Ingestive and Digestive Responses to Dietary Fiber and Nitrogen by 2 Macropodid Marsupials (Macropus-Robustus-Erubescens and M-R-Robustus) and a Ruminant (Capra-Hircus)

Abstract

It has been hypothesised that large macropodids can maintain greater intakes of increasingly fibrous diets than ruminants of similar body size because of the tubular nature of the macropodid forestomach compared with the sac-like rumen. Responses to changes in levels of dietary fibre (plant cell walls) and nitrogen were examined in captive euros (Macropus robustus erubescens), wallaroos (M. r. robustus) and goats (Capra hircus). When all animals were fed pelleted isonitrogenous diets of 40, 60 and 80% barley straw, the macropodids compensated for decreasing dry matter (DM) digestibility by increasing DM intakes of the 60% and 80% straw diets. In contrast, the goats were unable to fully compensate for falling DM digestibility. Consequently, in the goat, digestible DM intakes tended (P<0.12) to decline from 43 g DM per kg0.75 per day on the 40% straw diet to 35 g DM per kg0.75 per day on the 80% straw diet. In comparison, digestible DM intakes only declined from 28 to 25 g DM per kg0.75 per day in the macropodids. When euros and goats were offered pelleted or chopped 80% straw diets, with and without dietary nitrogen (N) supplementation, pelleting improved DM intakes in both species. N supplementation had no effect on intakes of the chopped diets, but improved intakes of the pelleted diets. When euros, wallaroos and goats were fed chopped 50 and 80% straw diets, with and without N supplementation, DM and digestible DM intakes fell equally among the species on the 80% straw diet compared with the 50% straw diets. N supplementation had no effect on intakes of the 80% straw diet, but improved digestible DM intakes on the 50% straw diet. It was-concluded that the macropodids can maintain relatively greater intakes of increasingly fibrous diets if the constraint of mastication is removed by grinding and/or pelleting the feed on offer. Intakes were equally depressed among the species by increasingly fibrous chopped diets. On the 50% chopped-straw diet, maintenance dietary N requirements were 273, 364 and 413 mg N per kg0.75 per day, and truly digestible N requirements were 160, 251, 250 mg N per kg0.75 per day, in the euro, wallaroo and goat, respectively. These N requirements support the conclusion that the euro has a particularly low requirement for N, and that macropodid N requirements are often lower than those of eutherian grazers.