Mycotoxins in Pig Feed
Feed is the mostly costly component of a swine operation. The nutritive value and relative cost of feed plays a crucial role in maximizing returns. However, the producer also needs to consider the condition of the feed. Feed contaminated by moulds can make a serious dent in the productivity of a given operation, with effects ranging from feed refusal to serious illness.
Though moulds may diminish the nutritive value of strain on which they grow, the more serious concern is with mycotoxins. Mycotoxins are secondary metabolites produced by the moulds. These vary among the species of mould, which in turn may thrive under different conditions and favour different grains, in the field and in storage.
Aflotoxins are produced by the common soil microorganism Aspergillus flavus. This mould is prevalent among feeds in the southern U.S., particularly in corn, but may occur in other grains when conditions are hot and dry, growing best between 30 and 40 ° C. A. flavus commonly grows during the storage of summer crops, but can also affect wheat and barley. Aflotoxins may be produced within 2 to 6 weeks of infestation.
A pig that has consumed aflotoxins does not exhibit characteristic clinical signs. Symptoms may include feed refusal, reduced growth, paleness and jaundice. The latter is an indication of liver damage which may occur at levels above 100 ppb in the feed. Feed refusal may be observed at levels as low as 20 ppb, depending on the variety of aflotoxin.
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Ergot is of growing concern to Alberta farmers, with infections arriving from the eastern prairies. Wind blown spores infect flowering grains and grasses. Rye and triticale are most susceptible but barley, wheat and oats may also be infected. Ergot favours cool, wet weather. This parasitic fungus of the Claviceps genus does its damage in the field, replacing one or more kernels in a mature grain head with a hard, dark mass called a sclerotia.
In addition to the reduced yields, numerous poisonous alkaloids and amines are produced. There are four primary alkaloids that are toxic to pigs. In common with other mycotoxins depressed, feed intake and, subsequently, growth are initial symptoms of ergot alkaloid poisoning. Additional effects may include, convulsions, lack of coordination, respiratory distress, rapid pulse, excessive salivation, vomiting and tremors. A high level of toxin intake results in vasoconstriction and subsequently dry gangrene of hooves, ears and tails. Rye ergot reduces lactation in sows. This may by caused by a reduction in plasma concentration of hormones such as prolactin. Ergot poisoning may also induce spontaneous abortions. As such, ergot infected grain should never be fed to brood sows. Caution is indicated for growing-finishing pigs.
Fumoisins are frequently found in corn infected with Fusariam moiliforme. However, fusarium is a very common organism that is often present with no production of fumonisin. This fungus occurs in most temperate regions of the world. It is most likely to be seen before harvest. Sprinkler irrigation may heighten risk of fusarium infection. Symptoms of fumonisin poisoning may be observed at levels above 10 ppm and include pulmonary edema and liver damage.
Trichothecens comprise a group of toxins that share a common chemical structure, though produced by a variety of fungi particularly Fusaria and related genera. Purple red moulds, occasionally detected in corn, wheat and soybeans, signal the presence of tricothecens.
Pigs are more sensitive to these mycotoxins than other farm animals, with levels as low as 300 to 500 ppb resulting in feed refusal, decreased weight gain and increased vulnerability to infectious diseases. The trichothecens include deoxynivalenol (DON), also known as vomitoxin because of its strong vomiting effect. At levels over 1 ppm, severe weight loss and vomiting occur. Weaner pigs are especially susceptible. Recent research suggests that male pigs may be more tolerant than females. Effects on reproduction are unknown, but it is best to avoid feeding breeding animals infected grain.
T-2 is a very potent tricothecen, found in barley and wheat, which can seriously impair fertility in swine.
Ochratoxins are produced by Aspergillus and Penecillium fungi. Ochratoxin A is a common contaminate of barley and occurs in cool, wet conditions.
Ochratoxin A produces depressed appetite and reduced growth rate. At concentrations greater than 5 to 10 ppm, a number of conditions may arise such as impaired kidney function, necrosis of lymph nodes and fatty liver changes.
Zearalenone is produced by several Fusarium molds under cool, wet conditions. It grows on grain before harvest, but can worsen in storage. Insect damage increases the susceptibility of crops. Infected corn may be identified by dark purple discoloration and affected wheat by pink tips.
Zearalenone is probably the mycotoxin most detrimental to swine with serious effects on the breeding stock. Prepubertal gilts are most sensitive. Toxicity results in the reddening and swelling of the vulva, increased size of mammary tissue, straining with subsequent rectal and vaginal prolapse, as well as pseudopregnancy and false heat. The piglets of affected sows may experience depressed piglet growth in utero, early embryonic mortality and be born with splayed legs. Fertility problems surface at 100 to 200 ppb. Zearalenone also produces swelling of the prepuce in boars.
Treatment & Prevention
There are currently no practical methods for decontaminating feedstuffs containing mycotoxins. Neither is it possible to entirely avoid exposure to fungi. Prevention is the key. It is important to recognize conditions that promote growth of fungi likely to contaminate the crop or grain in question.
When planting, scelortia (ergot) free seed should be used – zero tolerance is indicated. Stressed crops are susceptible substrates for fungal growth, included those damage by insects. Rotating crops and burning the stubble of infected fields is advised. Deep tillage and planting prevents ergot from developing spore forming bodies.
Managing temperature and moisture levels in stored grains is critical. Stored feed should be used quickly, particularly if already ground. Avoid high levels of moisture in the bins.
Fungal inhibitors, such as proprionic acid, can halt or slow fungal growth in stored grains but will have no effect on mycotoxins already present. Grain testing can help assess the level and type of contamination. However, care must be taken to collect a representative sample, as mold growth normally appears in hotspots in the bin. Recently blended grain should be tested. Alternately, one may collect and combine several samples from the stream when grain is moved between bins.
Good grain handling practices are essential. Avoid spillage when transporting. Dispose of contaminated by burial, burning or composting. Feed handling equipment should be cleaned regularly.
When feeding infected material is unavoidable, it should be directed to finishers, which are generally more tolerant of mycotoxins. Blending infected material with unaffected ingredients is an option, but presents economic and logistical challenges. It may be difficult to avoid feeding younger pigs and breeding herds when blending grains.
Toxin binding agents have been explored but their efficacy is variable. They typically include non-nutritive mineral agents like clays, such as sodium bentonite and certain zeolites. One must balance nutrient displacement against possible benefit. Potential detoxification methods such as acid/base treatment, separation and heat treatment are being investigated but practical approaches have yet to materialize. At present, prevention of fungal infection, from seed to feed, is the most effective approach for dealing with mycotoxins.
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