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Release of Soluble Protein and Nitrogen in Alfalfa. II. Infleunce of Potassium and Nitrogen Fertilizer on Three Cultivars¹

Reduced incidence of ruminant bloat was reported on legume pastures fertilized with high levels of potassium (K) or K plus nitrogen (N). Amount of soluble protein released from alfalfa (Medicago sativa L.) into sodium maleate buffer (pH 6.9) was proposed as possible estimate of ruminant bloat potential in comparative studies. Our objective was to determine whether K and N fertilization would influence concentration of several N fractions released from alfalfa.

‘Ramsey,’ ‘Saranac,’ and ‘520’ alfalfa cultivars were fertillzed in the fall with 0 (control), 350, and 700 kg as K₂SO₄/ha and in the following spring with these same levels of N as NH4NO(3₄ 3 x 3 factorial). The upper 10 to 15 cm of told-bud stage alfalfa in first and second crops was analyzed for several N fractions.

As N applied increased, mean concentrations of total N (TN), total soluble N (TSN), soluble nonprotein (SNPN), and soluble protein N (SPN) increasedm first-harvest herbage. Mean concentrations of SPN increased 10.8% at 300 kg N/ha and 13.3% at 700 kg over that in alfalfa not fertilized with N. As the K level increased, mean concentrations of TN, TSN, and SNPN decreased in first-harvest herbage. Fertilization with K did not affect concentration of SPN. Mean concentrations of TN, TSN, and SNPN in Saranac and TN, TSN, and SPN in 520 first-harvest herbage were slightly but significantly greater than those in Ramsey. Cultivars did not affect concentrations of N fractions in secondharvest herbage.

We found no evidence that fertilization of alfalfa with K or N reduced its apparent bloat potential, as estimated by SPN release. Instead, we showed that alfalfa fertilized with high levels of N had a slightly increased apparent bloat potential.

Key Words: Medicago sativa L. • Total nitrogen • Total soluble nitrogen • Soluble nonprotein nitrogen • Soluble protein nitrogen • Legume pasture bloat

² Formerly graduate research assistant (now assistant professor, Dep. of Agronomy, Univ. of Illinois, Urbana, IL 61801; research agronomist, AR. SEA, USDA, and professor of agronomy; and professor of soil science.

Received for publication June 30, 1980.