A planning tool for integrating crop choices with weed management in the Northern Great Plains
Abstract Crop production in the Northern Great Plains is rapidly changing because of no-till practices; producers now grow a diversity of crops with winter wheat to increase production as well as to manage weeds. With the multitude of crops available, producers are asking for guidelines to sequence crops in rotations that help weed management. We developed a planning tool that lists various choices with crops when designing a rotation; the choices are arranged by impact on weed dynamics as quantified by research in the region. The tool includes choices among crops, such as varying crops with different life cycles, planting dates, or row spacing, and choices within an individual crop, such as varying cultivar or planting date. Choices among crops impact weeds the most, whereas choices within an individual crop are less effective and usually lead to lower crop yield. For example, rotations comprised of two cool-season crops followed by two warm-season crops can reduce weed populations six- to 12-fold. In contrast, replacing a standard-height cultivar with a taller cultivar that is more competitive with weeds, is often inconsistent for weed management, whereas crop yield may be less. Producers associated with the Northern Plains Sustainable Agricultural Society felt that the planning tool would encourage long-range planning as well as help integrate weed management with the design of cropping systems. The purpose of the tool is to encourage ecologically based weed management, which can reduce herbicide inputs by 50% for Northern Great Plains producers.
Cultural Systems Can Reduce Reproductive Potential of Winter Annual Grasses
Feral rye and jointed goatgrass are winter annual grasses that infest winter wheat in the western United States. Currently, no herbicides are available that selectively control these weeds in wheat. Because of this constraint, producers need cultural practices that reduce the seed densities of these two grasses in the soil seedbank. This research shows that applying nitrogen 5 mo before wheat planting and increasing the seeding rate with a tall wheat cultivar reduces seed production per plant of either species by > 40%. However, tall wheat cultivars usually yield less grain than semidwarf cultivars. Producers can avoid this yield loss by combining narrow rows with time of N application and increased seeding rate of semidwarf cultivars. This combination reduces weed seed production similarly to cultural systems with tall cultivars.
Chemical and Biological Control of Downy Brome (Bromus tectorum) in Wheat and Alfalfa in North America
As an agrestal weed, downy brome (Bromus tectorum L. # BROTE) is most troublesome in winter wheat (Triticum aestivum L. # TRIAE) and alfalfa (Medicago sativa L. # MEDSA) because of the ability of downy brome to reproduce prior to crop harvesting (70, 76). Geographically, downy brome is most common in the western half of the United States where annual rainfall totals 15 to 55 cm and autumn rainfall ranges from 5 to 12 cm (10, 11, 86, 98). In contrast, cheat (Bromus secalinus L. # BROSE) is more common in the eastern portion of the hard red winter wheat producing regions and in the more humid soft red winter wheat region. Behrendt and Hanf (12) state that cheat has been more common on acid sandy or loamy soils while downy brome commonly occurs on dry, mostly alkaline, sandy to loamy soils. Since soils are frequently alkaline in the relatively dry climate of the western regions (61), it is not clear exactly what role pH plays in affecting geographical distribution of the various Bromus spp. Recent serious problems with downy brome in the New England nursery industry (69) and orchards (57) indicate that downy brome could thrive under a relatively wide range of edaphic and climatic conditions. However, as a weed problem in wheat, the common prevalent conditions of low annual precipitation and alkaline soils should not go unnoticed in the development of chemical weed control practices.