How does silage get contaminated?
Contamination of forage with soil, dead animals or straw and rank grass during harvest should be avoided. Undesirable bacteria may be introduced that will adversely affect the silage fermentation, aerobic stability of the silage at opening, and the health of animals fed the silage.
Soil
Soil-borne bacteria (e.g. clostridia) can cause undesirable fermentations or lead to diseases in livestock (listeriosis, caused by listeria)
Dirt and mud may be carried into the stack as clods picked up by the harvester, from the wheels of unloading trailers or the rolling tractors. Rolling paddocks after sowing to break up or bury large clods can reduce soil contamination of the mown forage.
Setting the tedding and raking machines at the correct height will also reduce soil contamination. A cement apron in front of the stacks will prevent the forage coming in contact with the soil during loading and unloading.
Ideally, the tractor rolling and spreading the forage should remain on the stack surface until filling is completed.
Contaminated soil
Old straw and rank grass
Old straw, rank or rotting stems of previous crops and lodged plants are usually contaminated by a range of bacteria, yeasts and moulds. Harvesting this material can adversely affect fermentation and reduce aerobic stability at feedout.
Ensiling a significant proportion of this inferior quality material will also decrease the energy (ME) content of the silage.
Dead animals
Animals are at risk from botulism if they eat silage that contains dead animals trapped in the forage at harvest. All animal remains should be picked up before mowing, although it is often difficult to see bird, snake or rodent carcases as they are picked up during harvest.
There is also a risk of botulism when burrowing animals die in the stored silage. The risk of botulism increases with lower DM silages.
Effluent
Risk of contamination from animal effluent (e.g. from piggeries, dairies or feedlots) used on silage crops or pastures can be minimised if it is not applied within six weeks of the crop being harvested.
The risk is further reduced if it is applied onto bare ground, before the crop is sown or while the crop is very short.
Contamination risks increase if the effluent contains large particles that may be picked up by the harvesting equipment.
Toxic weeds
There are inadequate Australian data on the impact of ensiling on the poisoning risk of toxic weeds. The level of risk will vary with the type of weed, the amount fed to the animal and the concentration of weed in the silage. The type and class of animal is also likely to affect the risk level.
Weeds suspected of being toxic should be controlled, or infested portions of the paddock avoided at harvest. Producers should seek appropriate advice on weeds of concern.
Also to be considered when harvesting broadleaf weeds is the potential for reduced quality and the effect on silage fermentation.
