Drench resistance and soil fertility

Fert Options
with Robin Boom
Agronomic Advisory Services

Drench resistance is becoming a more widespread issue on many sheep and beef properties in particular, but it also occurs on dairy farms among calves and yearlings.

Older sheep and cattle tend to develop immunity to internal parasites and it is mainly in the first year that drench resistance can be particularly devastating on animal performance in young stock. One of the reasons for drench resistance is drenching too frequently, and another cause is under-dosing at lighter rates than label recommendations, resulting in worms developing resistance and tolerance to various drench families.


Certain soil fertility factors can feed into drench resistance problems, such as pasture quality and grazing height. Low fertility properties with poorer, lower growing pasture species such as browntop, danthonia, chewings fescue, sweet vernal and flat weeds result in animals grazing lower to the ground where there is a greater chance of animals ingesting high numbers of larvae. Higher protein and ME species improve animal performance and grow taller and more vigorously resulting in less worm larvae being ingested per kg DM consumed.


Applying spring nitrogen and/or gibberellic acid can benefit in that the grass is taller so again the larvae numbers consumed are lower, but in the case of nitrogen, if pastures are too lush with excessive nitrates and not enough minerals and sugars, animals scour more and the farmer thinks it is parasites so drenches stock more frequently than they need to be. When these artificial growth stimulants are applied to pasture, the mineral levels in the pasture can get diluted. Clovers have higher mineral levels and are of greater feed quality than grasses, so improving clover content of pastures will also improve resilience in livestock to internal parasites.


Summer or winter crops, or using deferred grazed pastures will result in lower parasite numbers consumed as animals will not graze these as low as normal pastures. Although sheep and goats share many of the same internal parasites, cattle and horses don’t, so integrating sheep and cattle grazing on the same pasture can help dilute larvae numbers. Breeding animals resistant to parasites, changing drench types, and putting drenched animals back onto recently grazed pastures rather than into a fresh paddock so a ‘refugia’ population of non-resistant worms will interbreed with resistant ones, lowering their tolerance to drenches are also techniques to avoid developing drench resistance.


Copper deficiency in particular can often be confused with worm burdens, resulting in daggy sheep and scouring cattle, in which case drenching animals for parasites will be a waste of money and time, increasing the risk of developing drench resistance. Copper also has an anthelmintic effect and before modern drenches, copper nicotinell and CNA (copper, nicotine and arsenic) were used as worm drenches. Because it was easy to over-drench sheep with copper, resulting in copper poisoning, farmers would drench test a couple of sheep with a certain rate, leave them for half an hour, and if they were still alive and looking bright, then would carry on drenching the rest of the flock at that rate.


Applying copper sulphate to the soil can be an effective treatment for copper deficiency on soils low in copper, and its physical application can temporarily lower worm larval numbers as well as fungal spores, but also gives animals which would otherwise be deficient in copper, greater resilience to larval challenges. Other trace element deficiencies can also make animals more vulnerable to parasite challenges, and although some worm drenches contain minerals, these only last a short time in the animal and a more permanent solution is to address soil deficiencies. To asses the need for applying trace minerals, soil and/or herbage tests which look at these should be conducted. High nitrogen, phosphorus, sulphur, molybdenum and iron can all impede the absorption of copper in plants and/or animals, so getting the correct balance is important when it comes to fertiliser inputs. With the current high prices for some fertilisers and the comparatively low cost of trace elements, getting the soil mineralogy right can be a good start for improving animal resilience to parasitic challenges long term.


ROBIN BOOM Member of the Institute of Professional Soil Scientists


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