Nutrient Intake of Deer in Arizona Chaparral and Desert Habitats

Abstract

Chemical analyses and in vitro digestibility of hand-harvested forages comprising seasonal diets of desert mule deer (Odocoileus hemionus crooki) and Coues white-tailed deer (O. sirginsnus couesi) were combined with diet-composition data, to estimate qualitative nutrient intake. Despite differences in habitat and composition of diet, the two deer species have similar intakes of protein, phosphorus, calcium, and fiber. Digestibility is somewhat more variable. Phosphorus appears inadequate in early winter, although selection by deer for higher-quality plants and plant parts may coampensate for this factor. Browse and fruits of woody plants exceed herbaceous forages in percentage of annual composition of diet, but the latter contribute disproportionately more phosphorus. Other nutrients tested appeared to be adequate, but quantitative intake was not measured. Deer populations on chaparral and semidesert ranges in central Arizona, seldom high in density, have declined sharply in recent years. The decline on the Three Bar experimental area in central Arizona was tentatively attribluted to hunter harvests in excess of low yearling recruitnent ( Smith et al. 1969)) although the basic problem here, and generally in Arizona, aplpeared to be extremely high fawn mortality. Nutritional deficiencies have long been suspected for adversely influencing fawn survival and population levels (Hanson and McCulloch 195S, Swank 1956 ) . However, studies of forage quality have been restricteld to proximate analyses of important browse species ( Swank 1958, Reynolds 1967 ) . Levels of phosphorus and protein in most browse species appear inadequate except during periods of active growth. McCulloch ( l963, 1967 ) has shown that deer depend upon browse and fruits of woody plants for a major proportion of 1 Rocky Mountain Forest and Range Experiment Station, Forest Service, U. S. Department of Agriculture. Central headquarters is maintained in cooperation with Colorado State University at Fort Collins. 2 Present address: Tonto National Forest, Federal Building, Phoenix. yearlong diets, but, they also consume significant quantities of herbaceous forages when these are available in late winter, spring, and fall. These forages vere believed,to supplement nutrients belosv minimum requirements in dormant browse. The problem of estimating Ithe quality of the diet is partially resolved by combining the nutritive value of individual forages, a function of chemical composition and digestibility, with the intake of forage ( Barnes 1965). Recently developed in vitro techniques for obtaining coefficients of digestibility (Tilley and Terry 1963) offer a relatively inexpensive and fast alternative to laborious in vivo methods. Tilley and Terry report generally high correlations betveen in vitro and in vivo estimates of digestibility. The present study was conducted by the Rocky Mountain Forest and Range Expleriment Station in cooperation with Arizona State University. Seasonal quality of the diet was estimated to reveal possible deficiencies in nutrients that could account for poor reproductive performance of deer in central Arizona. H. A. Pelarson, Project Leader for range research at the Southern Forest and Range Experiment Station, pro-