FEED QUALITY BEYOND THE EYE: Managing Mould, Yeast and Mycotoxin Risk

Owen Rees
Apr 16
3 min read

By Owen Rees, TRAC Senior Ruminant Productivity Consultant

UMMER 2026

When buying hay and silage that has been made under suboptimal conditions, we can see that animals have different reactions depending on the quality of the feed. Hays that have had some rain on them between cutting and baling, silage that has been exposed to air, or even feed by-products that have been improperly stored can all have high mould and yeast counts. Feeds that look and smell good to us can still contain levels of moulds and yeasts that decrease palatability and reduce the animals’ willingness to consume them. Not only will this reduce production of milk, meat, and wool, but it can also have negative effects on reproduction.

Cows have 2.5 times more taste buds than humans and are estimated to have 20 times more scent receptors than we humans do. What might smell and taste acceptable to humans may be a different story for a cow or sheep. The only way to confirm this is through a mould and yeast count test.

Testing has confirmed that in any feedstuff, even fresh pasture, there is a reasonable likelihood of the presence of yeasts, moulds, and mycotoxins. It is not necessarily the presence of mould and yeast that is the issue, but rather the quantity of them.

While high levels of all three can reduce overall feed intake, mycotoxins are the most likely to cause direct harm to the animal.

There are a number of different mycotoxins, each requiring a separate test to determine both their presence and their level in a given feed. As such, testing for overall moulds and yeasts is faster and more cost-effective, and the environmental conditions and type of forage can often give a good indication of the type of mycotoxin that may be present.

Yeasts While not harmful in themselves (other than affecting DMI), elevated yeast counts indicate that aerobic spoilage has begun and that mould counts are also likely to increase, bringing a greater toxin risk. As a general guide, dry hay and grain should be less than 1 million CFU, and silages no higher than 4–5 million.

Moulds Moulds, also called fungi, are the organisms that actually produce mycotoxins, so their levels are indicative of potential risk. Table 1 shows the likelihood that a given feed may be a source of a mycotoxin challenge and suggests recommended actions. It should be noted that clinical signs may still occur at lower levels.

Table 1. Mould CFU count and relative risk

10 – 10,000	Relatively safe
10,000 – 100,000	Transition zone
100,000 – 500,000	Relatively safe – watch for clinical signs, consider feed management options
500,000 to 1 million	Discount nutrient value by 5%, feed with caution,
1 million to 5 million	Dilute feed, and include feed management options
Over 5 million	Avoid feeding
Over 10 million	DO NOT FEED

Adapted from Hoffman (2009), Adams (2013) & Penn State Extension

The colour of a given mould can also provide some indication of the potential type of mould, although using colour as a sole guide is unreliable and can be dangerous.

White mould, for example, can be harmless and not a producer of mycotoxins, or alternatively, it can be Fusarium, of which certain strains produce extremely potent toxins. The old assumption that white moulds are safe is not true.

In Southern Australia, we typically see a lot of Fusarium, particularly through winter and the cooler autumn and spring seasons. Aspergillus, which produces aflatoxin, is more common in the southern summer and year-round in Northern Australia.

Toxin management tools

While dilution is normally the best option with high yeast and mould count feeds, this is not always possible. In some cases, we simply must utilise these feeds due to limited options. In these situations, we need to ensure we are selecting animals with the lowest risk of production and/or growth losses. Feed additives can also be used as a management tool for mycotoxins. Simple binders are effective against Aspergillus-type toxins but are significantly less effective against some Fusarium-produced toxins. Products that combine toxin-binding and toxin-deactivation actions (such as Elitox®) are effective against a wider range of mycotoxins.

If you have any questions, please do not hesitate to talk with one of our TRAC Ruminant Experts on 08 8733 1888 or email us at info@totalresult.com.au.