The effect of pasture availability on food intake, species selection and grazing behaviour of kangaroos.
Food intake, grazing behaviour and diet of red and western grey kangaroos were monitored as they progressively depleted an arid zone pasture from 1000-1200 kg DW/ha to an ungrazable residue. Food intake declined progressively over the trial for both species but red kangaroos were better able to maintain their intake at low pasture biomasses. Red kangaroos reduced the pasture to a residue of <20 kg/ha of halophytic chenopod subshrubs, but grey kangaroos were unable to reduce it below 180 kg/ha; the residue consisted mainly of chenopods and the lily Bulbinopsis semibarbata. Both species preferred grasses (Lophochloa cristata, Enneapogon avenaceus, Eragrostis dielsii) and forbs to chenopods and lily. Red kangaroos harvested grasses, forbs and B. semibarbata faster than did western grey kangaroos. Red kangaroos responsed to declining plant biomass and food intake by increasing grazing time by 0.5 h/day for each 100 kg/ha decrease in available vegetation. The grazing time of grey kangaroos was higher than that of reds from the commencement of the trial and less reactive to changes in pasture biomass. Females grazed for longer than males. Grazing activity of both species peaked between dusk and dawn with animals spending >60% of their time grazing during this period. Biting rate of kangaroos while harvesting and chewing food did not differ between species, averaging 56 bites/min. Red kangaroos were significantly more mobile while feeding, taking more steps/min and fewer bites/step.