The Effect of Pasture Availability on Food Intake, Species Selection and Grazing Behaviour of Kangaroos

Abstract

(1) Food intake, grazing behavior and diet of red and western grey kangaroos [Macropus rufus and M. fulginosus] were monitored as they progressively depleted an arid zone pasture from 1000-1200 kg ha-1 to an ungrazable residue [Australia]. Food intake declined progressively over the trial for both species, but red kangaroos were better able to maintain their intake at low pasture biomasses. (2) Red kangaroos produced the pasture to a residue of < 20 kg ha-1 of halophytic chenopod subshrubs, but grey kanagaroos 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 Bulbinopsis simibarbata at a significantly faster rate than did western grey kangaroos. (3) 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 commencement 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 or longer than males (14.6 v. 10.0 h day-1 at 800 kg ha-1). Grazing activity of both species peak between dusk and dawn with animals spending > 60% of their time grazing during this period. (4) 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 sites per step. (5) Kangaroos appear able to harvest and process their daily food intake in substantially fewer bites than do ruminants. This may enable kangaroos to be more selective of food items, as they can devote more time to choosing each bite.