The Effects of Brassica Cover Crops on Weed Dynamics

Abstract

Concerns about the health and environmental impacts of herbicide use have led farmers and researchers to seek alternative methods of weed management. One such alternative is the use of allelopathic cover crops, which release chemicals into the soil environment that can contribute to weed management through suppression of weed seed germination, seedling emergence and establishment, and seedling growth, in addition to providing other soil quality benefits to farmers. Brassicas, or members of the Brassicaceae, contain glucosinolates, sulfur-containing molecules that degrade to form compounds toxic to a variety of organisms. Glucosinolate content varies between brassicas grown as cover crops; mustards typically have very high glucosinolate content, whereas others, including canola and rapeseed, contain lower amounts. Brassica residues have strong phytotoxic effects; weed density and biomass has been observed to be lower in crops following incorporation of brassica residue. In addition to a literature review, this thesis describes two experiments that investigated the mechanisms behind this observed weed suppression by brassicas. The first experiment examined seedling establishment following incorporation of brassica residues and residues of other short-season cover crops. We hypothesized that all cover crop residues would decrease and delay seedling emergence compared to fallow and that emergence following the brassica residues would be lowest and slowest, particularly following the high-glucosinolate mustard. Based on the theory that smaller seeds are more susceptible to stress such as those imposed by allelopathic cover crops, we expected that smaller-seeded species would be affected more than larger-seeded ones. While average emergence was indeed lower following all of the cover crop residues compared to fallow, emergence was similar following the brassicas and following the other cover crop residues. Seed size was a poor predictor of species emergence following the cover crops. While emergence was slower following all the cover crop residues, emergence rates were generally similar between the brassicas and the other cover crops, although delayed emergence following mustard was observed in one year. The second experiment examined the effects of mustard and canola on the growth of established redroot pigweed and green bean plants and, in turn, on interspecific competition. Redroot pigweed and green bean, both alone and in mixture, were seeded into incorporated mustard and canola residues and fallow plots; plants were sampled throughout the season to assess the effects of the cover crop residues on growth. We hypothesized that growth of the smaller-seeded redroot pigweed would be harmed by the brassica residues, particularly early in the season, but that growth of the larger-seeded green bean would not be affected. In turn, we expected that the brassica residues would mediate interspecific competition, through differential effects on growth, thereby conferring a competitive edge to green bean plants growing with redroot pigweed. As expected, brassica cover crop residues did not affect green bean height, biomass, leaf area, relative growth rate, or final marketable yield. Contrary to expectations, these parameters for redroot pigweed were also unaffected. While the presence of competition reduced growth of both species, no interaction with cover crop residue was found.

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