Agronomic practices for nitrogen fixation in alfalfa seed production and the establishment year
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Alfalfa (Medicago sativa L.) seed production in western Canada involves establishing the crop during the first year and harvesting seed during the subsequent years. Therefore, seed production has been restricted almost exclusively to winterhardy, fall dormant cultivars. Moderately dormant and non-dormant cultivars rarely survive the winter in western Canada, therefore seed production from these cultivars may not be possible under current management practices. The development of a production system that allows seed production during the year of establishment is required if these cultivars are to be grown for commercial seed production in western Canada. There is a lack of information on N2-fixation and the agronomic practices for establishment year seed production. Field experiments were conducted in 1992 and 1993 to determine how cultivar, seeding rate and stage of plant development affect seed yield, dry matter production and N2-fixation. An additional experiment was designed to determine the effect of cultivar, seeding rate and clipping management on alfalfa seed yield components when managed for establishment year seed production. The first experiment was established in a split plot design with five alfalfa cultivars: Algonquin, Saranac, Saranac-In, Nitro and CUF 101 at two seeding rates (3.36 kg ha-1 and 16.8 kg ha-1 ). N2-fixation was determined using the difference method on intact excavated plants at different stage of plant development. A wheat crop was seeded over the 1992 experimental plots to estimate the actual N contribution to subsequent crops. Results from the first experiment showed that the high seeding rate showed better agronomic production than the low seeding rate for most of the traits measured including seed yield. The dormant cultivar, Algonquin, and the moderately dormant cultivar, Saranac, had the highest seed yield. The total N yield ranged from 97 to 139 kg N ha-1 and root plus crown N yield ranged from 42 to 60 kg N ha-1 during the mature seed stage. During both years all cultivars had an equivalent quantity of N2-fixed. Furthermore, depending upon stage of growth, N2-fixed comprised 37% to 59% of the total N in the crop, corresponding to 23 kg N ha-1 to 74 kg N ha-1 • The average biomass in the root plus crown portion at the mature seed stage ranged from 1,841 to 2,485 kg ha-1 dry matter, and the root plus crown N yield available for fall incorporation was 44 to 66 kg N ha-1 • Wheat yield was not increased by planting it following any of the alfalfa cultivars at either seeding rate. The second experiment was established with three cultivars (CUF 101, Cimarron VR, and Algonquin) at two seeding rates, 1.12 kg ha-1 and 3.36 kg ha-1 and subjected to two clipping treatments (clipped and unclipped). This experiment showed that flowering percentage and racemes per metre of row were highest for Cimarron VR. Furthermore, there was no influence of clipping treatment or seeding rate on plant height, number of racemes per metre of row and number of pods per raceme and seed yield. Clipping reduced lodging, but also delayed flowering. Cultivars produced similar seed yields, but the overall yield was much lower than long term averages in Manitoba. Seed yield components are predictors of the potential seed yield. Measurements of seed yield components was important in both years of this study because adverse environmental conditions directly influenced pollination, seed set, seed development, seed maturation and final seed yield. In conclusion, the adverse environmental conditions during 1992 and 1993 affected N2-fixation and seed production. N2- fixation continued during alfalfa seed production and provided supplemental nitrogen to subsequent crops. Although establishment year seed production was low in this research, actual yields were equivalent to the provincial average on established stands in 1992 and 1993. Therefore, further research should be conducted to determine if establishment year seed production is possible in western Canada.