Alfalfa benefits from reducing weeds before planting ByBob Johnson
“Weed competition at establishment can affect the alfalfa crop for years,” said Sarah Light, UC Cooperative Extension farmadvisor in Sutter and Yuba counties. Light described her trials of different preplant and in-season treatments as researchers discussed alfalfa weed con- trol during the UC Small Grains–Alfalfa/ Forages Virtual FieldDay lastmonth. “Both tillage andglyphosatewere effec- tive preplant treatments,” she said. “With
no treatment, thebroadleafweedscovered basically the entire plot.” In Light’s trial, plots were tilled, treated with glyphosate or left untouched before planting, and either left alone or treated withRaptor sevenweeks after planting. Though the application of Raptor im- proved weed control and increased yield in the first cutting, Light said themost dra- maticdifferencewasbetweenplots leftun- touchedbeforeplantingoreithercultivated
or treatedwithherbicide before planting. When nothing was done to control weedsbeforeplantingearly inMarch2020, she said, therewas next to no alfalfa at the first cutting in early June. “Alfalfa yields at first cutting were im- proved with both preplant and in-season weed control,” Light said. “There was al- most noalfalfaat first cutting inplotswith- out preplant weed control.” Tillage appears to have been slightly moreeffective thanglyphosateat reducing weeds and promoting early crop growth, she said, but the difference was not statis- tically significant. What was significant, however, was that failure to control weeds in someway before planting led to serious long-term crop damage. “Preplantweedcontrolmade thealfalfa stand more robust after the first cutting, even with high initial weed pressure,” Light said. “Alfalfa plants died with no preplant control.” TomGetts, UC Cooperative Extension farm advisor in the intermountain re- gion, reported he is looking for new al- ternatives, as resistance threatens some herbicides and regulations tightenon the widely used paraquat. “Weed competition at establishment can affect the alfalfa crop for years.” — Sarah Light UC Cooperative Extension “People rely after planting on chemical control, and there aren’t that manymate- rials,” Getts said. “Paraquat is a very effec- tivematerial, but a lotofpeoplearemoving away from it because of regulations.” Although chemical control in season does improveweedcontrol andyields, reg- ulation, economicsor, fororganicgrowers, a lack of effectivematerialsmaymake this option impractical. “Preplant weed control was critical for reduced weed pressure, yield and alfalfa stand counts,” Light said. “Highest yields at first cuttingwere in plots with both pre- plant and in-season control.” One option for organic farmers and others looking to reduce theiruseof herbi- cidesmight be tocultivatebeforeplanting, and figure the crop will grow out of weed competitiontoeventuallyproduceaccept- able yields, she said. “Stand counts in plots with preplant control onlywerestill relativelyhigh,”Light said. “If in-seasoncontrol options are lim- ited, itmaybepossible todopreplantman- agement to control weeds and establish a good stand. Yieldmay not be reduced for the first cutting, but the impact may not carry over to subsequent cuttings.” ( Bo b J o hn s o n i s a r e p o r t e r i n Sacramento. He may be contacted at bjohn11135@gmail.com.)
Although themethods may vary, some preplant weed control strategy—either mechanical or chemical—is essential to establish a viable alfalfa stand. In a recent University of California trial, failure to protect the crop from competi- tion with weeds before planting resulted in virtually no harvestable crop at the first cutting—andhadan impact onyields that could last indefinitely.
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20 Ag Alert June 9, 2021
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