Most animals have a very keen sense of smell, which they use to keep themselves safe. In addition to their eyesight and hearing they use their noses to warn of predators approaching.

The likes of deer, hares and rabbits constantly sniff the air as they graze. They will often smell us approaching long before we can spot their camouflaged forms among trees or grass.

Smell is also used in the animal world to mark out territory, while many like cattle and sheep identify their young by scent.

Sadly our own sense of smell is not as good, and a scent has to be really strong for us to notice it, like the strange, musky odour of a fox that lingers in the air long after it has gone. Pigs, cattle, sheep and hens all have distinctive smells that most farmers recognise. While some of them may be strong, I don't find any particularly offensive.

Stockmen often carry the smell of the livestock they work with. Just as smokers smell like ashtrays, tractor-men and mechanics have a whiff of oil or diesel about them, and dairy-men can reek of the disinfectants they use to clean their milking equipment.

Mind you, there are some smells that townsfolk find unpleasant - like silage, slurry and farmyard manure (FYM), or muck as we call it.

Most farmers quite like the sharp tang of well-made silage, although it does tend to cling to work-clothes.

Recently the air around my bungalow was a bit smelly and prompted an urban visitor to complain about the "awful smell of slurry". I, with my superior knowledge of rural aromas, corrected her mistake and told her it was silage effluent and explained why a nearby farmer was spreading it on his fields.

Silage is actually pickled grass. The process involves consolidating large volumes of grass in an airtight pit where harmful bacteria that would cause the grass to mould or rot die as they use up oxygen and replace it with carbon dioxide. That's when anaerobic bacteria flourish and produce lactic acid from sugar in the grass, which in turn kills them off, leaving the grass pickled and preserved in a stable state.

One of the problems with wet silage is that it produces large amounts of effluent that needs to be contained and properly disposed of. Silage effluent contains a lot of highly-nutritious sugars from the grass that gives it a high biological oxygen demand (BOD). If it gets into watercourses bacteria flourish on it and use up all the oxygen in the water. That's why silage effluent kills fish and other aquatic creatures - they are literally suffocated to death.

Nowadays farmers have systems for collecting and storing this deadly liquid, before safely spreading it over a wide area of land so that it seeps harmlessly into the soil - and that's what the neighbouring farmer was doing.

Slurry and FYM are usually spread on the land in the spring as a valuable source of plant nutrients like nitrogen and potash.

Slurry is a mixture of dung, urine and water that is collected from cattle and pig sheds to be stored before spreading as a liquid fertiliser. FYM is a mixture of dung, urine and straw that is stacked to compost before being spread. In addition to nourishing the land it helps to condition soil by making it more friable and improving its ability to retain moisture.

At one time both were considered a nuisance that were spread in the autumn and winter just to get rid of it - but not anymore. With the soaring cost of chemical fertilisers, slurry and FYM are now regarded as precious commodities to be spread at times of the year when grassland can get the maximum benefit. Where there's muck there's money.

Different slurries and FYMs have different smells. Dairy cow slurry has a sharp smell compared with FYM, pig slurry is definitely "piggy", while poultry muck smells strongly of ammonia.

Some farmers are now using anaerobic digesters (ADs) to produce methane from slurry for use in heating systems or to generate electricity. For those who can afford the hefty capital cost of installing the technology, AD is a perfect environmental solution for handling slurry from large livestock enterprises.

Odour from the material left after slurry has been "digested" is reduced by 90 per cent compared to untreated slurry, and that could be important to those farmers forced to spread it within close proximity to villages and towns.

Not everyone enjoys country smells, but most rural dwellers learn to tolerate them.