Date of Graduation

12-2019

Document Type

Thesis

Degree Name

Master of Science in Crop, Soil & Environmental Sciences (MS)

Degree Level

Graduate

Department

Crop, Soil & Environmental Sciences

Advisor/Mentor

Norsworthy, Jason K.

Committee Member

Scott, Robert C.

Second Committee Member

Gbur, Edward E. Jr.

Third Committee Member

Spurlock, Terry N.

Fourth Committee Member

Roberts, Trenton L.

Keywords

Canopy coverage; Herbicide; Herbicide resistance; Pathogen; Soybean; Weed control

Abstract

The rapidity in evolution of herbicide-resistant weeds and the resulting cost to U.S. farmers demonstrate the need to responsibly steward the limited number of herbicides available in agricultural systems. To reduce weed emergence and likewise added selection pressures placed on herbicides, early-season crop canopy formation has been promoted. However, impacts to soybean following a potentially injurious herbicide application have not been thoroughly evaluated. Therefore, field experiments were conducted to determine whether: 1) soybean injury from metribuzin or flumioxazin delayed canopy formation or changed the incidence of pathogen colonization; 2) residual herbicides applied preplant reduced the potential for soybean injury and achieved the same longevity of weed control as herbicides applied at planting; 3) POST-applied acetolactate synthase (ALS)- and protoporphyrinogen oxidase (PPO)-inhibiting herbicides alone and in combination with glufosinate delayed canopy formation or impacted grain yield. Few interactions between herbicides and soil-borne pathogens were observed. Results from various experiments showed that soybean canopy formation was delayed after an application of preemergence (PRE)-residual herbicides and postemergence (POST)-foliar-active herbicides. However, delays in crop canopy formation caused by a PRE application of metribuzin and flumioxazin were only observed in varieties with sensitivity to the herbicide. Soybean injury caused by PRE applications were mitigated by applying herbicides 14 days prior to planting. Treatments that were applied 14 days prior to planting and contained an effective herbicide with a half-life greater than 70 days suffered no reduction in longevity of Palmer amaranth control when compared to the same herbicide applied at planting. POST-applied herbicides delayed soybean canopy formation relative to the amount of injury present following application. Delays in canopy formation can result in a lengthened period of weed emergence, subsequently increasing the need for additional weed control and increasing selection pressure on sequentially applied herbicides.

Nomenclature: Flumioxazin, glufosinate, metribuzin, Palmer amaranth, Amaranthus palmeri (S.) Wats., soybean Glycine max (L.) Merr.

Key words: Acetolactate synthase (ALS)-inhibiting herbicides, canopy formation, half-life, herbicide-resistance weeds, POST foliar-active herbicide, preplant, protoporphyrinogen oxidase (PPO)-inhibiting herbicides, PRE-residual herbicide, soil-borne pathogen, soybean injury

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