Mycotoxins are natural substances produced by moulds and fungi. They cannot be seen with the naked eye, and they cannot be tasted — but they are toxic.
More than 500 different mycotoxins have been identified to date, and most animal feedstuffs are likely to be contaminated to some degree, including grazed grass, conserved forages (silage, wholecrop, maize, straw), plus home-grown and bought-in concentrate feeds.
One example of a harmful mycotoxin is penicillic acid, which, according to silage specialist Dr. Dave Davies, is now one of the most common forms of mycotoxins produced in Irish grass silages.
Dr. Davies attributed the increased threat posed by this, at least in part, to the impacts of climate change – adding that the issue of mycotoxins would not even have been on his radar a decade ago.
“Today, we find that the threat of both in-field and in-store mycotoxins represent a major challenge for farmers, and the entire feed industry.
“Management decisions taken by farmers while crops are growing and being harvested will have a major impact on the development or otherwise of in-store mycotoxins subsequent to harvest,” Davies said.
Davies spoke at a recent mycotoxins workshop which was hosted by Alltech. The event was attended by farmers and feed industry representatives from across Northern Ireland.
He went on to say that silage that is free of mould, is not necessarily free of mycotoxins. Fungal attack in the field can led to the production of mycotoxins at that stage, which are subsequently brought into the silage.
Conversely, silages containing moulds may not contain mycotoxins. Dr. Davies gave the example of Co. Tipperary blue cheese, which is inoculated with a microorganism that does not generate mycotoxins.
“This is a complex story, one which we are learning about as scientists,” he said.
He went on to point out that mycotoxins are secondary metabolites, produced by moulds, but not yeasts.
Moreover, they are toxic at very low concentrations, and are hard to denature in a physical sense – e.g., mycotoxins are very resistant to both chemical attack and the impact of heat.
Moulds
Dr. Davies attributed the onset of milder winters as being a core reason behind the growth in the threat posed by mycotoxins.
“As a consequence, there is a lower kill-off of the moulds and yeasts that live in the decaying material found at the bottom of all grass swards.
“Extremes of temperature, and the increased threat of drought throughout the growing season are creating the conditions that facilitate the growth of Penicillium fungi in all feedstuffs. So, the reality is that much lower levels of fungi are being killed in the field.
“Yeasts, on the other hand, do not produce mycotoxins. However, they do initiate aerobic spoilage within forages. The onset of aerobic conditions will facilitate the growth of moulds that produce in-store mycotoxins,” he said.
The forage specialist outlined several steps that can be taken, both in the field and while ensiling a clamp, to minimise the threat of mould growth and subsequent mycotoxin production.
The key driver in-field is to secure the required level of wilting as quickly as possible, while minimising soil contamination of the subsequent silage – which is a key source of both fungal and clostridial inoculation.
Dr. Davies recommended a grass cutting height of 10cm, saying: “This provides the cut grass with a cushion of stubble to lie on, which facilitates the circulation of air around it.
“Cut grass should also be kicked out as quickly as possible, again to hasten the wilting process. Cutting to 10cm also encourages subsequent sward re-growth.”
“It is also important to ensure that all cut grass is lifted. Forage that is left in the field will start to decay, thereby representing a possible fungal source for second-cut crops,” he added.
Where the ensiling process is concerned, Dr. Davies recommends the use of side sheets, an oxygen barrier film and a top cover. Tyres and/or gravel will further aid this process.
In this way, it is possible to secure an extremely effective seal along the top and sides of the silage clamp. In addition, grass should be consolidated in 15cm layers.
“Providing totally anaerobic conditions within the silo is critical,” Dr. Davies explained.
“The introduction of air, no matter in how small a quantity, will encourage the growth of moulds and yeasts.”
Limiting the amount of time that forage is exposed to oxygen at feed-out reduces the chance of mycotoxin development as well.
According to Davies, silage covers should only be removed at point of feeding, with the aim to move across the face as quickly as possible and, if needed, to take half blocks.
“A sharp shear grab or block cutter should be used to keep the face as clean and even as possible,” he added.
