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Effects of Light Intensity on Growth of Some Tropical Forage Species. I. Interaction of Light Intensity and Nitrogen Fertilization on Six Forage Grasses¹

In order to determine the response of some tropical grass species to low light situations such as under plantation crops, seasonal cloud cover, etc., six tropical forage grasses were evaluated over a 20 month period on an Oxic Haplustoll in Hawaii (100 m above sea level) under four light regimes (100, 70, 45, and 21% daylight using polypropylene netting) in the field.

The forage grasses evaluated were: Brachiaria brizantha, B. miliifirmis, Digitaria decumbent, Panicum maximum, Pennisetum clandestinum, and P. purpureum. Dry matter yields of N-fertilized (365 kg N ha^–1 yr^–1) grasses were highest at 100 and 70% daylight (16 to 40 metric tons of dry matter (DM) ha^–1 yr^–1), with P. maximum and P. purpureum having highest yields. Under 27% daylight, yields were 8 to 15 tons, with P. maximum, B. brizantha, and B. miliiformis having highest yields. When no N was added maximum yields in tons ha^–1 yr^–1 were B. miliiformis, 9.2 at 27% daylight; D. decumbens, P. maximum, and B. brizantha, 13.5 to 15.0 at 45% daylight; and P. clandestinum, 9.2 at 70% daylight. P. purpureum without N yielded 30 tons at full daylight, apparently because its very extensive root system invaded adjacent N-fertilized plots. Percentage of DM hi the forage decreased with shading and N fertilization. Percent N increased with decreasing light intensity (from 1.0 to 1.6% hi the minus-N and from 1.2 to 1.9% hi the plus-N treatments). Slight acetylene reduction activity was found in soil cores beneath all species, except B. brizantha, indicating that the plants were almost entirely dependent on soil and fertilizer N for growth. Sward height increased significantly with decreasing light intensity and N fertilization. Concentrations of P, K, Ca, Mg, S, Cu, and Zn tended to be higher in shaded forage, higher in N-fertilized forage (except for P and Zn), and generally higher during the cool season.

Thus under N-deficient conditions, most yield and forage quality parameters were enhanced under moderate shade. Conversely, the tropical grasses studied generally responded to N fertilization only under conditions of moderate to high solar radiation.

Root weight data and observations on the rate of recovery after clipping indicate the shaded pastures would require careful management to avoid excessive depletion of root reserves, either by lenient grazing to maintain high leaf areas or by allowing an extended recovery period.

Key Words: Shading • Acetylene reduction • N-fixation • N percentage • Root weight • Dry matter content • Height • LAI • Brachiaria brizantha • Brachiaria miliiformis • Digitaria decumbens • Panicum maximum • Pennisetum clandestinum • Pennisetum purpureum

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

Received for publication February 12, 1980.

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