Mycotoxins in silage

Introduction

Moulds and their toxins in feed are an increasing global challenge, and multiple environmental and pre-harvest and post-harvest agricultural practices may contribute to this increase. Mouldy feed and mycotoxins have been associated with lower feed intake, reduced digestibility and health disorders in ruminants. An array of mycotoxins can be present in mouldy silage, some of which are not normally found in concentrates. The mycotoxins of greatest concern are those produced by Penicillium, such as PR toxin, mycophenolic acid, roquefortine C and patulin. The clinical signs of mycotoxicosis are often subtle and the diagnosis is, therefore, often missed. The inclusion of a mycotoxin binder is commonly used in dairy production to safeguard against the potential production depressive effects of mycotoxins.

Moulds found in silage

Moulds are found everywhere and over 80 different fungal species have been identified in corn and grass silage. Silage is invaded by moulds that are adapted to the micro-aerobic, acid conditions of the silage (Storm et al., 2008). Air frequently invades silage and supports the growth of acid-tolerant yeast and moulds. Penicillium moulds are commonly found because they are acid-tolerant and have a low oxygen requirement. Moulds can produce mycotoxins depending on environmental conditions. Distribution and extent of spread of the moulds and toxin formation in silage is highly variable. There is an uneven growth of moulds in the silage and the growth rate, production and stability of various mycotoxins varies. Penicillium roqueforti is able to invade silage and considered an indicator of mould invasion. They can be seen as blue balls in corn silage and darker areas in grass silage. P. roqueforti destabilises silage and results in an increased silage temperature. The mould paves the way for other potentially pathogenic organisms. In the instable silage, the toxin-producing moulds and yeasts (Monascus ruber, Aspergillus fumigatus, Byssochlamys) may be found, together with potentially pathogenic bacteria, such as Listeria monocytogenes and Clostridia butyricum. P. roqueforti and A. fumigatus produce a wide range of secondary mycotoxins with antimicrobial or immunosuppressive effects. P. roqueforti comes later in the silage period, than some of the other mould toxins. This uneven temporal and spatial distribution of toxins creates challenges when samples are taken with the intent to detect a mycotoxin challenge.

The Mycotoxins

PR toxin has an antimicrobial effect and has been considered a marker for problem silages (Sumarah, et al., 2005). PR-toxin has been associated with reduced feed intake, rumen stasis, intestinal irritation, abortion, retained placenta and reduced fertility in cattle.

Penicillic acid has antimicrobial effects and can destabilise the rumen microbial flora

Roquefortine C is a neurotoxin. When P. roqueforti mould is used in cheese production the potential toxins are bound and inactivated by milk proteins and, therefore, cause no harm when ingested. However, in silage, these nerve toxins can have serious, adverse consequences.

Mycophenolic acid is a very strong immunosuppressant toxin, so strong that it has been used in therapy for kidney transplants as the strongest immunosuppressant drug available.

Patulin is a common mycotoxin in silage. It affects ruminal fermentation and is a nerve toxin (neurotoxin) and a cell toxin (cytotoxic). It has been implicated in deaths of cows but has received little attention in the literature.

The risk is that cattle may be exposed to the negative effects of a cocktail of mycotoxins that interact with each other. This risk is not taken into account by current legislation and recommendations, which are based on individual mycotoxins and not multi-contamination.

Rumen health effects

Cows are exposed to various moulds and mycotoxins both through inhalation and ingestion. The rumen has long been considered a very strong buffer against all these toxins because the rumen microflora were assumed to naturally detoxify mycotoxins (Schiefer, 1990). This ability makes ruminants relatively resistant to these toxins compared with non-ruminants. However, this is not the case for all mycotoxins. High-producing animals, such as lactating dairy cows have an increased rumen passage rate and this faster processing of contaminated feed may overwhelm the rumen microflora, so that they will not be able to denature all the toxins (Kiessling et al., 1984). Furthermore, young calves do not have fully developed rumens and are therefore, also more susceptible to mycotoxins. Mycotoxins create a cascade of events by destabilising the rumen environment, leading to endotoxin formation and ruminal wall leakage. Toxins with antibiotic effect can disturb the rumen microbiota to the point that they cannot properly handle other toxins. The silage-derived mycotoxins may, through their antibiotic impact on the rumen microflora, result in common pre-harvest mycotoxins, such as deoxynivalenol (DON), zearalenone and tremorgens becoming a health problem.

