Optimising Survival, Production and Immunity of Twin-Bearing Ewes and T heir Lambs

Summary

This project was conducted to determine if prepartum supplementation can substitute for tissue reserves of fat and skeletal muscle to regulate survival and production of twin-bearing Merino ewes and their lambs. Two replicated factorial field experiments were conducted which investigated the treatment effects of mid-pregnancy fat score (FS), prepartum supplementation and worm infection. The commercial relevance of these treatments was to assess the likely impact of managing twin-bearing ewes at FS targets below industry standards as a means of increasing whole-farm stocking rates. The work was conducted in the Northern Tablelands region of NSW which has a summer dominant rainfall pattern and severe winters and pasture, during the prepartum months of winter, is generally of high mass and very low ​quality. 

In both experiments, twin-bearing Merino ewes were deliberately managed to attain a low FS by day 100 of pregnancy and then given the nutritional opportunity to restore tissue reserves by supplementation with cottonseed meal pellets. The compensation displayed by low FS ewes in the last 35-40 days of pregnancy resulted in a very large increase in ewe and lamb survival to mirror that of high FS ewes. The compensation of low FS ewes in response to prepartum supplementation indicated a metabolic shift towards tissue anabolism which is a useful feature for sheep producers to manage. For example, sheep producers have the flexibility to run ewes during pregnancy at FS targets that are below current industry standards, for the Northern Tablelands region, with the caveat that they subsequently provide the ewe with the opportunity to replenish tissue reserves and control infections of gastrointestinal nematode parasites. Uncontrolled infections of T. colubriformis reduced birth weight and H. contortus reduced milk yield. In contrast to effects on survival, treatments applied during pregnancy had a variable effect on subsequent lamb growth with the duration of the treatment effect into lactation accounting for this variability. 

These observations indicate that the metabolic predisposition of the periparturient ewe to replenish tissue reserves provides the opportunity to maintain greater ewe stocking rates while safeguarding animal welfare. The key elements of adopting these actions are the ability and willingness of sheep producers to fat score ewes and to plan appropriate feed resources (eg. supplements, pasture, crop) to allow ewes to gain condition in the last 50 days of pregnancy. The magnitude of effects of lower ewe FS on whole-farm stocking rates will vary among farms depending on factors such as, pasture availability and quality and time of lambing and this precludes an specific estimate of the size of the effect. However, in relation to maintaining FS during pregnancy, it seems reasonable to conclude that these actions alone may allow producers to increase ewe stocking rate by approximately 10% which is an effect additive to those benefits for ewe and lamb survival.