Relationships between pasture availability, milk supply, lamb intake and growth.

Abstract

RELATIONSHIPS BETWEEN PASTURE AVAILABILITY, MILK SUPPLY, LAMB INTAKE AND GROWTH C.M.J. WILLIAMS*+, P.E. GEYTENBEEK* AND W.G. ALLDEN* Summary Competition for herbage between ewes and their lambs was measured over a wide range of pasture availabilities at one stocking rate during a 33 day period. Sucking lambs had significantly better growth rates than weaned lambs at all levels of pasture availability. The relative contribution of milk and grass to the total intake of the lamb varied with the tiller length of pasture. At each level of pasture availability, weaned lambs consumed at least twice as much grass as the sucking lambs, but this greater intake of grass did not compensate for the lack of milk resulting from early weaning. Removal of weaned lambs and the weaning of sucking lambs from pastures of very short tiller length (less than 5 cm) onto pastures of 10 cm or longer would result in an increased lamb growth rate from unsatisfactory low levels to reasonable gains of about 200 g day-1. This gain is approximately 70% of that achieved by sucking lambs grazing with their dams on pastures of similar tiller length. However, the plant pool was rapidly depleted below its initial level when ewes and lambs were grazing together, whereas weaned lambs had little effect on the amount of plant material present in spring. If pastures of greater length than 10 cm are not available for weaning, it is preferable to leave the lamb sucking the ewe, to ensure lamb survival. I. INTRODUCTION The quantitative definition of the relationship between the herbage available, its effect on milk supply and the growth of the lamb are of The intake of digestible energy importance to the .prime lamb industry, (DE) is a major determinant of lamb liveweight gain (Joyce and Rattray Also lamb intake and growth in grazing situations are dependent 1970) on the amount of plant material present (Arnold 1964; Allden and Whittaker 1970). l The results of early weaning experiments are mixed and inconclusive (Watson and Elder 1960; Spedding, Large and Brown 1961; Jagusch et al, 1971). This is most likely due to a confounding of pasture andanimal factors in these relatively long term experiments. Also few workers have considered the quantitative importance of the level of pasture quality and availability in relation to competition for feed between ewe and lamb (Langlands 1973)0 These factors could have a major influence on the outcome of early weaning practices. Competition between ewesand their lambs for herbage was measured at five levels of pasture availability and at one stocking rate at the Mortlock Experiment Station, Mintaro, South Australia. * The University of Adelaide, Waite Agricultural Research Institute, Glen Osmond, South Australia, 5064. + South Australian Department of Agriculture, South East Regional Headquarters, Box 618, Naracoorte, South Australia, 52710 333 II. MATERIALS AND METHODS (a) Pastures Pure swards of Wimmera ryegrass (Lolium rigidum Gaud.) at five levels of pasture availability were prepared as described by Allden and Whittaker (1970). Pasture availability as tiller length (TL, as defined and estimated by Allden and Whittaker, 1970) was measured weekly. The relative rate of change of tiller length was calculated for Phase 1 as the difference between the logarithms of tiller length at day 2 and day 16 divided by the time in days. (b) Animals Wether lambs from Dorset Horn rams joined to Border Leicester x Merino ewes were divided into ten groups of four on the basis of weight and age and the groups allocated at random to the weaned or suckling treatments. Half the lambs were weaned at an average age of six weeks, one week before treatments began. These lambs became treatment 1, their ewes became treatment 2 and,the remaining lambs with their ewes became treatment 3. A constant stocking rate (80 animals ha-l) was maintained over all availabilities, i,e. flock types (2) and (3) each grazed half the area available to flock (1). (C ) Animal Measurements Ewes and lambs were weighed, after fasting overnight, at weekly intervals during the experiment. The energy content of liveweight changes during the experimental period was estimated from the relationships of body weight to body energy calculated for lambs by Allden (1970) and for ewes by Hutchinson (1969). Ewe lambs (either we wethers and fistulated at ment to obtain dietary pa lambs were estimated over phase. The daily intake was estimated by methods that the digestibility of by three units. The ener (1,6 kcal) per gram of fr of ryegrass was taken as organic matter (Kellaway aned or sucking) from the same mating as the oesophagus were used six to ten per ,sture samples. Herbage and milk intake a 4-day and a l-day period respectively of milk and grass organic matter by the similar to those of Langlands (1972), ex the grass consumed by lambs was not red gy content of ewes milk was taken as 6,5 esh milk (Perrin 1958) and the energy co lg.7 kJ (4.7 kcal) per gram of digestibl 1969). III. RESULTS the treaAof al.l in e ach lambs cept uced kJ ntent e (a) Growth rate and body energy Sucking lambs had significantly better growth rates and estimated body energy gains (at least 50 g day' and 1000 kJ da@, respectively) 334 than did weaned lambs at .a11 pasture availabilities (Figure la). However, lactating ewes lost more weight and body energy than did the dry ewes at low pasture availabilities. Only at high levels of available pasture did lactating ewes show similar liveweight and body energy changes to those of dry ewes (b) Pasture changes The ewes with or without lambs substnntia!.!.y reduced the pasture available, whereas the weaned lambs had little effect (Fig. lc). The figure also shows that when an area was divided equally between a ewe and her weaned lamb, the grazing pressure of the ewe was greater and of the . lamb less than when the ewe - lamb association grazed the whole area together. (c) Lamb intake The digestibility of the grass selected by the fistulated lambs was similar at each level of pasture availability irrespective of whether they had been weaned' or were suckling (Table l), Table 1 shows that at each level of pasture availability weaned lambs consumed twice as much grass as the sucking lambs, but this increase in grass intake did not compensate for the lack of milk resulting from early weaning, Also, a.lthough greater liveweight losses were recorded for ewes on short pasture, milk production was equal to that of ewes grazing abundant pasture. 335 TABLE 1 The digestibility ($) and grass DE intakes of lambs, the milk DE intakes of sucking lambs and the total DE intakes of lambs grazing pastures of different availabilities. All DE intakes are expressed in terms of kJ kg-1 WO.73 dav-1. IV. DISCUSSION The results of this short term study indicate that only under situations of low pasture availability is there any merit in weaning the young lamb, and if any growth advantage is to be obtained the lamb must go onto pastures of higher availability (Figure la). In the current study there was no suggestion of any advantage of weaning 8 weeks old lambs when the tiller length of pastures was greater than 10 cm; indeed, there was a loss in lamb growth rate resulting from the separation of ewe and lamb. Thus, in the ewe - lamb unit the ewe sacrificed her own body tissue at low pasture availability in order to sustain the growth of'her lamb. Under such conditi,ons the weight loss of the ewe when separated from her lamb was substantially less than that of the suckling ewe, but her lamb grazing alone grew more slowly than the sucking lamb. The use of body energy to sustain milk production in the short term was an important compensatory mechanism under conditions of adverse nutrition, The results in Table 1 show that there were no consistent differences in the total DE intakes of the lambs at each level of availability, although the partition of intake between milk and grass energy differed with treatment, but the sucking lambs grew significantly faster than the weaned lambs (Figure la). If the higher efficiency of use of milk compared to grass is taken into account the differences between the treatment groups can be eliminated (Jagusch and Mitchell 1971). VI REFERENCES ALLDEN, W.G. (1970). Aust. J. a&c. Res. 21 : 261. ALLDEN, W.G. and WHITTAKER, I.A. McD. (1970).Aust. J, agric. Res. 21 : 755. ARNOLD, GOW. (1964). Proceedings of Aust. Soc. of Animal Prod. 2 :275. HUTCHINSON, K.J. (1969). Ph.D. Thesis, University of New England. JAGUSCH, K.T. and MITCHELL, R.M. (1971), N.Z. J. of Agric. Res. 14 : 434. 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