Weed-species dynamics and management in no-till and reduced-till fallow cropping systems for the semi-arid agricultural region of the Pacific Northwest, USA

Abstract

Weed management is an important consideration in implementing new cropping systems. In the semi-arid region of the Pacific Northwest, grower interest is increasing in no-till spring cropping systems because of wind erosion from traditional winter wheat Triticum aestivum L./dust-mulch fallow (WWF). However, no-till represents a major shift in production practices and is likely to produce new weed management challenges. A 6-year study was initiated in 1995 to develop no-till spring cropping systems and to examine associated weed management strategies for the region. Large field-size plots were delineated in two adjacent WWF fields designated west and east sites. Rotations in each site were WWF, no-till spring wheat Triticum aestivum L./chemical fallow (SWF), no-till continuous hard-red spring wheat (CSW), and no-till hard-red spring wheat/spring barley Hordeum vulgare L. (SWSB). Weed density and richness were surveyed three times each year and included late-winter, spring in-crop, and prior to crop harvest. Because of previous cropping histories, initial weed density was higher in the east than in the west plots. Weed management for east WWF was more intense than for west WWF and reduced Bromus tectorum L. density without a subsequent increase of other species. In contrast, weed management in west WWF was less intense and B. tectorum increased. No difference in weed density or richness was detected between CSW and SWSB rotations within each field site; therefore, data from these two rotations were combined and analyzed as a single continuous spring cereal (CSC) rotation. After 6 yrs, weed density was lower in east SWF and east CSC rotations at all three assessments. Weed density in the west CSC rotation was low throughout the research except for an occasional increase in volunteer cereal, but problems in chemical-fallow management increased west SWF weed density. Species richness in no-till increased in late-winter assessments after the first year as B. tectorum population declined and dicot species appeared. However, species richness was low at harvest assessments as herbicides controlled dicot weeds better than annual grasses. Weed populations in no-till rotations declined because of late-winter herbicide control of winter-annual weeds, in-crop herbicide control of dicot weeds, and postharvest herbicide applications to control Salsora iberica. The presence of wind-disseminated seed of Lactuca serriola L. and Conyza canadensis L. Cronq. in west SWF suggests these species may be future problems for long-term no-till.