Dietary overlap of grazing herbivores including Damara sheep and kangaroos during ongoing dry conditions in the north-eastern goldfields of Western Australia

Knowledge on what grazing animals eat and how nutritious the fodder is during rainfall deficient periods or drought conditions in the Southern Rangeland of Western Australia is limited. An exploratory study was undertaken on a pastoral lease 90 km north of Kalgoorlie from November 2005 to November 2007 to gather information on short term seasonal condition of vegetation to determine potential diet quality and dietary overlap between grazing herbivores including Damara sheep, feral goats and kangaroos during ongoing dry conditions. Monitoring sites were installed at approximately 1 km intervals from the water point in both a stocked paddock and unstocked/rested paddock. The monitoring sites were used to collect data on short term seasonal condition of vegetation using time series photographs of the sites and target plants. The data were combined with rainfall information and normalised difference vegetation index (NDVI) for the study sites. Dung collection areas were established at the monitoring sites to determine the presence or absence and the trend over time of Damara sheep, feral goats and kangaroos. Limited data on the liveweight, body condition score (BCS) and tail length and circumference of the sheep were collected to assess if there was an impact of the ongoing dry conditions on livestock production. The extent of dietary overlap between the grazing herbivores was investigated by DNA fingerprinting target plants and establishing animal faecal DNA profiles. Plant selection was based on the known palatability to herbivores, then determining which of these target plants appeared in the dung of livestock or kangaroos. The nutritive value of the selected plants, which included all of the plants collected for DNA profiling as well as additional plant species that were locally known to be palatable to herbivores, was also assessed to investigate the impact of the ongoing dry conditions on the quality of the diet available to grazing animals.
The lack of sufficient winter rainfall over the study period resulted in no evidence of germinations of annual grasses, perennial grasses, forbs and annuals and no green new growth on existing plants. The site-based, time-series pictures taken at each site over six months monitored how the size and presence of the plants responded as the water stress continued. While more than 50% of the target plants grew in height, they all looked visually drier and the mass of the plant less dense. Many plants had smaller leaves, often curling and dropping off as rainfall deficiencies increased and rainfall effectiveness decreased, as supported by the NDVI data. Several of the target low shrubs appeared to be less resilient to the ongoing dry conditions than the other shrubs and trees. The height difference of the target plants varied between species and paddocks. Of the target plants found in both paddocks, Atriplex vesicaria, Frankenia setosa and Eremophila maculata grew in the rested paddock but decreased in height and width in the stocked paddock.
Dietary overlap of target plants occurred between sheep and kangaroos; however, the extent of this dietary overlap declined as the dry conditions continued, possibly due to kangaroos moving to a different location(s).
Animal production data was minimal with two measures in close succession and the third measure 22 months later. Each measure was not precisely the same; only BCS was consistent for each of the three measures. Nonetheless, it was still apparent the sheep were not thriving, having lost body condition including a reduction in tail diameter within the first two months of the study and gaining less than 1 kg per month, indicating available feed supported low levels of production.
If the plants selected in this study were to represent the sole component of the diet, none of the plants was of appropriate nutritive value to meet the maintenance requirements of sheep.
The results from this study have provided further information and insight into the effects of ongoing dry conditions on vegetation and livestock on a pastoral station located in the southern rangelands of WA. However, predominantly, the project became an observational study as the statistical strength of the findings was weak.