Date of Graduation

8-2024

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

Butts, Thomas R.

Second Committee Member

Fernandes, Samuel B.

Third Committee Member

Drescher, Gerson L.

Keywords

Competitiveness; Cultural control; Herbicides; Interference; Remote sensing; Weed control

Abstract

Palmer amaranth (Amaranthus palmeri S. Wats) is the most troublesome and problematic weed in most mid-southern U.S. row crop production systems. With the increased adoption of furrow-irrigated rice (Oryza sativa L.), there is a greater occurrence of Palmer amaranth due to sustained moist conditions allowing continual emergence of the weed throughout the growing season. Palmer amaranth has previously been shown to negatively affect crop yields when left unmanaged. Therefore, there is a high potential for rice yield loss due to the weed’s ability to emerge over an extended period and interfere with crop development. However, there is a lack of research evaluating Palmer amaranth interference in rice. Prior research has also highlighted the effectiveness of cultural weed control tactics, such as the use of cover crops or altering crop competitiveness, on Palmer amaranth suppression. Fluridone, a widely used preemergence residual herbicide in cotton (Gossypium hirsutum L.) for Palmer amaranth control, was recently registered for use in rice. However, concerns about crop injury and the ability of fluridone to be integrated with other commonly applied rice herbicides that may also cause injury have led to the need for additional research. Research conducted included experiments evaluating rice yield loss as a function of Palmer amaranth time of emergence relative to rice and weed density, the effect of rice density and cover crops on Palmer amaranth emergence, and Palmer amaranth control and rice tolerance to preemergence- and postemergence-applied fluridone in combination with other rice herbicides. Palmer amaranth emerging before rice caused 50% or more rice yield loss up to 0.40 m away from the weed. Rice yield loss was 41% with Palmer amaranth densities of 5 plants m-2 interfering season-long with rice, indicating that the weed is extremely competitive with the crop even at lower densities. As rice density increased, total Palmer amaranth emergence decreased, with crop densities of 50, 100, and 150 plants m-2, allowing 27, 13, and 7 weeds to emerge m-2. However, Palmer amaranth emergence did not cease with any of the evaluated rice densities, suggesting that additional Palmer amaranth suppression will depend more on crop canopy formation than crop density as rice matures. In most instances, the cover crops evaluated did not negatively affect rice emergence, groundcover, or grain yield. In general, Palmer amaranth emergence was lowest with the use of legume cover crops compared to cereals. However, cereal rye (Secale cereale L.) exhibited some weed control benefit, reducing Palmer amaranth emergence by 19 to 35% in three of five trials. Late-postemergence applications of fluridone at 84 and 168 g ai ha-1 with florpyrauxifen-benzyl resulted in 92% and 94% Palmer amaranth control 14 days after treatment for the single and multiple fluridone application experiment, respectively. Based on these findings, Palmer amaranth has the potential to cause severe rice yield loss, yet cultural weed control methods have the potential to reduce early season Palmer amaranth emergence and herbicide programs utilizing labeled fluridone applications for effective control of the weed without causing long-term harm to rice.

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