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The graminoid layer forms an important forage resource for the extensive livestock production system of northern Kenya. However, in recent years, increased grazing pressure resulting primarily from exogenous influences has had severe impact on this very basic resource. The result is land degradation and a threat to the very existence of the nomads and their herds. To assess this impact, investigations were conducted on the perennial grass Leptothrium senegalense, to determine its production in relation to grazing pressure. Peak standing aboveground production was 0.2-46.8 kg/ha and 16.7-256.7 kg/ha on grazed and protected sites, respectively over a 3-year period. Rainfall also influenced production on the protected and seasonally grazed sites. The direct effect of gazing was a reduction in plant size. Based on height measurements, 70.6-78.2% and 85.3-100% was estimated to be removed by seasonal and year-long grazing, respectively. Simulated defoliation confirmed that grazing had its most significant impact on residual mass and reproduction capacity, leading to a reduction in the survival fitness of this species. Results also indicated that production of L. senegalense is water-limited, and that water stress may interact with intense grazing to limit its production and recruitment success. It is suggested that to optimise yield to grazers while ensuring reproductive success, grazing should be moderate to leave stubbles of about 15 cm. At this stubble height, adequate residual mass necessary for regrowth is also assured. In the light of this study, absence of L. senegalense can be considered a bio-indicator of overgrazing in the arid lowlands of the Rendille ecosystem. (C) 1997 Elsevier Science B.V.