Effect of selection for lean tissue growth on muscle fibre characteristics in lambs, and the implications for welfare
Coombs, Tamsin Margaret
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In the UK annual lamb mortality rates range between 10-30% with the majority of deaths occurring within the first three days of life, however research has shown that lambs that stand and suck quickly are more likely to survive. Modern breeding strategies have led to breeds of sheep, such as the Suffolk, which despite greater lean muscle growth, show slower behavioural development and are less able to thermoregulate in the neonatal period than relatively unselected breeds, such as the Scottish Blackface. The reason for this is unclear however fast growing strains of pigs and cattle have been shown to have a greater proportion of fast-twitch fibres to slow fibres in their muscles, but it is still unknown as to whether these changes in fibre proportions affect muscle function and behaviour. Thus the aim of this project is to investigate whether selection for lean muscle growth in sheep has altered muscle development by affecting the proportions of different fibre types and determine what effect this may have on the animal’s ability to perform certain behaviours, such as neonatal progression to standing and sucking. As muscle fibre development occurs very early in gestation (starting around day 32) it was also hypothesised that there may also be a relationship between muscle fibre characteristics, and foetal behaviour and presentation at birth. A further hypothesis to be addressed was that maternal undernutrition of 75% of requirements for ewe maintenance and foetal growth for the first 90 days of gestation would have a greater negative effect on muscle fibre development in genotypes selected for lean muscle growth. It was found that Suffolk foetuses (genotype selected for lean growth) were significantly less active at days 56 and 77 of gestation than Blackface foetuses (genotype relatively unselected for growth) while nutritionally restricted foetuses were more active at day 56 than control foetuses. A subsequent study found that there was a negative relationship between foetal activity at day 56 and neonatal activity while activity at day 98 of gestation was positively associated with neonatal activity. A relationship was also found between foetal activity and presentation at birth with malpresented lambs being less active as foetuses at day 77 of gestation than normally presented lambs. Suffolk foetuses had lower proportions of slow twitch (SO) fibres and higher proportions of fast-oxidative-glycolytic (FOG) fibres in the soleus (postural muscle) than Blackface foetuses and SO fibre proportions were positively correlated with foetal activity at days 56 and 77 of gestation while fast twitch (FOG and FO) and transitional (Trans) fibre proportions were negatively correlated with foetal activity at each scanning period. Suffolk lambs showed significantly slower neonatal behavioural development than Blackface lambs and there was an interaction between breed and nutritional treatment with prenatally undernourished Suffolk lambs consistently being less active and prenatally undernourished Blackface lambs being more active than all other groups of lambs. At slaughter at 164 days old, Suffolk lambs had lower proportions of SO fibres and higher proportions of fastglycolytic (FG) fibres in the soleus muscle while also having a lower proportion of FG fibres in the plantaris (muscle involved in movement of the limb) than Blackface lambs. SO fibre proportions in the soleus muscle were found to be positively correlated with total duration standing and walking in the early neonatal period while proportion of FG fibres in the plantaris was negatively correlated with duration of lying laterally following birth. The results from this study indicate that divergent breeding strategies have led to differences in muscle fibre proportions within certain muscles in sheep and also that there may be a relationship between muscle development and both foetal and neonatal lamb behaviour. This research has added to our understanding of the consequences of selection for growth on the function of the animal and it is hoped that it will lead to the development of broader breeding goals which incorporate welfare characteristics.