Feeds and pet foods

4 Feeds and pet foods

I Introduction

A feeding-stuff may be defined as any component of a ration that serves some useful function (Church, 1979). Most ingredients of feeding-stuffs provide a source of one or more nutrients although some may

be included to improve acceptability or as preservatives. Feeding-stuffs are usually classified into six categories (National Research Council, 1972):

r roughages include pasture and feed from green plants; r dry forages and roughages (hay) and silage; r concentrates used primarily for energy such as cereal grains, milling by-products, seed and mill

screenings, animal, vegetable and marine fats, molasses, and others; r protein concentrates containing more than 20% protein, including plant-protein sources (oilseed meals,

corn, gluten, distillery and brewery by-products, and dried legumes); proteins of animal origin (meat meal, meat and bone meal, miscellaneous animal and poultry by-products, fish meal, milk and milk by-products); and single-cell sources;

r mineral and vitamin supplements; and r non-nutritive additives (e.g. antibiotics, antioxidants, buffers, colors and flavors, emulsifying agents,

enzymes, and bacterial preparations). Feed is the major cost of intensive animal production systems and, therefore, the conditions of feed

preparation and processing are controlled to supply an adequate nutritional diet with high efficiency of feed conversion while encouraging consumption without waste. Feed may be processed simply to improve the capability of machinery to handle it or to add other ingredients. Feeds also may be processed to alter the physical form or particle size, isolate specific components, improve palatability, detoxify poisons, or prevent spoilage (Church, 1979). Feedstuff and feed processing may involve a large variety of mechanical methods (e.g. grinding, crumbling or milling, blocking, cake making, pelleting, flaking, extruding), thermal processing (e.g. conditioning, dehydrating, heat processing, steaming, toasting), chemical processes (e.g. solvent extraction, acid preservation), and microbiological fermentation (e.g. silage making).

The quality of feeding-stuffs depends on available energy, digestibility, nutrient content, and absence of toxins. In addition, quality depends on microbiological condition because spoilage and decay of feedstuffs can impair the efficiency of feed utilization.

Feeds containing pathogens or toxins, especially mycotoxins, can be direct sources of animal dis- ease. Epidemiological data have occasionally linked contaminated feed to human health problems as a result of handling animals or consuming animal-derived products. Human salmonellosis is the principal pathogen of concern. Reported outbreaks of listeriosis, botulism, or bovine spongiform encephalopathy (BSE) among livestock further emphasize the potential of contaminated animal feeds for disseminating pathogens (Hinton and Bale, 1990; Anonymous, 1996; Brown et al., 2001).

The probability of human disease arising from toxicity of animal products as a result of mycotoxin ingestion by animals in feeds is dependent on both toxin and animal type. Ruminants, in particular, are able to detoxify mycotoxins quite effectively by microorganisms in their rumen. Levels of aflatoxin that must be ingested by various domestic animals to produce a residue level in edible portions of 1 µg/kg total aflatoxins are given in Table 4.1. This level, 1 µg/kg, is considered to be acceptable and is rarely

251 Table 4.1 Levels of aflatoxin in feeds resulting in detectable levels

FEEDS AND PET FOODS

(1 µg/kg) of aflatoxin residue in edible portions of domestic animals Animal

Residue (µg/kg) Beef steer

Tissue

Level in feed (µg/kg)

Dairy cow

Laying hen

Modified from Stoloff (1977).

reached. In particular, poultry are quite sensitive to aflatoxins, so many poultry-producing countries have monitoring programs for aflatoxins in poultry feeds, particularly when contaminated maize is a problem, and consequently, levels in poultry meat and eggs are usually low.

The major cause for concern is the possibility of aflatoxin M1 excretion in milk. This toxin is produced by hydroxylation of the dominant naturally occurring aflatoxin, B1, when it is ingested in feeds. Aflatoxin M1 has been detected in many countries where animals are fed compounded feeds (Van Egmond, 1989), particularly in cows with a high milk production (Veldman et al., 1992a). Consequently, levels of aflatoxin permitted in feeds for dairy cows are very low in developed countries (Van Egmond, 1989). The potential also exists for mycotoxins to be present in other dairy products, especially cheese, as a result of mycotoxins in feeds (Scott, 1989).

