Nutrition of Sheep Grazing Crested Wheatgrass versus Crested Wheatgrass-Shrub Pastures during Winter

Abstract

Grazing sheep on improved pastures during winter offers an economically attractive alternative to supplementation in sagebrush steppe ecosystems. We studied diet selection and nutrition of sheep grazing in crested wheatgrass (Agropyron dksertorum) and crested wheatgrass-shrub (Kochia prostrata, Atripkx canescens, Purshia tridentata, Artemisia tridentata, Chrysothamnus nauseosus, Ceratoides lanata) pastures during early-January (period l), mid-January (period 2), and late-January (period 3). Diet selection by esophageally fistulated sheep differed during each of the 3 periods because the amount of available forage changed with snow depth, trampling, and utilization. Sheep grasing crested wheatgrass (CW) pastures consumed diets that were about 55% mature grass and 45% green vegetative growth during periods 1 and 2, and 93% mature grass and 7% green vegetative growth during period 3. Sheep grazing crested wheatgrass-shrub (CWS) pastures consumed diets that were about one-half grass and one-half shrub during all periods. Organic matter intake (g . kg BW-.076), determined from total fecal output and in vitro digestibility estimates, were higher (P = 0.036) for sheep grazing CWS pastures than for sheep grazing CW pastures during periods l(38 vs. 28)and 3 (31 vs. 27), but were similar (FKO.10) during period 2 (UI vs. 26). Diets of sheep grazing CWS pastures contained more (P = 0.002) crude protein (%) than diets of sheep grazing CW pastures during periods l(9.0 vs. 5.g), 2 (7.3 vs. 6.6), and 3 (7.9 vs. 4.6). In vitro organic matter digestibilities (%) of diets of sheep in CW and CWS pastures were similar during period 1(45 vs. 48), but higher (P = 0.001) for sheep grazing in CW pastures during periods 2 (46 vs. 29) and 3 (32 vs. 24). We stocked pastures heavily to accentuate differences between sheep diets in CW and CWS pastures during period l-3; we believe results from period 1 best represent the potential nutritional benefits of shrubs on snowy winter ranges. Harvesting hay during summer and feeding it during winter accounts for over half of the variable costs of ranching in the Intermountain West (Simonds 1980). The machinery, irrigation, fertilizer, seed, fuel, and labor associated with winter feeding of livestock represent a major expense for the livestock industry. Substantial savings would result if the amount of hay fed to livestock during winter could be reduced through enhanced management of winter ranges. Salt desert shrub ecosystems, which constitute the traditional winter ranges in the Intermountain West, cannot support all animals that must be fed during winter. However, the sagebrush semidesert (West 1983a) and steppe (West 1983b) ecosystems of the region are much larger, and offer alternative wintering areas. About 4 million ha of these rangelands have been seeded with crested wheatgrass (Agropyron desertorum and A. cristatum). These species are renowned for their ease of establishment and tolerance of spring grazing, drought, and cold. Unfortunately, they tend to become fibrous at maturity, usually in early June. Thereafter, their palatability and nutritional quality decline rapidly (Murray et al. 1978). Knipfel (1977) concluded that mature crested wheatgrass was nutritionally inadequate for pregnant ewes. Two properties of crested wheatgrass give it potential as a winter