Insect pests and parasites thrive in moist manure, particularly in warm weather. Intestinal worms and flies, particularly the stable fly (Stomoxys calcitrans), are the biggest concern to horses and horse owners.
Stable flies are blood-sucking insects that feed on all short haired mammals, including humans.
Fly bites can be stressful for livestock and can also spread animal diseases.
The irritation from stable flies may result in reduced grazing time, causing a decrease in daily weight gains.
Horses and cattle reject areas of a paddock which are contaminated by manure as a way of minimising their parasite burdens. When large areas are rejected for this reason, the paddock is referred to as being ‘sick’.
In this situation the pasture close to the manure thrives, becoming tall and rank and causing it to be even less desirable, while the unaffected areas are well grazed or often overgrazed.
Constant movement around the areas of contaminated pasture eventually causes the ground to become bare, increasing the risk of land degradation and enabling weeds to establish.
Stores of weed seeds may already exist in the soil or they may be introduced through the manure itself.
Consider possible weed invasion when selecting purchased fodder.
The end result of leaving manure undisturbed is a decrease in the available grazing area.
This is less of a problem with sheep and goats since their faeces are smaller and more scattered. Alpacas tend to have ‘toilet areas’ within a paddock and the grass in such areas may become rank.
Manure is a source of nutrients, including nitrogen and phosphorus, which can be transported in run-off and cause pollution of lakes, streams and other water resources if manure is not managed carefully.
Manure can also be used as a valuable source of plant nutrients. A 450kg horse will produce 5-9 tonnes of manure a year, containing around 50kg nitrogen, 5-10kg phosphorus, 40kg potassium, 45kg calcium, 8kg magnesium and 8kg sulphur.
Cattle produce 50-130kg nitrogen, 15-30kg phosphorus and 40-65kg potassium per year.
These amounts are sufficient to maintain the fertility of 1-2ha of pasture for a year.
Horses and cattle will also produce 15-40L of urine per day, but this is less of a problem as it infiltrates into the soil, unlike manure which remains on the surface harbouring flies and worms.
However it can add to the nitrogen concentration in groundwater which can be a problem for those that rely on groundwater for stock and house use.
Managing fly and worm problems
The most effective way to combat problems from flies and worms is to interrupt their breeding cycle. As their breeding cycles are very fast in warm, moist conditions, manure should be removed from stables and yards at least once a day.
Design your stables and feedlots to facilitate thorough cleaning and to prevent the continual accumulation of waste. This will make them relatively worm and fly free. Be sure to meet the conditions required by your local government authority for stables and keeping livestock.
If it is possible to remove manure from paddocks this will be beneficial. Collected manure should be disposed of, used or stored in ways that exclude flies.
For example the fresh manure can be composted or buried in garden beds. When manuring garden beds, remove the top 4cm of soil, spread the manure and replace the topsoil evenly over the area before watering.
Do not mix the topsoil and manure together. Having a layer of soil over the top of the manure prevents fly infestations.
Manure that is expected to contain many weed seeds (e.g. horse manure) – due to what the livestock have been fed – should be either well composted to kill the seeds or legally disposed of away from the property.
Manure which contains a large amount of wood shavings or sawdust from stables should be composted before applying to the garden bed or pasture. If these are applied fresh to the soil they can reduce available soil nitrogen.
For manure to be composted successfully, it requires regular turning to maintain aerobic conditions.
If it is not possible to collect manure from paddocks, harrowing or slashing the paddocks are options to spread the manure thinly and evenly and aid its drying.
In addition slashing will remove the build up of tall, rank grass clumps.
These operations are normally undertaken when livestock are removed from the paddock. Therefore implementing a rotational grazing system is a natural addition to pasture and manure management.
Certain conditions aid the harrowing operation:
- the pasture should be grazed to a height that permits contact between the harrows and the manure pats
- the ground should be dry enough to minimise damage to the soil by the harrowing
- the manure pats should be moist enough to break up (for example, following heavy dew or slight rainfall)
- forecast hot, dry conditions after harrowing will help to desiccate parasite larvae
- harrowing should be confined to those areas containing manure (to prevent contamination of ‘clean’ areas)
- spelling the paddock after harrowing or slashing until the worm risk is reduced. (Note: This can be four to six weeks in hot conditions and ten to twelve weeks in cold, frosty conditions.)
In and around Perth, the conditions described above are most likely to occur following early autumn rains.
In areas which remain warm and moist throughout the year, harrowing is not recommended as a means of worm control since it can take six to eight months to reduce the worm risk.