THE WATER ENVIRONMENT AND IMPLICATIONS FOR PRODUCTIVITY

Water deficits constrain productivity of herbivores on savanna grassland in 2 ways. The lack of soil water in the dry season causes a nutritional stress due to the low protein concentration and low dry matter digestibility of dead tropical perennial grass tissue. Less frequently, lack of rain in the growing season causes feed shortages in the subsequent dry season. The 1st part of this paper deals with simulation of these effects using weather data and describing the variation which occurs in N Australia. Options for reducing the impact of the annual dry season include sowing a pasture legume, since dry legume leaf is relatively nutritious. The 2nd part of the paper examines variation imposed by the water climate on legume production potential and on the risk of spoilage due to out-of-season rainfall. The paper then examines the effects of N and P deficiencies and the consequences of mis-management of the soil surface on the water constraint. Whereas formerly the main impact of nutritional stress was high cattle mortalities, now genetically better adapted cattle survive by consuming a greater proportion of the low-quality herbage with the aid of mineral supplements. The sowing of legumes increases average carrying capacity and increased pasture utilization. Both innovations increase the risk of overgrazing in years of below-average pasture production. -from Authors