Abstract:
Proc. Aust, Soc. Anim, Prod. Vol. 18 MOBILISATION OF BODY FAT RESERVES BY EWES IN EARLY LACTATION K.P. JOSEPH* and J.Z. FOOT** Indirect estimates of body fat were made in lactating ewes of three genotypes (Corriedales and two types of Merinos) immediately post lambing and three and Measurements were made in similar non-lactating ewes at five weeks later. times equivalent to post-lambing and week five of lactation. Lactating ewes suckling twins lost an average of 3.2 kg of body fat in the first five weeks of lactation and two-year old ewes with their first single Fat Dry ewes gained body fat in the same period. lambs lost 1.5 kg of fat. These loss was related to initial fat weight, lamb growth rate and genotype. ewes appeared to derive about 10% of their calculated energy requirements from mobilised fat in the first five weeks of lactation. INTRODUC!l!ION During early lactation ewes may suffer nutritional stress when their intake of At this food does not satisfy the high nutrient demands of milk production. stage they usually need to draw on body reserves to make up the deficit (Bryant and Smith 1982). The extent to which they are able to do this may substantially affect the efficiency of a pasture-based wool producing enterprise and has important implications for management decisions. This experiment investigated the effects in the first five weeks of lactation in Ewes most likely to utilise body reserves year olds with their first lambs or those of breed on mobilisation of body fat predominately wool-producing sheep. These were either two were studied. suckling twins. Animals The ewes were of three genotypes:- Corriedale (Corr), small-framed Western District (WD) Merinos and large-framed South Australian (SA) Merinos. Pregnant ewes had been mated by rams of their own genotype. Two age groups were represented in the Corr and SA Merinos, these were two years old with their first lamb and four years old with twins. The WD Merinos were all two years old. Each age/genotype was represented by five unmated ewes and originally the intention was to have eight pregnant ewes in each treatment group but this was not achieved (Table 1). Management and measurements Pregnant ewes which had been housed in individual pens in They remained housed until Non-pregnant ewes were housed grazing together since mating in late March, were an animal house from about day 143 of gestation, their lambs were approximately seven weeks old. for an equivalent period. While housed, all ewes were fed lupin grain and grass hay. Lupin grain was offered at 1.0 kg/day, 0.75 kg/day and 0.5 kg/day for ewes with two lambs, one lamb and zero lambs respectively. Hay was offered ad libitum to all ewes. * ** Victorian College of Agriculture & Horticulture, Glenormiston Campus, Glenonniston South, Vic, 3265. Department of Agriculture and Rural Affairs, Pastoral Research Institute, PO Box 180, Hamilton, Vic. 3300. 264 Proc. Aust. Soc. Anim, 'Prod, Vol. 18 This Body composition was determined indirectly using tritiated water (TOH). method makes use of the close inverse relationship between body water and body After a preinfusion blood sample had been taken, a fat (Foot et al. 1979). dose of 200 uCi was injected intramuscularly and the ewes were weighed immediately. Feed and water were withheld until blood samples were taken, six hours after the injection. The preinfusion and six hour samples were analysed for TOH concentration using the method'described by Bird et al. 1982 and TOH space was estimated. TOH space and live weight at infusion were then used to estimate body fat. Estimates of body fat were made three to four days postpartum and in the third and fifth week of lactation in the lactating animals, and at equivalent times to post-partum and the fifth week of lactation in the non-mated animals.- The equations used to calculate body fat have been derived from lactating and dry Merino ewes at the same stages of lactation, in the same year as this trial. For example: Lambs were weighed at birth and weekly and lamb growth rate (LGR) was determined. To allow for a difference between sexes, LGR for female lambs was multiplied by 1.069, so that their growth rates were comparable with those of male lambs (Atkins and Thompson 1979). Equivalent dietary energy of body fat loss (MJ/d) was calculated by multiplying The value the change in body fat by 32.8 and dividing by the number of days. of 32.8 is the number of megajoules (MJ) of usable energy for lactation in each kilogram body fat. It is calculated from gross energy of fat (ARC 1980) and its efficiency of utilisation for lactation (Robinson 