Achieving mold growth control

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 cause 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

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 aerial mycelium, 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. Molds evolve 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.

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. Molds proliferate when moisture levels are above 17%. Humidity can 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.

Molds can grow at wide range of temperatures from 30 to 135 oF (Table 1). Molds show their best growth at optimum temperatures, but they can still grow, just at a slower rate, when surrounding temperatures are low (minimum) or high (maximum). Unless a grain is frozen or subject to high temperature, mold growth may take place. 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 space around 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.

Molds need nutrients for growth. The availability of nutrients is restricted by the integrity of grain kernels. Broken kernels in grains or processed feeds will speed up mold growth because more nutrients in these grains or feeds are available. Molds are aerobic microorganisms, i.e. they require oxygen to grow. However, unless a silage is kept in a air-tight silo, presence of oxygen is unavoidable in practical storage of grains or feeds.

Table 1. Growth Temperatures of Common Molds

		TEMPERATURE F
MOLDS	MINIMUM	OPTIMUM	MAXIMUM
Aspergillus glaucus	46	75	100
A. flavus	45	98	113
A. fumigatus	52	99	135
A. ochraceus	30	55	80
Fusarium roseum	39	75	90
F. graminareum	39	75	90
Penicillium cyclopium	28	73	86
P. viridicatum	28	73	97

POTENTIAL FOR MOLD GROWTH IN GRAINS AND FEED

Molds are synonymous with mycotoxins in the practical world. Molds not only produce mycotoxins but also damage and reduce the nutritional value of a grain or feed. Actively growing molds utilize 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 component of energy sources 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 the ones fed unmolded corn. To obtain the same animal performance as that achieved with unmolded corn, the moldy corn required the addition of an extra 3% fat in the diet.

Research carried out at the University of Minnesota and Penn State University demonstrated 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). 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 the 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 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 dramatic changes in vitamins and amino acids content when grains were molded. In this experiment Thiamine, Niacin, Lysine and total amino acids in the moldy grain were reduced 49, 25, 45 and 21%, respectively. Additionally, health problems e.g., mycotic abortions and respiratory diseases may result when a considerable amount of moldy feed is consumed by an animal. This may be due to a high content of mold and mold spores in the air or in the ingested feed. Reductions in production performance and increases in health problems from moldy grain or feed are often moderate if mycotoxins are absent. However, mycotoxin contamination can lead to greater losses in production and health problems.

Mycotoxins are a collective term used to describe highly toxic substances produced by molds as a result of their secondary metabolic processes. It is known that molds can produce more than 300 mycotoxins, of which aflatoxin, vomitoxin (deoxynivalenol), zearalenone, and t-2 (trichothecenes) are most commonly found in animal feeds. Mycotoxins have direct effects on animal performance. They may reduce blood vitamin or amino acid levels resulting in vitamin or amino acid deficiencies. They may decrease antibody titers leading to less resistance to disease and virus infections. Brown et al reported that mycotoxins seriously hinder the ability of a cow to resist mastitis. Mycotoxins may reduce digestive enzyme activity resulting in of poor digestion and absorption. Mycotoxins may also cause reproductive problems including cystic ovaries and long breeding back intervals. Research has shown that the addition of zearalenone to dairy heifer rations reduced the conception rate from 87% to 62%. The extent of mycotoxicosis varies with mycotoxin type and concentrations as well as animal type and preexisting health and nutritional status.

Adams and coworkers presented the dangerous levels of mycotoxins in grains or total ration (Table 2).

Table 2. Levels of Mycotoxins of Concern in Production Animal Husbandry

			*POTENTIALLY*	*HARMFUL*
MYCOTOXINS	FEEDS	WARNING	CATTLE	SWINE
Aflatoxin ppb DM	Major ingredients	22	22-333	22-111
	Total ration	20-40	20-132	20-102
Vomitoxin ppm DM	Major ingredients	0.56	2.0-4.0	0.8-1.4
	Total ration	0.56	1.0-4.0	0.6-1.0
Zearalenone ppm DM	Major ingredients	0.56	5.6-10.0	1.1-5.6
	Total ration	0.56	3.9-7.0	0.6-3.9
T-2 toxin ppm DM	Total ration	0.25	0.7-1.5	0.7-1.5
Ochratoxin ppm DM	Total ration	0.25	5.0-9.0	0.7-1.5

CONTROL OF MOLD GROWTH

Effective control of mold growth is the key to preventing deterioration and/or mycotoxin formation in stored grains or feeds. There are many chemicals available on the market which can 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).

Table 3. Minimum Concentration of Organic Acids For Inhibition of Common Molds

MOLDS	Propionic, %	Formic, %	Acetic, %	Sorbic, %
Aspergillus flavus	0.25	0.50	0.50	0.25
A. niger	0.25	0.50	0.50	0.50
A. versicolor	0.25	0.25	0.50	0.13
Fusarium moniliforme	0.25	0.10	0.25	0.10
F. nivale	0.13	0.25	0.25	0.05
F. oxysporum	0.13	0.13	0.25	0.10
Penicillium roqueforti	0.13	0.10	0.13	0.10
P. spinulosum	0.10	0.10	0.25	0.05

An experiment was conducted to compare the efficacy of different organic acids (propionic, acetic, butyric, sorbic and formic acid) at equal application rate of 3 lb/ton in the preservation of a finished feed with a moisture of 14%. Results showed that the feed treated with propionic acid held 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 for mold growth only serves to reduce the efficacy of the product for the job it is being used for.

Although propionic acid is effective in the inhibition of mold growth, handling of the pure acid is dangerous because of its pungent, offensive odor, corrosiveness to mild steel and relatively short residual time due to its volatile nature. Thereby, propionate salts including ammonia, sodium, magnesium and calcium propionate are commonly used in the industry. The efficacy of propionic acid and salts is closely related to their solubility in water,. The stronger the bond between the acid-base the less soluble is the product and therefore less effective in inhibiting molds. Research showed that among these salts ammonium propionate is the most soluble in water (90%) followed by sodium propionate (25%), magnesium propionate (10%) and 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 level (0.1%) based on propionate content. The results showed that ammonium propionate kept the grain fresh for 19 days, sodium propionate for 13 days and calcium propionate only preserved the grain for 10 days before it molded. It was concluded that ammonium propionate is more effective than other salts.

Profresh Plus is a granular ammonium propionate salt. As an mold inhibitor it has the following characteristics: high efficacy of mold inhibition, low to non corrosion of equipment, low volatility which translates into long shelf life, stability through pelleting, and more user friendliness. However, Profresh Plus as a chemical should be handled with care and applied by carefully following the manufacturer's instructions.

Profresh Plus can be added at low volumes (1-3 lbs) per tonne of TMR to the mixer wagon to achieve effective protection against mold growth and nutritonal damage to the TMR. It keeps the ration cooler, fresher and retains more nutrients, thereby maintaining palatability and maximizing production. Profresh Plus can also be used as an effective and economic way of minimizing spoilage losses that occur on the top and along the shoulders of silos and bins where air ingress can occur and stimulate mold activity.