Abstract:
Milk Yield of Australian Merino Ewes and Lamb Growth under Pastoral Conditions H. L. D AVIES * Summary Milk yields of Merino ewes were determined at Dickson ` Experiment Station, Canberra, over a 10 weeks lactation on native and improved pastures under varying systems of management. Ewes rearing twin lambs gave an average of 231 lb of milk in 10 weeks. Ewes rearing one lamb averaged 153 lb in a 10 week lactation. These results suggest that :(a) Milk yield is influenced by the number of lambs suckled, but not influenced by the number of lambs born. (b) The potential milk yield of Australian Merinos is not a limiting factor in lamb growth. In the Australian environment, reliance for the ewe' nutritive s requirements is often placed solely upon pasture, therefore knowledge on the capacity of pasture as the raw material for milk production is needed. Low milk production by the mother can not only limit the expression of the growing animal' full genetic potential s for an economic product but in the first days of life the survival of the lamb may be jeopardized. There is no published information on the milk yield of the grazing Australian Merino. Pierce (1934, 1936) in South Australia under pen conditions determined milk yields for 24 hr periods in the 3rd and 9th weeks of lactation in one experiment using 6 Merino ewes and in a further experiment using 5 ewes for the 2nd, 4th and 6th weeks of lactation. Several workers (Fuller and Kleinheinz, 1904; Neidig and Iddings, 1919; Bonsma, 1939; Wallace, 1948; Hugo, 1952; Thomson and Th,omson, 1953; Guyer and Dyer, 1954) have carried out milk yield trials under pen-feeding conditions. Barnicoat, Logan and Grant using Romneys (1949, 1956) and Owen using Welsh Mountain sheep (1957) determined milk yields under pasture conditions. The present work sought to determine the levels of milk production of Australian Merino ewes on different pastures available at C.S.I.R.O. Experiment Station, Canberra. MATERIAL AND METHQDS Thirty-two 4 to 7 year old Merino ewes were randomly selected from the Station flock, which was of medium-wool Peppin origin. Subsequent to mating with medium-wool Peppin rams the ewes were divided into six groups, 4 groups of six ewes and 2 groups of four, on the basis of body weight. They were placed in descending order of bodyweight and allotted at random within blocks of six. Four of the six groups were put in appropriate plots on May 6. Th,e two remaining groups were put on their appropriate plots on Jul*y 21. The times, types and rates of stocking of the pastures are shown in Table I. *Division of Plant Industry, C.S.I.R.O., Canberra, A.C.T. 15 16 Lambing started on 14.viii.55. Table II ment, the number of lambs born and reared born as twins but reared as singles, due to within 24 hr of birth, and the number born TABLE II. shows for each treatas twins, the number the death of one lamb and reared as singles. Type of Birth and Number of Lambs Reareld. (i) Liveweight of Ewes.-The ewes were weighed to the nearest 0.5 lb on a platform scale at intervals during pregnancy and lactation. (ii) Milk Yields .--Most of the methods quoted in the literature for measuring the milk yield of sheep are unsuitable under natural grazing conditions. An undesirable feature of many methods in separation of the ewes and lambs. Objections to separation are as follows :- . (1) Separation of ewe and lamb could interfere with normal grazing behaviour and increase the possibility of lower feed intake by the ewe which could affect the ewe' milk s yield. (2) The distress to the ewe caused by separation could upset the neuro-hormonal equilibrium of the ewe which could have a marked effect upon milk yield. The method used to overcome this difficulty was that used by Owen (1957) ,+' which involves placing an udder cover on the ewes so that suckling is prevented except when the experimenter wishes but grazing is not disturbed. (iii) Liveweight of Lambs.- The lambs were weighed to the nearest 0.5 oz. on a spring balance. The liveweight for the week was taken as the weight prior to the fifth suckling of the weekly test period. (iv) Pasture Production.-The pasture information in this paper is a measurement of herbage available to the animal at intervals during the grazing season. Open quadrats were cut to ground level and the yields expressed in lb per acre on an oven-dry weight basis. Willoughby (1958) has shown that in a pasture of green and dry components, animal production is strongly related to the amount of green material expressed on a dry-weight basis, and only *The only modification made was that the period for determining milk yields extended over 30 hours each week instead of 24. The milk yield for each recording period was computed from the last four weights of amount suckled: the first weight in each period was ignored because the state of the ewes was unknown when the cover was first placed. Recordings were made over a 19 week lactation. 17 slightly related to the quantity of dry material presented. The pasture yields given in this paper refer only to green feed available to the grazing animals. (i) Liveweights of the Ewes.-The average liveweight changes of the ewes showed large differences in liveweight gain during pregnancy as a result of differing levels of green feed available during the autumn and winter. There were no significant differences between any of the sown pastures. Table III shows the liveweight gains of the ewes when the sown-pasture groups are put together as one treatment. TABLE III. Liveweig' Gain.s of the Ewes from May-Augnst. ht R ESULTS There were no significant differences in liveweight gain of the ewes between any treatments during the post-lambing periods. (ii) Milk Production of the Ewes.