Monogastric animals are less able to detoxify mycotoxins than ruminants. Pigs, in particular, are sensitive to mycotoxins in feeds, as little as 1–5 mg/kg of the trichothecene toxin deoxynivalenol (vomitoxin) can cause feed refusal and vomiting (Vesonder and Hesseltine, 1981; D’Mello et al., 1999). Rapid metabolization and low transmission rates into tissues ensure that carry-over of trichothecene toxins into edible portions is usually not a problem (Prelusky, 1994; Bauer, 1995). However, ochratoxin

A, which is commonly produced in grains in north temperate climates by the growth of Penicillium verrucosum during storage, accumulates in kidneys and depot fats in pigs (Hald, 1991a). In areas of Europe where the consumption of pig meat is high, the potential for human disease has to be controlled (H¨ohler, 1998); many Europeans carry detectable levels of ochratoxin A in their blood (Hald, 1991b; Zimmerli and Dick, 1995).

It is not feasible to cover all types of feeding-stuff that may be used for animal feeding due to their tremendous variety. Rather, the text will focus on a few major ingredients, which may be indirect sources of human pathogens or mycotoxins. Categories of feeding-stuffs considered will include roughage (particularly silage), ingredients of animal origin (meat meal, meat and bone meal, fish meal, and other specific animal by-products), compounded feeds, and pet foods.

II Roughages

Roughage consists of plant material, excluding the seeds and roots, which is used for feeding grazing and browsing animals such as domestic ruminants and horses. Roughage is a bulky feed that has low weight per unit of volume. Roughages are highly variable in physical and chemical composition and nutritional quality. They range from very good (lush young grass, legumes, and high-quality silage) to very poor nutrient sources (straws, hulls, and some browse) (Church, 1979). Plant materials may be used with no preparation (e.g. pasture, grazed forage) or collected and fed loosely on the ground or in feeding troughs.

MICROORGANISMS IN FOODS 6

Roughages (e.g. hay) can be stored dry for feeding animals when grazing is not possible, i.e. in winter or during times of drought. In traditional hay making, herbage is cut at an optimal stage of development, field dried, and then moved to storage either in loose form or, more often, in bales. Chopping, grinding, pelleting, or cubing can facilitate transportation, handling, and feeding. In some instances, drying may

be completed in the barn by circulating hot air through the hay. Other crops (e.g. alfalfa, maize, and beet pulp) are also dried for storage and subsequent feeding. Some chopped herbage is dried in rotating drums with hot air (800–1000 ◦

C in modern high-temperature processes). After drying, the material is ground and stored in bulk, under inert gas, pressed or pelleted.

A large quantity of herbage is converted to silage by anaerobic fermentation. Whole maize plants, grass, and legumes are widely used to make silage. Ensiling can be also used for a large variety of herbaceous material such as waste from canneries, processing food crops (sweet corn, green beans, green peas), or vegetative residues (e.g. beet tops).

A Effects of processing on microorganisms When making hay, the moisture content is reduced to permit storage without marked nutritional change

or microbial activity. When cut, herbage usually contains 70–80% moisture that must be reduced to 15– 20% for storage. Rapid drying to 14–15% moisture results in the least change in chemical composition and microbial activity. Drying, however, does not inactivate bacterial or fungal spores and many non- spore-forming microorganisms may survive.

Ensiling involves a natural anaerobic fermentation by lactic acid producing bacteria to convert soluble carbohydrates to lactic acid and, thereby, decrease the pH and inhibit enzyme and microbial activity (McDonald et al., 1991; Driehuis and Oude Elferink, 2000). If the pH is low enough (about 3.8–4.5, depending on crop properties) and the silage is stored under oxygen-limiting conditions, its longevity increases. While making silage, the herbage is harvested, chopped, and placed into silos where it gets packed and sealed to restrict oxygen and encourage a desirable fermentation.

