Ninety-five percent of the worm population is on pasture
On any single farm, only 5% of the worm population are adult worms within the bellies of the grazing livestock; 95% are immature stages out on the pasture. Concentrating on a drench strategy is only attacking control of 5% of the worms on your farm. Big gains in stock performance are possible where management addresses ways to reduce exposure to that 95% out on pasture. The real benefit from this approach is not only reduced drench inputs, but improved production performance.
Cattle in their first year of life are the main contaminators
Before 9 months of age, cattle have very poor immunity to internal parasites, and if grazing contaminated pastures, will allow large numbers of worms to establish in their gut. Once these worms are mature, they produce many eggs each day, which are passed out of the animal and further contaminate the pasture.
From about 9 months of age, cattle begin developing immunity to worms; preventing establishment of new worm larvae in the gut, repelling resident adults, and limiting the egg producing ability of those adults remaining.
This is one of the reasons that faecal egg counting is not a good indicator of parasite burdens in older cattle; there may be significant numbers of worms in the gut, but the immune system of the animal is preventing significant egg production.Larval challenge is more important than resident adult worms
The establishment of adult worms in the gut of animals can be the cause of depressed liveweight gain or weight loss. Such worm burdens can show externally as scouring, poor coat and hollow gut.
However, significant depressions in growth rate (and clinical weight loss) will occur purely through exposure to high levels of worm larvae on pasture. Worm larvae ingested with pasture by an animal are treated as foreign infectious agents. The body reacts to that intrusion by mounting an immune response. This attack on the parasites uses both protein and energy from the diet (thus it is not available for growth). When the larval challenge is very high, this diversion of nutrients and fluid loss can cost a lot of liveweight gain. Such losses can often occur with no external symptoms at all.
When a worm infection becomes clinical (visually obvious), growth is already being seriously impaired. As well as the immune response described above, fluid is excreted into the intestine, which we see as scouring (diarrhoea).
Drenching does not stop larval challenge
Bull beef systems with cattle-only grazing that take weaners through to 18 months, or blocks where young dairy heifers are grazed year after year can end up with extremely high levels of pasture contamination. This leads to overall depressed liveweight gain performance. Regular drenching will only reduce the effect of this larval challenge, it will never remove it.
There is no better demonstration than when young cattle graze "clean" pasture and are compared to those grazing contaminated pasture, but are regularly drenched. Cattle grazing "clean" pasture will always grow better, no matter how many drenches the others get.
Other stresses, such as cold wet weather, concurrent infections such as BVD, Coccidiosis or Yersiniosis can all depress the immune system of young animals and make the effects of larval challenge more severe.
There are various ways of avoiding high pasture larval accumulation. Cropping/pasture renewal is one of the approaches being used on many farm units. This is extremely effective and has returned life to many bull blocks in particular. On many farms there are grazing management options where alternate stock classes can be used to provide ‘low contamination' feed for young cattle.
At Totally Vets we have several experienced sheep and beef veterinarians with a particular interest in parasite management who can help you plan to get the best out of your young cattle.
