Assessing the impact of pasture resting on pasture condition in the extensive grazing lands of northern Australia

Abstract

Pasture resting (spelling) during the early wet season, has been identified as an ecologically useful tool for restoring degraded rangeland pastures. Resting has been included within a suite of promising grazing land management practices being evaluated within the Northern Grazing Systems program, coordinated by Meat and Livestock Australia. The GRASP pasture simulation model has been used to investigate many aspects of grazing land management, from estimating safe stocking rates through to assessing the impact of projected climate change. The investigation of pasture response to resting required some important modifications to the GRASP model. Change in pasture condition as simulated by GRASP was previously based on annual utilisation rates of pasture growth. As such, the actual timing of grazing had no impact on change in condition and this is an appropriate means of calculating change in condition when paddocks are continuously grazed throughout the year. However, pasture resting means that grazing is undertaken for only part of the year. Tropical grasses are most sensitive to grazing during the early part of the growing season - December to March. If ungrazed during this period, pastures will generally improve in condition; if heavily grazed, pastures will deteriorate markedly. We describe changes made to the GRASP model to enable simulation of pasture resting regimes. Three applications of the modified GRASP model are reported. The first deals with the pasture response to different frequencies and lengths of rest periods, with this information being averaged across 20 different climate windows (each 30 years) to overcome the possibility that a particular climatic period produced an 'unusual' response. The second covers the change in pasture condition for a recommended resting regime where animals are agisted off-property, and the third is a variant of the second application with the displaced animals being distributed across the other paddocks involved in the rotational resting regime. Pasture resting resulted in marked improvement in both pasture condition and production. As the frequency and duration of rest periods increase, so does the condition and productivity of pastures. Importantly, frequent, shorter rests produced better responses than less frequent, longer rests. A rotational rest grazing system involving four paddocks, with each paddock receiving a 6-month rest on a one-in-four year cycle, improved pasture condition in the system as a whole, especially when the cattle from the rested paddocks were agisted off-property. When the system involved those stock from the rested paddock being distributed across the other three paddocks being grazed, one of those paddocks commonly showed deterioration in condition. This is due to the three consecutive years in which the last paddock in the cycle to receive a rest has a stocking rate which was 33% higher than the long-term stocking rate during the sensitive summer growing period. Limitations to the use of these modelling results are considered and practical changes to the simulation approach are discussed.