Comparative nutrient extraction from forages by grazing bovids and equids: a test of the nutritional model of equid/bovid competition and coexistence

Abstract

Ruminants are unevenly distributed across the range of body sizes observed in herbivorous mammals; among extant East African species they predominate, in numbers and species richness, in the medium body sizes (10-600 kg). The small and the large species are all hind-gut fermenters. Some medium-sized hind-gut fermenters, equid perissodactyls, coexist with the grazing ruminants, principally bovid artiodactyls, in grassland ecosystems. These patterns have been explained by two complementary models based on differences between the digestive physiology of ruminants and hind-gut fermenters. The Demment and Van Soest (1985) model accounts for the absence of ruminants among the small and large species, while the Bell/Janis/Foose model accounts both for the predominance of ruminants, and their co-existence with equids among the medium-sized species (Bell 1971; Janis 1976; Foose 1982). The latter model assumes that the rumen is competitively superior to the hind-gut system on medium quality forages, and that hind-gut fermenters persist because of their ability to eat more, and thus to extract more nutrients per day from high fibre, low quality forages. Data presented here demonstrate that compared to similarly sized grazing ruminants (bovids), hind-gut fermenters (equids) have higher rates of food intake which more than compensate for their lesser ability to digest plant material. As a consequence equids extract more nutrients per day than bovids not only from low quality foods, but from the whole range of forages eaten by animals of this size. Neither of the current nutritional models, nor refinements of them satisfactorily explain the preponderance of the bovids among medium-sized ungulates; alternative hypotheses are presented.

Voluntary intake and digestibility in horses: effect of forage quality with emphasis on individual variability

“…Further, the fibre content of the forages was a better predictor of DMD than CP; this regression for fibre content (DMD (%) 5 102.8 2 0.81 3 NDF, P , 0.05, R 2 5 0.71, n 5 45) is close to the one obtained from the review by Duncan et al (1990), (DMD (%) 5 93.3 2 0.64 3 NDF (%), P , 0.001, R 2 5 0.87, n 5 25) on the same fibre range (40% to 80%), though our intercept and slope are higher. This difference could be due to the forages used, which were hays in Duncan et al (1990), whereas we also included fresh forages, which were more digestible. If we consider only hays in our study, the regression becomes DMD (%) 5 100.3 2 0.78 3 NDF (P , 0.05, R 2 5 0.71, n 5 38), which is even closer to the regression obtained by Duncan et al (1990).…”

“…This difference could be due to the forages used, which were hays in Duncan et al (1990), whereas we also included fresh forages, which were more digestible. If we consider only hays in our study, the regression becomes DMD (%) 5 100.3 2 0.78 3 NDF (P , 0.05, R 2 5 0.71, n 5 38), which is even closer to the regression obtained by Duncan et al (1990). The relation between DMD and NDF in ruminants is also negative, but cattle digest fibrous forages better than do horses, so the slope of the regression for cattle is shallower over the same range of forage quality, DMD (%) 5 86.6 2 0.49 3 NDF (%) (P , 0.001, R 2 5 0.36, n 5 54, Duncan et al, 1990).…”

“…Duncan et al, 1990). For intake, however, most studies have found no significant relationship with forage quality (Martin-Rosset and Doreau, 1984;Cymbaluk, 1990;Duncan, 1992).…”

“…Equids are grazing herbivores that are believed to be better adapted for the consumption of high-fibre low-quality forages (Duncan et al, 1990). Compared with ruminants, these hindgut fermenters are not limited by the need to reduce the particle size of food for passage out of the digestive tract, and thus their food intake should be less affected by declines in forage quality, and they may even be able to increase their intake to maintain rates of energy and nutrient absorption (Janis, 1976).…”

Soil ingestion, nutrition and the seasonality of anthrax in herbivores of Etosha National Park

“…Dietary thresholds of fecal nutrition indices determined for ruminant species are unlikely to apply to non-ruminant herbivores (e.g., elephants, Woolley et al 2009), since they only take into account the proportion of nitrogen or phosphorus per unit feces. Ruminants have lower intake rates, greater digestive efficiency, and select higher quality forages than nonruminant herbivores of similar body mass (Foose 1982, Duncan et al 1990). Thus, these thresholds alone are unlikely to provide concrete evidence of whether Etosha's herbivores are in nutritional stress.…”