The water environment and implications for productivity

Water deficits constrain productivity of herbi- management of the soil surface on the water constraint. vores on savanna grassland in two ways. The lack of soil It is concluded that the substantial progress in reducing water in the dry season causes a nutritional stress due to nutritional stress of the annual dry season has changed the the low protein concentration and low dry matter nature of the problem of periodic droughts. Whereas digestibility of dead tropical perennial grass tissue. Less formerly the main impact of nutritional stress was high frequently, lack of rain in the growing season causes feed cattle mortalities, now genetically better adapted cattle shortages in the subsequent dry season. The first part of survive by consuming a greater proportion of the low- this paper deals with simulation of these effects using quality herbage with the aid of mineral supplements. The weather data and describing the variation which occurs in sowing of legumes increases average carrying capacity and northern Australia. increased pasture utilization. Options for reducing the impact of the annual dry season Both innovations increase the risk of overgrazing in include sowing a pasture legume, since dry legume leaf is years of below-average pasture production. Avoidance of relatively nutritious. The second part of the paper examines accelerated land degradation as a result of loss of buffering variation imposed by the water climate on legume produc- capacity inherent to the former extensive system will tion potential and on the risk of spoilage due to out-of- require skilful management. season rainfall.