Starch is an important nutrient in corn, which is highly coveted by livestock nutritionists, wet millers and dry-grind ethanol manufacturers. High starch concentration is usually indicative of good kernel maturation and/or filling conditions, and often results in high kernel densities. Today’s new corn hybrids have been selected to contain more starch. A 2016 Export Cargo Report conducted by U.S. Grains analyzed 430 yellow corn samples from 3 regions of the U.S. (Figure 1). These samples were collected from corn export shipments as they underwent the federal inspection and grading processes performed by the USDA Federal Grain Inspection Service (FGIS). Results from these analyses showed that the U.S. aggregate corn starch concentration was 72.4% for the 2016/17 crop year. Previous test-years have shown even higher starch concentrations of 73.9% for 2015-2016, and 73.8% for the last five-year average. Usually around 4-5% (Kim et al. 2008) starch remains in distillers dried grains for a fermentation efficiency of roughly 94%. In addition, another step (usually centrifugation) has been added to the corn fermentation process that removes more than half of the oil present in the mash. Ethanol bio-refineries have developed the technology to extract oil from the distillers’ grains mash to be sold separately. The 2016 U.S. Grains Export Cargo Report shows that US corn had 4.0% oil (3.8% for the 5-year average). As corn starch is fermented to ethanol, and oil is removed after fermentation the non-starch nutrients and even deleterious substances are concentrated approximately threefold in distillers dried grains. As a result, there have always been concerns with regards to the mycotoxin concentration in this co-product mostly derived from corn fermentation.
Figure 1. Sampling Regions
Mycotoxins of Concern
The main fungi that produce toxins during storage belong to three genera: Aspergillus, Fusarium, and Penicillium. When dealing with cattle diets, it is not easy to correlate the presence of mycotoxins to that of molds. The same types of molds can produce different types of mycotoxins, and different types of molds can produce the same mycotoxin. When addressing mycotoxicosis, the fact that multiple ingredients usually make up a dairy cattle diet can be viewed both positively and negatively. On the one hand, multiple feeds dilute the toxins from any given feed, resulting in a safer diet. On the other hand, because the effect of toxins can be additive, if there are multiple contaminated feeds, the toxic effect of the feeds will be compounded.
The primary toxins of concern are aflatoxin, zearalenone, trichothecene, fumonisin, ochratoxin, and patulin. Aflatoxins are produced by the fungi Aspergillus flavus and A. parasiticus. They produce four toxins, of which aflatoxin B1 is considered the most potent natural carcinogen. Rumen microorganisms can degrade up to 42% of aflatoxin B1 (Santin, 2005), but they are also capable of producing aflatoxicol. Another metabolite, aflatoxin M1, is produced from B1 in the liver and can end up in the rumen through the rumino-hepatic circulation. The toxicity of aflatoxicol and M1 is similar to that of B1, and they are readily absorbed by the intestine. Therefore, even when B1 is degraded in the rumen to aflatoxicol and transformed in the liver to M1, the toxic end-result is similar. The metabolite M1 circulates from the liver into the blood and ends up in milk or urine. Zearalenone is degraded by protozoa to α-zearalenol, a product with high estrogenic activity, and to β-zearalenol, a product toxic to the endometrium (Tiemann et al. 2003). The main effects of zearalenone in cattle are thus related to reproductive problems such as embryo survival, infertility, hypertrophy of the genitalia, and feminization of young males (decreased testosterone). Trichothecenes derive from the fusarium group of molds, which include diacetoxyscirpenol (DAS), T-2 toxin, and deoxinivalenol (DON); these molds have been associated with gastrointestinal lesions in dairy cows.
Trichothecenes have been well known for their impact on the cow’s immune system. Fumonisins seem to be better tolerated by cattle than by monogastrics, although feed intake and milk production can be negatively affected in dairy cows. Ochratoxins, which are rapidly degraded in the rumen, are considered of little consequence to ruminants. Patulin is commonly found in silages, and sudden exposure to patulin may result in reduced feed intake and milk production (Santin, 2005). Depending on weather conditions during growing and harvest seasons, corn grain may contain high concentrations of molds that are detrimental to livestock.
