Masters Theses

Date of Award

12-1976

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant, Soil and Environmental Sciences

Major Professor

Larry S. Jeffery

Committee Members

Laurence N. Skold, Elmer L. Ashburn

Abstract

Controlling johnsongrass [Sorghum halepense (L). Pers.] in such agronomic crops as corn (Zea mays L.) and soybeans [Glycine max (L.) Merr.] presents a major problem to many farmers. Although several preemergence herbicides have been developed to control seedling johnsongrass, in general rhizome johnsongrass is primarily responsive only to foliar-applied chemicals. One such postemergence chemical which gives excellent control of rhizome johnsongrass is glyphosate (N-phosphonomethyl glycine). If glyphosate is applied in the spring, a waiting period is required both before spraying to allow johnsongrass foliage to develop and then for trans location of the herbicide following application. As this waiting period may cause an undesirable delay in planting date, a fall application would be more favorable. The objectives of this study were (1) to determine the effectiveness of fall applications of glyphosate for controlling rhizome johnsongrass, (2) to determine the response of corn and soybeans to glyphosate applications made immediately prior to crop maturity, at maturity, and following maturity. A field experiment was conducted at Ames Plantation, Grand Junction, Tennessee, and at the Knoxville Plant Science Field Laboratory, Knoxville, Tennessee. The corn treatments were applied at various grain moisture levels beginning at about 45 percent and continuing through 15 percent. Soybean treatments were applied at one week intervals beginning at three weeks prior to crop maturity and continuing through two weeks past maturity. Johnsongrass treatments corresponded to soybean treatments, except for one early fall treatment applied on August 29. Due to calibration error, the rates of glyphosate application varied between locations. The resulting rates were 1.68 kg/ha at Ames Plantation and 2.24 kg/ha at Knoxville. The glyphosate application made at Knoxville on corn at the 47 percent grain moisture level caused a significant reduction in seed moisture and weight at harvest. However, when the corn was treated at grain moisture levels of 40 to 15 percent, the glyphosate had no effect on lodging, yield, seed moisture, or seed weight at either location. Corn progeny germination was not affected by any of the glyphosate applications. Nevertheless, treatments at grain moisture levels of 35 percent and higher caused serious reductions in progeny seedling emergence, vigor, and weight at 21 days. The emergence of abnormal progeny seedlings was also increased by these same treatments. In addition, there tended to be a reduction in primary root length of these injured seedlings. The glyphosate application made three weeks prior to soybean maturity at Ames reduced seed weight. This treatment also produced drastic reductions in progeny seedling emergence, vigor, weight at 21 days, and primary root length. Furthermore, this treatment increased dry matter content of the seedlings. The subsequent treatments at Ames, as well as all the treatments at Knoxville were applied after the onset of soybean senescence. Consequently, these treatments caused no effect whatsoever on the treated soybeans or their progeny. Protein and oil content of the corn and soybean seed from treated plants was not affected by any of the fall applications of glyphosate. The earliest glyphosate treatments produced the best control of rhizome johnsongrass. Applications made between August 29 and October 18 resulted in 80-95 percent rhizome control. Treatments made on October 24 and 31 gave only fair rhizome control, while no control resulted from a November 7 application. From these data, the optimum treatment stage for controlling johnsongrass in corn or soybeans would appear to be (1) when the corn grain moisture level is below 35 percent, (2) after the onset of soybean senescence, and (3) prior to mid October if johnsongrass is to be controlled in either crop.

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