Herbicides, Dryland Farming Practices and Research on Seedbanks for Weed Management
Each year herbicides are applied to over 90% of the major field crops in the USA, and atrazine alone is applied to approximately 70% of the US corn crop (USDA, 2000b). This constitutes a substantial production cost. Management of weeds in agricultural fields with herbicides is under intense public scrutiny because of their potential negative environmental effects (especially on water resources). Public safety and ecological concerns have resulted in the banning of some pesticides (Moffat, 2001).
Herbicides, more than any other type of pesticide, are being found in water quality studies and atrazine is often the principal contaminant (USDA, 2000a). In addition, the development of resistance by numerous weed species to acetolactate synthase (ALS) inhibitors is a major concern. ALS is the target site for 44 different herbicides within 5 different chemical families (Mallory-Smith and Retzinger, 2003). In the MonDak region, ALS resistant kochia readily outcompetes seedling sugarbeets and other crops (Mesbah et al. 1994; Thompson et al. 1994; Weiner and Fishman, 1994). Other weeds such as green foxtail now have high levels of resistance to ALS inhibitors (Wolf et al. 2000) and other commonly used herbicides (Cranston et al. 2001). Much is known about ALS resistance effects on kochia physiology, genetics and pollen flow (Dyer et al. 1993, Christoffoleti et al. 1997, Gutierri et al. 1992, Gutierri et al. 1995, Mengistu and Messersmith 2002, Stallings et al. 1995), but very little is known about the impact of ALS resistant weeds on sugarbeet production, which relies on a limited number of herbicides. Research has documented that ALS-resistant kochia seeds differ biochemically from non-ALS-susceptible seeds, with resistant accessions having elevated levels of branched chain amino acids (Dyer et al. 1993).
Weed management is the highest cash expenditure in most dryland farm operations in the Northern Great Plains. Wheat-fallow systems under conventional tillage also have significant problems with weeds such as green foxtail (Spandl et al., 1999), wild oat (Lueschi et al., 2001), herbicide resistant kochia (Christoffoleti et al., 1997), and numerous other annuals and perennials (Thompson et al., 1994; Derksen et al., 2002). As farming systems become more complex (zero-tillage, diversification, intensification of crop rotations), herbicide options become fewer and less reliable, further complicating weed management (Anderson, 1999; Anderson et al., 1998).
Weed management is also the single greatest impediment to the adoption of dryland conservation (low-till or no-till) farming systems in the MonDak and Great Plains regions (Vigil and Nielsen, 1998; Nielsen, 1998; Norwood, 1999). A crucial topic with minimum till integrated crop production systems (ICPS) is the evaluation of ecological relationships between the target crops and weeds as a basis for developing ecologically safe (cultural and biological) and affordable weed control practices. Farming practices such as different planting dates (Spandl et al. 1999) and fertilization practices (Anderson et al., 1998; Blackshaw et al., 2003) can increase a crop’s ability to out compete weeds, increase water-use efficiency, and improve crop yield and quality. Therefore, new and less problematic long-term solutions are being sought.
One approach is to develop and integrate cultural and biological weed management options (Anderson, 1999; Blackshaw et al., 2003) into viable farming systems (Derksen et al., 2002) for the MonDak region. This will require an ecological understanding of weed/crop competition and biologically based integrated pest management (BIPM.) However, much of this work is still in the developmental stages (Holtzer et al 1996; Derksen, et al., 2002).
Recent research stresses crop cultural management and crop diversity as important components in controlling weeds (Derksen, et al., 2002). Additional studies have shown three or more crop cultural practices (e.g. seeding rate, modified row spacing, and crop variety selection), when used together, can greatly assist in managing weeds and reducing herbicide use (Anderson, 1999; Anderson, 2000; Anderson et al., 1998; Nielsen and Anderson, 1993; Westfall et al., 1996; Tanaka et al., 2002).
A limited amount of research has focused on the soil weed seedbank and its response to different management strategies, but it is known that weed seedbanks are influenced by crop rotation (Entz et al. 1995; Kegode et al. 1999) and tillage systems (Webster et al. 2003). Proper soil management can be used to increase the percentage of weed seeds that germinate and decrease the time period of weed emergence (Larson et al. 1958; von Polgár 1984, as cited in Hokansson 2003), thus reducing the number of times herbicides need to be applied and the timeframe (years) needed to effectively control many annual weeds.
New soil/seed sampling approaches are now available that make it possible to process sufficient samples to determine the influence of ICPS on weed seedbanks and monitor weed population dynamics (Derksen et al 1998). These tools increase our understanding of soil seedbank dynamics, which in turn aids in the development of farming practices capable of controlling weeds without a disproportionate reliance on pesticides.
Contributing Scientists: Andrew Lenssen (Weed Ecologist), Upendra Sainju (Soil Scientist), Jed Waddell (Soil Scientist) and TheCan Caesar-TonThat (Microbiologist) Latest Research Findings/Reports Preplant Weed Management and Planting Date Influence Barley Forage Yield and Quality
Latest Research Findings/Reports
Preplant Weed Management and Planting Date Influence Barley Forage Yield and Quality