Implications of Spatio-Temporal Variation in Forage Production and Utilization for Animal Productivity in Extensive Grazing Systems.

Abstract

In this paper we examine patterns of utilisation at the level of individual plants and forage availability at the paddock scale on five commercial sheep grazing properties that all experienced drought during the course of the study. Specifically, patterns of forage availability and pasture utilisation in large paddocks are examined against the assumptions of established models of animal productivity and grazing management practice. Forage resources in the rangelands can have dynamics independent of stock density and are not distributed evenly in time or space. Therefore, the extrapolation of known plant-animal interactions across scales is questionable and it is yet to be demonstrated how measures of animal productivity on small trial paddocks relate to production outcomes in large paddocks. A model is proposed for understanding animal production outcomes that incorporates variation in both pasture biomass and stock density. The scales, both spatial and temporal, at which pastoralists and scientists integrate ecological and animal productivity information usually differ. The understanding that each holds of the processes that limit animal productivity and the relationships between different elements of the production system is a product of the perspective from which they view the system. It is concluded that the difference between the 'scale of exploitation' and the scale at which heterogeneity is sufficient for survival of livestock is probably crucial to determining animal production outcomes in grazing systems subject to a high degree of temporal variation in forage availability. In these systems it seems reasonable to suggest that grazing experiments that vary the 'spatial scale of exploitation' while maintaining stock density constant may give as many insights into the limitations on animal productivity as the more typical experiment that only varies stock density. Key words: grazing models, animal productivity, heterogeneity, scale, utilisation, forage availability, stocking rates.