Masters Theses

Date of Award

5-1992

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant, Soil and Environmental Sciences

Major Professor

William A.Krueger

Committee Members

G. Neil Rhodes, Robert M. Hayes

Abstract

Field experiments were conducted at Springfield and Spring Hill, Tennessee in 1990 and 1991 to determine when to apply DPX-79406 to achieve maximum johnsongrass control. Experiments were established in areas naturally infested with seedling and rhizome johnsongrass. Treatments were replicated four times in a randomized complete block design. DPX-79406 at 6, 9 and 11 g ha-1 was applied postemergence at three stages of johnsongrass growth. Nicosulfuron at 35 g ha-1 and primisulfuron at 40 g ha-1 were included at each application. A handhoed check and weedy check were also included.

Johnsongrass control with DPX-79406 did not differ when applied to 20- or 50-cm johnsongrass. DPX-79406 did not control 75-cm johnsongrass. Nicosulfuron and primisulfuron performed similarly to DPX-79406 at each timing. Corn grain yield did not differ among herbicide treatments applied to 20- or 50-cm johnsongrass. Corn grain yields were always higher than the weedy check when treatments were made to johnsongrass ≤50 cm. Corn grain yields were reduced when DPX-79406 was applied postemergence to 75-cm johnsongrass.

Similarly, no differences in johnsongrass control was observed among herbicides within an application time. DPX-79406 gave excellent control when applied to 20-cm johnsongrass; control decreased by approximately 10% for each successive timing. In general, corn grain yields with the early postemergence treatments did not differ from those of the weedfree check.

Field studies were conducted at Springfield, Spring Hill, and Knoxville, Tennessee in 1990 and 1991 to compare DPX-79406 to DPX-E9636, primisulfuron and nicosulfuron for johnsongrass control. The experiments were established in areas naturally infested with seedling and rhizome johnsongrass. The experimental design was a randomized complete block with treatments replicated four times. In 1990 DPX-79406 (6, 9 and 11 g ha-1) was compared to DPX-E9636 (9, 13 and 17 g ha-1), nicosulfuron (9, 13, 17 and 35 g ha-1), and primisulfuron (40 g ha-1). In 1991 the following treatments were utilized: DPX-79406 at 6, 9, 11, 18, 27 and 35 g ha-1; nisosulfuron at 6, 9, 11 and 35 g ha-1; and primisulfuron at 40 g ha-1. Treatments were applied both years to 25±5- cm and 70±10-cm johnsongrass. All experiments also included a hand-hoed check and a weedy check.

All treatments controlled 25-cm johnsongrass the first year. Only primisulfuron controlled less johnsongrass than the hand-hoed check. DPX-79406 at 6 and 9 g ha-1 failed to control 70-cm johnsongrass. Nicosulfuron and DPX-E9636 at 9 g ha-1 failed to control 70-cm johnsongrass. Nicosulfuron at 13 to 35 g ha-1 controlled 70-cm johnsongrass ≥87%. DPX-E9636 at 13 to 17 g ha-1 gave ≥93% control of 70-cm johnsongrass. Corn grain yields from treatments applied to 25-cm johnsongrass did not differ from the hand-hoed check. Treatments to 70- cm johnsongrass showed considerable variability among grain yields. DPX-E9636 applied to 70-cm johnsongrass injured corn and contributed to lower yields. Both herbicides controlled 25-cm johnsongrass at Springfield in 1991. DPX-79406 at 6, 9 and 11 g ha-1 controlled johnsongrass better than nicosulfuron at the identical rates. No treatment controlled 70-cm johnsongrass, although yields did not differ from the hand-hoed check. This was probably due to light johnsongrass pressure at Springfield. Control of 25-cm johnsongrass did not differ among herbicide treatments at Spring Hill in 1990. All treatments applied to 70-cm johnsongrass resulted in control lower than the handhoed check. Corn grain yields were not different among treatments to 25-cm johnsongrass, with the exception of primisulfuron at 40 g ha-1 and DPX-79406 at 6 g ha-1. Corn yields from plots with 70-cm johnsongrass treated with nicosulfuron and primisulfuron were higher than those with DPX-79406 and DPX-E9636 both years. All treatments to 25-cm johnsongrass with the exception of primisulfuron provided >87% control prior to harvest. No herbicide applied to 70-cm johnsongrass gave control equal to the hand-hoed check. Corn yields were less in plots treated with primisulfuron than in those maintained weedfree by handhoeing. DPX-79406 at 35 g ha-1 injured 4-leaf corn and contributed to low yields.

At Knoxville in 1990, johnsongrass control did not differ among treatments, regardless of when applied. There were no differences in grain yield among treatments to 25-cm johnsongrass. Corn yields were less than the hand-hoed check when nicosulfuron at 17 g ha-1 was applied to 70-cm johnsongrass. In 1991 trials, DPX-79406 controlled 25-cm johnsongrass. No treatment controlled 70-cm johnsongrass. Corn yields from all treatments to 25-cm johnsongrass were greater than the weedy check. Corn yields from plots treated at the 70-cm johnsongrass height were variable.

A field study was conducted at Knoxville in 1991 to assess corn injury from DPX-79406. The experiment was established in a relatively johnsongrass-free area. Treatments were replicated four times in a randomized complete block design, DPX-79406 at 18, 27, 35 and 54 g ha-1 was applied to 4- to 5-leaf corn and to 8- to 9-leaf corn. Nicosulfuron (35 g ha-1) and primisulfuron (40 g ha-1) were also applied at both growth stages. An untreated plot was also included.

DPX-79406 injured 4- to 5-leaf corn at rates of 27 g ha-1 or greater. Despite the injury, grain yields were not different from the untreated plot. No treatment caused any injury to 8- to 9-leaf corn.

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