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WEED MANAGEMENT

Interference between Spring Cereals and Kochia scoparia Related to Environment and Photosynthetic Pathways

^a Vegetable Crops Dep., Univ. of California, Davis, CA 95616 USA
^b Dep. of Plant Sciences, N. Dakota State Univ., Fargo, ND 58105-5051 USA

ajfischer{at}ucdavis.edu

Kochia [Kochia scoparia (L.) Schrad.; syn. Bassia scoparia (L.) A.J. Scott] is a weed that infests cereal crops in the Great Plains of the USA, often severely reducing yields. Herbicides have controlled kochia, but recently kochia has developed resistance to many herbicides. Nonherbicide alternatives are therefore needed for the integrated management of kochia. Greenhouse and growth chamber competition studies were conducted between kochia, a C₄ weed, and barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) to determine the environmental conditions that would render kochia most vulnerable to competition by a small-grain crop. Replacement-series experiments between kochia and wheat or barley were conducted under various temperature, soil moisture, and light conditions. Unlike wheat, kochia growth and photosynthesis were suppressed under cool temperatures. Barley suppressed kochia more than wheat did because of its larger canopy, despite its lower photosynthetic rates. Under high radiation conditions and warm temperatures, growth and photosynthesis were greater for kochia than wheat. Warm temperatures also increased dark respiration and reduced water use efficiency under low radiation conditions, however, thus limiting kochia's competitiveness under a closed canopy. Water stress did not affect competition, although net photosynthetic rates of kochia were greater at photosynthetically active radiation (PAR) values > 400 µmol m^-2 s^-1. Growth and CO₂ exchange rates varied among four different kochia accessions, but growth of all accessions was reduced by shade. Results suggest that a leafy, cold-tolerant crop or cultivar, grown early in the season to produce necessary ground cover, should provide opportunity to suppress kochia.

Abbreviations: DAE, days after emergence • PAR, photosynthetically active radiation • RCC, relative crowding coefficient • RYT, relative yield total(s)

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