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Nitrogen and Rhizobium Effects on Establishment of Legumes via Strip Tillage¹

The success of interseeding pastures with forage legumes via striptillage is limited by the rate and degree of seedling establishment. Before biological nitrogen (N) fixation is initiated, the legume plant must depend on combined N from the soil or seed for establishment. Soils under permanent grass sod are known to contain very low levels of combined available N.

Field studits were initiated to determine the effects of seed rhizobium inoculum level and N fertilizer level on the establishment of three forage legumes introduced into grass pastures via striptillage. 'Ramsey' alfalfa (Medi cago sativu L.), 'Arlington' red clover (Trifolium patense L.), and 'Carroll' birdsfoot trefoil (Lotus corniculatus L.) were interseeded into pastures at two locations (L 1 and L 2) with differing soils (Typic Haplaquoll and Typic Hapludoll, respectively). Location 1 had predominantly Kentucky bluegrass (Pou pratensis L.) , and L 2 had a mixture of Kentucky bluegrass and smooth bromegrass (Bromus inermis Leyss.). The grass was temporarily suppressed by a broadcast application of glyphosate [N-(phosphonomethyl)glycine] at 0.84 kg/ha. Inoculum treatments consisted of 4,000 and 40,0000 rhizobia/ seed. The four fertilizer levels applied at seeding were 0, 30, SO, and 90 kg N/ha.

Legume establishment was not affected by seed rhizobium inoculum level. Seedling vigor, first cutting leg ume, and total herbage yields increased significantly with added N fertilizer at both locations. Legume percentage in the sward followed a quadratic pattern with N level in the second cutting of the first year at L 2 and in the first cutting of the second year at L 1; legume percentage inmeased at the intermediate N levels and decreased at the highest N level in these cases. Increasing the N level resulted in either negative linear or quadratic changes in crude protein concentration in the 1st year, depending on location. Neutral detergent fiber concentrations in the total forage increased linearly with inoreased N at both locations in the first year. Herbage in vitro dry rnatter digestibility concentration was quadratically influenced by N level in the regrowth of the first year only at L 1.

We conclude that the application of 30 to 60 kg N/ha before interseeding legumes into a grass sod having low soil N availability may enhance early legume seedling growth and increase the initial legume contribution to the sward herbage yield, as well as total herbage yield.

Key Words: Legume Interseeding • Alfalfa (Medicago sativa L.) • Red clover (Trifolium patense L.) • Kentucky bluegrass (Poa pratensis L.) • Smooth bromegrass (Bromusinermis Leyss.) • Glyphosate • Pasture

² Research assistant (now at Iowa State Univ.); associate professor, Dep. of Agronomy and Plant Genetics, Univ. of Minnesota, St. Paul, MN 55108; and research agronomist, USDA-SEA.4R and professor, Dep. of Agronomy and Plant Genetics.

Received for publication October 26, 1979.

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