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A recently published article in the journal Weed Science just how devastating Palmer amaranth (Amaranthus palmeri) can be for furrow-irrigated rice production, even in small amounts. The findings show that successful, in-furrow rice production greatly hinges on Palmer amaranth-free fields—before, during, and after the growing season.

"Palmer amaranth that emerged one week before the crop caused the greatest yield reduction in the study, with grain yield loss of 50% where rice was within 40 centimeters, or about 15 inches, of the weed," says Tanner King, weed science Ph.D. student at Mississippi State University, Weed Science Society of America (WSSA) member scientist, and corresponding author for the study. "Yet, even a single Palmer amaranth plant emerging within three to four weeks after rice emergence still has the potential to negatively impact rice growing within 1.2 square meters, or almost 13 square feet."

King and his team conducted their research during 2022 and 2023 at the Milo J. Shult Agriculture Research and Extension Center in Fayetteville, Arkansas. Their goal was to determine how Palmer amaranth's emergence time impacted rice grain yield and Palmer amaranth seed production in a furrow-irrigated system.

"As herbicide options for Palmer amaranth control become less available and furrow-irrigated rice production increases, it's critical to understand the weed's biological effects on rice yields and incorporate successful weed management programs into this system," says King.

"Based on the Palmer amaranth seed production data from our study, female Palmer amaranth plants that emerge three to four weeks after rice can still successfully contribute offspring to the soil seedbank. As a result, producers must focus on minimizing returns to the soil seedbank, which will subsequently help mitigate the additional emergence of Palmer amaranth seedlings in future growing seasons."

Fields free from Palmer amaranth prior to planting proved critical for successful grain yield in the study, as did controlling the weed throughout the season.

"Rice yield loss was most severe when Palmer amaranth emerged one week before the crop, which captures a worst-case scenario for a producer in the event a burndown application fails to control all weeds before planting," points out King. "Yet, Due to the consequences of allowing Palmer amaranth to compete with rice throughout the growing season, and the need to prevent weed seed dispersal at harvest, producers should place extreme emphasis on the 'zero tolerance' approach regardless of when the weed emerges."

Season-long Palmer amaranth weed control is no easy task, but the rewards are justifiable.

"Preventing Palmer amaranth from reaching reproductive maturity will require a combination of control measures throughout the growing season, including the use of residual preemergence herbicides, considering effective weed control options decline once weeds become established in a field," says King.

"Although not assessed in this study, the monetary losses associated with Palmer amaranth interference in furrow-irrigated rice are exacerbated when harvest efficiency and quality are negatively impacted and the economic impact of Palmer amaranth escapes in rice will extend well beyond the year in which plants are allowed to compete and produce seed."