A.N. Thompson, M.B. Ferguson, S.E. John, G. Kearney, G. Rose and J. Young School of Veterinary and Biomedical Sciences, Murdoch University, Murdoch, WA 6150 Sheep Industries Group, Department of Agriculture and Food Western Australia, South Perth, WA 6151

Introduction

Merino ewes are the backbone of the Australian sheep industry and this is likely to be the case for some time. Stocking rate will remain a key profit driver in Merino enterprises and to maintain or improve profitability producers will need to continually adapt their production systems to deal with even larger changes in feed supply between seasons and years. The reproductive performance of the Merino ewe also needs to improve, largely through improving the survival of twin born lambs, to rebuild flock numbers and meet market demand for lamb and sheep meat. Increasing both stocking rates and reproductive performance need to be achieved in the context of producers wanting to run more sheep per person with less intervention and increased consumer demand for welfare friendly products. Improving genetics and matching sheep genotype to the production and management system will inevitably become more important. We believe this will include defining traits to more easily identify Merino sheep that are more robust, that lose less liveweight when faced with sub-optimum nutrition and that produce more progeny with higher survival rates both pre- and post-weaning.

Increasing genetic fat is the prime candidate for increasing the robustness of Merino ewes and their progeny as the storage and mobilisation of fat is an important mechanism for all animals to cope with fluctuating environments. Fat is stored during favourable times and then mobilised to provide energy for fundamental functions when requirements exceed supply, such as during periods of limited nutrition or during late pregnancy and lactation. The amount of fat stored in fat depots in sheep can be increased by selection for higher subcutaneous fat depth, using Australian Sheep Breeding Values (ASBVs) from MERINOSELECT. However, from a genetic perspective, reducing the fatness of lamb to improve its appeal to the consumer has resulted in a general focus on selection for less fat in Australian sheep breeds. Merino sheep have also become leaner as a result of selection for higher fleece weights and the genetic association between higher fleece weight and reduced fatness (Huisman and Brown 2009). Defining the true value of fat requires an understanding of the effect it has on the value of lamb carcasses as well as its effects on the productivity of the sheep production system in different environments. In this paper we have reviewed published papers and our own unpublished work to test the hypothesis that Merino sheep that are genetically fatter will have improved performance especially under more restricted nutritional conditions.

Ewe liveweight loss to restricted nutrition

Sheep producers across southern Australia, especially those located in more marginal and variable environments, rank selection and breeding of sheep that are more resilient to poor nutrition and that can survive and produce under these conditions as a priority (M.B. Ferguson unpublished data). Data from the Sheep CRC Information Nucleus Flock indicate that ewes from some sires lost 5-kg of liveweight during summer and autumn when the supply and quality of paddock feed is limiting whereas ewes from other sires gained liveweight (John et al. 2011). Current estimates suggest that the heritability of this trait in Merinos is low (0.10-0.15; Rose et al. 2011), but nevertheless it could be possible to breed adult Merino ewes that are more tolerant to variations in feed supply. More information is needed on the genetic correlations between 2 liveweight loss during summer and autumn and other production traits and the biological mechanisms responsible for differences in liveweight loss to determine the true value of the trait within different sheep production systems.

Ewes that lose less liveweight when paddock feed is limiting may be in better condition at mating and or throughout pregnancy resulting in higher fertility, fecundity and lamb survival. Rose et al. (2012) has shown that Merino ewes that lose less liveweight during mating on poor quality dry pasture have a higher probability of giving birth to and weaning lambs. The genetic correlations between liveweight change during mating and the probability of having a lamb ranged from 0.22 to 0.81 and the probability of weaning a lamb ranged from 0.56 to 0.93 across ewe age groups. It is known that liveweight loss during pregnancy also influences lamb birth weight and survival. Merino ewes that lose 10 kg during pregnancy produce lambs that are 0.3 to 0.5 kg lighter and this can reduce the survival of twin born lambs by up to 20% (Oldham et al. 2011). The risk of ewe mortality also increases rapidly below condition score 2 (A.N.Thompson unpublished data). It is therefore reasonable to expect that ewes which are genetically more robust and lose less weight during summer and autumn will wean more lambs, but the effect is likely to depend on time of mating and lambing in relation to seasonal changes in feed supply.

We have recently demonstrated that adult Merino ewes that lose less weight when nutrition is restricted are genetically fatter (S.E. John unpublished data). Ewes were fed rations formulated to achieve liveweight maintenance when offered ad libitum or an average liveweight loss of about 100 g/day when intake was restricted to 50% of estimated maintenance requirements. The results indicate a significant interaction between diet and estimated breeding values for yearling fat (YFAT, Fig. 1a). If nutrition was restricted, liveweight loss was reduced by about 25 g/day for every mm increase across the range of YFAT values (-1.28 to 1.01). By contrast, when fed around maintenance, albeit not statistically significant, weight gain was reduced by 19 g/day per mm increase in YFAT. Extrapolation of these data suggests that ewes with a 2-mm higher YFAT value could be up to 5-kg heavier if a restriction on nutrition similar to this study was imposed for 3 months. This association between liveweight loss and an easily measured trait like YFAT suggests that YFAT could potentially be used to select more robust Merino ewes.

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