--A preliminary analysis of variance by fitting constants was carried out on the total milk yield from the 2nd to the 8th week of lactation. The numbers of animals on pastures I and II were too small for adequate treatment comparisons, and no significant difference between pasture treatments could be demonstrated. There was a marked difference in milk yield, however, between ewes rearing single lambs and ewes rearing twin lambs, the latter having a much higher yield. The lactation curves for ewes rearing twins and ewes rearing singles for a 10 week lactation is shown in Figure 1. The total milk yield for a 10 week lactation for ewes rearing one lamb was 152.6 lb; the total milk yield for ewes rearing twins was 231.2 lb-a difference of 78.6 lb which was highly significant. (iii) Liveweight Gains of th,e Lambs.-The growth. curves of single and twin lambs are shown in Figure I. The difference in weight between the single and twin lambs at IO weeks old was 3.04 lb. The figures for the singles include the four lambs whose mothers were on pasture I. The differences between singles and twins were statistically significant. The differences in lamb growth as a result of pasture treatment were not significantly different. However, the actual gains on each treatment suggested that growth of lambs at a stocking rate of 7 sheep to the acre from mid July until December 31 in the year of this study (1955) on a Phlaris tuberosalsubterranean clover pasture compared very favourably with lamb growth on natural pasture at I sheep to the acre. Several authorities, e.g., Hammond (1938)) Belschner (1950)) Hawkins (1956, have stated that Merinos are unsuitable for fat lamb production because of low milk production by the ewe. However, these preliminary results of milk yields of Australian Merino ewes under normal grazing conditions compare quite favourably with milk .yield of ewes of recognised fat-lamb breeds. Table IV gives milk yields obtained by other workers. A stud.y of this table would suggest that criticism of the Australian Merino ewe as a fat-lamb mother on the basis of insufficient milk yield is not valid. However, this experiment does not suggest that the pure-bred Australian Merino is at present a suitable fat-lamb breed; what is implied is that the milk yield per se of the Australian Merino is not a limiting factor in lamb growth. The large differences between the milk yields of ewes rearing twin lambs and those rearing singles confirm the findings of W*allace (1948). Th,e differences between the milk yields of ewes rearing single lambs born as singles and ewes rearing a surviving twin lamb when its sib had died within 24 hr was small (7 lb of milk in 8 weeks) and not statistically significant. The contrast of ewes rearing twins with those rearing singles suggests that the amount of milk withdrawn in ear1.y lactation has a profound effect upon subsepresquent milk yield. These findings are in accordance with the ' ture theory' of milk secretion propounded by Hammond (1936). He suggested that the pressure created by accumulated milk slows down th,e rate of secretion and eventually causes the cessation of milk production. Thus the ewes with twin lambs had had much. more of the milk formed in the first 48 hr removed, thus stimulating an increased secretion rate. The inability of single lambs in very early lactation to withdraw all the available milk could cause an increased pressure in the udder from accumulated milk which could then decrease the rate of milk secretion and hence reduce total milk yield. An interesting study arising from these observations would be to see what effect different breeds of lambs would have upon the milk yield of the ewe. For instance, there is the possibility that a heavier and more rapidly-maturing lamb, with presumably a higher feed requirement, would withdraw greater amounts of milk in the early stages of lactation than a lighter and slower-maturing lamb; th,is greater withdrawal could thus stimulate the secretion of further amounts of milk. Some evidence from growth rates of the lambs of Merino ewes crossed with Border Leicester rams having a liveweight gain of 21 lb between birth and 28 da.ys (Davies, unpublished) would still further suggest that milk yield of Merino ewes is not a limiting factor in lamb growth. 19 D ISCUSSION The data reported here would suggest that a re-assessment of the Australian Merino be made in the role of fat lamb mother provided she was crossed with, a ram of suitable breed so that the progeny could fully exploit her genetic potential for milk production. The author is particularly indebted to Mr. A. Axelsen for invaluable technical assistance at all stages of the experiment, to Messrs. J. Harding and E. Cooper for assistance with the milk-.yield determinations, to Messrs. G. A. McIntyre and M. L. Dudsinski, Division of Mathematical Statistics, C.S.I.R.O., for much assistance with statistical treatment of the data. Finally, without the continuous help, stimulating discussion and encouragement of Mr. W. M. Willoughby, this work could not h,ave been carried out. Barnicoat, C. R., Logan, A. B., and Grant, A. I. (1949) .-T. agric. Sci. 39: 44 and 237. . Barnicoat, C. R., Logan, A. B., and Grant, A. I. (1956) .-J. agric. Sci. 48: 9-33. . Belschner, H. G. (1950).-'Sheep Management and Diseases' p. 57. (Angus and Robertson : Sydney). Bonsma, F. N. (1939). -Univ. Pretoria Publ. Ser. I. Agric. No. 48. Burris, M. J., and Baugus, C. A. (1955) .-J. Anim. Sci. 14: 186. Fuller, J. G., and Kleinheinz, F. (1904) .-Wisconsin Experiment Station Report 21: 48. Guyer, P. A., and Dyer, A. J. (1954) .-Univ. Missouri College of Agric. Res. Bull. No. 58. Hammond, J. (1956).-Vet. Rec. 16: 514. A Hammond, J. (1938) .-' Report on the Conditions of Animal Production in Australia' C.S.I.R. Aust. Pamphlet No. 79, . p. 18. Hawkins C. J. (1956)-Agtic. Gax. N.S.W. 67: 67. Hugo, W. J. (1952) .-Farming in South Africa 27: 503. 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