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

Part 2

In part 1 of this series we began a discussion on the effects of mycotoxins in beef cattle. To review, remember that 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.

In the previous part of this series we considered an overview of several mycotoxins of concern. We completed with beginning a discussion on aflatoxin, the most problematic of the mycotoxins. Below, we will pick up on this discussion and expand on the issue of aflatoxin affects at greater length.

Aflatoxin Effects on Calves

Young calves are especially susceptible to these toxic effects. This may be largely due to the underdevelopment of the rumen which, in its fully developed state, may have at least some detoxifying effects. The clinical symptoms of aflatoxicosis in younger cattle are similar to more mature animals and include decreased growth rate and feed intake, rough hair coats, anorexia, reluctance to move, grinding of the teeth, apparent blindness, diarrhea and rectal prolapse.

Obviously the research has shown that the effects of aflatoxin on cattle, beef and dairy, are profound. The cost is staggering once the level of lost production in terms of reduced feed efficiency, reduced gains and reduced milk production, not to mention the loss of the animals themselves in many cases, are considered. A relatively unmeasured expense is that of production losses due to long-term low-level ingestion of aflatoxin contaminated grains. This is an area which requires substantial research.

How can this problem be managed?

As with many situations, the best defense is a good offense. A number of steps can and should be taken to reduce potential aflatoxin contamination in feed and grain supplies. They include:

1) Make sure that grains and feeds do not contain excessive moisture. If the grain samples test higher than 14 to 15 percent moisture the load should not be accepted.

2) Set up an extensive quality control procedure which utilizes appropriate and detailed sampling. This should be standard in most feedmills, especially those located in the southern United States. Aflatoxin contamination is not an analytical problem. Quick, relatively inexpensive assays exist which can effectively be used to quantify aflatoxin levels in a sample. The root of the problem is in sampling. This is primarily true because aflatoxin is not uniformly distributed throughout a field or a load. Improper or inadequate sampling tends to be one of the greatest problems in most grain elevators and feed mills. Simply taking one or two "hand-grab" samples will not provide a representative sample for testing. Even use of an approved grain probe will not effectively provide a representative sample when only probing a load two to three times. Loads of grain should be probed multiple times with a standard grain probe and these subsamples combined to produce a gross sample which can be tested. This will help to locate and target any "hot spots" which may be found within a load of grain. Although not fool proof by any means, this method greatly reduces the chance of missing highly contaminated areas in the load. Finally, elevators and mills need to employ testing procedures which are quantitative, meaning the test gives a fairly accurate indicator of the level of aflatoxin in the sample, not just an indicator that something is there. A number of testing kits and procedures are available at varying cost of set up and per sample. Blacklighting, which was once the industry norm should no longer be considered.

3) Keep bins and equipment (mixers. etc.) clean and free from grain and dust build-ups where moldy feed might accumulate and subsequently contaminate clean grain or feed.

4) Any feed or grain which smells or looks musty or moldy should be rejected. These outward signs provide a strong indicator for other more serious conditions.

5) Retain samples of grain and feed for a period of time. Different facilities will retain samples from 3 months to two or more years. This simply provides a way to go back and double check a given load should a problem arise.

6) If a problem with aflatoxin is found, especially in stored grains, the grain can still be used as long as it is fed in a diluted manner. In beef cattle (cows, finishing animals) the level of aflatoxin intake needs to be kept below 300 ppb. In younger or growing cattle, this level should be even lower (<100 ppb) or eliminated altogether if possible. In newly received cattle or stressed cattle, obviously it is best if affected grain is not used. This means that if you have a supply of affected grain, depending on what the level is, you should feed another, clean source of grain along with the tainted product to dilute the overall amount down as far as reasonable. More details on this in a moment.

More on Mycotoxins

Some generalities to remember on aflatoxin and other mycotoxins: These compounds can have considerable effects on beef cattle although the problems are usually less critical than for swine and poultry. Consumption of feeds highly contaminated with aflatoxin may reduce growth rate and increase the amount of feed required per pound of gain. Calves are generally more sensitive to feed contamination than adult cattle. In affected calves, some cases have revealed severe rectal straining and a prolapsed rectum. Lactating cows show a significant reduction in milk yield. Research has shown that high levels of aflatoxin can also cause liver damage in adult cattle. Feeding a high level of aflatoxin may also depress immune function, resulting in disease outbreaks. These facts alone emphasize the significance of the problem.

Based on the feeds available, those contaminated with aflatoxin should be fed at the lowest level possible and for the shortest period of time practical. The effects of aflatoxin fed to cattle depend on the level of aflatoxin in the ration, the length of the feeding period, and the age of the animal. If aflatoxin-contaminated feeds must be fed to beef cattle, follow these guidelines (on a dry matter basis):

1) Creep feeds and diets for gestating and lactating beef cows should contain less than 20 ppb of aflatoxin.

2) Unstressed, growing-finishing cattle in excess of 400 pounds may be fed diets containing up to 100 ppb of aflatoxin.

3) Diets for stressed feeder cattle should contain no more than 20 ppb of aflatoxin. Stressful conditions include weaning, shipping, extreme heat or cold, diseases, and parasites.

4) Animals destined for slaughter should receive aflatoxin-free diets for at least three weeks before slaughter.

Since cattle in much of the country (excluding feedyards) are typically fed high forage diets, they are usually fed grain only as a supplement. Thus a relatively high level of aflatoxin can occur in the grain before it exceeds the tolerable dietary level. In general, cattle will eat about 2.5 percent of their body weight as dry matter. Of course this varies by individual. This can be used to calculate the contribution of grain to their total ration, and the tolerable level of aflatoxin in the grain. For example, growing calves weighing 600 pounds will consume about 15 pounds of total feed (600 lb. multiplied by 2.5 percent equals 15 lb.). If they are fed 3 pounds of grain plus forage-to-appetite, the grain will make up about 20 percent of their total diet (3 lb. divided by 15 lb. equals 20 percent). In this case the grain could feasibly contain up to 500 ppb of aflatoxin (100 ppb divided by 20 percent equals 500 ppb) and still remain within tolerances. Aflatoxin levels allowable in the grain, given different rates of inclusion in the beef ration, are illustrated in the following table.

This table assumes that aflatoxin is contained only in grains. It can be found in silages and in products like whole cottonseed, or a variety of by-products. Each dietary component should be tested for aflatoxin prior to use of any contaminated grains.

Also remember that heat stress, 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.

A Final Point - Mechanisms Involved in Toxic Effects on the Immune System

Little information exists on how mycotoxins produce immunotoxicity (toxic effects to the immune system). Some mycotoxins, such as those we have discussed, inhibit protein synthesis and cell division and growth. This inhibition may not be the primary mechanism involved in their immunotoxic effects since both have selective effects on various subpopulations of cellular components of the immune system. Several mycotoxins are cytotoxic to lymphocytes as shown in the lab, perhaps because of their effects on membranes or interference with synthesis and function of particular macromolecules such as the proteins mentioned previously.

Mycotoxins can indirectly influence the immunologic functions. Some of the compounds are neurotoxic or cause other negative effects to organs, and these compounds may activate the endocrine mechanisms. The stress-induced release of corticosteroids inhibits immune functions.

Mycotoxins or their metabolites in cattle and other mammals and avians may be highly reactive and may destroy tissues.


In a nutshell, mycotoxins should be avoided if at all possible in cattle diets. Conclusive results of long term effects of feeding mycotoxins have not been determined but it is known that significant effects are seen on performance and growth. Producers need to be familiar with these compounds and have an understanding of how to avoid, as best possible, their inclusion in feeding and supplementation programs.

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


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