A trial was conducted on a farm comprising a flock of approximately 300 Mutton Merinos on which the FAMACHA© system was in use. Seventy five maiden and multiparous ewes were blocked by class before being ranked by weight and then randomly allocated using block randomization, with due regard to approximately equal apportioning of the two classes of ewes, to the following three trial groups: (i) FAMACHA© (FMCH) group, in which only animals evaluated to be in FAMACHA© categories 4 and 5 (overtly anaemic) were treated with levamisole HCl 2,5% (Nemasol NF, Intervet) (ii) Strategically dosed (STRAT) group, blanket treated every six weeks with levamisole HCl 2,5% (Nemasol NF, Intervet) (iii) Suppressively dosed (SUPPR) group, blanket treated at the same intervals with injectable moxidectin 1% (Cydectin, Bayer AH) The trial was set to take place during the period of high haemonchosis risk (December to April) but data was recorded from November 2006 to July 2007. However, the deworming schedule of the trial only commenced in February 2007, due to Cydectin being out of stock until that time. All the trial animals were evaluated once weekly according to the FAMACHA© system, and Faecal Egg Counts (FECs) were performed on all groups prior to commencement of the trial, as well as during the trial period. Initial analysis of the results of all ewes, regardless of class, appeared to show that the FAMACHA© group gained, on average, 3-4 kg less (P<0.05) than the other two groups. However, these results compared groups which contained both pregnant and nonpregnant ewes. Within each treatment regimen the multiparous ewes, which accounted for most of the pregnancies, were responsible for the largest difference in weight gains. Data from ewes that became pregnant during the trial period was separated from the data of non-pregnant ewes, and analysed because the pregnant ewes did not lamb down in the same week and cannot therefore be used as an accurate comparison as they were in different stages of pregnancy at any given time. The pregnant ewes’ data was then ranked according to weeks before lambing and re-analysed. When the data was analysed separately for ewes that conceived during the trial and ewes that did not conceive, the results showed that there was no significant difference in weight gains amongst the three treatment regimens (Figure 4.2). There were no significant differences in lamb weights (Table 4.3.3) nor average daily gains (ADG), as can be seen from Tables 4.4.1 to 4.4.3. Wool production was not analysed in this trial due to faulty sample collection at shearing, but ideally it should have been included. Economic evaluation of the data initially showed that there appeared to be a benefit to dosing more frequently. During the trial period the total cost of deworming the SUPPR group was R163.51 and the liveweight gain gave an additional income of approximately R2758.00 resulting in a calculated financial gain of R2594.49 for 25 ewes when compared to the FMCH group. The cost of deworming the STRAT group for the trial period was R 104.65 and the liveweight gain gave an additional income of approximately R2261.00 which resulted in a financial gain of R2156.35 in comparison to the FMCH group for 25 ewes. The cost for deworming the FMCH group was R10.96 for the duration of the trial. However, these calculations were based on both pregnant and nonpregnant ewe data combined and therefore do not accurately reflect the cost and return for the farmer of meat had the pregnant ewes data been analysed together with those that did not fall pregnant during the trial. Once the pregnant ewes were separated from the rest, there was no significant difference between the three groups regarding liveweight gain. Therefore there was no financial benefit to the farmer in deworming either suppresively or strategically and in fact resulted in an economic loss due to the cost of anthelmintic when compared to using the FAMACHA© system. A shortage of feed due to unseasonal downpours leading to poor Body Condition Scores (BCS), forced the farmer to supplement the grazing and the FMCH group was able to make use of compensatory growth so that by the end of the trial, the was no significant difference in BCS between the groups. The FAMACHA© system allows for selective targeted use of anthelmintics, and studies to date are contradictory on whether or not production is significantly affected by applying the FAMACHA© system to control H. contortus. However, this trial concluded that there is no significant difference in selected production parameters when using the FAMACHA© system as opposed to other methods of anthelmintic use in a Mutton Merino flock in a semi-intensive farming system. The FAMACHA© system is therefore the preferred method of worm control, where the major parasite problem is Haemonchus contortus, as other methods compared in this trial are not sustainable with regards to the worldwide increase in anthelmintic resistance (AR) and now with the increase in multiple anthelmintic resistance (MAR) on certain farms. It is also evident from this trial that the FAMACHA© system cannot be used in isolation as nutrition also plays a vital role in resistance and resilience of individual animals. Therefore if nutrition and other management practices are poor, the FAMACHA© system cannot be blamed for financial and production losses. / Dissertation (MMedVet)--University of Pretoria, 2010. / Production Animal Studies / unrestricted