Neonatal lamb mortality in flocks of Merino ewes selected for and against fold development.

Abstract

NEONATAL LAMB MORTALITY- IN FLOCKS OF MERINO EWES SELECTED FOR AND AGAINST FOLD DEVELOPMENT ByR.B. D UN * and T. A. W ALL * Summary A survey, using reciprocal matings, has been made of neonatal lamb mortality in two Merino flocks, Folds Plus and Folds Minus, which have been selected for and against skin fold development since 1951. Results showed that death of single lambs was concentrated in the group where Folds Plus rams were mated to Folds Plus ewes. This mortality was almost invariably associated with dystocia or difficult birth due to the presentation of over-size lambs. I. INTRODUCTION Two Merino flocks selected for and against skin folds were established at Trangie Agricultural Research Station in 195 1. Selection has been based entirely on a score visually estimated after the weaner shearing. The photographic standards developed by Carter ( 1943) have been used, a total score being given each sheep by addition of separate scores for neck, side, and breech. A rapid response to selection has been obtained, the Folds Minus flock now being tight skinned while the Folds Plus flock carries slightly more fold than stud Peppins (average fold scores on 1959 drop: Folds Minus 5 1; Folds Plus 17 02; Trangie Stud 16.6). In examining the production record of these flocks, the poor lambing performance of the Folds Plus ewes was soon noted. Over the series of matings from 195 1 to 1960, these ewes showed 10 per cent. less twins mothered and 12 per cent. more wet ewes without lambs at foot. In addition, the annual death rate of ewes (principally from mortality at lambing) averaged 9 per cent. for the Folds Plus flock as against 4 per cent. for the Folds Minus flock. We therefore decided to make a detailed examination of mortality in these flocks, making the initial hypothesis that the Folds Plus ewes were poor mothers of weak constitution. l II. MATERIALS AND METHODS As an annual routine, each folds flock of 100 ewes is mated to 3 hogget rams for 3 weeks by artificial insemination. For a further 3 weeks, each flock is then backstopped to 2 hogget rams, individually mating 1 ram to 50 ewes. This general plan was adhered to in the present experiment, and in addition reciprocal matings were employed. Ewes varied in age from 2 to 8 years, the age distribution being approximately the same in each flock. Both flocks- were lambed together in an open 90.acre paddock, a constant watch being kept on the ewes during daylight hours. Dead lambs were examined using the technique of MacFarlane ( 1955). * New South Wales Department of Agriculture, Research Station, Trangie. 187 III. RESULTS AND DISCUSSION Table I shows the pattern of lamb losses. Losses of twin lambs (desertion and starvation) were equally distributed between mating groups. Growth rate of lambs also indicated little difference between flocks in mothering ability. T ABLE 1 PATTERN OF NEONATAL LAMB LOSSES FROM EWE FLOCKS SELECTCD FOR AND AGAINST SKIN FOLD DEVELOPMENT Single Lambs Fate of Lamb Folds Plus Ram Folds Plus Ewe 2 4 1 7 28 Folds Minus Ewe 0 1 0 1 28 Folds Minus Ram Folds Plus Ewe 1 1 0 Twin Lambs Folds Plus Ram Folds Plus Ewe 0 1 2 3 Folds Minus Ewe 0 0 1 1 ! Folds Minus Ram Folds Plus Ewe 0 0 4 4 i Folds Minus Ewe 1 1 3 5 31 Died prior to birth Died during birth Died after birth Total deaths Survived ._- Folds Minus Ewe 0 0 0 0 27 - - - - 2 26 19 3 ! 20 I Difference between flock combinations in losses of single lambs, significant o-025 > P > O-01 There was a marked concentration of single lamb deaths in the Folds Plus ram x Folds Plus ewe group. A closer examination of these losses showed that they were invariably associated with dystocia or a difficult birth, caused by the presentation of over-large lambs. Four ewes from the Folds Plus x Folds Plus group died from unresolved dystocia-the only ewes which died during the experiment. Further examining the cause of these losses, birth weights were analysed, results being listed in Table 2. There were highly significant sire and dam effects, such that very large lambs were concentrated in the Folds Plus x Folds Plus flock. These striking birth weight differences were not associated with heavier body weight of Folds Plus ram or ewes, nor with an increase in length of gestation. IV. CONCLUSION Our theory that heavy lamb and ewe mortality, in a flock bred for increased fold development, was due to poor mothering ability of ewes has not been substantiated by experiment. From the data presented we now hypothesize that selection for increased fold development has been associated with selection of sheep 188 of heavy birth weight. When single lambs were carried this resulted in an increased incidence of dystocia, with death of lambs and ewes sufficient to explain our previous field observations. TABLE 2 MEAN BIRTH WEIGHTS FOR LAMBS BORN TO EWES SELECTED FOR HIGH AND LOW WRINKLE DEVELOPMENT. MEANS WERE CALCULATED AFTER CORRECTING FOR TYPE OF BIRTH Birth Weight (ib) Flock Age of Ewe Folds Plus Rams Ram Lambs Maiden-2 yrs Folds Plus Ewes 3-4-5 yrs . 6-7-8 yrs . Weighted Means Maiden-2 yrs Folds Minus Ewes 3-4-S yrs . 6-7-8 yrs . Weighted Means 8.20 8*91 10.78 8980 10~00 10.39 10*03 964 7.78 7*74 954 8.31 8941 8.62 8.40 Ewe Lambs 9.03 Se97 9.83 Folds Minus Rams Ram Lambs 8.28 9*43 9.75 9.03 7969 8904 8.97 Ewe Lambs 8920 8.65 9.15 Summary of analysis of variance 1. No. 1st or 2nd order interactions significant 2. AI1 the following treatment effects highly significant, P < 0.0 1 Type of Ram Type of Ewe Sex of Lamb Age of Ewe V. ACKNOWLEDGEMENTS Our thanks are due to Professor David MacFarlane of the Sydney University Veterinary School for demonstration of, and discussion relative to, the postmortem examination of lambs. VI. REFERENCES H. B. (1943) .-Studies in the biology of the skin and fleece of sheep. Bull. Coun. Sci. Indust. Res. Aust. No. 164: 59. MACFARLANE, D. (1955) .-Neonatal lamb mortality in the Gisborne area. Proc. N.Z. Soc. Anim. Prod. 15: 104-119. C ARTER , 189