The initial dominant microbial population of standing or freshly harvested forage crops are various aerobic microorganisms, but they do not contribute to the silage fermentation and their growth is inhibited soon after the silo is sealed. During the early phase of ensiling, different obligate or facultative anaerobes (lactic acid bacteria, enterobacteria, clostridia, and yeasts) compete for available nutrients. In well-preserved silage, lactic acid bacteria rapidly dominate the fermentation. Lactic acid bacteria are

present in relatively low numbers (usually 10 3 –10 5 cfu/g) on green forage. Under normal processing conditions, they multiply rapidly and in less than 8 days reach levels up to 10 10 cfu/g. These bacteria are mainly lactobacilli (facultatively heterofermentative Lactobacillus plantarum, Lb. casei, Lb. curvatus or obligate heterofermentative Lb. brevis, Lb. buchneri) and to a lesser extent species of Enterococcus, Leuconostoc, and Pediococcus. They convert the soluble carbohydrates to lactic acid (up to 10% of total dry matter in well-preserved silage), and to a lesser extent, to acetic acid and propionic acid. The pH decreases to about 3.8–4.5, depending on crop properties, which eventually stops the fermentation. The presence of volatile fatty acids (e.g. acetic, propionic acid, and butyric acid) in silage assists in inhibiting fungi when silage is exposed to air (Moon, 1983; Driehuis et al., 1999).

B Spoilage When making hay, appreciable changes may occur if drying is slow in the fields, stacks, or bales.

Excessive moisture in the hay results in a microbial fermentation causing an increase in temper- ature, browning, and development of thermophilic fungi. Severe toxicity from the consumption of moldy hay sometimes occurs, more commonly in monogastric animals such as horses (Lacey, 1991).

253 When making silage, an abnormal fermentation may occur resulting in a slow or insufficient decrease

FEEDS AND PET FOODS

in pH, thus permitting abundant growth of enterobacteria, yeasts, or clostridia. This may occur, for example, when ensiling a very wet material that is low in water-soluble carbohydrates or a highly buffered plant material (e.g. legumes). The two main sources of spores of clostridia in silage are soil and animal manure. During cutting and harvesting of a silage crop, contamination of the crop with soil particles is unavoidable. Spores of clostridia can survive the passage through the alimentary tract of livestock consuming the silage, leaving the animal in feces. In many situations, animal manure is used as organic fertilizer for silage crops. This practice adds to the pool of clostridia spores in the soil and on the crop. However, the initial level of spores is of only minor influence on the final level of spores under conditions that permit growth of clostridia in silage. Clostridium species typically associated with silage include both saccharolytic species (e.g. Clostridium tyrobutyricum and Cl. butyricum) and proteolytic species (e.g. Cl. sporogenes and Cl. bifermentans) (McDonald et al., 1991). Growth of clostridia in silage is associated with a high pH, which may lead to instability of the silage, and relatively large amounts of butyric acid, ammonia, and biogenic amines, such as tryptamine and histamine, causing losses in nutritional value and low palatability (Van Os, 1997).

Another type of spoilage relates to the proliferation of (facultative) aerobic microorganisms as a result of infiltration of air. Aerobic spoilage of silage is manifested in two ways. Firstly, the deterioration of surface layers, often visible by the development of molds. Secondly, deterioration of material deeper inside the silo, usually observed as heating of the silage. Acid-tolerant yeasts, oxidizing residual sugars, and lactic acid as substrates usually initiate this deterioration process. As this process proceeds, the pH rises, which in turn allows the growth of many other spoilage microorganisms, including molds, bacilli, enterobacteria, and Listeria (Woolford, 1990; McDonald et al., 1991).

C Pathogens Hay and silage may contain relatively high levels of Cl. botulinum, particularly when containing soil

or when the herbage has been collected from fields treated with sewage sludge and particularly poultry waste. There is evidence that Cl. botulinum is commonly found in cultivated or manured soil or sludge (Mitscherlich and Marth, 1984). Another possible origin of Cl. botulinum in hay or silage is the presence of a cadaver (e.g. of a mouse or bird), which entered the hay or silage during the harvesting operation. As Cl. botulinum does not grow below pH 5.3, growth of this microorganism in well-fermented silage is unlikely. However, growth in deteriorating parts cannot be excluded. Spores and toxins of Cl. botulinum have been detected in the outer layers of wrapped big bale silage (Ricketts et al., 1984; Wilson et al., 1995). Botulism has been diagnosed in cattle and horses fed “big bale silage” (Hinton and Bale, 1990). Botulism traced to silage, particularly in cattle, has been reviewed in depth by Roberts (1988) and by Kehler and Scholz (1996).

