Abstract
Food intake, grazing behaviour and diet of red (Macropus rufus) and western grey (M. fuliginosus) kangaroos were monitored as they progressively depleted an arid zone pasture from 1000-1200 kg ha-1 to an ungrazable residue. Red kangaroos reduced the pasture to a residue of <20 kg ha-1 of halophytic chenopod subshrubs, but grey kangaroos were unable to reduce it below 180 kg ha-1; the residue consisted mainly of chenopods and the lily Bulbinopsis semibarbata. Both species preferred grasses and forbs over chenopods and lily. Red kangaroos harvested grasses, forbs and B. semibarbata at a significantly faster rate. Red kangaroos responded to declining plant biomass and food intake by increasing grazing time by 0.5 h day-1 for each 100 kg ha-1 decrease in available vegetation. The grazing time of grey kangaroos was higher than that of reds from the start of the trial (13.8 v. 10.8 h day-1 at 800 kg ha-1), and less reactive to changes in pasture biomass. Females grazed for longer than males (14.6 v 10.0 h day-1 at 800 kg ha-1). Grazing activity of both species peaked between dusk and dawn with animals spending >60% of their time grazing during this period. The biting rate of kangaroos while harvesting and chewing food did not differ between species, averaging 56 bites min-1. Red kangaroos were significantly more mobile while feeding, taking more steps per minute and fewer bites per step.