Systemic health effects

Mycotoxins can produce a variety of symptoms in dairy cattle that are vague and nonspecific. Mycotoxins absorbed into the systemic circulation will have various effects and result in an activation of the immune system (Sharma, 1993). Mycotoxins are considered one of the most important dietary factors that suppress immunity. Symptoms may include reduced feed intake, feed refusals, unthriftiness, rough hair-coat, poor body condition, lower fertility and increased susceptibility to disease. Conditions, such as altered rumen fermentation, digestive upsets, diarrhea, intestinal irritation, rumenitis, high somatic cell counts, mastitis, laminitis, skin erosions, , and reproductive problems are all associated with a mycotoxin challenge. Often, there are no clinical signs, but sub-clinical production losses, which have a serious financial impact on farm profitability. Mycotoxins in feed will often cause 10 to 15% loss in milk production and, in serious clinical cases, up to 50% loss in production can be seen.

Diagnostic methods

As mentioned, detection of moulds and toxins in feed is difficult and not very informative. A lot of research has been carried out to find diagnostic markers for a mycotoxin challenge in the animals (Zheng et al., 2006). Liver enzyme tests are only of value to detect an aflatoxin challenge and, although they have now been found in feed in southern Italy, they are not the major concern in Europe. Various markers of metabolic distress are not unique to mycotoxins but can assist in making a diagnosis. Research is on-going to find biomarkers, and researchers are currently developing a bovine gene signature that will be used to assist in the diagnosis of mycotoxicosis. Gene expression analysis may also be a good approach to present biomarkers of effect when animals are exposed to multiple mycotoxins.

When a mycotoxin challenge is suspected, a practical diagnostic method is to perform a diagnostic treatment with a broad-spectrum mycotoxin binder, such as Mycosorb®. If the clinical symptoms decrease or disappear following mycotoxin-binder administration, then it is very probable that there may have been a mycotoxin challenge through the feed. This approach avoids the issue with uneven sampling of materials and the difficulties to interpret the risks associated with mycotoxin levels when they are found in combination.

Silage management

Silage is a preserved feed where microbial processes have depleted the oxygen supply and lowered pH due to production of organic acids. Silage management recommendations are designed to achieve an anaerobic and acidic environment to prevent further microbial growth. Under these perfect conditions, undesirable spoilage microorganisms will not grow. Good silage management is very important to prevent moulds and mycotoxins. Plant stress needs to be minimized through optimised planting and harvesting times and proper moisture levels, packing and sealing are essential to ensure the exclusion of air. Inclusion of a silage inoculant, such as Sil-All (Alltech), optimises the fermentation and preservation of the forage. The feeding face should be cut cleanly to avoid deterioration and 15-30 cm should be removed daily to prevent deterioration of the feeding face and face cutters are recommended. Silage should be fed directly after removal from the silo and mouldy silage should be discarded.

Interventions to stabilise rumen function

Stable rumen fermentation is important to optimise mycotoxin degradation in the rumen. General recommendation to minimize the effect of mycotoxins on the rumen is to increase roughage inclusion in the feed. The use of live yeast product, such as Yea-Sacc® contributes to maintain a stable rumen pH and thereby optimises rumen fermentation. A good quality broad-spectrum mycotoxin binder, such as Mycosorb® (Alltech), should be used to bind mycotoxins. Mycosorb® is a yeast extract with ability to bind a broad range of mycotoxins in the feed. In a study determining mycotoxin-associated oxidative stress, 85% of the cows been supplemented with Mycosorb for two weeks showed reduced levels of oxidative stress, lower somatic cell counts and increased milk production (Santos & Fink-Gremmels, 2010)

Summary

Moulds in silage feeds and mycotoxin challenges to cows are common but are often neglected in terms of optimising cow health and production. Detection of moulds in feed is challenging due to the uneven spatial and temporal distribution of moulds and toxins in the feed. The antibiotic effects of some of these silage moulds can disturb the rumen microbial environment paving the way for other mould toxins, yeasts and bacterial hazards. Symptoms of mycotoxin exposure are non-specific and often sub-clinical but production losses can be significant. Moulds should be kept in mind dealing with any type of sub-optimal performance in ruminants and inclusion of an appropriate mycotoxin binder is recommended. For further information, the website www.knowmycotoxins.com is highly recommended and frequently visited by those involved in food animal production.

Written by Dr. Anna Catharina Berge, Berge Veterinary Consulting, Belgium, following Pr Johanna Fink-Gremmes's talk at Dairy Solutions Symposium 2010. Published in Dairy Solutions Newsletter, issue 7, 2011