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Release od Soluble Protein and Nitrogen in Alfalfa. III. Influence of Shading¹

Changes in leaf anatomy, morphology, and chemical composition due to different amounts of solar radiation could affect the apparent bloat potential of alfalfa (Medicago sativa L.) mediated via release of soluble protein into rumen fluid in the presence of ample available carbohydrates. Our objectives were to determine whether shading during growth would influence production characteristics as well as concentration of several N fractions and total nonstructural carbohydrates (TNC) released from alfalfa.

Alfalfa was shaded with saran shades [0 (control), 47, and 73% shade] from 12 May to 1 June [Period (P)l], 30 June to 7 July (P2), and 9 to 16 August 1978 (P3). Total nitrogen (TN), total soluble N (TSN), soluble nonprotein N (SNPN), soluble protein N (SPN), total nonstructural carbohydrates (TNC) were determined.

The 73% shade reduced yields 42% for P1, 30% for P2, and 24% for P3, while the 47% shade reduced yields 29% for P1, 22% for P2, and 10% for P3 compared to the respective control yields. In P1 (20 days of shade), concentration of SNPN in herbage was 10.1 mg/g dry wt for control, 10.5 mg for 47% shade, and 11.5 mg for 73% shade, while that of SPN was 14.8 mg for control, 13.8 mg for 47% shade, and 12.6 mg for 73% shade.

In P2 (7 days of shade), concentrations of TN, TSN, and SNPN in herbage increased as shade increased. Shade did not significantly influence the herbage concentrations of SPN in P2. In P3 (7 days of shade), shade did not significantly affect herbage concentrations of any of the N fractions. Shade of 73% reduced the TNC concentrations in herbage 16% for P1, 44% for P2, and 19% for P3, while shade of 47% recuded TNC 8% for P1, 28% for P2, and 10% for P3, compared to that in control herbage.

Reduced apparent bloat potential of alfalfa, as estimated by SPN release, occurred only in plants grown under low solar radiation for a continuous 20-day period. However, both short and long periods of decreased solar radiation caused decreased TNC concentrations in herbage. Because an abundant supply of TNCis essential for the microbial activity that leads to gas production in the rumen, even relatively short periods of shade could be considered as potentially bloat suppressing. We conclude that the usual incidence of cloudy weather is not likely to be a major mediator of bloat incidence in ruminants that graze alfalfa, although periods of high solar radiation maybe considered bloat-promoting rather than bloatsuppressing.

Key Words: Medicago sativa L. • Total N • Total soluble N • Soluble nonprotein N • Soluble protein N • Total nonstructural carbohydrates • Rate of forage digestion • Legume pasture bloat

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

Received for publication June 30, 1980.

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