Better Crops/Vol. 98 (2014, No. 3) 21 MIDWEST UNITED STATES P roduction of flax (Linum usitatissimum L.) has increased in North America over the past decade, due to markets driven by the recognition of the potential health benefits of including flax seed in the human diet. Knowledge of ap- propriate conventional and organic production practices for production in the Midwest, U.S. is lacking, as much of the research in North America originates in North Dakota, Mani- toba, Saskatchewan and Alberta on the Northern Great Plains. Midwestern research is necessary to develop N recommenda- tions for flax growers in this region. Additionally, adequate management of weeds is of particular concern for growers of flax due to its lack of competitiveness (Franzen, 2004). Researchers evaluated the response of flax to three N sources applied at three N rates in the presence and absence of weed competition. This study occurred in 2007 and 2008 at the Iowa State University Agronomy and Agricultural En- gineering Farm located near Ames in central Iowa. In both years, the experiment took place in fields planted to soybeans the previous year. Composted swine manure, fresh liquid ma- nure, and urea were surface applied by hand to plots at 0, 30, 60, and 90 lb N/A and immediately incorporated into the soil. Nutrient analyses and applied rates of compost and manure are provided in Table 1. Each N source x rate combination was replicated four times. Flax was drilled into plots at 8-inch row spacings at 50 lb/A accompanied by red clover (Trifolium pratense L.) broadcasted at 15 lb/A. Flax was underseeded with red clover to mimic a common management strategy used for production of small grains. Plots were split into randomized subplots that either contained uncontrolled weeds (‘weedy’) or were weed-free. Throughout the growing season, weeds were removed by hand as needed from weed-free subplots. At flax seed maturity in late July, aboveground shoot material of flax, red clover, and weeds was hand-harvested at the soil surface from four randomly located 1 ft 2 areas in each subplot. The linolenic acid (omega-3 fatty acid) concentration of seed oil was assessed upon harvest. Impact of Weed Competition and N on Flax In both 2007 and 2008, flax seed yield was significantly reduced by weed competition regardless of N source or rate applied. In 2007, mean seed yield was 709 and 995 lb/A under weedy and weed-free conditions, respectively. In 2008, mean seed yields under weedy conditions were 197 lb/A compared to 385 lb/A under weed-free conditions. Nitrogen source was not a factor in either year. Under weed-free conditions, seed yield responded to N rate in 2007 and 2008 (Figure 1). Re- gardless of N source, seed yield response to each incremental increase in N rate under weed-free conditions was linear (p = 0.05) with greatest yields occurring at the 90 lb N/A rate. Under weedy conditions, maximal seed yields were observed at the 60 lb N/A rate in 2007 and no response to N rate was By Stefan R. Gailans and Mary H. Wiedenhoeft Expansion of flax into the Midwestern U.S. has created a gap in regionalized knowledge on N source and weed manage- ment for this crop. Recent experiments in central Iowa found good responses across selected N sources, but results varied between the two very distinct growing seasons. Composted manure had the largest impact on reducing harvest index relative to other N sources in the initial year, but not the following, more challenging growing season. Weed competition had the most pronounced effect on flax yields compared to the effects of N source and rate. Weed biomass also increased with N rate, emphasizing the need for effective weed management in flax production systems. Adapting Management of Nitrogen Sources and Weeds in Flax Systems of Central Iowa Abbreviations and notes: N = nitrogen. Table 1. Nutrient analyses of composted swine manure and liquid swine manure and the amount applied to meet target N rates in 2007 and 2008. Ames, Iowa. Nutrient analysis Target N rate, lb N/A Total N Total C Moisture 30 60 90 Compost - - - - - - - - % - - - - - - - - ton/A 2007 2 19 31 7 14 21 2008 2 34 77 12 25 37 Manure lb/1,000 gal % gal/A 2007 28 - 98 1,053 2,000 3,053 2008 26 - 96 1,158 2,316 3,474 Amount of compost and manure applied was based on the availability of 10% of the total N in the compost and 98% of the total N in the manure. Mr. Stefan Gailans, Iowa State University.