Variation in perinatal mortality of lambs with age and purity of ewes.

Abstract

VARIATION IN PERINATAL MORTALITY OF LAMBS WITH AGE AND PARITY OF EWES J. W. MC DONALD* Summary This paper presents data on the incidence and cause of lamb deaths from 3, 4, 6 and 7 year old ewe groups on one property in south west Victoria. Postmortem examinations were performed on 390 lambs. The perinatal mortality rate was highest among the lambs of young and aged ewes, being associated with a higher incidence of lambs dying from dystocia and starvation respectively. This variation in perinatal mortality is discussed in relation to the dead weights of the lambs. I. INTRODUCTION Barton (1947), Kennedy and Bettenay (1950), Purser and Young (1964) and Turner and Dolling ( 196*5) all observed higher mortality among lambs of young and aged ewes than among ewes of intermediate ages. Purser and Young (1964) found that there were more stillbirths in the young than the older ewes. Otherwise these various authors did not provide information on the causes of mortality. In the present paper, results are presented on cause as well as incidence of perinatal lamb death in four groups of ewes of different ages within one co,mmercial flock. II. MATERIALS AND METHODS The property concerned is in southern Victoria 65 miles west of Melbourne. A closed Polwarth flock is run on the property. Approximately 30% of all lambs born had been picked up dead at lambing in 1964, and there had been a history of poor reproductive performance for 16 years. Onion grass was thought to be implicated in the poor performance (Gorrie 1962). The observations were undertaken in 1965. The ewes in the four age groups studied were 3, 4, 6 and 7 years old at lambing and the groups were grazed in similar adjacent paddocks at the rate of about two sheep per acre. There were approximately 400 ewes in each group. Rams were joined for mating from mid December to early April, but observations were confined to lambs born during May and June. Dead lambs up to seven days of age were picked up daily and forwarded to the laboratory three times weekly, where post mortem examination was carried out. Cause of death was assessed at the time of post mortem examination using such criteria as those set out by McFarlane ( 1955). *S. S. Cameron Animal Research Laboratory, Department of Agriculture, Werribee, Victoria. 60 TABLE 1 Cause of lamb mortality in relation tu age of ewe III. RESULTS AND DISCUSSION The results are set out in Table 1 and Figure 1. The numbers of lambs born in the ewe groups were 100, 374, 290 and 300 lambs for the 3, 4, 6 and 7 year old ewe groups respectively. The incidence of lamb mortality was high in all ewe groups. Nevertheless, it varied significantly between the different groups (x2 = 26.96; P<O.OOl ) . The Fig. 1 .-Incidence and cause of lamb mortality in relation to age of ewe. 61 variation - higher mortality in the 3 and 7 year old ewes than in the 4 and 6 year old ewes - is similar to that found by Barton ( 1947), Kennedy and Bettenay (1950), Purser and Young ( 19614) and Turner and Dolling (1965). The high level of lamb mortality in the maiden 3 year old group and 7 year old group was associated with a significantly higher incidence of lambs born which died from dystocia and starvation respectively than in the three other groups combined (x2 dystocia = 54.9; P<O.OOl; x2 starvation = 20.0; P<O.OOl). Th e mean weight of lambs which died of starvation in the 7 year old group, 3.02 t0.13 kg, was similar ' to that of the combined other groups, 2.74 t- 0.09 kg. On the other hand the lambs which died of dystocia in the 3 year old group were significantly lighter, 2.88 t- 0.20 kg, than the mean weight of the combined The higher incidence of dystocia, other groups, 3.51 t- 0.14 kg (P<O.O5). associated with lower birth weight in the 3 year old group contrasts sharpl y with the results of Alexander, McCance and Watson (1955) who found that dystocia was associated with high birth weight of lambs, although lamb weights were generally low in the present study. The incidence of dead lambs which had not utilised any of their fat reserves and showed no other lesions except antemortem mutilation by predators was high in all groups (Table 1). The predators involved were foxes (vulyes v&es) and crows (Corvus spp.). In general there was little evidence of infectious disease and it consisted almost entirely of enterotoxaemia. These observations serve to indicate the variation in incidence of loss of different types that may occur in flocks of different ages on one property in one year. IV. ACKNOWLEDGMENTS The author is indebted to Dr. R. H. Watson of this laboratory for advice in the preparation of this manuscript. V. REFERENCES , G.9 MCCANCE, I., and W ATSON , R. H. (1955). Some observations on losses among meri no lambs. A ustralian Veterinary Journal 31: 85. B ARTON , R. A. (1947). Fertility in the Romney ewe, with particular reference to the Twotooth. Proc. 10th annual meeting of Sheep Farmers, Massey Agriculture College, New Zealand. GORRIE, C. J. R. (1962). Ovine Abortion in Victoria. Australian Veterinary Journal 38: 138. K ENNEDY , J. F., and B ETTENAY, R. A. (1950 Mating and lambing in an experimental of merino sheep. A ustralia n Journal o f A gricultural Research 1: 76. A LEXANDER MCFARLANE, D. (1955). Neonatal lamb mortality in the Gisborne area. Proceedings of the New Zealand. Society of Animal Production 15: 1014. P URSER , A. F., and Y OUNG , G. B. (1964). Mortality among twin and single lambs. Animal Production 6: 321. I T URNER , H. N., and D OLLING , C. H. S. (1965). Vital Statistics for an experimental flock of merino sheep. Australian Journal of Agricultural Research 16: 699.