B. Mirshekari - Department of Agronomy and Plant Breeding, Tabriz Branch, Islamic Azad University, Tabriz, Iran.

To study interferential effect of lamb s-quarters (Chenopodium album L.) on marigold (Calendula officinalis) yield and lamb s-quarters seed production two factorial field experiments were conducted in Tabriz, Iran, during 2010-2011. Treatments were densities of lamb s-quarters (0, 2, 4, 6 and 8 plants per meter row) and relative times of its emergence (0, 10, 20, 30 and 40 days after marigold emergence). Marigold could tolerate4 lamb s-quarters plants per meter row without any significant yield reduction, when it emerges 40 days after that of marigold. A suitable fit to model of Cousens et al. was obtained for all marigold flower yield data sets at interference with lamb s-quarters. Results also indicate that controlling lamb s-quarters early in the season may reduce its seed production capacity.