SUNFLOWER COMPETITION IN WHEAT

Abstract

Sunflower (Helianthus annuus L.) is grown in rotation with wheat (Triticum aestivum L.) in the upper midwest of the U.S.A. However, volunteer sunflower is often a problem in wheat planted the year following sunflower. Wheat yields as influenced by wheat seeding date and sunflower density, duration of sunflower competition and rate of sunflower control were determined in the field. Season-long sunflower competition at densities of 3, 9, and 23 plants per square metre reduced yield of the following wheat crop by an average of 11, 19, and 33%, respectively, averaged over seeding date and location. Sunflower was more competitive with wheat seeded in late than in early May, particularly at the lower sunflower densities. Wheat yield was reduced 22% when 24 sunflower plants/m2 were allowed to compete until the wheat flag-leaf stage. Wheat yields obtained were similar when volunteer sunflower was controlled by postemergence MCPA [[(4-chloro-o-tolyl)oxy] acetic acid], bromoxynil (3,5-dibromo-4-hydroxybenzonitrile), or bromoxynil plus MCPA despite the slower rate of control with MCPA compared to bromoxynil or bromoxynil plus MCPA. This research indicates that wheat following sunflower should be planted early and sunflower densities of nine plants per square metre or higher should be removed before the flag-leaf stage to prevent yield reductions.Key words: Density, duration, bromoxynil, MCPA, Helianthus annuus, Triticum aestivum