MYCOTOXINS ARE A SIGNIFICANT ISSUE IN GRAINS AND FEEDS

by: Stephen B. Blezinger
Ph.D., PAS

Part 1

Every year about this time cattlemen are taking a serious look at how they are going to feed/supplement their cattle through the winter. In many if not all cases there is a focus on how this can be done as cheaply as possible. This just makes good economic sense as long as attention is given to performance. Often this process exposes the cattle producer to a variety of feeds and ingredients from which to evaluate and ultimately choose. With this evaluation it is important that a number of factors be considered when assessing the value of the product. One of these factors is the potential presence of mycotoxins.

Mycotoxins are chemical agents (toxins) produced by a wide variety of naturally existing fungi. The word mycotoxin comes from “myco” which refers to fungi and the metabolites it may produce that are toxic to other organisms. Penicillin is derived from a fungus and is toxic to certain bacteria and therefore could be considered a mycotoxin. There are about 100 types of fungi that grow on standing crops or stored feeds and produce toxic substances. Approximately 20 of these have been associated with naturally-occurring diseases. Toxigenic (toxin producing) fungi spores are present almost everywhere, and they can germinate, grow, and interject their toxins into a variety of substrates when moisture, temperature, and aeration conditions are favorable. Optimal conditions for toxin production by different fungi may be quite variable. In many cases, the production of one of the most well-known of these toxins, Aflatoxin, is associated with drought stress in corn and cotton (cottonseed and other cotton products). We will discuss Aflatoxin at length in a bit.

In addition to Aflatoxin, other mycotoxins include T-2 fusarium, Zearalenone, Fumonisin, Vomitoxin and DON (Deoxynivalenol). This is only a short list of the variety of fungi produced toxins which can affect livestock. In general, most of these mycotoxins affect monogastrics (pigs, chickens) more severely than cattle. However, their affects on both beef and dairy cattle can be extensive and profound.

Mycotoxins affect cattle in a variety of ways. The fact that there presence is wide ranging implies that their occurrence in feeds cannot be overlooked. Additionally, the fact that profound reductions in performance as well as the adverse affects these compounds can have on animal health also supports the need to recognize their potential presence in grains and feeds. While a large amount of research has been performed on a number of these toxin producing fungi, their metabolites and the subsequent effects on the animal after ingestion, there is a great deal we do not know about these compounds and their activities. The affects of mycotoxins can be seen in reproductive function, immune response and animal growth and performance (gains, milk production). As noted previously these compounds can affect the animal in different ways. Aflatoxin and T-2 have been shown to inhibit protein synthesis and cell replication, growth and development. These compounds and others have been shown to have significant suppressive effects on the immune system at a variety of levels but in general, this is related to this effect on cell proliferation upon which the immune system is dependent.

Mycotoxins may can be highly reactive in mammals and may destroy different tissues. This is most true of liver tissues. The liver is the central organ in the body and the presence of different mycotoxins has been shown to be highly toxic to the liver.

Discussion of Some of the More Common Mycotoxins

Fumonisins are a group of mycotoxins produced by fungi in the genus Fusarium. The fungus Fusarium moniliforme (Fusarium verticillioides) is a common pathogen of corn, so common in fact that it is found wherever corn is grown. Fusarium moniliforme usually appears white to salmon colored, although it may not be visible on the corn kernel. This fungus often produces a symptom on the corn kernels referred to as "starburst," or a white streaking of the kernel. The visual absence of mold, however, does not mean that kernels do not contain the toxin. Intact corn kernels may contain the fungus and the toxin but show no sign of the fungal contamination.

In severe cases, the corn shucks will become "glued" to the kernels in the cob. Fumonisins have been implicated as a possible cause of human esophageal cancer, equine leukoencephalomalacia (ELEM), a serious disease in horses, and porcine edema – a disease in swine. Poultry and cattle are not especially susceptible to fumonisins. However, caution should be used in feeding moldy corn to these animals as other mycotoxins may also be present in rotted or moldy corn.

Fumonisins were found at levels above 5 ppm in 23.5 % and 31.0% of suspect grain samples in North Carolina in 1992 and 1993 respectively. Contamination with fumonisins was more severe in 1998, and corn with fumonisin levels in excess of 15 ppm was rejected by some buyers. Currently, no levels for fumonisins have been set by the FDA, but they will likely be less than 5 ppm for human consumption and horses. Buyers of corn used in feed may accept grain with higher levels of fumonisins than those recommended depending on what portion of the feed ration is corn. Also, corn and corn products shipped to Europe may be regulated at a lower level in the future.

Currently there are no corn hybrids resistant to the fungus F. moniliforme, which is the principle producer of fumonisins. Some hybrids may be more susceptible to Fusarium than other hybrids, but no hybrid is known to be completely resistant. Dry weather early in the season, followed by wet weather during silking of the corn plant, and insect infestation increase the amount of fungal infection of corn kernels. Typically, infection by F. moniliforme will not greatly affect the yield of corn. However, if conditions favorable for fungal growth continue up to harvest, fumonisin levels in harvested corn may exceed recommended levels for certain animals. Factors that influence fumonisin production in corn are not well understood at this time. Certainly, insects provide an avenue of infection for both Aspergillus and Fusarium. Hybrids genetically engineered to resist insects may have lower levels of fumonisins. Also, research is under way in the corn industry to engineer plants with an enzyme to degrade fumonisins.

