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.

Authors: Young, Francis; THORNE, MARK - WASHINGTON STATE UNIV.

Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2004
Publication Date: 7/1/2004
Citation: Young, F.L., and M.E. Thorne. 2004. 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. Crop Protection Journal. 23:1097-1110.

Interpretive Summary: The traditional, tillage-intensive, winter wheat/fallow system of the low rainfall zone in the Pacific Northwest (PNW), is characterized by winter annual grass weed species, severe wind erosion, and soil-borne diseases. Computer models have predicted that reduced-tillage fallow and no-till annual spring crops will reduce wind erosion susceptibility by an estimated 55% and 95%, respectively. However, weed management implications are unknown for these erosion-saving systems. In 1995, a 6-yr multi-/interdisciplinary, long-term field study was initiated to evaluate the agronomic and economic feasibility of no-till spring cropping systems to replace or supplement the century-old rotation of winter wheat/fallow. The study compared three no-till spring cropping systems to a winter wheat-conservation tillage fallow system. Weed management strategies targeted major weed species problems including downy brome, a winter annual grass, and Russian thistle, a summer annual broadleaf weed. By utilizing a combination of mechanical, cultural, and chemical weed management options, downy brome populations in the winter wheat-conservation tillage system were reduced from 55 plants ft-2 to about 1 plant ft-2. After 6 years of planting no-till spring crops, which interrupted the lifecycle of downy brome, the initial downy brome population of 85 plants ft-2 was decreased to > 0.5 plants ft-2. An intense weed management program including in-crop and post-harvest herbicide applications for 2 to 3 yrs kept Russian thistle populations very low in no-till spring cereals and further post-harvest control was not required. In contrast, the population of wind-dispersed weeds such as prickly lettuce and horseweed increased by the end of the 6 yrs in no-till spring cereals. Successful weed management for conservation tillage systems in the low rainfall region of the Pacific Northwest should assertively focus on preventing weeds from producing seed, both during the crop-growing and summer fallow seasons.

Technical 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 effect new weed management challenges. A six-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 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 diversity 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 censuses. 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 censuses after the first year as B. tectorum population declined and dicot species appeared. However, species richness was low at harvest censuses 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 post-harvest herbicide applications to control S. iberica. 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.