Linda Benedict, Strahan, Ronald E., Griffin, James L.
Ronald E. Strahan, James L. Griffin and Kathy R. LeJeune
Itchgrass, often referred to as Raoulgrass, is a major weed problem in south Louisiana. Yield reductions attributed to itchgrass competition have been as high as 43 percent in sugarcane and 30 percent in corn and soybeans. The spread of itchgrass has been linked to birds, flood water, rodents and contaminated farm equipment. Itchgrass infested south Louisiana rapidly during the 1970s and 1980s when road matting materials were used in oil exploration. The detrimental effects of itchgrass were not known at that time. Consequently, the weed was generally ignored until it had infested much of the area. Itchgrass is pale green with brace roots that descend from lower nodes, much like what is present on corn, and hairs on the leaf sheath that cause an itching sensation. The weed is aggressive and may reach a height of 8 feet at maturity.
Unlike the perennial weeds johnsongrass and bermudagrass, itchgrass depends solely on seed production for regeneration. A single itchgrass plant can produce more than 2,000 seeds. Weed seeds have a built-in dormancy or survival mechanism. Dormant seeds do not germinate, even when environmental conditions are favorable for germination. Seed dormancy assures that weeds will be a problem year after year, even if new seeds are not produced every year. Weed seeds can have strong or weak dormancy characteristics. Seeds with a strong dormancy mechanism can remain viable in soil for many years, sometimes as long as 40 years. In contrast, seeds with weak dormancy either germinate within a few years or deteriorate. Weeds with weak seed dormancy would be easier to eliminate from fields, if plants are not allowed to set seed to replenish the soil reservoir. An understanding of seed dormancy of itchgrass would be useful in developing long-term management programs. Our research was conducted to determine the effect of weed management programs over a single growing season on reducing the itchgrass seed reservoir in soil.
The study was conducted over two years in fallowed fields where itchgrass had set seed for the two previous years. Treatments included monthly tillage to a 4-inch depth, monthly application of Roundup at 1.5 quarts per acre and no weed removal (undisturbed control). The rationale was that tillage would stimulate seedling emergence, resulting in more rapid depletion of the seed soil reservoir when compared with the herbicide-only treatment. Experiments began in May. Tillage and herbicide treatments were continued through October.
Before each tillage or herbicide application, the itchgrass seedling population was determined in selected subplots. The itchgrass seed population was determind from soil core samples collected at a zero to 4-inch depth in mid-May and in late November. A full seed was one that was solid and hard to the touch. Itchgrass seed viability was not determined. Rainfall was monitored each year.
Initial itchgrass seed population in soil at the zero to 4-inch depth was similar for the tillage, herbicide-only and undisturbed control each year, indicating that seeds were uniformly distributed within the experimental areas. Soil seed population when experiments were initiated was calculated to be 8.3 million and 3.1 million seeds per acre for the first and second year, respectively. Total itchgrass seedling emergence across the entire growing season was stimulated by tillage in one of two years.
Itchgrass seedling emergence closely followed rainfall. When drier conditions prevailed, seedling emergence was reduced, and no advantage was observed for the tillage treatment over that of the Roundup-only treatment. Previous observations have shown itchgrass emergence to be enhanced by successive periods of soil wetting and drying. When itchgrass was not allowed to produce seed over a single growing season (May - November), itchgrass seed reserves were depleted 91 percent to 95 percent with tillage and 88 percent to 89 percent with Roundup, suggesting that itchgrass seeds possess weak dormancy. The fact that seed depletion was equivalent for the tillage and Roundup-only treatments indicates that seed loss was caused not only by germination and emergence, but also by seed rot.
Other research has shown that approximately half the itchgrass seeds present in soil were lost each year because of rotting. In the present study, when itchgrass was not disturbed throughout the growing season and allowed to set seed, seed reserves increased by almost twofold in the first year and fourfold the second year.
Results clearly show that timely herbicide application and frequent tillage, both of which destroy emerged plants and prevent seed production, are equally effective in reducing the itchgrass soil seed reserve over time in fallowed fields. The use of Roundup would be especially effective when field conditions preclude tillage operations. An intense management program can reduce the itchgrass soil seed reserve in a single growing season by around 90 percent. Even so, a significant quantity of itchgrass seed would remain in the soil to cause problems the next year. It would be imperative that control methods be implemented in crops planted in successive years to prevent seed production. Successful efforts over time would further reduce the itchgrass seed reservoir and could eventually eliminate itchgrass as a problem weed.
The authors are appreciative of the Louisiana Soybean and Grain Research and Promotion Board and the American Sugar Cane League for providing funds to support this research.
Ronald E. Strahan, Extension Associate in Plant Pathology; James L. Griffin, Professor; and Kathy R. LeJeune, former Graduate Research Assistant, Department of Plant Pathology and Crop Physiology, LSU Agricultural Center, Baton Rouge, La.
(This article was published in the summer 1999 issue of Louisiana Agriculture)