Partitioning of Nutrients in Merino Ewes. I. Contribution of Skeletal Muscle, the Pregnant Uterus and the Lactating Mammary Gland to total Energy Expenditure

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Abstract
The contribution of leg muscle, pregnant uterine tissue and lactating mammary gland to overall energy utilization was determined in Merino ewes. Ewes were offered 1 of 3 diets based on chaffed oaten hay (7.9 MJ metabolizable energy per kilogram dry matter); chaffed lucerne hay (8.6 MJ/kg); or a 50:50 (wt/wt) mixture of chaffed oaten and lucerne hay (8.2 MJ/kg). Measurements were made during 5 different physiological states: dry (non-pregnant), at 94 and 125 days, after mating, and at 20 and 50 days after lambing. Tissue energy use was calculated from O2 uptake and CO2 output obtained from measurement of blood flow and arteriovenous difference. Whole-body energy use was calculated from CO2 energy rate. Energy use by leg muscle was 144 .+-. 8 (mean .+-. SE) kJ kg-1 day-1, and unrelated to metabolizable energy intake, but leg energy use increased with ewe body wt. On the basis that leg muscle was representative of all muscle, total muscle energy use accounted for 26 .+-. 4% of whole-body energy expenditure in dry ewes. Uterine energy use per unit wt was respectively 348 .+-. 53 and 254 .+-. 23 kJ kg-1 day-1 at 94 and 125 days after mating. Milk production was highly correlated with wt of secretory tissue, and with blood flow to the mammary gland. The ratio of blood flow to milk produced was 473:1 in ewes producing from 200 to 1000 ml of milk per day. The mammary gland used energy to produce milk with an efficiency of 0.90 .+-. 0.01, a value close to the theoretical estimate of 0.89. On the basis that metabolic rate does not increase durng lactation, the efficiency of use of metabolizable energy for milk production was 0.51 .+-. 0.05. Examination of energy use by different tissues indicated that energy use by muscle was related to weight, but energy use by remaining tissues (whole body less muscle, uterus and mammary gland) was related to metabolizable energy intake. The results reveal an increase in energy use by the remaining tissue in lactating ewes (8500 .+-. 569 kJ/day) compared with dry (5634 .+-. 216 kJ/day) and pregnant ewes (5815 .+-. 393 kJ/day).