling date, red morning-glory emergence was equal whether soil was tilled or not tilled; emergence averaged 9.7 plants per square foot.

In August, weed emergence was 45 percent greater when plots had been tilled around four weeks earlier as compared with plots that had not been tilled (9.3 versus 6.4 plants per square foot). On the September sampling date, only 2.1 plants per square foot emerged in the non-tilled plots compared with eight plants per square foot where plots were tilled. The decrease in red morningglory emergence as the season progressed is probably caused by depletion of the seed reservoir in the soil.

Soil samples taken in October clearly showed a decrease in seed from the initial sampling, but no difference in seed population was noted between tilled and nontilled treatments.

Shade and Red Morningglory

Another study looked at red morningglory tolerance to shade. Shade boxes were used to simulate zero, 30 percent, 50 percent, 70 percent and 90 percent shade. Weed emergence and growth data were collected just before each tillage operation. Red morning-glory emergence decreased both years as the season progressed. But even though weed emergence decreased for some of the sampling dates in response to shading, plant growth for individual dates was equivalent regardless of shade. Results clearly showed that, unlike most weed pests, red morning-glory is tolerant of shade.

Atrazine versus Spartan

Typically in sugarcane production, herbicide is applied at the last cultivation to keep fields clean until harvest. Herbicides are directed underneath the crop canopy to avoid contact with the young leaves in the sugarcane whorl. Atrazine applied in this manner has provided inconsistent red morningglory control.

Another herbicide, Spartan, is also recommended and provides consistent control of red morningglory. Spartan, however, has little grass activity and should be applied in conjunction with Prowl or one of the Treflan products. Prowl can be applied to the soil surface, but Treflan products must be incorporated into the soil to avoid herbicide loss.

One question often asked is if Spartan can be incorporated into the soil with Treflan rather than applied to the soil surface following incorporation of Treflan. Research has shown that at around eight weeks after treatment, Spartan applied at 8 ounces per acre and soil-incorporated controlled red morningglory 80 percent. When Treflan was soil-incorporated and Spartan was applied at 4 ounces per acre to the soil surface, however, control was 94 percent. When Spartan was applied to the soil surface and Treflan was not used, red morningglory was controlled 79 percent with 6.7 ounces per acre and 91 percent with 8 ounces per acre. Findings clearly show that to maximize red morningglory control, Spartan should not be incorporated into the soil and that Treflan followed by a surface application of Spartan is superior to that of Spartan applied alone.

Use of 2,4-D

When red morningglory growth threatens harvest, then a late-season herbicide must be used. Late-season herbicide applications are most often made by airplane or helicopter and involve use of 2,4-D products. If sugarcane is standing upright, a high clearance sprayer can be used and, if desired, herbicide can be directed underneath the crop canopy.

Because of restrictions on use of 2,4-D in sugarcane production areas, alternative herbicides for red morningglory control were evaluated. Complete control of 12- and 24-inch red morningglory was obtained 21 days after treatment with 2,4-D at 1 pint per acre, Weedmaster at 1 pint per acre, Atrazine at 2 quarts per acre and Spartan at 6.7 ounces per acre.

Red morningglory at 6 feet tall was controlled 100 percent with 2,4-D at 1 quart per acre 28 days after treatment in 2001, but control was only 78 percent the next year. In 2002, when herbicides were applied three weeks earlier than in the previous year and when weed growth was more vigorous, red morn-ingglory was controlled 87 percent with 2,4-D at 1.5 quarts per acre. Directed applications to the lower 18 inches of 6-foot red morningglory plants with Atrazine at 4 quarts per acre and Spartan at 6.7 ounces per acre provided at least 96 percent control the first year, but control was 23 percent to 30 percent less the second year.

Tough to Control

Results of this research show that red morningglory is not only capable of germinating throughout the growing season but is able to thrive under the sugarcane canopy. This suggests that soil-applied herbicides that provide long residual control are needed to prevent red morningglory emergence later in the growing season. Since there can be at least four months between the last cultivation and harvest, growers may choose to apply a grass herbicide at the last cultivation and delay application of Spartan or atrazine until after a rain has settled the soil. By delaying the herbicide application, the weed control period is extended.

If the grower has a high clearance sprayer, this application can be delayed even more. Spartan at the recommended rates of 6.7 to 8 ounces per acre of the dry formulation has been effective in controlling red morningglory, but long-term control can be rate dependent. In fields with heavy soil and where morn-ingglory vines have typically been a problem of concern in sugarcane harvested early, growers should lean toward the high end of the rate range. The higher cost associated with a higher use rate of Spartan can be offset if an application of 2,4-D is avoided. Note that Spartan is not as effective on the pitted morningglory (smooth leaf species). For this weed, the higher end of the rate range should be used.

When red morningglory begins to climb and wrap sugarcane stalks, control with herbicides other than 2,4-D is more variable and difficult. 2,4-D remains the treatment of choice for late-season control of morningglory vines and other broadleaf weeds in areas where its use is not restricted. Growers should be aware that applying 2,4-D in late season to sugarcane used for seed can affect seed germination and yield of the plant cane crop. Research conducted over two growing seasons has shown that a seven-week period should be allowed between 2,4-D application and harvest of LCP 85-384 for planting material as either whole stalks or billets.