Liquid feeding of pigs : potential for reducing environmental impact and for improving productivity and food safety.

Abstract

49 Liquid feeding of pigs: potential for reducing environmental impact and for improving productivity and food safety P.H. Brooks, J.D. Beal and S. Niven Seale_Hayne Faculty of Land, Food and Leisure, University of Plymouth, Newton Abbot, Devon, TQ12 6NQ, UK phbrooks@plymout.ac.uk Summary Liquid feeding creates an opportunity to recycle liquid residues from the human food industry as animal feed. Such residues are often low in dry matter and variable in composition. However, if diets are reformulated frequently to take account of such changes there is no loss of productivity and profitability can be increased. Modern liquid feeding equipment facilitates frequent changes in diet composition, delivery of different diets to different pens of pigs and enables pigs to be fed ad libitum or accurately rationed. These features provide an opportunity to use `step' and `phase' feeding regimes that can reduce the nutrient content of effluent and reduce environmental loading. Current research is investigating ways of increasing the efficiency of utilization specific raw materials by the targeted use of exogenous enzymes. Surveillance studies have shown that liquid feeding reduces Salmonella incidence. This has been particularly associated with the use of acidic residues derived from the food industry. More recently, and particularly because producers wish to feed liquid diets ad libitum, there has been much interest in the concept of feeding fermented liquid feed (FLF) to pigs. Natural, uncontrolled fermentation has produced very mixed results on commercial units. However, when selected lactic acid bacteria inoculants are used and fermentation conditions are carefully controlled, an acidic diet is produced that rapidly and effectively excludes enteropathogens. Such feed is readily accepted by pigs and it has been shown to enhance post_weaning growth and reduce coliform levels in the lower gut. Because of this FLF may be a useful alternative to antibiotic growth promoters. Although results in experimental units are impressive, more research is needed before we can provide Standard Operating Procedures relevant to different conditions. These are needed to enable the transfer of this exciting technologyto commercial pig units. Keywords: pigs, liquid feeding, environment, health, food safety Introduction Liquid feed delivery systems for pigs have tended to be most common in areas where liquid residues from food processing are readily available and where the size of the production unit can justify the capital expenditure involved. Although historically the availability of food industry liquid residues may have been the main incentive for installing a liquid feed system, there are now additional, compelling, reasons why liquid feeding should be considered. This paper considers some of the environmental, production and food safety benefits that may become available to producers who adopt liquid feeding. It is important to define liquid feeding and differentiate it from other feeding systems. Liquid feeding involves the use of a diet prepared either from a mixture of liquid food industry residues and conventional dry food, or from dry raw materials mixed with water. Generally, the diets are mixed at a central point in a pig unit. If the pigs are ration_fed a single pipeline can be used to transfer the mixed feed direct to target pigs wherever in the unit they may be. If the pigs are to be fed ad libitum the feed is moved to `satellite' tanks from which specific groups of pigs are fed. Thus in a modern system, one feed preparation area can produce a range of diets to match the nutrient requirements of pigs of different ages and stages of production. A liquid diet will typically contain 200_300 g dry matter (or dry ingredients) per kg. This type of feeding system should not be confused with wet/dry feeder systems where water and feed are kept separate up to the point of delivery to the pig. A key difference between these two feeding systems is the length of time that the dry matter fraction of the diet is in a liquid medium before it is consumed by the pig. This has important implications for the microbiology and nutrient availability from the feed, which are discussed in this paper. Recent Advances in Animal Nutrition in Australia, Volume 13 (2001) 50 Brooks et al. Potential advantages of liquid feeding Traditionally producers have perceived a number of advantages in using liquid feeding. These include: � � � � reduction of food loss, as dust, during handling and feeding improvement in the pig's environment and health due to the reduction of dust in the atmosphere improved pig performance and FCR flexibility in raw material use (opportunity to utilize more economic food sources and reduce cost per kg gain) improved materials handling (system can act as both a feed mixing and distribution system) increased accuracy of rationing (computer control brings a degree of accuracy to the system that it is difficult to emulate with dry feeding systems) improved dry matter intake in problem groups (e.g. weaners and lactating sows) improved intakes at high ambient temperatures. Utilization of liquid co_products from the human food industry Liquid co_products of many sorts are used for pig feeding around the world. However, the Netherlands provides the best example of co_product utilization, where it has been estimated that annually about 6.5 