Masters Theses

Date of Award

6-1983

Degree Type

Thesis

Degree Name

Master of Science

Major

Plant, Soil and Environmental Sciences

Major Professor

Robert M. Hayes

Committee Members

Larry S. Jeffery, Elmer L. Ashburn

Abstract

Several experiments were conducted in 1981 and 1982 for evaluating different methods for controlling johnsongrass [Sorghum halepense (L.) Pers,] in no-till double cropped soybeans [Glycine max (L.) Merr.] planted in wheat [Triticum aestivum (L.)] stubble. The objectives of these studies were to (1) evaluate the effectiveness of several overtop grass herbicides for johnsongrass control in no-till soybeans, (2) determine the optimum time to apply sethoxydim [2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen- 1-one] overtop johnsongrass in no-till soybeans, (3) evaluate the effectiveness of different herbicide and wheat stubble management systems for johnsongrass control in soybeans, (4) evaluate the effectiveness of single as compared to sequential applications of sethoxydim for johnsongrass control in different wheat stubble man agement systems and (5) compare the efficacy of glyphosate [N-(phosphonomethyl)glycine] plus alachlor [2-chloro-2',6'-diethyl- N-(methoxymethyl)acetanilide] (BroncoTM1), to a tank mixture of paraquat plus alachlor as preemergence applications for johnsongrass control in no-till soybeans planted into wheat stubble.

Experiments were established either as randomized complete block designs or a randomized complete block design in a split plot arrangement with 4 to 6 replications. Parameters tested included weed control, crop injury, johnsongrass dry weight and soybean yield.

All of the new overtop herbicides gave fair to excellent initial johnsongrass control; however, regrowth occurred in all treatments containing single herbicide applications. DPX Y 6202 [2-[4-[(6- chloro-2-quinoxalinyl)oxy]phenoxy]propionic acid] and Dowco 453 [methyl 2-(4-( [3-chloro-5-(trifluoromethyl)-2-pyridinyl)oxy)phenoxy)- propanoate] gave better johnsongrass control than sethoxydim, fluazifop butyl [(±)-butyl 2-[4[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]- propanoate], CGA-82725 [2-propynyl 2-(4-([3,5-dichloro-2-pyrindinyl] oxy)phenoxy)propanoate] or HOE 33171 [ethy1-2-[4-(6-chloro-2- benzoxazolyloxy)-phenoxy]propanoate]. CGA-82725 and HOE 33171 were the weakest compounds on johnsongrass control at the rates tested. Differences in soybean yields among these herbicides were recorded, although none were found to be significantly different according to Duncan's New Multiple Range Test. Sequential applications of fluazifop-butyl and sethoxydim provided excellent control of john songrass regrowth but did not consistently increase soybean yields over single applications.

Fair to poor johnsongrass control was obtained with single and sequential mefluidide [N-[2,4-dimethyl-S-[[(trifluoromethyl)- sulfonyl]amino]phenyl]acetamide] treatments as well as split applications of mefluidide plus acifluorfen [5-[2-chloro-4- trifluoromethyl)phenoxy]-2-nitrobenzoic acid]. Glyphosate applied with the ropewick applicator did not give acceptable control due to the amount of johnsongrass below the crop canopy at the time of application. Soybean yields from this treatment were not significantly lower than yields from the overtop herbicides.

Fair to good initial johnsongrass control was obtained when sethoxydim was applied from 2 to 3 weeks after soybean planting. Severe johnsongrass regrowth occurred in these treatments; however, most optimum soybean yields were produced when sethoxydim was applied during this time. A sequential sethoxydim application resulted in excellent control of johnsongrass regrowth, but did not give higher soybean yields than single treatments applied 2 to 3 weeks after planting.

No differences in johnsongrass control or soybean yields were found among 10 cm wheat stubble with paraquat, 30 cm wheat stubble with paraquat and burned wheat stubble. Thirty centimeter wheat stubble with glyphosate plus alachlor (BroncoTM) was superior to the other no-till treatments. It provided better initial johnsongrass control, lower johnsongrass dry weights and higher soybean yields. Conventionally-tilled double-cropped soybeans with trifluralin [α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine] at 1.7 kg/ha plus a layby cultivation gave greater than 80% johnsongrass control throughout the growing season, had lower johnsongrass dry weights and gave higher soybean yields than any no-till treatment. Johnsongrass control declined in all no-till treatments due to johnsongrass regrowth. Sequential sethoxydim applications gave excellent control of johnsongrass regrowth in all stubble management treatments but did not significantly increase soybean yields.

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