Mouldy Silage, Listeriosis and Sheep!

Frank Mickan is the Pasture and Fodder Conservation Specialist with the Department Environment and Primary Industries (DEPI) at Elinbank in Victoria.

"Sheep and mouldy silage do not mix!” says Frank Mickan, Pasture and Fodder Conservation Specialist, Department of Natural Resources and Environment, Ellinbank. All forms of silage (pits, bun stacks, round and square bales) are being more commonly used in the extensive grazing industries of beef, sheep, goats, horses, alpacas and even ostriches. This silage has the potential to produce high performance from all classes of animals if conserved early in the season when the pastures are leafy and contain some clover but must be harvested and sealed airtight quickly,

However shortcuts, lack of experience, lack of attention to detail, and not repairing holed plastic, has resulted in many storages, particularly round baled silage, becoming mouldy. There have a few cattle losses, and a worrying number of sheep being affected, including deaths. Most sheep deaths have been due to Listeriosis. This won’t occur if the silage is made and stored in such a way that mould does not occur!

Listeriosis is caused by a soil borne bacteria, called Listeria monocytogenes. It is an opportunistic pathogen affecting animals with a depressed immune system such as pregnant ewes. Although widespread in the environment it is usually in such low numbers as not to be a problem normally.

The listeria bacteria can be problematic in three ways. Consult your vet for the correct treatment.

  1. Nervous disease: This is the most common occurrence and is characterised by meningitis, abortion in late pregnancy, and conjunctivitis, referred to as "silage eye". It is thought that the organism gains access to the animal's nervous system via damage around the gum or teeth. It quickly becomes sick, unsteady on its feet, drooling at the mouth, and if not treated promptly, will lie down and die.
  2. "Circling disease":The bacteria affects the brain and causes the animals to often walk in circles, hence the name.
  3. Septicaemic form:The sheep will have a listless appearance, often having diarrhoea, and dragging the hind legs.

When grass is contaminated with soil the risk of listeriosis (and other diseases) is higher, and usually corresponds to when the silage pH, or acidity, is approximately pH 5.0 and above.

The mould which causes Listeriosis usually only occurs in poorly preserved silages where air has gained access slowly to the silage. This results in aerobic deterioration, i.e. the silage starts to deteriorate, and all sorts of nasties, such as yeasts, bacteria and moulds, rapidly increase in numbers. Listeria bacteria are usually confined to the outer surfaces of bales and clamps where the air has entered slowly and the pH has started to rise.

Aerobic deterioration can be due to:

  • insufficient wraps of stretch-wrap plastic film (a minimum of 4 is essential) on bales
  • incorrect plastic overlap (at least 50%  overlap for individually wrapped bales)
  • excessively dry and/or rank plant material has been ensiled
  • poorly compacted bales or stack of silage
  • plastic sheets sealing stacks not weighted sufficiently
  • holes in the plastic film not being patched quickly enough or effectively when done
  • spike holes when moving bales not being filled with grass (or other filling) once moved.
  • plastic film degradation.

Most of the Listeria contamination is in the visibly spoiled material. Removal of this before feeding to sheep will greatly reduce any possibility of listeriosis.  Listeria bacteria are usually not present in completely deteriorated (i.e. composted) silage because of competition from the putrefying bacteria.

The best method of preventing listeriosis is to ensure that well shaped, tight bales and stacks of silage are quickly and well-sealed as soon as possible after harvest with an airtight seal. If the seal is punctured, it must be sealed as soon as possible.  Air = mould = possible Listeriosis.