The Changes in the Distribution of Pasture and Weed Species in a Grazed Tropical Pasture

Abstract

This pasture ecological study was conducted at Hawaii Agricultural Experiment Station, Kauai Branch, from July 1978 to October 1978. The pasture area covered two steep-sided slopes facing north and south. A valley which rims east and west separates the two slopes. No fertilizer was added to the pasture since 1970 and the area was continuously grazed since 1973. The objective was to find the changes in species distribution and soil factors comparing results of the same area studied in 1971 (Nicholls, 1972). Data were collected using the belt transect method. Of the dominant pasture species of 1971, intortum [Pesmodium intorturn (Mill.) Urb.] had disappeared from the pasture, green panicgrass [Panicum maximum Jacq. (var. trichoglume Eyles)] was only found on middle sections of both slopes in areas where it was protected from grazing. Stylo [Stylosanthes gaianensis (Aubl.)] and pangolagrass [Pigitaria dec^umbens Stent.] had spread to lower parts of the slope. In 1978, the pasture was dominated by weeds. The species present in greater frequency at all locations in the pasture were Hawaiian elephantfoot [Elenhantopus mollis H. 3. K.], Boston fern [Nephrolenis exaltata (P.) Schott.], glenwood grass [Sacciolepis indica (Wight & Arn.) Hitchc.] and sour paspalum [Pasnalum con.jugatum Bergius ] . Hawaiian elephantfoot had spread to the valley bottom. Indicator weed species of pasture deterioration such as lantana [Lantana camara L.], melastoma [Melastoma malabatricum L.], nettleleaf vervain [Stachytarpheta urticaefolia (Salisb.) Sims] had increased in numbers, while American burnweed [Erechtites hieracifolia (L.) Raf.] decreased in number. Other weed species present in 1978 were knotroot foxtail [Setaria geniculata (Lam.) Beauv.] and ricegrass paspalum [Pasnalum orbiculare Forst.] dominating the middle and top sections of the slope, and guava [Psidium guajava L.] dominating the valley bottom. Soil factors were also measured. Soil extractable P decreased while exchangeable K, Mg and Ca increased since 1971. The deteriorating pasture of 1978 produced very low dry matter production for cattle feed.