FEED Business Worldwide - December, 2011
Using ammonium propionate to prevent mold growth in feed storage bins
by Dr. David PARFITT, Chief Executive Officer, Micron Bio Systems
Microbial deterioration of grain, feedstuffs and animal feed is of great concern to all segments of the production animal industry. Molds are a group of microorganisms that use deterioration of grains or feeds, various infections in livestock and intoxication due to the formation of mycotoxins in feedstuffs.
It is reported that as much as 25% of the world's cereal grains may be contaminated with mycotoxins. Since mycotoxin production and feedstuff deterioration are caused by the growth of molds, it is critical to understand mold growth and know how to control it.
Mold growth & environmental resistance
Molds reproduce by forming spores on a specialized structure known as a mycelium. This structure serves to raise the spores above the material upon which the mold is growing. The spores are then carried by air currents to other environments where spore germination occurs, followed by rapid mycelial growth and sporulation can take place. On a single aerialmycelium, literally thousands of spores can be produced. Due to the widespread distribution of these spores, molds are regarded as ubiquitous, that is, they are present in virtually all niches in our environment.
Mold spores are considered resistant because they can remain viable under extremely dry conditions. On the other hand, mold spores are relatively susceptible to heat. For example, mold spores in a sample of feed will drastically reduced due to exposure to typical pelleting conditions used in the manufacture of animal feed.
Although the spores will not be eliminated completely from the feed, this reduction points to the relative susceptibility of mold spore to moist heat. Even though mold spores can be destroyed during pelleting, recontamination of feed with mold spores can occur in feed manufacturing equipment, storage bins, bulk feed trucks and on-the-farm feed bins. When this recontamination occurs, subsequent germination and molding can occur at a rapid rate. 
Molds are widely spread in crops, grains and forages, evolving through several stages during their life span.  The vegetative stage is where a mold is actively growing and reproducing. It is this stage that is the most dangerous to feed and animals. The vegetative mold is relatively sensitive to its surrounding environments. The spore stage is where a mold is dormant, allowing the mold to endure very harsh environments, e.g. hot or cold temperature, dry conditions. When the environment is suitable, a mold spore will germinate and become a vegetative mold.
Role of moisture & temperature
Molds are living organisms and therefore their growth is influenced by many factors including moisture, temperature, oxygen, and substrates. Among these factors moisture is most critical. Molds cease growth when the moisture level in grains or feed is lowered to below 12%. When the moisture level is increased to above that level, molds will start to germinate and grow. In general, they proliferate when moisture levels are above 17%.
In this situation, humidity can also affect grain moisture. It has been reported that at high humidity levels (more than 70%) a dry feed (11% moisture) is quickly, within 3 hours, rehydrated to dangerous moisture levels. Therefore it is of paramount importance to keep moisture levels as low as possible during feed storage and avoid exposure to high humidity environments during storage of grains or feeds made from them.
Molds can grow at wide range of temperatures from -1°C to 57°C (see Table 1). Although they show their best growth at optimum temperatures, they can still grow (though at a slower rate) when surrounding temperatures are low (minimum) or high (maximum). Hence, unless a grain is frozen or subject to high temperatures, mold growth may take place, provided that there is also sufficient moisture.
In addition, temperature also indirectly affects mold growth by influencing moisture content of a grain or feed. In a bin where dry grains are stored, when temperatures fall during the night, the wall of the bin becomes cold and water molecules in the grains and spaces between them will start moving toward the cold surface to condense.
The colder the temperature at night, the faster the water molecules moves. After a short of time the grains or feed next to the bin wall will contain substantially higher levels of moisture.  This may become sufficient to promote spore germination and subsequent mold growth.
Nutrient availability & grain qualityAlong with suitable temperatures and sufficient moisture, molds also require nutrients for their growth. This in turn is very much a matter of grain quality, as the availability of nutrients is restricted by the integrity of grain kernels. The more damaged grain is, the more exposed its nutrients are to possible decomposition by germinating mold spores. Consequently, broken kernels within grains or processed feeds accelerate mold growth because more nutrients in these grains or feeds are accessible to germinating spores. 
In addition, molds are aerobic microorganisms; that is, they require the presence of oxygen to grow. However, unless silage is kept in an air-tight silo, the presence of oxygen is unavoidable in practical storage of grains or feeds. 
Mold growth & nutrient degradation
Insofar as the damage they cause, molds are synonymous with mycotoxins; as much of their damage occurs through their production of these contaminants. In addition to actively producing mycotoxins, molds also damage and reduce the nutritional value of a grain or feed.
Actively growing molds utilise carbohydrates, particularly the water soluble ones such as sugars, to produce carbon dioxide, water and heat. Consequently the energy value of a moldy grain or feed is reduced, as carbohydrates are an important livestock energy source and the heating process decreases nutrient digestibility.
Bartov (1983) reported an experiment in which growing chicks were used to evaluate the effect of mold infection on corn energy. He found that chicks fed moldy corn had significantly less weight gain (7.3% less) than a control group fed unmolded corn. In fact, to obtain the same animal performance as that achieved with unmolded corn, moldy corn required the addition of an extra 3% fat in the chick diet.
Similarly, research carried out at the University of Minnesota and Penn State University demonstrated that the energy value of a corn for ruminants was reduced by 5% after it had molded. Assuming that net energy for lactation for normal corn is 0.9 Mcal/lb DM, moldy corn with same test weight only contains 0.86 Mcal/lb DM (0.9 x 0.95).
