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Managing the Growth of Weaner Cattle: The impact of internal parasites

Despite beef production expanding over the last decade into the most important grazing enterprise on the NSW tablelands and worms being identified as a major health issue for beef production, at the beginning of this PDS project we had little quantitative information on whether current worm control as practiced in the region was effective in eliminating production loss in well managed herds. Previous estimates were done during the 1970s prior to the release of the effective mectin drenches. 

To update this situation, 6 group member herds were monitored in Year 1, by comparing the growth of worm-free heifers, produced by repeated treatment with long acting macrocyclic lactone (mectin) drenches, with heifers treated with the latest drenches and worm control practices. As a result of this, in Year 2, a modified strategic drenching program, applied at weaning, was evaluated. The major findings of these trials were:

  • Undrenched weaner heifers were 60 kg lighter than the suppressed weaners 12 months after weaning, while the heifers drenched using normal commercial practice with short acting drenches were 30 kg lighter than the suppressed weaners at the same time. 
  • This difference had developed by 6 months pw suggesting that the developing immune system can effectively control worms after about 12 months of age. 
  • Improving worm control in the 6 months after weaning by a single treatment with a long acting mectin drench recouped these losses. 
  • While WECs and plasma pepsinogen levels were elevated in worm affected heifers these indicators were low and would be difficult to use as predictors of the need to drench. Hence generic strategic drenching recommendations would be more useful for tablelands beef producers than tactical drenching following monitoring. 
  • Because long acting drenches have been implicated with drench resistance the group conducted drench effectiveness tests in 6 member herds. No resistance to BZ or full dose ivermectin was noted but 1 herd had resistance to levamisole and 2 of 2 to half dose ivermectin suggesting that producers should manage drenches to limit the development of resistance. 


The results of this PDS will be of immediate benefit to the group as well as the local and wider southern beef industry. Similar production losses are likely to be occurring across the industry and the project results highlights the need for producers to control the impact of worms in the immediate post weaning period without compromising the effectiveness of the mectin drench family. Because of this, further demonstrations of the use of other drench classes in combination with worm safe pastures would be worthwhile.

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This page was last updated on 24/07/2017

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