Contaminated grains decrease productivity and negatively affect the health of the animal. As greater amounts of corn are used for fuel ethanol production, livestock producers are feeding lesser amounts of corn and greater amounts of the coproducts (known as distillers’ grains) resulting from ethanol production—these coproducts are primarily distillers dried grains with solubles (DDGS) and wet distillers’ grains (WDG). When concentrations of molds and mycotoxins are elevated in corn grown in a given year or region, there is a concern that these undesirable inhabitants of the grain will be transferred to the distillers’ grains. Mold spores can be present on surfaces previously used to store distillers’ grains and can inoculate new batches. Depending on the conditions in which distillers’ grains are kept at the plant prior to shipping and/or on conditions during transport and storage, both the initial concentration of mycotoxins and their profile might change. Molds may grow if under normal storage conditions a temperature between 68 and 86ºF can be maintained for several days or weeks. Moisture also plays an important role in mold development, with ideal conditions for growth ranging from 13 to 18% moisture. Muschen and Frank (1994) suggested that in grains with high levels of oil, such as peanuts (20-60%), molds can grow at a moisture concentration as low as 7%. Traditional, non-extracted DDGS have an oil content that ranges from 10 to 15%. This suggests that even when DDGS is kept under recommended dry conditions, it might have an increased susceptibility to fungal growth. Most fungi need oxygen present to grow (1 to 2% oxygen). In a normal fermentation process, enzymes that consume oxygen are inhibited by low pH (Woolford, 1974). It has been found that pH in WDG is commonly between 3.0 and 4.0 (Kalscheur and Garcia, 2005), which would be low enough to inhibit oxygen depletion by enzymatic activity. Intact corn kernels breathe and consume oxygen in any structure where they are stored. Distillers grains, on the other hand, undergo a heating process during the ethanol production process that basically transforms the once “breathing” grain into a collection of inert particles loaded with nutrients, and these particles can be a substrate for mold growth. In fact, high temperatures attained after ethanol processing can also denature the enzymes responsible for oxygen depletion by respiration (Puzzi, 1986). During aerobic respiration, fungi utilize grain fat and carbohydrates (Dixon and Hamilton, 1981b). Although there is little starch left in distillers’ grains, there are still plenty of structural carbohydrates and fat, both of which are readily available for fungal growth. The use of the carbohydrates and fat for fungal growth reduces the energy content of the distillers’ grains. DDGS are more exposed to mold growth than whole kernels because the pericarp that protects the grain has been completely disrupted. This disruption allows for an easier colonization of the remaining nutrients by mold spores. When whole kernels are stored, molds grow as a result of the moisture present. The amount of moisture present between kernels is determined by the equilibrium of the moisture both inside the grain and between the kernels. When warm and ground wet or dried distillers’ grains are confined in a container (e.g., bin, bulk grain wagon, silo-bag, etc.), free water vapor moves from the warm core towards the cooler area (the inner surface of the containment surface). There it condenses and increases the amount of free water, which thus allows further mold growth.
Mycotoxins in U.S. Corn During 2016
Molds are usually present in the grain’s pericarp and can result in high levels of mycotoxins. Consequently, when starch is fermented to ethanol, mycotoxins are also concentrated threefold. The quantity of mycotoxins in newly processed distillers’ grains is directly related to their presence in the original grain. Corn samples (n = 430) from the Federal Grain Inspection Service (FGIS) of the USDA were analyzed for mycotoxins by the Illinois Crop Improvement Association’s Identity Preserved Grain Laboratory (IPG Lab). Samples were analyzed for aflatoxins, and vomitoxin (deoxynivalenol; DON). All of the export samples tested below the FDA action level of 20 ppb for aflatoxins. All corn samples tested below the 5 ppm FDA advisory level for DON (same as in 2015-2016 and 2014-2015). The maximum export sample test result observed in 2016/2017 for DON was 1.3 ppm. That is to say that if this corn was used to produce ethanol, the resulting DDGS would have had a maximum DON concentration of at worst almost 4 ppm, still below the action level of 5 ppm suggested by the FDA.
Of the total samples, 80.0% (n = 344) had no detectable aflatoxins (< 5 ppb). Corn DDGS obtained from these shipments would still have DDGS below the FDA’s action level of 20 ppb. Aflatoxin concentrations between 5 ppb and 10 ppb, were found in 16.5% (n = 71). Only 3.5% of the samples (n = 15) had aflatoxin concentrations equal or above 10 ppb, but always below or equal to the FDA action level of 20 ppb.
