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RESEARCH ARTICLE
Contents Vol 28(2)

Death model for tussock perennial grasses: thresholds for grazing-induced mortality of mulga Mitchell grass (Thyridolepis mitchelliana)

Ronald B. Hacker A C , Kenneth C. Hodgkinson B , Gavin J. Melville A , Judith Bean A and Stephen P. Clipperton A
+ Author Affiliations
- Author Affiliations

A NSW Department of Primary Industries, Trangie Agricultural Research Centre, PMB 19, Trangie, NSW 2823, Australia.

B CSIRO Sustainable Ecosystems, GPO Box 284, Canberra, ACT 2601, Australia.

C Corresponding author. Email: ron.hacker@dpi.nsw.gov.au

The Rangeland Journal 28(2) 105-114 https://doi.org/10.1071/RJ06001
Submitted: 3 August 2005  Accepted: 4 November 2005   Published: 9 November 2006

Abstract

We investigated the relationship between grazing intensity and death of mulga Mitchell grass [Thyridolepis mitchelliana (Nees) S.T. Blake] plants in semi-arid wooded grasslands of eastern Australia. The study (from July 1993 to March 1997) involved factorial combinations of intensity and duration of sheep grazing before rest from grazing. Grazing intensity varied considerably among plants within the small plots and logit analysis of the binary responses of individual plants (a plant was judged to be alive or not) was used to establish the relationships between grazing history and death. Average residual (shoot) biomass over a moving window of 10–12 months was a good predictor of either general plant death or death during summer drought. Death over summer increased as average residual biomass dropped below 70%, and increased rapidly when the average fell below 50%. Variables based on residual shoot biomass generally provided better predictors of death than variables based on foliar height. However, as a predictor of death over summer, current foliar height was as good as average residual shoot biomass over the extended period. Summer death increased rapidly as foliar height fell below 10 cm. Environmental conditions were much less important than grazing in determining death rates, indicating that grazing management can have important benefits in maintaining productive grasslands even in more or less ‘normal’ seasons.

Additional keywords: grasslands, survival, tactical grazing.


Acknowledgments

The assistance in the field, and in data compilation, of colleagues in NSW Department of Primary Industries (formerly NSW agriculture) and CSIRO is gratefully acknowledged, particularly Sharon Hawke, Ivan McManus, and Steve Marsden. Financial support for part of the study was provided by the Australian Wool Research and Promotion Organisation (now Australian Wool Innovations Limited). Leigh Hunt provided helpful comments on the manuscript.


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1Although ‘utilisation’ is the term most commonly used with graziers to express intensity of grazing the concept is best thought of in terms of residual biomass relative to the ungrazed plant. This tends to avoid ambiguities associated with the grazing of regrowth and directly measures the amount of tissue available for photosynthesis.