Lamb mortality and growth to weaning in commercial Merino flocks in Western Australia

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Eighteen farm flocks of four-tooth Merino ewes (average of 371 ewes at lamb marking, range 278 to 511) were studied in 1987/88 ( n =8 ) and 1988/89 (n=10) over the period from 2 to 4 weeks pre-joining until 12 weeks post-lambing. All farms were located in the south-west of Western Australia, and the flocks started lambing between mid-April and mid-July. The average liveweights of ewes pre-joining ranged over 40.2 to 61.8 kg, and average condition scores ranged from 2.2 to 4.2. Mean liveweight changes of the ewes ranged over losses of up to 8.5 kg to gains of up to 10.4 kg between consecutive recordings at 2 to 3 month intervals up to 12 weeks post lambing, and condition score changes ranged over minus 1.2 to plus 1.2. The greatest losses in liveweight and condition score in many flocks corresponded with the period from mid-pregnancy to marking, the time of greatest nutrient demand of the ewe. There was considerable variation between flocks in the percentage of dry ewes (2.9 to 17.7%), lambs born per ewe present at marking (83.0 to 147.3%), lambs alive per ewe present at marking (66.3 to 123.2%), and lamb deaths as a percentage of total lambs born (10.9 to 29.8%). For single and twin-born lambs, the percentage of lamb deaths between flocks ranged from 6.0 to 19.9% (mean = 11.0%) and 19.1 to 63.2% (mean = 33.1%) respectively. Of the ewes giving birth to twins, an average of 9.6% lost both lambs. The mean birthweights were 4.4 kg (range 3.6 to 5.1 kg) for singles and 4.0 kg (range 3.3 to 4.5 kg) for twins, and were highly correlated (r = 0 - 74, n = 13 flocks, P<0.01). Lamb growth to marking averaged 232 g head-1 day-1 and 205 g head-1 day-1, and from marking to weaning 198 g head-1 day-1 and 193 g head-1 day-1 for single and twin-born lambs respectively. The mortality rates of both single and twin-born lambs were highly correlated with mean liveweight of the ewes at mid-pregnancy (r = -0.64, P<0.01; r = -0.75, P<0.001; respectively). A kilogram increase in mean liveweight at mid-pregnancy was associated with declines in the mortality rates of single-born lambs of 0.7% units (s.e. = 0.23), and of twin-born lambs of 2.2% units (s.e. = 0.57). Mortality rates of the singles were highly correlated with that of twins (r = 0.86, P<0.001), with a 1% unit change in death rate of singles being associated with a 2.5% (s.e. = 0.41) unit change for twins. Mortality rates of twins were associated with mean cotyledon diameter measured in mid-pregnancy (r = -0.48, n = 13, P<0. l0), and mean umbilical resistance to blood flow (r = 0.78, n = 9, P<0.02). Growth from birth to weaning of single-born lambs was associated with the mean liveweight of the flock in mid-pregnancy (r = 0.72, P<0 .001). Clean fleece weight and mean fibre diameter of the fleeces from single.bearing and rearing ewes were correlated with mean ewe liveweight in mid-pregnancy (r = 0.56, P<0.05; r = 0.70, P<0.01; respectively). Staple strength was correlated with mean ewe liveweight at marking for both single and twin producing ewes (r = 0.72, n = 18, P<0.01; r = 0.74, n = 9, P<0.05; respectively). It was concluded that liveweight of the ewe in mid-pregnancy, which represents the sum of starting liveweight and liveweight change, is the most important single and practical criterion that can be used by a farmer to set the goals for nutritional management of the flock over pregnancy to improve lamb survival.