Grazing in Herds: When are Nutritional Benefits Realized?

McNaughton (1984) showed that feeding in groups can convey significant utritional benefits to large herbivores by enhancing food yield from the grasslands they feed upon. However, the nutritional benefit of gregarious behavior was subsequently questioned by Westoby (1985), who reviewed empirical evidence showing that increases in herbivore density frequently compel deterioration i their nutritional status. McNaughton (1986) argued that this discrepancy was due to differences inthe evolutionary history of domestic and wild grazers. We believe there is a more parsimonious explanation. Here, we offer a simple model that reconciles the seemingly conflicting observations of Westoby and McNaughton, showing them to be specific examples of a more general case. Gregariousness i nutritionally advantageous when the capture of nutrients by herbivores feeding in grazed patches exceeds nutrient capture in ungrazed patches (McNaughton 1986). The expected rate of nutrient intake from grazed and ungrazed patches can be predicted from the functional response of the herbivore, the standing crop of the grazed and ungrazed patches, and the difference innutrient concentrations ofthe grazed and ungrazed forage. The rate of nutrient intake (F, in g/min) by a feeding herbivore is a product of its dry-matter intake rate (I, in g/ min) and the concentration of nutrients (D, a decimal fraction) in the mass of forage eaten: F = ID. The dry-matter intake rate of large herbivores is an asymptotically increasing function of the mass of available forage (Allden 1962; Allden and Whittaker 1970; Wickstrom et al. 1984; Short 1985, 1986; Hudson and Watkins 1986; Renecker and Hudson 1986), which can be represented in the form I = axl(b + x), where x is the standing crop of food; a is a constant describing the maximum intake rate; and b is a constant describing the rate of approach to maximum intake. Assuming that the standing crop of ungrazed grass exceeds the standing crop of regrowth in a grazed patch, the biomass of regrowth (R, in kg) following razing can be expressed as a fraction (fi) of the original, ungrazed biomass (U, in kg): R = fi U, where 0.0 < fi < 1.0. Similarly, the concentration of a nutrient in regrowth (Dg) can be expected to exceed its concentration i mature, ungrazed forage (Du). It follows that the nutrient content of regrowth can be expressed as a multiple (f2) of the nutrient content of mature forage: Dg = f2Du, where'2 > 1.0. The expected intake of nutrients ingrazed (Fg) and ungrazed (F") food patches can then be calculated as a function of the standing-crop biomass and of the nutrient concentration of the ungrazed food:

[1]  M. J. Montgomery,et al.  REGULATION OF FOOD INTAKE IN RUMINANTS. 1. PELLETED RATIONS VARYING IN ENERGY CONCENTRATION. , 1965, Journal of dairy science.

[2]  W. G. Allden,et al.  The determinants of herbage intake by grazing sheep: The interrelationship of factors influencing herbage intake and availability , 1970 .

[3]  D. Minson,et al.  The relationship between apparent retention time in the rumen, voluntary intake, and apparent digestibility of legume and grass diets in sheep , 1973 .

[4]  H. Lippke.  Regulation of Voluntary Intake of Ryegrass and Sorghum Forages in Cattle by Indigestible Neutral Detergent Fiber , 1986 .

[5]  P. Jarman,et al.  The Social Organisation of Antelope in Relation To Their Ecology , 1974 .

[6]  Jeff Short.  The effect of pasture availability on food intake, species selection and grazing behaviour of kangaroos , 1986 .

[7]  M. Westoby.  Does Heavy Grazing Usually Improve the Food Resource for Grazers? , 1985, The American Naturalist.

[8]  R. Bell.  A grazing ecosystem in the Serengeti , 1971 .

[9]  A. J. Belsky.  Does Herbivory Benefit Plants? A Review of the Evidence , 1986, The American Naturalist.

[10]  R. Hudson,et al.  Foraging rates of wapiti on green and cured pastures , 1986 .

[11]  S. McNaughton,et al.  Grazing as an Optimization Process: Grass-Ungulate Relationships in the Serengeti , 1979, The American Naturalist.

[12]  J. M. Forbes,et al.  Control of feed intake and regulation of energy balance in ruminants. , 1974, Physiological reviews.

[13]  Jeff Short.  The functional response of kangaroos, sheep and rabbits in an arid grazing system , 1985 .

[14]  T. A. Hanley,et al.  Food Intake and Foraging Energetics of Elk and Mule Deer , 1984 .

[15]  R. L. Cowan,et al.  Dry Matter and Energy Intake in Relation to Digestibility in White-Tailed Deer , 1973 .

[16]  J. Ellis,et al.  Composition and quality of elk winter diets in Colorado , 1981 .

[17]  S. McNaughton,et al.  Compensatory Photosynthetic Responses of Three African Graminoids to Different Fertilization, Watering, and Clipping Regimes , 1984, Botanical Gazette.

[18]  T. Whitham,et al.  Overcompensation in Response to Mammalian Herbivory: The Advantage of Being Eaten , 1987, The American Naturalist.

[19]  N. Hobbs,et al.  Strategies of digestion: digestive efficiency and retention time of forage diets in montane ungulates , 1987 .

[20]  S. McNaughton,et al.  Serengeti Migratory Wildebeest: Facilitation of Energy Flow by Grazing , 1976, Science.