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Nitrate Levels in Vegetative and Floral Tillers of Western Wheatgrass and Green Needlegrass as Affected by Nitrogen Fertilization¹

In the northern Great Plains, nitrogen fertilization is one means of increasing forage production in the spring during the cow-breeding season. Our objective was to determine how much N can be applied without causing toxic quantities of nitrate-N (>2,080 ppm) in the forage. In late March 1973, ammonium nitrate was applied at 0, 40, 80, 160, 320, and 640 kg of N/ha to adjacent seeded fields of ‘Rosana’ western wheatgrass (Agropyron smithii Rydb.) and ‘Lodorm’ green needlegrass (Stipa viridulu Trin.). Plant nitrate levels were determined on 10 and 11 dates during 1973 and 1974, respectively, from late April through October. There was almost a complete removal of soil nitrate from the top 30-cm of soil by both grasses and a marked accumulation in the 30 to 60-cm soil depth on N fertilized plots. About 40 kg of N/ha was mineralized each year in the check plots. About 160 kg of the 640 kg of N applied in March 1973 could not be accounted for 2 years after application. N fertilization did not increase forage production in either 1973 or 1974, but added N caused a linear increase in nitrate-N of both vegetative and floral tillers of both species both years. Floral tillers of green needlegrass fertilized with 640 kg of N/ha contained toxic quantities of nitrate during June and July 1973, but not in 1974. Neither vegetative nor floral tillers of western wheatgrass fertilized with N rates up to 640 kg/ha contained toxic quantities of nitrate at any sampling date during either year. Floral and vegetative tillers of green needle grass contained 500 to 1,000 ppm more nitrate-N than did those of western wheatgrass in both 1973 and 1974. If green needlegrass has been fertilized with more than 320 kg of N/ha, it should be tested for toxic quantities of nitrates before it is grazed by or fed to calves.

Key Words: Forage yield • Soil water • Downy brome • Japanese brome • ‘Rosana’ wetem wheatgrass • ‘Lodorm’ green needlegrass • Nitrate poisoning • Soil nitrate • Evapotranspiration

² Range scientist and soil scientist, USDA-SEA, AR, Northern Plains Soil and Water Res. Ctr. P. O. Box 1109, Sidney, MT 59270.

Received for publication May 29, 1979.