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Doctor of Philosophy




There has been increasing interest in alfalfa (Medicago sativa L.) as a source of nitrogen because of increased cost and scarcity of fossil fuel used in the production of commercial nitrogen fertilizer. It is not known if the recommended practices geared to maximum yield and quality in alfalfa also affect favorably the rate of dinitrogen fixation and nitrogen percentages in that species;A field study was initiated in August of 1978 using two cultivars of alfalfa, 'Agate', fall dormant and winterhardy, and 'WL 318', moderately fall dormant and moderately winterhardy. Seeding rates were 6.7, 13.5, and 26.9 kg/ha. The alfalfa was either left uncut or cut three or four times during the 2 years following seeding (1979 and 1980);Acetylene reduction rates were determined on a per core (soil) basis (ARPC) and calculated on a per plant basis (ARPP) therefrom to estimate rates of dinitrogen fixation. Plant density had no effect on ARPC values in either year. Decreased ARPP values resulted from increased plant density. Cutting the alfalfa caused an initial decrease in ARPC and ARPP values within the first 6 days. Maximum acetylene reduction rates were recorded when plants were in a late vegetative to bud stage of growth. Average ARPC and ARPP values were highest under either a three- or four-cut management system the first year and under the no-cut management system the second year. No significant differences in average ARPC and ARPP values resulted from cutting managements when averaged over both years. Agate had better stand persistance than WL 318, resulting in higher ARPC values but no difference in ARPP values;Total dry matter yields were determined on forage harvested during the growing season and on root mass at the end of each growing season. Percentage nitrogen in the forage and roots was determined and forage nitrogen removal and root nitrogen accumulation were calculated. No differences resulted from plant density in any of the parameters measured on the forage or roots. Forage yield was higher under the three-cut management system in both years. Higher percentage nitrogen in the forage resulted from the four-cut management system in both years. Forage nitrogen removal was higher under the three-cut management system the first year and under the four-cut management system the second year, resulting in no difference when averaged over the 2 years. Agate had a higher forage yield, forage nitrogen percentage, and nitrogen removal in both years as compared to WL 318. Root yield and nitrogen accumulation were higher under the no-cut management system the first year and under the three-cut management system the second year. A higher percentage nitrogen in the roots resulted from the no-cut management system in both years. There were no differences in root dry matter yield, root percentage nitrogen, or root nitrogen accumulation as a result of cultivars used in this study;Data presented indicate that a crop producer can obtain the benefit of residual nitrogen for a following nonlegume crop while simultaneously harvesting alfalfa for forage.



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William H. Bohl



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127 pages