Investigating causes of lamb loss on farms and crofts within the Highlands and Islands of Scotland

Hill sheep farming is an important component of Scottish agriculture and comprises a significant land use in much of the Highlands and Islands. It is however, faced by significant challenges due to the natural constraints of the landscape. Hill sheep farming uses hardy traditional breeds, such as the Scottish Blackface and North Country Cheviot to graze extensive areas, where the sheep are not housed and tend to lamb on the open hill. Flocks are gathered several times a year for stock checks, husbandry and health treatments. Between these handling events, stock will disappear and be unaccounted for. These unexplained losses are known as blackloss in the Highlands and Islands. This thesis aimed to investigate the causes of blackloss in lambs on hill sheep farms and crofts. Previously reported figures for annual blackloss give an average of 18.6% . These losses are in addition to the known losses of lambs and represent a significant welfare and sustainability issue. High parasite burdens, predation, a photosensitisation disease known as plochteach or yellowses, and poor nutrition are often given as presumed reasons for blackloss. A questionnaire was developed to assess the experiences, impacts and understanding flockmanagers have of blackloss (Chapter 2). Typology analysis was used to cluster respondents into three distinct groups: 1- very large extensive farms and Sheep Stock Clubs, 2- small-scale crofts, and 3- medium sized farms. The responses of these groups were subsequently analysed to see if their experiences and perceptions of blackloss differed with relation to lamb health challenges and predation impacts. The groups reported similar health challenges, apart from Group 1 which had a higher plochteach challenge. In terms of predators, Group 1 also perceived white-tailed eagles (Haliaeetus albicilla) as a much higher threat to their lambs than the other groups. It was observed that many of the respondents felt that blackloss is inevitable and that predators pose a large threat to lambs, however most agreed that reducing these losses is important and that understanding the causes would enable them to do so. The insights from this study informed later chapters of this thesis. The second study of this thesis used historical production datasets to investigate the role that plochteach has in blackloss, as well as factors leading to photosensitisation (Chapter 3). Data were available for 3,071 lambs born between 2014 and 2021 in Scotland’s Rural College (SRUC) Auchtertyre (AT, 1,600 lambs), Corrie (AC, 350 lambs), and Twin (TW, 1,121 lambs) flocks. Lamb presence/absence was recorded using EID tags, and count data was used to calculate marking to weaning levels of blackloss. Plochteach was diagnosed through clinical signs of photosensitisation on the ears and/or back. The effect of lamb traits, plochteach, and dam and sire factors on blackloss was investigated. The eight-year average blackloss observed between tagging (birth or lambing) to post-weaning was 8.3%. During the eight-year period 5.1% of lambs were affected by plochteach. Prevalence of plochteach in the blackloss populations was 10.6%, indicating it as a cause of blackloss. Generalised Linear Mixed Model analysis found that marking weight and plochteach significantly impacted blackloss, with low weight lambs (<15Kg) and those affected by plochteach to be at a significantly greater risk of disappearing. It was also found that management group (heft), skin colour and litter size significantly impacted plochteach, with white skinned, single lambs raised in a hill environment at a greater risk of photosensitisation. These findings highlight the risk factors for plochteach and also the role both it, and low marking weights, play in lamb blackloss. The third and fourth studies of this thesis were undertaken on two hill farms and a croft in addition to Auchtertyre farm. Their flocks were followed over two production years (2021 and 2022), where the risks and challenges to lamb health were assessed (Chapter 4). This involved attending key handling events (marking, shearing, and weaning) to record signs of ill health in lambs, such as categorising tick (Ixodes ricinus) burdens, plochteach severity, infections, and rear end dag soiling. Faecal samples were regularly collected from ewes and lambs for parasitology testing, to quantify gastrointestinal nematode infections and determine liver fluke (Fasciola hepatica) presence. The number of lambs present at each handling event was recorded, so that the blackloss between gathers could be calculated. Marking to weaning blackloss averaged 8.7% (1.9-27.3%) across the two years, however there were difficulties in gaining accurate lamb counts due to crossover between flocks, and the impacts of weather and labour availability on gathers. This study found that although liver fluke was present on all of the holdings, it was unlikely to present a significant lamb health challenge before weaning. However, the nematode faecal egg counts were substantial for a hill flock setting and were likely to impact lamb productivity and potentially survival. Tick burdens were found to be significant for some flocks, and likely contributing to losses within those flocks. Finally, the flocks were found to have similar proportions of lambs affected by plochteach during both years, indicating that the findings of Chapter 3 may be applicable to the other holdings included in this study. During 2022, the pre-lambing body condition score, β-hydroxybutyrate (BOHB), urea and albumin levels of pregnant ewes within each of the holding’s flocks were assessed (Chapter 5). It was found that although the flocks were generally in good condition and had sufficient nutrition, there were some flocks and individual ewes with elevated BOHB and deficient albumin, indicating a negative energy balance and insufficient protein intake. This would likely have negative consequences on the vigour and subsequent survival of lambs within these flocks. It was also found that ewes that received supplementary feeding had higher albumin levels and were in better condition than those that did not, highlighting the importance of feeding during late pregnancy. Additionally, lambs within these flocks were assessed for their selenium and cobalt status at weaning. The majority of flocks had sufficient levels; however one flock was low in selenium/vitamin E, whilst another had both a selenium and cobalt/vitamin B12 deficiency. Lambs within this flock were also suffering from high tick burdens, nematode worms (Chapter 4) and poor ewe pre-lambing nutrition, highlighting the complex nature of lamb health challenges and causes of blackloss. This thesis has increased our current understanding of the lamb health challenges and contributing factors of blackloss on hill sheep farms and crofts within the Highlands and Islands of Scotland. The ‘Four Ps of Blackloss’, namely plochteach, parasites, predators and poor nutrition were identified. These findings can be utilised and applied to hill sheep enterprises to help reduce lamb losses, thus improving the productivity of hill sheep farming by reducing the financial and genetic losses to flocks, increasing the overall sustainability and welfare of hill sheep systems.