Forage pathology studies have focused on major diseases and fungi infecting alfalfa and other forage species (Liu, 1984; Liu and Hou, 1984; Nan 1985, 1986; Hou et al., 1984; Liu and Nan, 1987). Liu and Hou (1984) reported more than 100 fungal diseases infecting the legume family in northern China. Liu and Nan (1983) studied major forage diseases in the Hexi Corridor. The Peronospora aestivalis disease of Trigonella ruthenica was intensively studied by Liu (1976, 1978, 1989). Nan (1990) investigated the fungal diseases of cultivated grasses and forage legumes in the loess plateau of eastern Gansu, and evaluated disease resistance of various forage varieties. He also studied Uromyces striatus disease of Medicago sativa (Nan, 1987a); the effect of Uromyces orobi on the growth and nutritive value of Vicia sativa (Nan, 1987b); and Erysiphe folygoni, a disease of Melilotus officinalis, in Qingyang (Nan, 1987c). Nan (1986a,b) demonstrated that the occurrence of Pseudopeziza medicaginis and U. striatus of alfalfa was reduced 27.7–76.2%, and Botrytis fabae and Stemphylium botryosum of sainfoin 18–85% in mixed as compared to monocultural pastures.

MODELING

Based on results achieved with the Grassland Agricultural System, efforts have been made to build optimization models for the Hexi Corridor. Using IBM modeling, Jiang (1988) suggested that population control combined with the proper integration of desert, oasis, and mountain grasslands could transform the corridor into a base for grain and meat production. Zhang and Ge (1990) developed an optimization model for the agrosystem in the desert-oasis grassland of Jingtai County, which emphasized the importance of small ruminants. An optimization model for the Linze County agrosystem is currently being developed.

Other modelers have followed a different path. Ai and Gu (1984) and Gu et al. (1984) have used models of agroclimatic suitability analysis to study bioclimatic zonation of the Gansu loess plateau and concluded that most of the investigated areas belong to the steppe or arid steppe. They pointed out the significance of grass planting in protecting the fragile ecosystems from further deterioration. Using the Integrated Rate Methodology (IRM) modeling of farming ecosystems, Li (1987) showed that as the proportion of grasslands in the rolling hills and slopeland of the loess plateau increases, grain production per capita will show a slight decline in the first five years but will increase thereafter. When the grassland area reaches 50% of total agricultural land, per capita energy production will be much greater than under any other pattern of land use.

Ecological modeling has aroused great interest among Chinese scientists in recent years. Chen et al. (1981) built mathematical models for the retrogression of populations of three steppe species (Stipa breviflora, Agropyron cristatum, and Artemisia frigida) under grazing conditions in Gansu. Liu et al. (1986)



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