Worm control for weaned stock

During drought conditions and tight feed situations a viable option is early weaning of calves. As calves do not have a well-developed immune system, especially calves weaned early, it is common practice to drench calves at weaning and now is a good time to look at your internal parasite management regime.

The impact of parasite loads may not always be obvious and yet low levels of infection can still impact growth and productivity. In beef grazing systems of southern Australia, parasites have a high economic impact. Costs to industry have been estimated to be over $80 million; comprising $42.4 million in preventative costs and $39.6 million in impacts on production.

Gastrointestinal parasites are one of the major causes of disease and production loss in cattle in Victoria, particularly in higher rainfall areas. In Victoria the major parasites are small brown stomach worm (Ostertagia ostertagi) and liver fluke (Fasciola hepatica). Other parasites include black scour worm (Trichostrongylus spp.) and small intestinal worm (Cooperia spp.)

Worm burdens can hold back weight gains in beef cattle by up to 300 grams per head per day. This can equate to delaying turn off times by 30 to 40 days which can have a major impact on business profitability. Worm burdens can also impact heifers in reaching critical mating weights for joining.

A study by Roll and Webb Ware in Victoria in 2011 compared groups of weaner cattle that were drenched and not drenched. The drenched mobs received treatments in April/May (at weaning) and July. When weighed in November the ‘no drench’ groups were 14 to 26kg less than the drenched groups.

Other studies including Forbes et al. (2004), have shown that animals untreated for worm burdens had lower appetites compared to treated groups and treated animals grazed for up to 50 minutes longer than those untreated. Depending on the level of infection, gastrointestinal parasites reduce the animal’s appetite, reduce the absorption of nutrients and divert energy from growth.

Young cattle are particularly vulnerable to worm burdens. This includes stock classed as weaners, yearlings and those under two years. Production losses can be significant in weaner cattle and in these animals most of the impact from worms is likely to occur in the first six months post weaning.

The use of drenches

The use of anthelmintics (drenches) plays an important role in parasite control. In Victoria 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 Australia. Although some benzimidazole (BZ) 'white' 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.

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 burden. Similarly if egg counts are high an 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.

In Victoria fluke areas are generally the high rainfall, irrigation and coastal areas. The most common fluke is the liver fluke (Fasciola heptica). Stomach fluke can occasionally cause disease mainly in coastal areas.

Mature cattle have some natural resistance to liver fluke due to the development in the gut of a fibrous mechanical barrier (fibrosis) against reinfection. This impedes the migration of young fluke into the liver. However, on fluke affected farms, treatment of all cattle with a flukacide will be required one or more times a year. The most important treatment is in late autumn (April/May) when both immature and mature liver flukes are present. To slow the development of resistance, consider rotating flukicides.

Adult cattle generally develop a solid immunity to parasites (worms and fluke) by two years of age and under normal circumstances cows do not require treatment. However, some adult animals may be considered vulnerable (such as first time calvers and highly stressed animals) and become infected and therefore require drenching. Susceptible animals can also include bulls which suffer more from parasitic diseases and a routine summer-autumn treatment is advisable particularly for Ostertagia.

A quarantine drench is recommended when new stock are introduced to the farm.

The risk of drench resistance is emerging in beef cattle herds, and producers can check if drenches are working effectively on their properties by conducting a faecal worm egg count reduction trial. Producers should  use effective drenches  on new stock to ensure resistance is not introduced. A strategic, tailored approach to drenching decreases the risk of developing resistance and increases effectiveness as well as reducing drench and labour costs.

The use of worm egg counts (WEC)

Worm Egg Counts (WEC) can play a role in managing parasites. A WEC measures the number of worm eggs in a sample of cattle manure (expressed as the number of worm eggs per gram of faeces) which indicates the type and size of the worm burden present. A WEC is considered a valuable tool in assessing worm burdens in sheep but historically the technology has not been able to detect lower egg numbers in cattle, which may still have a significant impact of animal health and performance.

Although a WEC is a valuable tool, it should not be relied upon in isolation. Other measures of productivity such as weight gain and visual assessment should also be used in determining the impact of worm burdens.

An integrated approach

Parasites can also be managed through an integrated approach of drenching, nutrition and grazing management. Young weaned cattle should graze on low risk pastures (for example pastures grazed by sheep for the previous 6 months), fresh un-grazed pastures or  pastures where older cattle have grazed (who are less likely to contaminate pastures as much as young stock).

Good nutrition is also critical to minimise the impact of worms in cattle.  Measures of productivity such as weight gain and  visual assessment should also be used in determining the impact of worm burdens.

References

Page last updated: 01 Jun 2021