The free-roaming ruminant requires energy for the demands of vii grazing, traveling and thermoregulation that are not required by its confined counterpart. Literature estimates of these additional costs range from 10 to 170 percent above maintenance. The uncertain magnitude of this increased demand and the factors that contribute to it impede the ability of the rangeland ruminant nutritionist to establish guidelines for the energy requirements of the free-roaming herbivore. This study was designed to estimate the energy expenditure of yearling Angus heifers while grazing a declining supply of available crested wheatgrass forage (Agropyron cristatum) on rangeland in west-central Utah.
Free-ranging energy expenditure was estimated twice for four heifers during each of five ten-day periods during June, July August and early September, 1979. These estimates were obtained using the carbon dioxide entry rate technique. In addition, total fecal output, dietary crude protein and dietary in vitro organic matter digestibility were estimated for animals grazing the 20- hectare crested wheatgrass pasture. From these data, daily forage intake was calculated. The level of available forage during each period was estimated using the ocular weight-estimate technique applied on forty 1 m2 circular plots.
Energy expenditure was estimated as 161 (with a confidence interval of ±43) kcal·kg body weight-.75.d-1 (n=10), and was independent of the decline in available forage from 880 to 284 kg dry matter·hectare-1 that occurred over the course of the grazing season. Daily intake was 54.5 grams (organic matter basis) per unit body weight.75 for the 305 kg heifers. Daily intake was independent of the supply of available forage.
During early July, 1980, crested wheatgrass was harvested as hay and fed to 260 kg yearling Angus heifers in metabolism stalls in a thermoneutral and constantly illuminated laboratory. Daily feeding levels were set at 54.5 grams (organic matter basis) per unit body weight.75. Energy expenditure under these conditions was estimated as 111 (±12) kcal·kg body weight-.75·day-1 , 6 kcal per unit body weight.75 greater than the mean estimate of the fasting metabolism rate. The latter estimate was obtained following a 48-hour fast. These estimates of maintenance and fasting metabolism were combined to provide a mean estimate of 110 (±10) kcal·kg body weight-.75·day-1 (n=14).
Of the 45 percent (51 kcal·kg body weight-.75·day-1) increase in the estimated energy expenditures by heifers under free-roaming conditions, 50 percent was attributed to the energetic cost of grazing. A daily average 9.2 hours were spent in this activity. The energetic cost of grazing was assumed as 0.82 kcal·kg body weight-1·hour-1 spent grazing. Daily travel was estimated as 3.9 km at an assumed energetic cost of 0.58 kcal·kg body weight-1·km-1. This accounted for a 20 percent estimated increase in energy expenditure. Average daily temperatures were generally between 12°C and 30°C and thermoregulatory demands were not considered as a substantial energetic expense. The remaining 30 percent (12 kcal) of the additional increment due to free-roaming conditions could not be explained
