Reducing Lamb Mortality by Enhancing Placental Efficiency

Secondary Supervisor(s): Mrs Nicky Naylor, Dr Robert Wilkinson, Dr Sarah Morgan

University of Registration: University of Birmingham

BBSRC Research Themes: Sustainable Agriculture and Food (Animal Health and Welfare)

Project Outline

With 7.1 million lambs born in the UK in 2023 and lamb mortality rates ranging from 7 to 21%, identifying the causes of, and adapting management practices to reduce neonatal lamb mortality is critical. Up to 50% of lamb losses occur within 48h of birth, and lamb survival is strongly associated with lamb birth weight (LBW).

Birth weight is primarily dependent on nutrient transport and supply to the growing foetus, particularly during late gestation. In sheep, nutrient exchange occurs in the placentomes of the placenta which are comprised of maternal caruncles and foetal cotyledons, with the number and/or weight of cotyledons being positively correlated with placental efficiency (PE), LBW and neonatal vigour (Dwyer et al., 2005). Sex of offspring, litter size and maternal factors including parity, breed, body condition score and nutrition also impact LBW.

To facilitate nutrient exchange, the cotyledon vasculature increases 12-fold during mid-late gestation (Reynolds et al., 2005) illustrating the importance of the function of the placentome in foetal growth. The effect of maternal nutrition on LBW and placental function is not fully elucidated and whilst it is generally accepted that nutritional restriction reduces LBW, the impact on PE is less established with changes in placental blood flow, placental weight, and size of cotyledons being variable. Supplementation of the diet with amino acids and antioxidants improves PE and increases LBW. Previous research at HAU determined that dietary vitamin E increases LBW though the mode of action is unknown. It is proposed that antioxidants may reduce oxidative stress through their ability to scavenge reactive oxygen species (ROS), and thereby enhance PE and foetal growth. There remains a lack of research investigating the effect of specific nutrient interventions on PE including those that may affect angiogenesis, oxidative stress or cotyledon size.

To better manage LBW and hence survival, there is a need to identify the maternal, offspring and nutritional factors that affect PE and LBW. This can aid the development of management practices to reduce mortality of neonatal lambs, and thereby improve productivity and welfare in the sheep industry.

Objectives

There are 3 main objectives:

Determine the animal factors affecting lamb birth weight: Using retrospective HAU Future Farm data, maternal and offspring factors affecting LBW and survival will be identified. Factors explored will include litter size, sex of offspring, ewe parity, ewe breed, ewe BCS at parturition, lambing date, sire and sire breed.

Determine the relationship between placental efficiency, lamb birth weight and neonatal lamb vigour: Following a live field trial, lambs will be weighed at birth, assessed for neonatal vigour, and survival rates recorded. Placentae will be collected from ewes at birth and assessed for gross anatomy to determine placental efficiency, cotyledon efficiency and cotyledon density. Histology and immunohistochemistry will determine cotyledon function, particularly blood flow. Data analysis will identify any maternal or offspring factors that affect PE and function.

Investigate the potential of specific nutrient interventions to increase placental efficiency, improve lamb birth weight and reduce neonatal mortality: A systematic review of the literature will determine the effect of antioxidants and/or amino acids on placental function in sheep to highlight nutrients that are worthy of further investigation. Using controlled experiments, identified nutrient interventions will be added to the diet of ewes at a pre-determined stage of pregnancy. Maternal and offspring factors identified in objectives 1 and 2, LBW, survival rate, and neonatal vigour will be recorded. Placentae will be examined in the laboratory as described in objective 2. Data analysis will determine the effect of nutrient supplementation on PE and LBW, and the interaction of the nutritional effect with both maternal and offspring factors.