Grazing Pressure From Kangaroos
All kangaroos are herbivores with grasses and forbs being the main component of their diet, though they will utilise bushes and tree browse when conditions deteriorate. Kangaroos digest through a forestomach fermentative process to break down fibrous vegetation. However, unlike ruminants they do not chew the cud, grinding their food finer before swallowing.
At any one time, the diet composition of macropods, unmanaged goats and livestock is often quite different. However, in rangeland regions all species rely on the same broad groups of forages (annual and perennial grasses, ephemeral and perennial forbs) for the large majority of their feed. Scientific literature tends to indicate that direct competition between macropods and livestock is limited and that kangaroos contribute little to total grazing pressure. However, as feed availability or herbage diversity declines, such as during periods of extended drought, dietary overlap and competition increases. Direct competition becomes more apparent when pasture biomass falls below particular thresholds. Thresholds are estimated as 300 kg/ha in the chenopod rangelands of western NSW but thresholds may be 400 to 500 kg/ha in other rangeland regions.
Dry sheep equivalent (DSE): There has been much research and debate on the DSE that should be attributed to kangaroos. Earlier predictions put the value at about 0.7 but a more conservative estimate is currently being used of 0.35 DSE for all macropods.
Daily dry matter intake (DDMI): When compared on a per kg metabolic body weight basis, the DDMI of average sized macropods is estimated to be around 0.46 – 0.49 of that of sheep when fed a low fibre forage such as young grass leaf. However, when forage contains high levels of fibre, such as grass stems, macropods of similar size consume similar amounts to sheep. Macropods also have the capacity to substantially increase their DDMI by expanding their gastro-intestinal tracts, meaning their food intake rates fall less slowly than ruminants who require a longer passage time with high fibre diets.
Kangaroos are likely to eat the dry stems of perennial grasses for longer than sheep, goats or cattle as browse is less attractive to them, and will even dig up and eat grass bases and roots before switching to browse.
Macropods are also generally more mobile than domestic stock and are able to take advantage of patchy and localised growth of grasses and ephemeral forbs resulting from isolated rain events. In addition kangaroos can access pastures where domestic stock have been removed to allow for rest and recovery. Accounting for kangaroo grazing is a major issue for landholders trying to budget feed and manage landscapes for long term health and production.