These findings bring up the question of 'at what level of molds should a grain or feed be discounted for its energy value?' Adams and coworkers at Penn State University recommended that feed energy value ought to be discounted when the mold spore count in a grain or feed reaches more than 1 million per gram.
Additional side effects of mold infestation include caking of feed, discoloration of grains and reduced palatability. Molds can also reduce the content of other nutrients. Research conducted at Kansas State University showed that dramatic changes in vitamins and amino acid content occurred when grains turn moldy. In this experiment thiamine, niacin, lysine and total amino acids in the moldy grain were reduced 49%, 25%, 45% and 21%, respectively.
Mycotoxins' impact on animal performance
Along with nutrient degradation, livestock health problems including mycotoxin-induced abortions and respiratory diseases may result when a sufficient quantity of moldy feed is consumed by an animal. This may be due to a high concentration of mold growths, spores and mycotoxins in the ingested feed or surrounding air.
The reduction livestock performance and animal health problems from mold-infested feed grain are often moderate if mycotoxins are absent or in low concentrations. However, mycotoxin contamination can lead to greater losses in production and a large number of serious health problems. A collective term used to describe highly toxic substances produced by molds during their secondary metabolic processes, mycotoxins come in over 300 varieties. The  most commonly found mycotoxins in animal feed include aflatoxin, vomitoxin (deoxynivalenol), zearalenone,  and  t-2  (trichothecenes) .
Various mycotoxins produce a wide range of impacts and symptoms on animal performance. Some may reduce blood vitamin or amino acid levels resulting in vitamin or protein deficiencies. Others impact livestock immune systems, resulting in fewer antibodies and less resistance to disease and viral infections.
Brown et al reported that mycotoxins seriously hinder the ability of a cow to resist mastitis. Mycotoxins may also reduce digestive tract enzyme activity, thereby resulting in poor nutrient digestion and poor animal growth and inefficient feed conversion ratios.
In parent and grandparent stock animals, mycotoxins can cause a variety of highly damaging reproductive problems. These include fetal abortions, cystic ovaries, reduced fertility and long inter-breeding intervals.
Research has shown that the addition of zearalenone to dairy heifer rations reduced their conception rates from 87% to 62%.
In general, the extent of mycotoxicosis varies with mycotoxin type and concentrations as well as the animal type, its pre-existing health and nutritional status.
Using organic acids to control mold growth
Clearly, effective control of mold growth is the key to preventing deterioration and/or mycotoxin formation in stored grains or feeds. In this respect, there are many chemicals available on the market which inhibit mold growth.
Among them, propionic acid has been widely used in both food and feed industries as a mold inhibitor because of its broad spectrum of mold inhibition and low toxicity (Table 3). 
An experiment was conducted to compare the efficacy of different organic acids (propionic, acetic, butyric, sorbic and formic acid) at an equal application rate of 3 lb/ton in the preservation of a finished feed with a moisture content of 14%.
Results showed that the feed treated with propionic lasted the longest time (25 days) before molding, followed by acetic acid (18 days), butyric acid (15 days), sorbic acid (12 days) and formic acid (6 days).
The experiment demonstrates that propionic acid is by far the most effective organic acid for mold control. The addition of organic acids other than propionic acid to a product designed to control mold growth only serves to reduce its efficacy.  
Superiority of ammonium propionate
Although propionic acid is effective in the inhibition of mold growth, handling of this acid in its pure form is dangerous because of its pungent, offensive odor and corrosiveness to steel. Its relatively volatile nature also means that it has a relatively short residual time in the feed into which it is applied.
To mitigate these problems, propionate salts including ammonia, sodium, magnesium and calcium propionate are commonly used in the industry. The efficacy of these propionic acid and salts is closely related to their solubility in water. In general, the stronger the molecular bond between the acid-base pair, the less water-soluble and less effective it will be in inhibiting  mold growth. 
Research shows  that  among  propium salts  ammonium propionate is the most soluble in water (90%), followed by sodium propionate (25%) and magnesium propionate (10%), with calcium propionate being least soluble at only about 5%.
An experiment was conducted to compare the efficacy of different propionate salts in the preservation of high moisture grains. The propionate salts were used at the same concentration (0.1%) based on propionate content.
Results showed that ammonium propionate kept the grain fresh for 19 days and sodium propionate for 13 day while calcium propionate only preserved the grain for 10 days before it molded. From these results, it was concluded that ammonium propionate is more effective than other salts in preserving feed grain from mold growth and mycotoxin contamination.
Profresh: An ammonium propionate salt
Profresh Plus is a granular ammonium propionate salt. As a mold inhibitor, it has the following scientifically proven properties: High efficacy of mold inhibition, low to non corrosion of equipment, low volatility which translates into longer shelf life and preserving time, stability throughout the feed pelleting process and more user friendliness. However, Profresh Plus is a chemical, should be handled with care and applied in careful accordance with the manufacturer's instructions.
Profresh Plus can be added at low concentration volumes of 0.45kg to 1.36kg (1 to 3lbs) per tonne of total mixed ration to achieve effective protection against mold growth and nutritional damage to the feed. Through its inhibition of mold growth, it keeps feed rations cooler and fresher while preserving their nutrients. This helps maintains the feed's palatability, thereby helping it to optimize livestock productivity.
In all these ways, Profresh Plus provides a technically effective, yet economical means of minimising the spoilage losses that occur along the mold-prone tops and sides of feed storage containers.
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