Estimation of energy needs in the free-ranging goat with particular reference to the assessment of its energy expenditure by the 13C-bicarbonate method

Today, extensive systems of small ruminants production are considered the most appropriate choice from the ecological, social and economic points of view for the provision of animal products in the arid lands of Mediterranean ecosystems. An efficient utilization of available resources requires a deep knowledge of feeding behavior, digestive and metabolic processes, nutritive value of feeds and nutrient/energy requirements. Energy availability is the biggest limiting factor in the animal production systems. This paper deals with the assessment of the energy requirements of goats. Emphasis is given to problems involved in predicting total energy expenditure of animals in free-living conditions. Particular attention is paid to the CO2-entry rate technique (CERT), where the production of CO2 of the free-ranging animal can be measured using NaH13CO3. Our calorimetric results suggest that maintenance requirements (MEm) of goat may vary from 401 to 443 kJ k-0.75 per day. An average value of 422 kJ kg-0.75 per day for castrated adult males, growing females and lactating goats has been proposed. Also, a requirement of 4.20 MJ ME kg-1 of 4% fat-corrected milk produced was calculated. Average values for km and kl were 0.75 and 0.67, respectively. The energy costs of the main physical activities (walking and eating) have been determined as well. Estimates of the energy cost of walking were 3.35, 31.7 and -13.2 J kg-1 BW m-1 for horizontal, ascending and descending movement, respectively. Energy cost of eating ranged from 9.02 J kg-1 BW g-1 DM for roughages to 1.55 J kg-1 BW g-1 DM for concentrates. The usual procedure to predict the total energy expenditure by grazing animals is the factorial method, whereby the increase in energy expenditure above that measured in confinement is quantitatively estimated from calorimetric determination of the energy cost of various activities. For a direct estimation of total energy expenditure in the free-ranging animal the CERT has been used. The entry rate of bicarbonate (production of CO2) was calculated from the infusion rate and the measured dilution of the isotope at equilibrium in saliva. A respiratory quotient (RQ) value was assumed, based on feeding level (or productive status), to estimate heat production (HP). So far, only a few studies have used 13C to estimate the energy expenditure of ruminants, and only two studies have addressed free-ranging goats. The method has also been used to determine the effect of cold exposure and stocking rate on total HP. Further investigation is needed to develop and improve new and existing techniques to quantify the total energy expenditure of animals in open range systems.