Wattle, wethers and weight gain
Loss of biodiversity and increasing land degradation has prompted many land managers in south east NSW to undertake broad-scale direct seeding of native trees and shrubs. It is known that sheep benefit from native revegetation through shade and shelter, and that those with access will feed on acacia foliage and pods (referred to as acacia browse). What we don’t know is whether this seasonal foraging makes for healthier, fatter sheep.
It was this uncertainty that led Greening Australia (Australian Capital Region) to ANU in search of a student willing to take up an investigation that might provide an answer. Enter Graham Fifield, an Honours student from the School of Resources, Environment and Society (SRES). With the assistance of Dr John Field from SRES, Mr Fifield set up three separate field trials at Binalong, Boorowa and Ginninderra that examined changes in sheep weight on traditional pasture paddocks versus revegetated paddocks. In so doing he soon found himself driving across remote paddocks; fencing, wrestling and weighing sheep; pruning wattles and even getting bogged.
Deep-rooted acacias have the advantage of being less susceptible to seasonal climatic variations than traditional
pasture plants. These pasture plants may contain as little as 8% protein at maturity whereas acacia browse commonly contains between 10% and 20%. Unfortunately, it also contains high levels of tannin, which limits the digestion of protein by sheep. Animal trials undertaken by other researchers have shown that 44-69% of foliage and seed pods are typically digested by sheep.
Using acacia browse as a protein supplement to complement low quality pasture is a relatively new avenue of research. A small number of studies have shown that acacia browse can improve the performance of sheep relative to those without the supplement. The protein in the browse improves the digestion of poor quality roughage and stimulates intake.
At one of his main sites at Binalong, both pasture and ‘acacia’ sheep displayed similar growth rates during the spring months, suggesting that pasture growth and availability were not limiting to animal growth during this time and that acacia browse had a negligible impact. During the summer months, both groups also displayed similar growth rates per unit area (though the experiment was run in a time of above average rainfall and this might not apply in a more typical year).
Interestingly, during this trial, two sheep from the pasture paddock became ill due to parasite infestation, while no sheep in the acacia paddock displayed these symptoms. After the trial, 13 sheep from the pasture paddock died from Barber Pole worm. Sheep in the acacia paddock displayed some symptoms of illness, however none were fatally affected. This experience suggests that sheep with access to acacia browse are less susceptible to illness or death due to internal parasites. This is supported by laboratory and animal trials in the literature, which indicate that condensed tannins, such as those contained within acacia browse, reduce the number of viable parasite larvae in sheep.
An important finding of the study was that previously saline degraded land, rehabilitated through direct seeding, is equally productive as non degraded pasture land during an above average spring/summer rainfall period. The observed pasture loss due to shading and competition from trees did not reduce the relative productivity of sheep in treed paddocks. Losses in pasture production may have been offset by the provision of seed pods, which sheep were observed consuming, or through the beneficial affects of improved shade and shelter. The quantity of dried acacia pods that fell as litter at one study site (2.6-4.0kg/day/ha) suggest that they may constitute a valuable addition to the diet of sheep during the summer and autumn months.
This research supports the conclusion that the forage from local acacia species is palatable, sought after and not toxic to Merino sheep. In addition to this, sheep with access to acacia browse were less vulnerable to internal parasites. Anecdotal evidence of land managers support the notion that sheep consuming supplementary acacia browse will display greater production compared to those consuming poor quality pasture alone. All of which adds to the growing pool of evidence that rehabilitating the land benefits both the environment and productivity.
The project was also a good example of research collaboration. In addition to inputs from ANU and Greening Australia, significant assistance was provided by CSIRO Plant Industry in the form of access to land, livestock and labour.
The authors wish to acknowledge Dr Hugh Dove for supervision, the Garry and Marsh families for their hospitality and support as well as staff at the CSIRO Ginninderra Experiment Station.
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