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Release of Soluble Protein and Nitrogen in Alfalfa. I. Influence of Growth Temperature and Soil Moisture¹

The influence of climatic and soil factors during growth on release of soluble protein from alfalfa (Medic ago saliva L.) has not been studied. Herbage proteins soluble in sodium male ate buffer (pH 6.9) have been proposed as possible estimates of ruminant bloat potential in comparative studies. Our objectives were to determine whether growth temperature or soil moisture availability at different growth temperatures would influence concentration of several N fractions released from alfalfa.

‘Ramsey’, ‘Saran ac’, and ‘Desert’ alfalfa cultivates were grown at 18/10, 26/18, and 34/26 C (day/night) with a 16 hour photo period (3 x 3 factorial). Total (TN), total soluble N (TAN), soluble non protein (SPN), and soluble protein N (SPN) were determined.

As the temperature increased, the mean concentration of TN, TAN, and SPN increased in leaves, stems, and total forage. The greatest concentration of SPN in leaves occurred at 26/18 C ( of 11.5 mg/g dry wt), compared to 18/10 C ( of 9.9) or 34/26 C ( of 10.4). The mean SPN concentration in stems increased as temperature increased from 2.7 mg N/g dry wt at 18/10 C to 3.0 mg at 26/18 C and 3.7 mg at 34/26 C. The total forage of any cultivate grown at any temperature contained approximately 6.6 mg SPN/g dry wt, except Desert grown at 18/10 C contained 5.6 mg SPN/g dry wt.

We conclude that the apparent bloat potential, estimated as SPN release, of alfalfa leaves of all cultivates grown at 26/18 C was greater than that of leaves grown at 34/26 and 18/10 C. Little difference in apparent bloat potential occurred in total forage regardless of cultivate or temperature during growth.

Ramsey and Saran ac alfalfa were grown at 18/10 and 26/18 C with a 16 hour photo period. Three soil moisture regimes (R) were imposed at each temperature; soil matrix potentials between –0.1 to –0.5 (R I), –1.0 to –6.0 (R 2), and –6.0 to –15.0 bars (R 3) were established before the soil was restored to field capacity (2 x 3 factorial within each temperature).

With a few exceptions, the concentrations of the N fractions in leaves, stems, and total forage of both cultivates generally increased as the soil moisture content decreased from R 1 to R 3 at each temperature. The leaves, stems, and total forage of both cultivates grown at 26/18 C contained greater concentrations of all N fractions than did those of cultivates grown at 18/10 C, which confirmed results of Experiment I.

Apparent bloat potential, as estimated by SPN release, of leaves, stems, and total forage increased slightly with moistur6 Stress at 18/10 C. At 26/18 C, apparent bloat potential only of stems increased with moisture stress. Insofar as moisture stress occurs in practice more often at warm temperatures, it is not likely to influence alfalfa bloat potentially instigated by soluble protein release.

Key Words: Medicago sativa L. • Total nitrogen • Total soluble nitrogen • Soluble nonprotein nitrogen • Moisture stress • Legume pasture bloat • Alfalfa leaf to stem weight ratio

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.