The ML (macrocyclic lactone) group of drenches are usually the most suitable treatment , especially where it is important to remove the larval stages of Ostertagia in late summer or autumn. As levamisole based drenches are ineffective against the inhibited stages, they are mainly used in northern areas of the country. Although some benzimidazole (BZ) drenches have a label claim for control of “arrested” stages of Ostertagia, the ML drenches are more reliable in this regard, and simpler to administer using the pour-on or injectable route of administration. A routine worm control program is useful, especially in high and medium rainfall areas.
In general calves should be drenched at weaning with an ML or BZ drench. If using a BZ drench, a follow-up drench one month later may be useful on farms with a worm problem. Similarly if egg counts are high and additional drench may be required in late autumn. In some herds two year old/ first calf heifers may also require treatment. In areas where Type II Ostertagiosis may occur a late summer –early autumn treatment is recommended. Additional treatments for liver fluke, usually given in late autumn (April/May) to control immature fluke and again in the spring to control adults and reduce pasture contamination are necessary where this parasite is present.
Adult cattle develop a solid immunity by 2 years of age and under normal circumstances cows do not require treatment. However bulls suffer more from parasitic diseases and a routine summer-autumn treatment is advisable.
A quarantine drench is recommended when new stock are introduced to the farm.
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