Escherichia coli O157 and other strains of E. coli that have been involved in cases or outbreaks of hemolytic enteritis and hemolytic uraemic syndrome (HUS) in humans may, in theory, be transmit- ted via grass and other unprocessed or re-contaminated feeding-stuffs. Survival for months and even multiplication of these strains in wet feed and feed-troughs has been demonstrated (Hancock et al.,

2001). However, the role of feed, and particularly different types of feed, has not been well established (Herriott et al., 1998; Buchko et al., 2000). Numbers of Enterobacteriaceae in silage decline sharply when the pH falls below 4.5. However, infiltration of oxygen during ensilage prolongs their survival (Donald et al., 1995), and extensive growth of Enterobacteriaceae during aerobic deterioration of silage was described by Lindgren et al. (1985).

The role of silage in the transmission of Listeria monocytogenes to ruminants was documented in 1960s (Gray, l960). Since then, others have found silage to be a major source of listeriae on the farm (Grønstøl, 1979). During normal silage production, the pH falls below 4.2, which is bacteriostatic for

MICROORGANISMS IN FOODS 6

listeriae. If air is excluded, the listeriae rapidly die. If the fermentation is not optimal L. monocytogenes can multiply. L. monocytogenes has often been reported to occur in poor quality silage at levels of more than 12 000 cells/g (Fenlon, 1986). Furthermore, L . monocytogenes can survive in silage for years (Dijkstra, 1975).

Insufficient acidification and aerobic deterioration may be the most important factors permitting multiplication of L . monocytogenes in silage (Hird and Genigeorgis, 1990; Perry and Donnelly, 1990). The frequency of listeriae isolations increases with increasing silage pH. In one study, L . monocytogenes was isolated from 22% of the samples with pH <4, 37% of the samples with pH 4–5, and 56% with pH >5 (Graostal, 1979). In another study, listeriae were isolated from 7.9% of the 114 silage samples with pH below 4, 52.9% of the 70 samples with pH 5.0–5.9 and all five samples with pH above 6.0. L . innocua and L . monocytogenes (84.3% and 15.7%, respectively) were the only listeriae isolated (Perry and Donnelly, 1990). Factors affecting silage pH (i.e. dry matter content, available carbohydrates, and buffering capacity), the rate of the decrease in pH, and the stability of the silage (i.e. thorough packing of the ensiled material and an air-tight seal) influence the growth of L . monocytogenes. If oxygen can enter the ensiled material, listeriae can withstand acidity as low as pH 3.8 for long periods (Fenlon, 1989). An experiment demonstrated that for a silage at 26% dry matter, the crucial level of oxygen that permits an increase of L . monocytogenes to levels significant for pathogenesis lies between 0.5% and 0.1%. At lower levels of 0.1–0%, L . monocytogenes perishes (Donald et al., 1993). Even in good-quality silage, oxygen may enter at the surface or along the sides, permitting growth of fungi, which may cause the pH to increase and, in turn, permit the growth of listeriae.

There are many sources for L . monocytogenes in silage. It can survive, and even multiply, in soil (Botzler et al., 1974), and is present in rivers, lakes, canals, and surface waters, flooding meadows, rivers, lakes, and canals (Brackett, 1988). Consequently, L . monocytogenes is often found on pasture grasses (Weishimer, 1968; Weiss and Seeliger, 1975) and is likely to be present when vegetation is harvested for making silage. L. monocytogenes also has been recovered from raw and treated sewage and vegetation treated with sewage sludge (Al-Ghazaii and Al-Azawi, 1986). Wild birds and scavengers may be vectors for contaminating silage with L. monocytogenes (Fenlon, 1985).

A variety of toxigenic fungi have been isolated from moldy silage, but outbreaks of mycotoxi- cosis attributable to silage are surprisingly rare (Lacey, 1991; Scudamore and Livesey, 1998). Most well-authenticated toxicoses are attributable to Aspergillus species, especially Aspergillus fumigatus (Yamazaki et al., 1971; Cole et al., 1977; Lacey, 1991). Aflatoxins may occur in silage, but are not considered to be a major problem (Lacey, 1991).