Abstract

As part of a trial to test the profitability of finishing weaner lambs over winter and spring on the Highveld of Gauteng province, an investigation was carried out over 4 consecutive years as to whether or not worm control was necessary while the lambs were on irrigated, improved pastures. Pastures that had not been grazed by sheep or other livestock for at least 5 years were planted to annual ryegrass (Lolium multiflora) that was ploughed under and replanted annually. Weaner lambs were randomly divided into either 2 (A and B) or 3 (A, B and C) groups and finished from April/May to November/December on a separate pasture per group. While Group A lambs were dewormed before being placed on pasture, those in Groups B and C were not. Faecal egg counts were performed at irregular intervals, and when the lambs were sold at the conclusion of each year's trial, the gastrointestinal tracts of 4-5 lambs per trial group were processed for worm recovery. Haemonchus contortus was the dominant worm species, while Trichostrongylus colubriformis, Ostertagia circumcincta and Trichuris spp. were encountered sporadically. From the results obtained it seems unlikely that there is appreciable 'carry-over' of the various worm species from year to year on pastures that are ploughed and replanted annually; thus it is improbable that there will be a build-up of anthelmintic-resistant worms. While on pasture, no drenching was required for the Group A lambs (dewormed before placing on the pasture), nor in 2 of the 4 years in Groups B or B+C that were not drenched and were lightly infected at the start. In the remaining 2 years Groups B or B+C were drenched once only during the course of each trial. Thus, under the conditions as in this study, little worm control is necessary, provided faecal worm egg counts are done to gauge the levels of infection before the lambs are placed on pasture, and to guard against the possibility of an increase in worm burdens thereafter in some years. Also, because of the apparent lack of carry-over between years, anthelmintic treatment at the time of introduction of the lambs or during the period of finishing should be safe as regards progressive selection for anthelmintic resistance. It is also likely to be cost-effective, considering the low cost of anthelmintics in relation to the price of lambs, to counter the possibility of a loss in production if lambs were to harbour relatively heavy worm burdens when introduced.

Highlights

  • Farmers in South Africa are inclined to drench lambs monthly while being finished on improved pasture, especiallyReceived: July 2002

  • In Year 4 the corresponding group was drenched in July, when there was a sudden increase in faecal worm egg counts (FECs) and it was considered likely that the worm burdens may have caused losses in production before the end of the trial, had the animals not been drenched at the time

  • In Years 2 and 3, when larger numbers of lambs were purchased, additional pasture had to be established adjacent to the former Group B pasture, and the lambs that were not predrenched were allocated to 2 separate pastures (Groups B and C), in order to obtain uniform grazing

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Summary

Introduction

Farmers in South Africa are inclined to drench lambs monthly while being finished on improved pasture, especiallyReceived: July 2002. Resistance of worms to anthelmintics has escalated to such an extent in South Africa[11,12,13,14,15], as in numerous other countries, that Van Wyk et al.[12] titled a recent article ‘Rampant anthelmintic resistance in sheep in South Africa – what are the options?’ These authors[12] described a strain of Haemonchus contortus with resistance to 8 of 10 anthelmintics tested, and surveys of resistance carried out mainly in Mpumalanga and KwaZulu-Natal provinces showed that such resistance was present on all of the 52 farms investigated[15] in these provinces.

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