Deoxynivalenol (DON or vomitoxin) is a mycotoxin produced by certain species of Fusarium, the most important of which is F. graminearum (Gibberella zeae). This fungus causes Gibberella ear (also known as red ear rot) or stalk rot on corn and head scab in wheat. The fungus itself appears reddish to pinkish. The fungus may cause a reddish dicoloration of the cob and kernels.

The DON causes reduced weight gain and suppresses animal feeding, especially in swine. At high concentrations (greater than 10 ppm) vomiting and total feed refusal may occur. FDA has recommended that total feed levels of DON not exceed 5 ppm for cattle and chicken, and 1 ppm for swine.

Other mycotoxins (DON, T-2, and zearalenone) present in grains, silages, and hays may cause problems with performance and immune status of beef cattle. However, little research is available on the levels of the individual toxins that may be tolerated by animals. In cases of disease outbreaks and reproductive problems, the feed should be tested for a full range of mycotoxins. Large producers should consider routinely screening feeds for mycotoxins.

Until further information is available, the producer should limit dietary mycotoxins to the levels listed in the table below.

Heat and other stress, age of the animal, marginal nutrient plane, crowding, disease exposure, the presence of more than one mycotoxin, and drug interactions, as well other factors, increase animals' susceptibility to mycotoxins.

What is Aflatoxin?

In order to understand how to manage this toxic compound, we must first understand what it is and where it comes from. Aflatoxin is a highly carcinogenic compound produced by several species of fungi. Aflatoxin B1 is considered to be one of the most highly toxic, naturally produced compounds known to man. The main causative fungal species are Aspergillus flavus (A. flavus) and Aspergillus parasiticus (A. parasiticus) during growth on some feeds or foods. Aflatoxin is a secondary metabolic by-product of the growth and development of these fungi in materials such as corn grain, grain sorghum, cottonseed, peanuts and dry beans. Corn has been found to be an especially good substrate for aflatoxin production because of its high carbohydrate and low nitrogen content. To a lesser extent it can be found in soybeans and small grains such as wheat, oats and barley. It can also be found in corn or other grain silages as well. A complicating factor is that the fungi of concern are normal, soil-borne microorganisms in the environment and are found on both living and decaying plant matter in virtually all crop production systems. Although mycotoxin contamination occurs across most of the U. S., aflatoxin contamination appears to be the most prevalent in the southeastern and southwestern states although midwest corn production is also susceptible. Although it is not understood what triggers the production of aflatoxin by the organism, environmental factors influence the production of aflatoxin in the field and during storage when conditions are favorable. Studies have shown that water activity, relative humidity, temperature, light and pH affect aflatoxin production. Presence of aflatoxin becomes much more prominent in drought years when crops are damaged by the dry weather conditions. The drought conditions allow access of the fungi to the susceptible parts of the plant such as the corn kernel, cottonseed, etc., by increasing the permeability of the seedcoat. Drought conditions are typically in conjunction with high temperatures as well as high relative humidity, all conditions conducive for the growth of the fungi which produce aflatoxin. As this statement makes clear, the development of aflatoxin is largely a matter of uncontrollable natural events. It has been shown however that grain which has been stored and remoistened increased in aflatoxin contamination. Grain that has been processed by rolling or grinding, in which the starchy interior is exposed is highly susceptible to growth and proliferation of the fungi.

Effects on Cattle

As stated, aflatoxin has been identified as one of the most toxic, naturally occurring substances known to man and is found in several forms, most commonly identified as B1, B2, G1, G2. This statement regarding the level of toxicity is quite profound and redeemed only by the fact that it is not terribly common to find aflatoxin in very high concentrations.

The effects of aflatoxin consumption are relatively similar in most animals. The animal's susceptibility to aflatoxin, however, varies by species, age and individual. Consumption of high levels of aflatoxin creates a condition referred to as aflatoxicosis. Specific liver conditions created by aflatoxicosis can include acute necrosis, cirrhosis and carcinoma as well as systemic effects such as blood cell fragility, immune system degeneration and other conditions which can lead to lost production and even death. In beef cattle it will commonly cause reductions in feed intake, rate of gain, feed efficiency, reproductive function, rectal prolapse and abomasal cavity edema. These symptoms are also found in dairy cattle in addition to suppressed milk production and the conversion of aflatoxin to aflatoxin M1 that is secreted in milk. It has been shown that consumption of even small amounts of aflatoxin by the lactating cow could lead to decreased milk production in addition to contaminated milk. Also, abortions of fetuses in breeding cattle have also been reported due to aflatoxin consumption.

Conclusions

With only a small amount of investigation it becomes obvious that mycotoxins are a significant issue in grains and feeds for cattle. In the next part of this series we will continue to discuss this topic and examine Aflatoxin, the most common of these compounds in greater detail.

Dr. Steve Blezinger is a management and nutritional consultant with an office in Sulphur Springs, Texas. He can be reached at 667 County Road 4711 Sulphur Springs, TX 75482, by phone at (903) 352-3475 or by e-mail at sblez@verizon.net.







Google
  Web CattleToday.com

Don't forget to BOOKMARK  
Cattle Today Online!