Interference and management of broadleaf signalgrass (Brachiaria platyphylla) in corn (Zea mays L.)

Author

Jason Lee Alford

Date of Award

5-2003

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant, Soil and Environmental Sciences

Major Professor

Robert M. Hayes

Committee Members

Thomas C. Mueller, G. Neil Rhodes Jr.

Abstract

Broadleaf signalgrass is a troublesome weed in the southern United States. However, with the development of glyphosate-resistant corn, glyphosate, a highly effective graminicide, can be used to manage broadleaf signalgrass.

Field experiments were conducted in the summers of 2000 to 2002 at University of Tennessee experiment stations in Jackson and Knoxville, TN. The focuses of the experiments were to determine the effect of broadleaf signalgrass competition on corn and the optimum time to apply glyphosate to prevent broadleaf signalgrass interference.

Glyphosate isopropylamine salt at 0.84 kg ae ha^-1 was applied as single applications at 2, 3,4, 5, 6, 7, and 8 weeks after corn emergence (WAE) and as sequential applications at 2 and 4 WAE; 2, 4, and 6 WAE; 2,4, 6, and 8 WAE at both locations. The test also included a weed-free and untreated check. Broadleaf signalgrass densities were ~30 plants m^-2 at Jackson and 150 to 300 plants m^-2 at Knoxville. Corn injury was evaluated at each subsequent treatment and prior to harvest. Broadleaf signalgrass control and biomass were determined prior to harvest.

While there were decreasing yield trends at Jackson, broadleaf signalgrass did not significantly reduce corn yields at (P=0.05), likely due to low densities and adequate soil moisture and nutrients. In 2001,corn yields increased by 248 kg ha^-1 wk^-1, when single applications of glyphosate were delayed from 2 to 5 WAE, due possibly to resurgence of broadleaf signalgrass following the early glyphosate applications. Corn yields with glyphosate treatments applied 6 to 8 WAE steadily decreased, apparently due to broadleaf signalgrass interference. Glyphosate treatments applied later than 5 WAE had an average yield decrease of 500 kg ha^-1wk^-1 the treatment was delayed. In 2002,corn yields from single applications of glyphosate between 2 to 5 WAE did not differ, while treatments applied 6 to 8 WAE had an average decrease of 300 kg ha^-1 wk^-1 delay in treatment.

Corn yields at Knoxville in 2000 to 2002 were reduced by broadleaf signalgrass interference, compared with the weed-free check. In 2000, the 7 and 8 WAE treatments and untreated check had lower corn yields than the weed-free check. Corn yields with single applications of glyphosate between 2 to 5 WAE did not differ, while yields from treatments applied 5 to 8 WAE had an average decrease of 360 kg ha^-1 wk^-1 delay in treatment.

In 2001, only the untreated check had lower corn yield than the weed-free check. Linear regression of 2001 corn yields suggested that treatments between 2 to 5 WAE had an average yield increase of 460 kg ha^-1 wk^-1.corn yield increases from 2 to 5 WAE were the result of optimizing control during the critical period (4 to 5 WAE) as opposed to early glyphosate applications (2 to 3 WAE), which allow broadleaf signalgrass resurgence in sufficient density and duration of interference to reduce yield. Treatments >5 WAE had an average yield decrease of 570 kg ha^-1 wk^-1 that glyphosate treatment was delayed.

Corn yields in 2002 from treatments made 6 to 8 WAE were lower than the weed-free check due to broadleaf signalgrass interference before the treatment was applied. In addition, the 6 and 7 WAE treatments had less yield reduction than the 8 WAE treatment and the untreated check. Further evaluations using linear regression determined that glyphosate treatments between 2 to 5 WAE did not differ. However, yields following the 5 WAE glyphosate application had an average decrease of 740 kg ha^-1 wk^-1 of delay. Corn yields from all sequential applications at Knoxville were similar to the weed-free check.

When examined over five field experiments, broadleaf signalgrass reduced corn yield by 490 kg ha^-1 wk^-1 (r²= 0.79) that glyphosate application was delayed beyond 5 WAE. Sequential applications of glyphosate at 2 and 4; 2, 4, and 6; or 2, 4, 6, and 8 WAE were effective in controlling yield loss, but in some instances four applications reduced corn yield. While broadleaf signalgrass can interfere with corn and reduce yield, glyphosate applied as a single application from 3 to 5 WAE or as sequential applications at 2 and 4 WAE can virtually eliminate yield loss. However, consideration must be given to the entire weed population in properly timing glyphosate applications to prevent weed interference and optimize corn yield potential.

Recommended Citation

Alford, Jason Lee, "Interference and management of broadleaf signalgrass (Brachiaria platyphylla) in corn (Zea mays L.). " Master's Thesis, University of Tennessee, 2003.
https://trace.tennessee.edu/utk_gradthes/6578