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Growth and N Fixation of Some Tropical Forage Legumes as Influenced by Solar Radiation Regimes¹

Decreased solar radiation due to cloud cover or shading by plantation crops or associated grass can severely limit the production of tropical forage legumes. We therefore evaluated the response of six legumes (three replicated and three not replicated) to four radiation regimes (100, 70,45, and 27% of unshaded solar radiation, hereafter termed "full sun") with polypropylene netting in the field.

The three replicated legumes had significant yield reductions at 27% full sun, with intermediate reductions at 70 and 45%. Dry matter (DM) yields at full sun [metric tons (mt) ha,^–1yr^–1 and proportional yields at 27%] were: Desmodium intortum cv. Greenleaf (20.0, 46%); Centrosema pubescens ‘centro’ (13.7, 44%); and Macroptilium atropurpureum cv. Siratro (12.9, 20%). Greenleaf was relatively tolerantof moderate shading; proportional yields at 70 and 45% of full sun were 93 and 75%, respectively. Centro and Siratro yields declined linearly as shortwave radiation (hereafter termed "radiation") decreased, but Siratro yields declined significantly more than centro. Marked seasonal differences were noted in the response of the legumes to shade, and this was attributed to differences in the ability of the legumes to utilize solar radiation during periods of cool temperatures. The three non-replicated legumes were evaluated similarly, except that yields were adjusted for replication effects. Dry matter yields and proportional yields at 27% full sun were: Leucaena Ieucocephala cv. Hawaiian Giant (23.5, 40%); Stylosanthes guianensis cv. Schofield (17.0, 17%); and Desmodium canum ‘kaimi clover’ (12.2, 32%). Hawaiian Giant and Greenleaf yielded similarly at 45 and 27% full sun. Kaimi clover DM yield tended to be slightly higher (13.8) at 70% full sun, but thereafter yields declined linearly with reduced radiation. Schofield stylo was the most sensitive to shading.

Dry matter and N concentrations were not significantly elevated by reduced radiation or cool weather except that N increased during the cool season. Concentrations of N differed among species being highest for centro and Hawaiian Giant (3.4%) and lowest for kaimi clover (2.6%). Total N yields were associated with DM yields. Nitrogen yields (kg N ha^–1yr^–1) of replicated legumes at full sun and proportional yields at 27% full sun were: Greenleaf (540, 45%), centro (461,44%), and Siratro (362,27%); and for the non-replicated legumes, Hawaiian Giant (751,38%), Schofield (496, 18%), and kaimi clover [340 (361 at 70% full sun), 38%]. Acetylene reduction rates by nodules in soil cores were highly correlated with radiation regimes (r = 0.92-0.995; except for centro, r = 0.71.) Correlation of acetylene reduction rates with DM yields ranged from 0.80 to 0.996.

The higher yielding legumes, Greenleaf, centro, and Hawaiian Giant (plus kaimi clover for 70 % full sun only) appear well-suited for areas of low solar radiation because they have relatively constant concentrations of DM and N and fixed significant quantities of N even under dense shade.

Key Words: Legumes • Solar radiation • Shading • Acetylene reduction • plant height • Nitrogen fixation • Nitrogen yield • Dry matter content • Desmodium canum • Desmodium intortum • Centrosema pubescens • Leucaena leucocephala • Macroptilium atropurpureum • Stylosanthes guianensis

² Former research assistant and agronomist, respectively, Dep. of Agronomy and Soil Science, Univ. of Hawaii, College of Agriculture and Human Resources, Honolulu, Hawaii, College of Agriculture and Human Resources, Honolulu, Hawaii 96822. Present address of senior author: Malling Agricultural College, 8340 Malling, Denmark

Received for publication October 27, 1980.

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