Only a few years ago, some worried that the modern drugs used by farmers to treat internal parasites in cattle would kill off the insects and worms that normally degrade the nutrient-laced dung that is the visible end product of a cow's extended ruminations. There were fears that this would lead to a decline in the number of pasture birds that feed on these dung insects, and of earthworms, which congregate beneath the cowpats; and also that fields would be knee-deep in smelly, undegraded manure. Fortunately, it hasn't happened. But the news, though mostly good, does have its downside.
The cause of the initial worry was a new group of anti-parasite drugs called avermectins, which were introduced in 1981. Of these, Ivermectin is regarded as the most effective. For cattle farmers they were revolutionary. Administered by injection or by mouth as a capsule that lodges in the rumen and releases its contents over a number of months, they kill both internal and skin parasites. Before avermectins, farmers had to inject a series of drugs to kill off a range of parasites which otherwise reduced the efficiency with which cows convert grass to body protein.
However, the drugs are not completely broken down by the body before they are excreted. They can still be present in the dung at a concentration which, while low, can kill or disrupt the development of a wide range of insects and other dung-degrading invertebrates. In experiments quoted by Dr Paul Green of the British Trust for Ornithology, half of the larvae of the common yellow dung fly died when exposed to just 0.05 parts of Ivermectin per million. Lower concentrations caused major disruption to the fly's life cycle. Cattle dung from bolus-administered cattle contains 10 times this concentration of the drug.
But once in the pats, avermectins do break down. Research by Les Strong and Richard Wall of Bristol University shows that beetle larvae were unable to develop in dung from Ivermectin-injected cattle seven days after treatment, but that they could 14 days after a dose. Cattle treated with moxidectin (another, less effective, avermectin) produced cowpats that were colonised as rapidly as drug-free dung.
Not all cattle are treated with avermectins anyway. "It's only worth treating young calves," says Bert Burns of MSD Agvet, manufacturers of avermectins. "Older cows develop natural immunity to most parasites. On a typical dairy farm there are perhaps five generations of cows, and it's only those of the youngest generation kept for future milking that are treated. So more than 90 per cent of the dung will be Ivermectin-free."
Most cattle are injected. Boluses are used to treat only about 3 per cent of cattle. Expense is one reason but, according to Mr Burns, the main reason is that the bolus-treated cattle have to be kept on the same pasture all season. This is because parasites picked up by the cattle as they graze are killed off when they get inside the cow, which slowly renders the pasture parasite-free. Moving them to another pasture would expose them to more parasites, and necessitate repeat doses with the boluses as previous ones became exhausted. Not many farmers can provide sufficient grazing for the herd in one place.
"At current levels of use, Ivermectin reduces insect populations in about 10 per cent of dung or less," says Dr Green. "It doesn't appear to affect earthworms, one of the main prey for many birds feeding on pastures."
The drugs may even provide some environmental advantages. Because they kill parasites living on the skin - including warble flies and ticks, for instance - use of the highly toxic organophosphate washes and dips (which have been implicated in some illnesses in farmers) has declined. Dr Green says that since ivermectins were first used to treat sheep scab in 1994, use of organophosphates against the disease has fallen by 20 per cent. The benefit is that, besides the risks it poses to farmers, dip is often allowed to wash away after use, frequently ending up in streams where it kills animal life.
To date there is no proven effect of ivermectin use on bird populations. Research has shown that insect populations around farms using them do decline, but such declines are local and short-lived: birds are likely to move elsewhere to feed. And you are no more at risk of stepping in a cowpat if you walk across a pasture than you were before these highly effective drugs were introduced.