Maize for Silage
Insight 349


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What are thermoduric bacteria?

Thermodurics are bacteria that can tolerate heat. They survive pasteurisation, and can cause spoilage in dairy products.

While pasteurisation is effective in reducing microbial risks, some bacteria survive pasteurisation; these are called thermoduric bacteria. Thermoduric bacteria isolated from processed milk products include strains of Microbacterium, Micrococcus, Enterococcus, Streptococcus, Arthrobacter and Lactobacillus; and spore-forming bacteria such as Bacillus, Paenibacillus and Clostridium.

Raw milk is routinely tested for a number of bacterial parameters to ensure that milk is harvested from healthy cows in a hygienic milking plant and is stored in a hygienic bulk milk tank under conditions that prevent bacterial growth. One of these tests is the Thermoduric Plate Count (TPC). Milk is pasteurised in the laboratory and an aerobic plate count is performed.  Increased levels of TPC (typically >1,500/ml) exceed the milk quality limits and will lead to demerit points which affect milk payout.

Thermoduric bacteria from silage

High thermoduric counts generally indicate unhygienic milking plant conditions, often associated with dry milk or protein deposits from ineffective cleaning systems.

It is now recognised that the black “gunge” from aerobically spoiled silage can contain large numbers of aerobic spore-forming bacteria which are part of the thermoduric group. These spore-formers pass through the cow’s gut and can contaminate milk through faecal soiling of teat skin. Milk is contaminated with the thermoduric bacteria during the milking process.

Aerobic spore-formers can also be found in sources such as soil, water, dust, vegetation and feed refusals, especially deteriorated leftovers. It is from these sources that they gain access to the milk and utensils.

The aerobic spore-formers can survive the laboratory pasteurisation procedure and contribute to an elevated TPC. They are readily recognised on the thermoduric agar plates and the testing laboratory (MilkTestNZ) will issue an interpretation of “Spore-formers” with the TPC, indicating that poor feed quality or environmental factors may be contributing the elevated count.

The thermoduric plate count is only affected by aerobic spores. These bacteria need air to grow, if air is kept from the silage by adequate covering, these aerobic spore-formers cannot grow and cannot cause an elevated TPC.

To reduce the risk of excessive levels of thermoduric bacteria in milk arising from silage:

  • Clean old silage residues from bunkers or stack site before the new crop is harvested.
  • Follow recommended guidelines for harvest timing and/or crop drymatter.
  • Use a quality Pioneer® brand silage inoculant to assist in attaining a low silage pH.
  • Ensure that the silage is well compacted. Follow recommended chop length guidelines to ensure that all air can be excluded from the bunker, stack or bale.
  • Fix any holes in silage covers or bales as soon as they develop. Rodent bait should be placed in bait stations around silage stacks and bales to minimise the risk of mouse or rat damage to plastic covers or wrap.
  • If black spoiled gunge has developed on the outside of grass silage stacks, or spoiled material is present on the surface of maize silage, make sure that it is safely disposed of.
  • Ensure feed bins are kept clean and free of silage residues. Good bin design and appropriate feeding rates will help ensure all the silage is eaten.

Spore-formers are also present in soil and faecal material. To minimise the risk of contamination from these sources, keep cows out of mud and prevent manure ponding on stock races, holding or feed pad areas.

Finally look for the spore alert on your daily docket to indicate whether silage, soil or faecal materials are likely to be contributing to the elevated TPC.


Thanks to Dave Malcolm (Dave Malcolm Dairy Consultants Ltd) for his assistance in the preparation of this bulletin.


Cornell University 2007: The Laboratory Pasteurization Count - Thermoduric Bacteria in Raw Milk;

D. Malcom 2012: Maize silage and its potential influence on milk quality – internal report

M. W. Woolford and J. H. Williamson 1998, Mesophilic Spores, DRC. DPDC Project Number 202

Quality Consultants of New Zealand QCONZ

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Revised: Dec 2019
Expires: Dec 2022