Interference between spring cereals and Kochia scoparia related to environment and photosynthetic pathways

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 k ochia has developed resistance to many herbicides. Nonherbicide alternative s are therefore needed for the integrated management of kochia. Greenhouse and growth chamber com petition studies were conducted between kochia, a C- 4 weed, and barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) to determine the environmental conditions that would render kochia most vulne rable 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 phot osynthesis were suppressed under cool temperatures. Barley suppressed kochi a more than wheat did because of its larger canopy, despite its lower photo synthetic rates. Under high radiation conditions and warm temperatures, gro wth and photosynthesis were greater for kochia than wheat. Warm temperature s also increased dark respiration and reduced water use efficiency under lo w radiation conditions, however, thus limiting kochia's competitiveness und er a closed canopy. Water stress did not affect competition, although net p hotosynthetic rates of kochia were greater at photosynthetically active rad iation (PAR) values > 400 mu mol m(-2) s(-1).. Growth and CO2 exchange rate s varied among four different kochia accessions, but growth of all accessio ns was reduced by shade. Results suggest that a leafy, cold-tolerant crop o r cultivar, gown early in the season to produce necessary ground cover, sho uld provide opportunity to suppress kochia.