1978). Statistical analysis Mean live weights, fat weights and changes in these during early lactation were compared between groups using a series of 'Student's' t test. The relationships between fat loss, initial fat and lamb growth rate were examined by linear and multiple regression. Apart from the twin-bearing animals, Corr ewes were heavier immediately post lambing or the equivalent time in non-lambing ewes, than the other breeds (P<O.Ol) (Table 1). They also tended to have a higher weight of body fat but this was only significant (P<O.OS) in the four-year old non-lambing ewes. Changes in live weight and in body fat during the first five weeks of lactation (or equivalent time in non-lambing ewes) are shown in Table 2 along with estimates of the energy value of fat losses expressed as the equivalent of dietary feed. All groups of non-lactating ewes gained live weight and body fat whereas on average all groups of lactating ewes lost. The differences between lactating and dry ewes were highly significant (P<O.Ol). The overall gain in body fat in the-dry ewes during five weeks was 2.9 kg in the three groups of two-year old ewes and 3.8 kg in the two groups of four-year old ewes. In the first five weeks of lactation ewes with twins tended to lose more fat (3.2 kg) than younger ewes with single lambs (1.5 kg) but the differences were not significant. 265 Proc. Aust. Soc. Anim. Prod. Vol. 18 Table 1 Post-partum live weight (kg) and fat weight (kg) in Corriedale W=r), Western District Merino (WD) and South Australian Merino (SA) lactating ewes of two ages and eguivalent measurements in nonlactating ewes Means within age and physiological groups, and within columns with dissimilar superscripts are significantly different, ab (P<O.OS), cd (P<O.Ol). Table 2 Change in live weight (kg) and fat weight (kg) during the first five weeks after lambing (or equivalent time in non-lactating ewes) and the equivalent dietary energy value of body fat loss (MJ/d) Means within age and physiological groups, and within columns with dissimilar superscripts are significantly different, ab (P<O.O5), cd (P<O.Ol). 266 Proc. Aust. Soc. Anim. Pr'od. Vol. 18 DISCUSSION Seventy nine per cent of the variation in fat change in all lactating Corriedale ewes during the first three weeks after lambing was related to the fat weight in ewes immediately post lambing (initial fat, kg) and to lamb growth (I&R,g/d) over that period. Virtually no relationship like this was seen with the Merino ewes. Changes in fat weight during the first five weeks of lactation differed from those during the first three weeks in that they were less well related to initial fat weight and more closely related to lamb growth, and the regression coefficient for LGR was significant for lactating ewes of all genotypes. Calculations of the mean contribution of mobilised fat in each ewe group to daily energy requirements (Table 2) do not include ewes which gained fat over the five weeks. These were all non-lactating ewes, one in each lactating mature and maiden SA Merino group, and two and three in the WD and Corr. maiden ewe groups respectively. Differences between groups of lactating ewes were not statistically significant. The contribution of mobilised body fat to energy requirements in the first five weeks of lactation was estimated to be approximately 9% of the total for young ewes with single lambs and 11% for ewes suckling twin lambs. The total energy requirements were estimated assuming maintenance of live weight. The energy intake of all the lactating ewes during the first five weeks of lactation fell between 27 and 36% short of the energy estimated to be required for supporting the measured milk output of these ewes and maintaining their live weight. The energy from mobilised fat supplied only about a third of this deficit, even though the ewes were in relatively good body condition at lambing. With leaner animals such deficits might have negative effects on milk supply. These were not observed here. We would like to thank our colleagues for their substantial help with this work. In particular we acknowledge the contribution of Peter Heazlewood. ARC (1980). 'The Nutrient Requirements of Ruminanet Livestock'. (CAB: London). ATKINS, K.D. and THOMPSON, J.H. (1979). Aust. J. Agric. Res. 30:1197. BIRD, P.R., FLINN, PC., CAYLEY, J.W.D. and WATSON, M-J. (1982). Aust. J-Agric. Res. 33:375. BRYANT, D.T.W. and SMITH, R-W. (1982). J. Agric. Sci. Camb. 99:319. FOOT, J-Z., SKEDD, E. and MCFARLANE, D-W. (1979). J. Agric. Sci. Camb. 92:69. ROBINSON, J.J. (1978). In 'Milk Production in the Ewe' EAAP Publ. No. 23:53, 267