Mould growth can occur in the field as well as during processing and storage of harvested products and feed (spoilage). What are the effects on feed quality, animal performance, and production economics? And which control measures can be taken?
There are a number of toxic compounds (mycotoxins) produced by certain species of fungi (moulds) and are capable of causing disease and death in animals. Mould causes liver damage, immune dysfunction, and numerous productivity problems in 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. In animals consuming mould-contaminated feed, the levels of pancreatic lipase, amylase, trypsin, and chymotrypsin are significantly reduced. This reduction in pancreatic enzyme levels results in insufficient digestion and absorption of protein, fat and carbohydrates. Vitamin A deficiency may also result if mouldy or spoiled forages are fed for long periods. Symptoms of a vitamin A deficiency include reproductive problems in both males and females. Pregnant cows may abort, retain their placenta and develop a uterine infection or give birth to weak, dead or blind calves. Bulls with a vitamin A deficiency produce semen with low sperm count and high numbers of abnormalities.
There are many different mycotoxin groups – this special tool gives up-to-date information on which mycotoxin species fall under each group.
Mould related health problems
Mycotoxins exert their effects through several means such as reduced intake or feed refusal, reduced nutrient absorption and impaired metabolism, altered endocrine and exocrine systems, and suppressed immune function. Much of these effects can be tolerated by ruminant in virtue of the microbial activity in the rumen. For example, ochratoxin A is rapidly converted into the less toxic ochratoxin α (lacking the phenylalanine moiety) by the rumen flora, and only very small amounts of intact ochratoxin A are absorbed. In vitro studies showed that ochratoxin A is mainly degraded by rumen protozoa and that in healthy cattle up to 12mg of ochratoxin A per kg feed could be inactivated. Pregnant cows and young heifers may, however, be more susceptible to mould and may experience one or more of the following health problems:
Mycotic infections (caused by mycotoxins) are a common cause of abortion in individual animals. The toxins in mouldy feeds will enter the bloodstream and the pregnant uterus, and abortions and infertility may result if the toxic elements from the mouldy feed enter and infect the placenta.
Badly moulded feeds contain large numbers of fungal spores. Cattle that inhale these spores may develop a respiratory allergy or bovine interstitial pneumonia. This disease prevents oxygen from getting into the bloodstream. An infected cow will gasp for breath, and the developing fetus may die due to a lack of oxygen.
Estrogenism occurs most often after livestock eat mould-damaged crops that have been put up during cool, wet weather. The ingestion of infected grains can result in the development of feminine characteristics in males, premature sexual development of young females, infertility in adults, abortion, stillbirth and the birth of deformed offspring.
Economic impacts of mould damage
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 come 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.
The following are the strategies to be adopted when attempting to control growth and development of the mould and hence reduce their effects on feed quality and animal performance.
Proper storage of feed is an important factor in preserving the quality of feed and protecting them from mould damage. 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. A microbial assay of feed at one farm has revealed a mould value of 22,000 cfu/g, as opposed to the normal level of only 5,000 cfu. With such a high level of contamination, poor feed efficiency, reduced growth rates, and increased susceptibility are often encountered. Much of these problems could have been alleviated if the feed was stored in silos. Moneywise, it was estimated that the cost of 20-ton silo can be recovered in just three years as a result of savings of up to 16,000 Egyptian Pounds per year (€ 758) being the value of the feed damaged under the current poor storage conditions.
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:
- Harvesting at the proper moisture content (30-35 %)
- Chopping uniformly at the proper length to allow for better compaction and exclusion of air
- Silo size should be matched to herd size to ensure daily removal of silage at a rate faster than deterioration can occur
- Filling the silo rapidly
- Packing the silage sufficiently
- Covering the horizontal silo immediately during or after filling
- Using silage additives (such as ammonia, propionic acid, microbial cultures, or enzymatic silage) may be beneficial in preventing mycotoxins because they are effective at reducing mould growth
Mycotoxin binders are substances that bind to mycotoxins and prevent them from being absorbed through the gut and into the blood circulation. Photo: Nico de Jong
Use of feed additives
Mould inhibitors are feed additives used to minimise mould contamination and prevent mould growth, thereby minimising the risk of having mycotoxin-producing moulds proliferate in grain or feed. Feed additives commonly used for this purpose 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. Once mould activity commences in pellets, however, it proceeds at a faster rate than in non-pelleted feed because the pelleting process that makes feed more readily digestible by animals also makes it more easily digested by moulds.
Mycotoxin binders are substances that bind to mycotoxins and prevent them from being absorbed through the gut and into the blood circulation. A variety of substances have the ability to bind mycotoxins. The most commonly used and most researched mycotoxin-binding agents are the aluminosilicates – clays and zeolites. 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.
Antioxidants are products added to animal feeds 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.