Livestock potential of Australian species of Acacia
Robyn A. Dynes and Anthony C. Schlink
CSIRO Livestock Industries, Private Mail Bag 5, Wembley, Western Australia 6913. Email: Robyn.Dynes@csiro.au
Trees and shrubs have long been considered important for the nutrition of grazing and browsing animals in Australia, particularly where the quantity and quality of pastures is poor for long periods The economic or feeding value of Acacia species for animal production will depend on when the nutrients are available (i.e. does foliage/seed/pod production match feed gap or drought?) and the concentrations of essential nutrients and secondary compounds in foliage, seeds or pods.
The feeding value of any forage is a function of a number of characteristics of the species, including its availability, accessibility, nutrient availability, chemical composition and presence or absence of secondary compounds. The accessibility of foliage and/or seeds and pods on acacia trees and shrubs is likely to limit the performance of grazing ruminants, although the potential exists to select lines more suited to grazing species. Forage digestibility is an important component of nutritive value, but only limited data exist for Australian Acacia species. The digestibility of Acacia anuera is reported to be between 39% and 64%, suggesting that there are lines suitable for animal production.
Mineral composition of Australian acacias varies considerably between species, ranging from toxic to inadequate for livestock production. Plants that grow in inhospitable environments commonly contain secondary compounds which play a role in plant survival. Acacias are reported to have a wide range of secondary compounds including tannins, oxalates, cyanides, saponin, amines, alkaloids, fluoroacetate and other unidentified toxins. The level and presence varies with both species and plant part. Options for reducing the impact of secondary compounds are discussed.
Utilisation of acacias in farming systems could be improved through selection of species to provide feed for livestock during feed gaps and droughts, as long as issues of fodder accessibility and secondary compounds are overcome. An effective selection process will require screening a very large number of samples to determine plant quality. Near-infrared reflectance spectroscopy (NIR) is a rapid, inexpensive and accurate method and requires only small samples of plant material.