In summary, all samples tested in 2016-2017 were below or equal to the FDA action level of 20 ppb (same as in 2015-2016 and 2014-2015). The US Grains Export Cargo Report shows that the concentration of aflatoxins in corn depends on where the grain was sourced from. The corn with less aflatoxins came from the Pacific Northwest rail (94.5% of all samples < 5 ppb; includes South Dakota, North Dakota, and Nebraska. Table 1) and the Southern rail (96.7% of all samples < 5 ppb) routes, compared to the Gulf route (71.6% of all samples < 5 ppb. Table 1)
Table 1. Aflatoxin concentration by region of sampling.
|< 5 ppb||≥ 5 to < 10 ppb||≥ 10 to ≤ 20 ppb||> 20 ppb||Total|
|Source: U.S. Grains 2017|
For DON, 57.7% (n=248) of the total samples had less than 0.5 ppm, 42.3% (n=182) had DON concentrations equal or above 0.5 ppm, but less than 2.0 ppm. No samples, had DON above 2.0 ppm. The maximum sample test result observed was 1.3 ppm (the FDA action level is 5 ppm).
The corn with less DON came from the Pacific Northwest rail (96.7% of all samples < 0.5 ppm; which includes South Dakota, North Dakota, and Nebraska. Table 2) and Southern rail (82.0% of all samples < 0.5 ppm) routes, compared to the Gulf rail (39.6% of all samples < 0.5 ppm. Table 2).
Table 2. DON concentration by region of sampling.
|<5 ppm||≥ 0.5 to < 2 ppm||≥ 2.0 to ≤ 5.0 ppm||> 5.0 ppm||Total|
|Source: U.S. Grains 2017|
Mycotoxins in Distillers Dried Grains
The maximum threshold for aflatoxins for dairy cattle is considered to be 20 ppb; at greater concentrations, M1 will appear in milk. According to results from this laboratory, vomitoxin (deoxinivalenol, or DON) is the mycotoxin that should be of borderline concern in DDGS. Its concentrations however were always below the FDA advisory level of 5 ppm. Table 3 shows DDGS mycotoxin analytical results published between 2000 and 2016 by the Dairy One forage laboratory (Ithaca, NY). The results observed in Table 1 warrant to remain vigilant of the concentration of DON in DDGS as well as the rest of the grains in the diet. The FDA suggests that levels above 2 ppm in the total diet may pose a potential hazard for lactating dairy cattle. For individual feeds destined for all animal species except for beef cattle, the FDA suggests not more than 5 ppm of DON for grain and grain byproducts (for beef cattle, the FDA suggests 10 ppm). If the 5 ppm concentration is found in commodities, those feedstuffs should not exceed 40 percent of the ration. A maximum of 7.7 ppm of DON was reported from 54 samples (Table 1) of DDGS tested between 2000 and 2016. This warrants caution and suggests the need to test either the diet or the individual feeds that may contribute to the total concentration of this mycotoxin. Nevertheless, SDSU research suggests DDGS should not be fed to lactating dairy cows at more than 20% of the diet (the University of Nebraska, Lincoln has tested 30% with success, albeit not for entire lactations). At these inclusion levels the maximum contribution to the total diet by DDGS containing 7.7 ppm DON would be roughly 1.5 ppm in the total diet, below the action level of 2 ppm suggested by the FDA. This stresses the importance of knowing the DON concentration (and other mycotoxins) of other grains in the diet since the effect of the mycotoxins is additive.
Table 3. Mycotoxin concentrations of distillers dried grains.
|Aflatoxin B1, ppb||346||0.541||0||3.468||2.928|
|Accumulated years 2000-2016; Source: DairyOne 2017|
Fumonisin does not appear to constitute a problem with DDGS in ruminant diets, considering the FDA maximum tolerance level is set at 60 ppm of total fumonisins in commodities and at 30 ppm in the total feed (50% inclusion rate). An earlier survey, this one conducted by BIOMIN (Rodrigues, 2008), indicated that out of 44 DDGS samples collected between October 2006 and September 2007, 14% tested positive for B1 aflatoxin, 77% tested positive for zearalenone, 80% were positive for vomitoxin, 88% tested positive for fumonisin, and no samples were positive for T2.
Mycotoxins are not destroyed during either the ethanol fermentation process or the distillers’ grains production processes; instead, they increase their concentration in the original kernels by nearly threefold. Inadequate storage conditions may also increase the concentration of mycotoxins (due to inoculation by mold spores present in the environment). The use of mycotoxin-contaminated distillers’ grains in dairy cattle diets poses a risk to human health because M1, an aflatoxin metabolite, transfers to milk. Even when the toxin level is within acceptable standards for distillers’ grains, their additive nature does not preclude the potential for toxicity. In the presence of borderline acceptable levels of aflatoxin B1 in distillers’ grains, testing the TMR and/or individual feeds is recommended to ensure that milk will not be contaminated.
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