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Development of improved weed manage ment systems requires more knowledge on how various weed species respond to changing agronomic practices. A long-term study was conducted to determine weed population responses to various tillage intensities and crop rotations in a winter wheat (Triticum aestivum L.) dominated cropping system. Weed density and species composition differed with tillage, rotation, year, and date of sampling within years. Weed community dynamics were most affected by year-to-year differences in environmental conditions, followed by crop rotation, and then tillage intensity. Russian thistle (Salsola iberica Sennen & Pau) and kochia Kochia scoparia (L.) Schrad. densities increased in years of low rainfall and above average temperatures. Winter annual weeds such as downy brome (Bromus tectorum L.) and flixweed Descurainia sophia (L.) Webb ex Prantl, as well as the perennial weed dandelion (Taraxacum officinale Weber in Wiggers), increased in years where higher than average rainfall was received in fall or early spring. Continuous winter wheat facilitated a dense downy brome infestation to develop over time. Trifluralin is not efficacious on stinkweed (Thlaspi arvense L.) or Canada thistle Cirsium arvense (L.) Scop. and its use in canola resulted in an increase in these species in a winter wheat-canola rotation. Total weed densities were often greater in zero tillage than in either minimum or conventional tillage. Russian thistle, downy brome, kochia, and redroot pigweed (Amaranthus retroflexus L.) were associated with zero tillage while wild buckwheat (Polygonum convolvulus L.), lamb’s-quarters (Chenopodium album L.), flixweed, and wild mustard (Sinapis arvensis L.) were associated with conventional tillage. Perennials such as dandelion and perennial sowthistle (Sonchus arvensis L.) were associated with zero tillage but Canada thistle was associated with conventional tillage. Information will be utilized to implement more effective weed management programs in winter wheat production systems. Key words: Conservation tillage, fallow, multivariate analyses, weed populations, weed shifts, zero tillage
Blackshaw et al. (Mon,) studied this question.