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

Scott, Robert C.

Committee Member

Norsworthy, Jason K.

Second Committee Member

Hardke, Jarrod T.

Third Committee Member

Gbur, Edward E. Jr.

Fourth Committee Member

Roberts, Trenton L.

Keywords

Barnyardgrass; Efficacy; Rice; Tolerance; Topramezone; Weedy Rice

Abstract

The rapid development of resistant weeds, particularly to postemergence-applied herbicides, is a growing concern to rice producers worldwide. Barnyardgrass, one of the most problematic weeds in rice cropping systems, alone has been found to be resistant to four herbicide sites of action (SOA) in certain populations, leaving growers no options for control. This unsettling fact has led to research on other SOAs that have not previously been used in US rice. One SOA that has garnered interest is the 4-hydroxyphenylpyruvate dioxygenase (HPPD)- inhibiting herbicides. The only HPPD-inhibitor that will soon be labeled for use in US rice is benzobicyclon. The major disadvantage benzobicyclon has when compared to many other herbicides is that it must be applied directly into water, causing the cost of application to be much higher due to the need for an aerial application. Therefore, topramezone, another HPPDinhibitor used in corn production was researched to evaluate the herbicide compatibility of mixtures containing topramezone, its efficacy on barnyardgrass and weedy rice, the effect of different mixtures, rates, and cultivar on rice tolerance, and the tolerance of soybean to low doses of topramezone. In field trials, topramezone applied alone provided comparable barnyardgrass control to quinclorac, fenoxaprop, cyhalofop, and imazethapyr. All mixtures containing topramezone resulted in additive effects with the exception of those containing saflufenacil or propanil, which had antagonistic effects. When tested in the greenhouse on accessions collected throughout Arkansas, topramezone exhibited varying levels of control when applied to most weedy rice but proved to be highly effective on barnyardgrass accessions. A wide level of variation in injury to rice occurred regardless of mixture, application timing, rate, or rice cultivar from site year to site year where some applications resulted in minimal injury and others resulted in near complete crop loss. Low doses of topramezone may also affect soybean yield potential even when plants have visibly recovered from injury. These data signify that while topramezone may be used to effectively control barnyardgrass, the risk for severe crop injury is substantial, thereby casting doubt on its potential use in rice cropping systems.

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