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

Decreased solar radiation due to cloud cover or shading by plantation crops or associatedass 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'yr' 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 tolerant of 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 leucocephala cv. Hawaiian Giant (23.5, 40 %); Stylosanthes guianensis cv gr. 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'yr') 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 (4%,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. Additional index 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. SOLAR radiation is usually high in the tropics and is often the least variable component of year to year differences in climate (Coaldrake, 1964). However, radiation varies with locations, seasons, and days depending on cloud cover. In forage grasslegume communities, competition for light between grasses and legumes is often an important factor influencing yields and stand composition (Santhirasegaram et al., 1966). There is excellent rationale for growing improved legumes and grasslegume mixtures under coconut palms and other plantation crops to suppress weeds, improve soil fertility and provide extra income from livestock (Hugh, 1972; Javier, 1974). Thus, tolerance of tropical legumes to reduced radiation can be important in their ability to produce and persist in competition with other plants. Moderate to heavy shading reduced yields of Greenleaf desmodium [Desmodium intortum (Mill.) Urb.] and centro (Centrosema pubescens Benth.), but yield reductions were less than for Siratro [Macroptilium atropurpureum (D.C.) Urb.] and stylo (Stylosanthes guianensis Swartz) (Ranacou, 1972). Poor growth of Siratro at reduced radiation has also been reported by Ludlow et al. (1974). Yields of Greenleaf desmodium previously recorded have ranged from 10 to 21 metric tons (mt) ha'yr' (Olsen and Moe, 1971; Plucknett and Fox, 1965; Whitney et al., 1967; Whitney and Green, 1969; and Younge et al., 1964). In Malaysia, yields of stylo, centro, and Siratro in mixtures with grass were less than 16 mt ha'yr' (Ng and Wong, 1976). Date (1973) estimated the N fixed by tropical forage legumes in grass association to be in the range from 100 to 200 kg N ha'yr'. In Hawaii Desmodium spp. grown alone and with grasses fixed between 47 and 407 kg N ha'yr' (Whitney et al., 1967; Whitney and Green, 1969; Whitney, 1970). Centro fixed about 270 kg N ha 'yr' in Hawaii (Whitney et al., 1967). Even higher N-fixation has been reported for the leguminous shrub Leucaena leucocephala (Lam.) de Wit. (Brewbaker et al., 1972). The acetylene reduction technique (Hardy et al., 1968) has provided useful information regarding N fixation of temperate legumes as affected by cutting, grazing, or fertilization (Moustafa et al., 1969; Sinclair, 1973; Chu and Robertson, 1974; Halliday and Pate, 1976) or by water stress (Engin and Sprent, 1973). However, there is little information on how defoliation or shading affects N fixation by tropical forage legumes. We therefore studied the influence of shading on the performance of six legumes commonly grown in the tropics. MATERIALS AND METHODS Six forage legumes were grown in the field on an Oxic Hapulstoll near Paia, Hawaii (20°55'N, 156°22'W and 100 m elevation) during July 1975 to February 1977. Legumes replicated three times were: Centrosema pubescens Benth. ‘centro’; Desmodium intortum (Mill.)