Dose of advice for treating infection
After a cold winter and a long dry spell, the expectation is that worm contamination of pasture is still relatively low for the time of year. With warmer and wetter weather forecast, the worm lifecycle will be speeded up and we can expect to see infection where calves are intensively reared.
When young calves go to grass they have no immunity against stomach worms and hoose. Where spring-born calves are intensively stocked on their own, there can be rapid build-up of pasture contamination. Due to the lack of immunity there is a large production of worm eggs from even a small population of adult worms in the intestine of young calves.
In warm, moist weather, the life cycle of the stomach worm can be down to about three weeks and therefore calves can become seriously affected with a heavy infection of stomach worms by mid-summer -- before they have time to develop a strong immunity. The process is accelerated where calves are grazed continuously on the same ground and separately from other stock.
Bucket-reared dairy calves are the ones most at risk since they will be depending on grass from an early stage and, as they are usually grazed as a group or on their own, there is no 'dilution' effect from older immune animals.
A leader/follower system helps reduce the pasture contamination where calves graze ahead of older stock in a rotation system. In this way, the stocking rate of calves per hectare is low, being spread over the full grazing area, and the build-up of pasture contamination is slower.
This grazing method also provides the calves with choice, high quality pasture and is essential to keep grass under control. Autumn-born suckled calves are also a high-risk group as they have no previous exposure to stomach worms at the time of turnout to grass, and will therefore have no immunity. At this stage, they are also relying on grass for the major part of their diet. Spring-born calves do not consume much grass for the first 8-10 weeks of their lives and this, combined with the dilution effect of the cows, should keep infection levels low until about mid-summer. However, cases of scouring in suckled spring-born calves should be investigated or test treated with an althalmintic as worm infection has occurred in spring-born calves in May.
Sometimes yearling cattle in their second season at grass also benefit from worm dosing if they are subject to a strong challenge (high pasture contamination due to high stocking rate of young cattle and suitable weather for the disease).
They are also at risk if their immunity is weak, perhaps due to a lack of exposure in the previous season or their immunity is impaired due to disease, mineral deficiencies or poor nutrition.