Treatment of the entire mob, which exposes the entire worm population on a property, is not recommended as it will ultimately lead to widespread drench resistance through the loss of ‘refugia’. Maintaining refugia keeps a proportion of worms in the population that are susceptible to drenches which means chemicals will continue to work against worms into the future.
Treatment schemes for cattle worms are based on the class of stock and/or the management system. See also the section on treating cattle entering feedlots.
Treatment according to an annual program. In regions where worms are a common cause of disease or production loss, an annual program should be developed to pre-empt the development of large populations of worm larvae on the pasture. Annual programs should be confined to classes known to be susceptible to worms (younger age groups such as weaners, yearlings and first and second calvers as well as bulls pre-joining). These strategic programs aim to remove adult worms before seasonal conditions begin to favour worm egg development, so the ‘worm season’ starts from a low base. (For example, drenching in late summer or autumn in winter rainfall regions prevents the deposition of large numbers of worm eggs onto pasture before seasonal rains and mild temperatures begin to allow worm egg development.)
Treatment of individual animals only. If one or a few animals show signs suggestive of worm disease, this may reflect an individual genetic susceptibility, and treatment to those individuals with a combination drench may be sufficient. This most commonly occurs in adult breeders, where a very small number of cows may show scours while the majority of the herd are not affected. However, it is possible that others are also carrying significant worm burdens and suffering ‘sub-clinical’ (invisible) parasitism, as well as contaminating the pasture with eggs. A worm egg count of the affected mob will show whether worms are involved. Where there is a lack of nutrition being provided, and lack of clean pasture available, then this should be addressed, rather than drenching.
Targeted treatment (TT). Signs of worms, or significant worm egg counts, may occur in many animals in a herd of a particular class (often weaners/yearlings), when others of the same age do not show signs. A targeted treatment approach involves treating a proportion of the mob to reduce worm numbers. Targeted treatments are more efficient and cost effective than treating all herds, when they do not need it. This treatment scheme reduces the risk of building up drench resistance by helping to maintain refugia.
Testing the benefit of a targeted worm treatment
The following example describes how to determine if a targeted worm treatment will provide cost benefits to managing worms in your mob.
Note that pour-on treatments can be shared among cattle through social licking, so they are unsuitable for conducting production or weight gain trials.
Example of a Targeted Treatment (TT) scheme using weight gain
This approach could equally be done using milk yield instead of weight.
For milk production, a representative portion of the herd can be treated with a drench, making sure the product has a zero milk withholding period. Milk production over a period of time (which has to be from cows in the same lactation status) can be compared with equivalent un-drenched cows. Comparisons can also be made between the benefit gained from treating cows at dry-off, or during the early part of lactation.