The agronomic and economic performance of flax in Iowa
Demand for flax (Linum usitatissimum L.) has increased due to the discovered health benefits of adding flaxseed to human diets. A facility in northwest Iowa that processes organic flaxseed for oil provides a viable market for organic growers in the Midwest. Organic growers are particularly concerned about adequate fertility and weed management when adding flax to their crop rotations. Our objective was to evaluate flax response to N rate and source as well as to ambient weed competition when following either soybean or corn in rotation. The particular response variables investigated were seed yield, straw yield, harvest index, oil concentration of flaxseed, and the linoleic acid and linolenic acid concentrations of flaxseed oil. The experiment was conducted in central Iowa in 2007 and 2008, on land previously sown to either soybean [Glycine max (L.) Merr.] or corn (Zea mays L.). Red clover (Trifolium pratense L.) was underseeded with flax at time of sowing. Nitrogen was applied as composted swine manure, liquid swine manure, or urea. Each source of N was applied at a rate to supply 30, 60, and 90 lb N acre-1. A control where no N was applied (0 lb N acre-1) was also included. Half of each plot was hand-weeded while ambient weed growth was allowed in the other half. Across both years, competition from weeds reduced seed yields by 32% to 96% compared to when weeds were removed. Weed competition reduced straw yield except in 2008 in the field previously sown to corn. In 2007, seed and straw yields increased with N rate, when weeds were removed. Response of seed yield was dependent on N source in the field previously sown to corn but not soybean. Response of seed and straw yield to N was reduced when weeds were not removed. No response of seed or straw yield to N was observed in 2008 regardless of weed competition, likely a result of late planting due to wet field conditions that year. The harvest index was reduced with increased N in 2007 in the field previously sown to soybean, indicating that N increased straw yield disproportionately to seed yield. Competition from ambient weeds reduced oil concentration, linoleic acid concentration, and linolenic acid concentration only in the field previously sown to soybean. Oil concentration of flaxseed was reduced by 0.8% when weeds were not removed. Nitrogen had no effect on oil concentration of flaxseed. Linoleic and linolenic acid concentrations of flaxseed oil were reduced with increasing N, particularly when weeds were not removed. Results indicate that sufficient weed management is necessary for maximum seed yield of flax. Sufficient weed management is also necessary when maximizing seed quality factors such as oil, linoleic acid, and linolenic acid concentrations.