Asparagus is a perennial crop that normally is established from one-year-old crowns and maintained for 12 to 20 years until the yield declines beyond profitability (Zandstra et al., 1992). Before introduction of all-male hybrids, volunteer asparagus was a serious weed problem. Traditional asparagus production included one or two tillings of asparagus fields each year for weed control and to incorporate previous year crop residue. However, it has been demonstrated that tillage reduces yield (Wilcox-Lee and Drost, 1991). In the 1970s, most Michigan growers converted to nontillage production of asparagus (Putnam, 1972; Putnam and Lacy, 1977).
Perennial weeds such as canada thistle (Cirsium arvense) and field bindweed (Convolvulus arvensis) often become serious problems in asparagus (Ogg, 1975). Other common biennial and perennial weeds in asparagus are quackgrass (Elytrigia repens), dandelion (Taraxacum officinale), common milkweed (Asclepias syriaca), spotted knapweed (Centaurea maculosa), and wild carrot (Daucus carota). Herbicide resistance in annual weeds is always a potential problem, and resistance to herbicides targeting photosystem II (PS II) has been confirmed for redroot pigweed (Amaranthus retroflexus) and powell amaranth (Amaranthus powellii) in Michigan asparagus fields (Heap, 2012). Other common annual weeds found in asparagus fields include field sandbur (Cenchrus incertus), large crabgrass (Digitaria sanguinalis), fall panicum (Panicum dichotomiflorum), common lambsquarters (Chenopodium album), russian thistle (Salsola iberica), horseweed (Conyza canadensis), hairy vetch (Vicia villosa), and common groundsel (Senecio vulgaris) (Zandstra et al., 2010).
Asparagus weed control programs have been based on herbicides that inhibit PS II for over 50 years. Simazine and monuron were registered for asparagus in the 1960s, followed by diuron, linuron, metribuzin, and terbacil (Boydston, 1995; Welker and Brogdon, 1972). Now, most fields in Michigan are treated with one or more PS II inhibitors each year (Zandstra, 2011). Several other preemergence herbicides have been registered for asparagus in recent years, including flumioxazin, halosulfuron, S-metolachlor, mesotrione, sulfentrazone, and pendimethalin.
In the past several decades, the life span of asparagus fields has been shortened by five to eight years (Morrison et al., 2011). This shortened life span has been largely attributed to the effects of the fungal pathogens Fusarium proliferatum and Fusarium oxysporum, which infect asparagus and produce soil toxins that inhibit asparagus growth (Keulder, 1999; Morrison et al., 2011). The application of herbicides has been suggested as a stressor that may facilitate Fusarium infection (Morrison et al., 2011) potentially leading to decreased yields. In addition, growers in Michigan have reported the development of lesions on spears from fields treated with flumioxazin that may be related to yield decline (J. Bakker, personal communication).
Welker and Brogdon (1972) conducted research on long-term effects of repeated herbicide applications on asparagus production and quality over a seven-year period. At the time of their research, there were few herbicides registered for weed control in asparagus. They concluded that some of the herbicides might have an adverse effect on spear quality but did not affect total yield.
Because the primary mechanism of preemergence herbicide selectivity in asparagus is differential placement (Monaco et al., 2002), there is a potential for crop injury if the herbicides leach into the crop root zone after heavy rainfall or flooding. This is a serious potential problem on light sandy soils, which are typical of Michigan asparagus growing regions. Several of the preemergence herbicides labeled for asparagus fields are moderately or highly soluble, so there is potential for crop injury after repeated use of these herbicides.
This experiment was established to determine if there are adverse effects on asparagus yield from single spring applications of the same preemergence herbicides repeated for seven years.
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