Mould growth can occur in the field as well as during processing and storage of harvested products and feed (spoilage). To prevent this from happening, knowledge on the growth conditions of the moulds and the control measures is needed.
Excess moisture is considered to be the most critical factor contributing to mould growth. Grains stored at high relative humidity readily absorb excess moisture from the air. Mites and insects utilise the nutrients in grain and produce water as a metabolic byproduct and hence produce additional moisture sufficient for mould growth. Insects also damage the protective seed coat of grains, which allows the damaged kernel to rapidly absorb moisture from the environment, 5 times faster than intact kernels.
The higher moisture level of the damaged kernel and easier access to the available nutrients promotes mould growth. Temperature has been reported to have a direct effect on the rate of mould growth and (myco)toxin formation. It has been generalised that the higher the temperature, the greater the rate of growth and the higher the level of mycotoxins produced. This assumption may appear to be correct for certain species of mould, such as Aspergillus. The optimum temperature for Aspergillus growth and aflatoxin production is reported to be 24 – 35°C.
Aspergillus can grow at lower temperatures but at a slower rate. Penicillium moulds also grow faster at high temperatures (The optimum temperature for Penicillium growth and toxin formation is 18 – 32°C). However, other economically important species of moulds, such as Fusarium, grow more rapidly at lower temperatures (4-15°C) and produce higher levels of toxins at less than 15°C.
Hay baled when slightly wet can either form mould, go through a heating process, or both. Heat damage has different implications for hay quality than mould. When the internal temperature of a bale stays in the 50 to 65°C range, mould spoilage typically results. Heat damage, also known as the Maillard reaction, occurs if the bale temperature exceeds 65°C. Protein and carbohydrates in the hay begin to react to the high temperatures, going through a browning process – the Maillard reaction. The browning reaction occurs within weeks of baling and essentially makes the nutrients unavailable to the animal, dramatically reducing the feed value of the forage.
The dust associated with the mould is also an important health issue. Some potential effects of feeding mouldy, dusty hay include reduced intake and respiratory health responses associated with inhaling the spores. On the other hand, mould counts in silage should be very low. High counts indicate improper ensilage (e.g. not properly packed, not enough moisture, too mature, opened pit < 50 days after sealing, excessive aerobic degradation). Improperly ensilaged forage can become heavily contaminated with yeast-like fungi (such as Candida) resulting in poor acceptability by livestock due to off flavour, poor palatability and/or nutrient depletion. Yeast contamination can also reduce keeping quality due to fermentation and spoilage, especially in warmer, humid conditions.
Mould causes liver damage, immune dysfunction, and numerous productivity problems animals. In addition, mould has been shown to affect nutrient availability by decreasing pancreatic and hepatic enzyme activity, decreasing nutrient absorption, and increasing nutrient excretion. When moulds start to produce mycotoxins, the effects can be worsened. Mycotoxins cause a reduced intake or feed refusal, reduced nutrient absorption and impaired metabolism, altered endocrine and exocrine systems, and suppressed immune function. Cattle have the ability to convert certain mycotoxins in less toxic variants. Monogastrics cannot, so the effects in pigs and poultry can be much more profound. However, pregnant cows and young can be more susceptible to mould and may experience abortion, respiratory allergy or bovine interstitial pneumonia or estrogenism (development of feminine characteristics in males, premature sexual development of young females, infertility in adults, abortion, stillbirth and the birth of deformed offspring).
The most obvious economic impacts will occur at the feed stuff producer level where losses will include yield losses, increased production cost, increased marketing risks and costs and increased post-harvest costs. Increased production costs could include increased expenses for irrigation to reduce drought stress and increased pesticide use to reduce plant pathogens that produce mycotoxins. Increased post-harvest costs could include sampling and testing, increased storage and drying costs, detoxification, increased transportation involved with limited markets and the disposal of unusable feed stuff. Price discounts could be substantial depending on the level of contamination and the supply of and demand for the contaminated commodity.
Producers feeding their own heavily contaminated crops to their own livestock, rather than taking price discounts or destroying or treating them, could compound losses to the farm operation. They can also suffer the loss of a customer or an entire market if the quality and dependability of their product comes into question. The consumer costs could include higher product prices as some increased costs faced by handlers and processors are passed on to consumers or as supplies are restricted. Consumers could also be faced with a less nutritious feed supply and debilitating effects of mycotoxicosis if the contaminated feed stuff were to enter the feeding system.
There are several strategies that can be adopted when attempting to control growth and development of the mould and hence reduce their effects on feed quality and animal performance. An important one is proper storage of feed. Field studies in Egypt, for example, have revealed that in most farms, feed is stacked under shelters where it can frequently be subject to mould damage and invasion from rodents, wild birds and/or insects. Much of the problems can be alleviated if the feed was stored in silos. A second prevention strategy is proper forage management.
If possible, the hay should be fed in a well-ventilated area and/or be mixed with wet feeds such as silages, beet pulp, wet distiller’s grains, or even liquid feed supplements, especially when feeding to pregnant cows and freshly weaned calves. Mouldy hay may also be fed with other forages to reduce exposure. The dilution ratio should start here at a half-and-half, with a higher proportion of good-quality hay to be used in the case of higher mould counts.
This will improve the overall safety of feeding mouldy hay. To prevent mycotoxins from developing in silage, production practices that preserve quality should be strictly followed. Accepted silage-production practices include for example: Harvesting at the proper moisture content (30-35 %), filling the silo rapidly and using silage additives (such as ammonia, propionic acid, microbial cultures, or enzymatic silage).
A third option to control moulds and mycotoxins is the use of feed additives. Mould inhibitors often include propionic acids and other organic acids. However, even if mould growth has been prevented, mycotoxins may still be present, because mould inhibitors have no effect on mycotoxins already present in the contaminated feed. Certain feed ingredients may affect mould inhibitor performance.
Protein or mineral supplements (for example, soybean meal, fish meal, poultry by-product meal, and limestone) tend to reduce the effectiveness of propionic acid. These materials can neutralise free acids and convert them to their corresponding salts, which are less active as inhibitors. Dietary fat tends to enhance the activity of organic acids, probably by increasing their penetration into feed particles. The widespread use of pelleted feeds in the feed industry is beneficial to the use of mould inhibitors. The heat that the feed undergoes during pelleting enhances the effectiveness of organic acids. Next to mould inhibitors. mycotoxin binders are sometimes needed to bind to mycotoxins and prevent them from being absorbed through the gut and into the blood circulation. Most of these products are efficient binders of aflatoxins but limited activity against other types of mycotoxins.
Cell-wall fraction β-glucan of yeasts such as Saccharomyces cerevisiae can alternatively be an effective substance in binding a wide range of mycotoxins. The use of antioxidants can also be used to prevent oxidation of fat or vitamins. Antioxidants found in commercial products include ethoxyquin, butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), and propyl gallate. Combinations of these antioxidants are normally found in commercially available products to take advantage of the different properties of each antioxidant.
References are available from the author upon request.