The aim of this study was to examine the use of Happy FactorTM weight based targeted selective treatment (TST) on several commercial farms in Scotland in combination with findings from a long term trial on a research farm to assess the potential for TST use in varying farming operations as an alternative to the current regimen of whole flock treatment. Lambs on each farm were regularly weighed and climatic conditions and pasture availability measured for inclusion into the Happy FactorTM model to calculate weight targets. Any lamb in the TST group failing to reach the weight target was treated with the anthelmintic of choice on that farm, while the remaining half of each flock was treated with anthelmintic as per regular practice on that farm (routine treatment, RT). The research farm (farm 1) hosted a long term trial using four anthelmintic treatment regimes over 6 years, and data from two regimes are presented here, alongside findings from three further farms: two commercial enterprises (farms 2 and 3) and a research farm operating as a commercial analogue with two breeds (farms 4a and 4b). The effect of TST strategy on lamb productivity and the number of anthelmintic treatments was investigated. There was no evidence (p<0.05) that mean bodyweight or growth rate was different between TST and RT groups on any of the farms. Growth rates ranged from 97.39 to 189.16g/day reflecting the varied nature of the farms. All commercial farms used considerably less anthelmintic treatments in lambs following TST, with up to 80% reduction in treatments. This study has shown that TST is a viable means of controlling parasitic disease without incurring production losses.