Keeping dairy feed free of mycotoxins is a challenge. However, there are some ways to stop mycotoxins from being passed on to milk.
Mycotoxins can reduce production efficiency of dairy cattle by altering both metabolism and behavior. Some mycotoxins cause damage to certain tissues of cattle, including liver, kidney and reproductive tract tissues. Other metabolic damage is less organ-specific. This would include the potential for mycotoxins to impair immunity in cattle, making animals more susceptible to infection and disease. The behavioral characteristic that is most often influenced by mycotoxins is feed intake. Reduced feed intake and subsequent reductions in milk production are commonly seen when feeds are contaminated with mycotoxins.
Another concern for dairy producers is the potentially harmful transfer of mycotoxins from feed into milk. This reduces milk quality and raises food safety issues when the mycotoxins in question are carcinogenic or otherwise harmful to human health.
Feed producers are now potentially faced with a two-fold challenge. First, they need to keep mycotoxins out of dairy feed. Failing that, they need to neutralize mycotoxins that do make it in.
The major mycotoxins influencing dairy production include aflatoxin and the various Fusarium mycotoxins.
Aflatoxin is produced by the fungus Aspergillus flavus. This is a tropical or semi-tropical mold that thrives at high temperatures, both in high humidity and under drought conditions. Aflatoxin targets the liver, causing reduced liver function and death. Aflatoxin also reduces the immune system, increasing the chances of infection. Reduced feed intake, reduced milk production and increased somatic cell counts can be seen.
Feed-borne aflatoxin appears in milk as the metabolite aflatoxin M1. This is of concern because aflatoxin is potentially carcinogenic, causing liver cancer following long term exposure to low doses. Alfatoxin content of dairy feeds is regulated, and milk exceeding maximal residues of aflatoxin M1 must be discarded. This is complicated by the fact that forages and silages can also be sources of aflatoxin and this is much more difficult to monitor. Dairy producers must be alert to the possibility of aflatoxin contamination of dairy feeds and monitor aflatoxin residues when poor production is detected.
Severe drought conditions in southern Europe in 2003 resulted in significant aflatoxin contamination of forages and silages and led to the discarding of an unprecedented quantity of fluid milk. Similar drought conditions existed across the Midwestern United States in the 2005 crop year, with a high level of aflatoxin contamination of corn crops seen in western Iowa and eastern Illinois for the first time in many years. This was again the case in 2007.
Fusarium molds thrive in soils in temperate climates and are common in the United States and Canada. Fusarium molds can produce a wide variety of mycotoxins with many different effects on dairy cattle. One large group of compounds is the trichothecenes. More than 100 trichothecenes have been chemically identified. The most common is DON, also known by the chemical name deoxynivalenol, and is sometimes referred to as vomitoxin. DON and the other trichothecenes affect dairy cows in three ways.
These compounds influence behavior causing reduced feed intake resulting in reduced milk production. A second effect is on the gastrointestinal tract. DON and the other trichothecenes can cause bleeding and ulcers in the digestive tract resulting in reduced nutrient absorption. The third effect of trichothecenes is immunosuppression and increased susceptibility to disease, including mastitis, and increased somatic cell counts in milk.
In a 2007 study, it was reported that the feeding of combinations of feedstuffs naturally contaminated with Fusarium mycotoxins altered immunity and reduced nitrogen utilization in dairy cows (Koresteleva et al., 2007; Table 1).
A second important Fusarium mycotoxin is zearalenone. This compound is estrogenic, and can cause infertility and abortions in dairy cows. The fumonisins are another family of Fusarium mycotoxins. These compounds can cause liver damage in dairy cows and, like aflatoxin and the trichothecenes, suppress the immune system.
Mycotoxins in silages
Silages contain mainly Fusarium, Mucor and Penicillium fungi, with lesser amounts of Aspergillus and Monila. The Penicillium mycotoxins of greatest concern in silages include PR toxin, patulin, citrinin, ochratoxin, mycophenolic acid and roquefortine C. These compounds are usually produced under aerobic conditions resulting from poor initial compaction and improper feed-out techniques. Little is known, however, about the toxicity of most of these compounds in dairy cattle.
Solutions to the mycotoxin problem
The only complete solution to the problems arising from mycotoxins in dairy feeds is to avoid feeding mycotoxin-contaminated feedstuffs. Efforts are continually being made to identify and produce strains of plants that are genetically resistant to mold infestation and mycotoxin production. Adverse climatic conditions such as those seen in the current crop year in the Great Lakes region are, however, beyond our control, so even if feed grains can be monitored by strict quality control, forages and silages cannot.
There are challenges in measuring the mycotoxin content of dairy feeds. The greatest single source of error is inadequate sampling techniques. It has also recently been reported, moreover, that conventional analytical techniques underestimate the DON content of barley produced in North Dakota by up to 88 percent (Zhou et al. 2007; J. Agric. Food Chem. 55:10141).
Use of adsorbents
One useful strategy is the feeding of mycotoxin adsorbents. These are non-nutritive feed additives that are not digested and not fermented in the digestive tract of the cow. Chemically, they are long polymeric molecules that pass like non-digestible fibers down the digestive tract and are excreted in the manure. While passing down the digestive tract, however, they can attract and bind small molecules like mycotoxins so they are not absorbed into the blood stream and cannot be carried to target tissues such as the mammary gland to be released into milk.
The absorbents can be classified as inorganic and organic polymers. The inorganic polymers are based on silica and are generally referred to as clays. The organic polymers are carbon-based and are the equivalent of plant fibers. Research by Diaz et al. (Mycopathologia 157:233 2004) has shown that a polymeric glucomannan adsorbent extracted from the cell wall of yeast, also known as GMA, was very effective in reducing the transfer of aflatoxin into milk. Bentonite, a silica polymer found in some types of clays, was equally effective, but it required a 25-times higher level of dietary inclusion to produce this effect.
GMA has also been shown to prevent the adverse effects of combinations of Fusarium mycotoxins in dairy cows (Table 1). Mycotoxin adsorbents are an effective way to minimize the adverse effects of mycotoxins on dairy production while ensuring minimal transfer of mycotoxins and metabolites into milk. They represent the most effective way to manage mycotoxin challenges in the short term until better, longer term solutions such as advances in analytical methodology and plant genetics can be developed.