Submitted to: Crop Protection Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2007
Publication Date: 7/1/2007
Citation: Thorne, M.E., Young, F.L., Yenish, J.P. 2007 Cropping systems alter weed seed bank in Pacific Northwest, USA semi-arid wheat region. Crop Protection Journal 26 (8): 1108-1120. Interpretive Summary: The traditional winter wheat/fallow system of the low rainfall zone in the Pacific Northwest (PNW) is characterized by intensive tillage, winter annual grass weeds, and severe wind erosion. Supplemental cropping systems of no-till annual spring crops and reduced-tillage fallow may reduce erosion susceptibility by an estimated 95% and 55% respectively. However, weed seed soil bank changes are unknown for these less erosive production systems. A 5-yr multi-/interdisciplinary, long-term, field study was initiated in 1995 to evaluate the agronomic, economic, and environmental feasibility of no-till spring cropping systems to replace or supplement the century-old winter wheat/fallow production system. The study was conducted in two adjacent fields and compared a winter wheat-reduced tillage fallow system to three no-till cropping systems. Weed management strategies targeted downy brome – a winter annual grass weed. Eleven weed species were identified from the soil seed bank assessments with downy brome being the most prevalent. Initial downy brome weed seed densities ranged from a high population of 85 seeds m-2 in the west field to 5,205 seeds m-2 in the east field. The majority of weed seeds were found at the 0-8 cm depth compared to the 8 to 15 cm and 15 to 23 cm depths. By utilizing a combination of mechanical, cultural and chemical weed management options, downy brome weed seed populations were reduced to almost 0 seeds m-2 on the east side plots in all cropping systems except spring wheat/chemical fallow (105 seeds m-2) after 5 years. In contrast, when postharvest disking was not conducted on the west side in the winter wheat/fallow rotation, downy brome weed seeds in the soil increased to 5,460 m-2 by the year 2000. The next most prevalent weed seed in the soil was slimleaf goosefoot – a summer annual broadleaf weed, followed by tumble mustard and flixweed. Preventing weeds from producing seed, both during the crop-growing and summer fallow seasons, should be the main focus for successful weed management strategies for PNW conservation tillage systems.
Technical Abstract: Weed management is an important consideration in implementing new cropping systems. Grower interest in no-till spring cropping systems is increasing in the semi-arid region of the Pacific Northwest because of wind erosion from winter wheat (Triticum aestivum L.)/tillage fallow (WWF). However, no-till spring crops and conservation tillage fallow represents a major shift in production practices that may affect weed management challenges. A 5-yr study was initiated in 1995 to develop no-till spring cropping systems and to examine the associated weed management strategies’ effect on weed seed in the soil. 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 seed in the soil was sampled at three depths in 1995 for baseline data and every year (1996-2000) after harvest. The two most frequently occurring and abundant species in this study were downy brome (Bromus tectorum) – a winter annual grass weed, and slimleaf goosefoot (Chenopodium leptophyllum) – a summer annual broadleaf weed. Of the 11 prevalent weed species, downy brome was the only grass weed, and the other 10 species were either winter or summer annual broadleaf weeds. Initial weed seed density was higher in the east field compared to the west field because of previous cropping histories. Weed management for east WWF was more intense than for west WWF. Downy brome seed in the soil was reduced from 5,205 seeds m-2 to almost 0 seed m-2 in 5 yrs in the reduced tillage fallow on the east side as well as continuous no-till spring cereals. On the west side, downy brome weed seed concentration in the soil was 85 seeds m-2. However, in 5 yrs downy brome weed seed populations increased to 5,460 seeds m-2. A combination of cultural, chemical, and timely mechanical control strategies reduced downy brome populations on the west side. In no-till rotations, weed populations decreased because of late-winter herbicide control of winter annual weeds, in-crop herbicide control for summer-annual dicot weeds, and postharvest herbicide applications to control Russian thistle (Salsola spp.)