million tonnes of co_products are used directly on farms (de Haas 1998). The quantity of material has increased dramatically in recent years and it is predicted that it will continue to increase. Currently, demand exceeds supply and, consequently, co_products are being shipped into the Netherlands from France, Poland and even the UK. Of the 6.5 million tonne total, approximately 35% (2.3 million tonnes) is fed to pigs (Table 2). Of this, 70% consists of carbohydrate rich materials (Scholten et al. 1999). The importance of these co_products to pig production is put into perspective when it is remembered that the net production of pork in the Netherlands in 1996 was 1.62 million tonnes (Meat and Livestock Commission 1998). Such detailed data are not available from other countries, but using information from trade sources we estimate that approximately 30% of pigs in Northern and Western Europe are fed liquid diets and that a majority of these incorporate at least some food industry co_products. � � � � Jensen and Mikkelsen (1998) reviewed nine recent studies in which the performance of pigs fed dry or liquid diets were compared (Table 1). Grow_finish pigs fed liquid diets generally had improved daily liveweight gain and feed conversion ratio. Table 2 Amount of liquid co_products (tonnes) from the food industry delivered directly to pig farms in the Netherlands (Scholten et al. 1999). Product Wheat starch industr y 1993 650,000 350,000 300,000 80,000 80,000 25,000 170,000 1,655,000 1996 885,000 525,000 300,000 120,000 100,000 50,000 360,000 2,340,000 Table 1 Improvement (%) in growth rate and feed conversion ratio in nine experiments conducted to compare liquid and dr y feeding for grow_finish pigs (Jensen and Mikkelsen 1998). Improved daily weight gain Mean � SD 4.4 � 5.4 Range _2.6 _ 15.0 Improved feed conversion ratio Mean � SD 6.9 � 3.5 Range 1.9 _ 12.7 Potato processing industr y Dair y industr y Fer mentation industr y Beer industr y Sugar industr y Other Total amount (tonnes) To this improved performance must be added other benefits that can accrue to the environment through: � the utilization of co_products from the human food industry which would otherwise incur a cost for environmentally acceptable disposal reduction in nitrogen loading through the easy adoption of `step' and `phase feeding' reduction in phosphate loading through activation of endogenous phytase in cereal grains and/or the addition of exogenous enzymes to diets. � � A major problem for the nutritionist and the pig producer wishing to use co_products is the variability in their composition (Table 3). This means that if they are going to be used efficiently, diets have to be continually reformulated to compensate for the changes that can occur in composition from one load to the next. Despite the variability of liquid products, they can be used efficiently and without detriment to pig performance if diets are formulated accurately. For example, Scholten et al. (1997) used combinations of liquid wheat starch, potato steam peel and cheese whey to replace 35% of the dry matter in growing pig diets and 55% in finishing diets at a water to feed ratio of Liquid feeding of pigs 51 2.6:1. The results of their study (Table 4) show that when the diet is properly balanced the inclusion of co_ products does not adversely affect pig performance. We have recently concluded a study in which `wheat bottom stills' (Greenwich GoldTM), a residue left after the production of ethanol, were substituted in diets for grow_finish pigs (Table 5). The basal diet was a conventional UK diet based on wheat and barley with extracted soyabean and rapeseed as the principle protein sources and substitutions were made using best_cost formulation. The growth rate and carcass quality of pigs was not affected by substituting conventional raw materials with up to 30% of this co_product. It was interesting to note that at the highest substitution rate growth rate was maintained even though DM intake was reduced. It is important that while solving an environmental problem for the human food industry the livestock industry does not transfer the problem to the farm. Some environmentalists have been concerned that liquid feeding may increase environmental loading. By their nature, liquid diets tend to increase effluent volume. Liquid diets may also increase water consumption. Many co_products have a high mineral content and it is essential that pigs be allowed access to a separate supply of water in order that they can maintain their homeostatic balance (Brooks and Carpenter 1990). This is a requirement of the UK Welfare Codes (HMSO 1991) and should be a mandatory whenever liquid diets are used. In addition to its need to maintain homeostasis, the pig appears to have a behavioural need for water separate from any that it consumes with its food. Producers should not see this as a disadvantage, for our studies have shown that the performance of grow_ finish pigs is improved if they consume more water with their feed, and as a result have greater total water intake (Table 6). Subsequently, Barber et al. (1991) demonstrated that increasing the water content of liquid diets fed to growing pigs increased dry matter digestibility (Table 7). If this finding is taken together with the improved FCR found when pigs are fed liquid diets (see data reviewed by Jensen and Mikkelsen (1998) summarized in Table 1) we can conclude that pigs are able to extract more nutrients from liquid diets than from dry ones. It follows that the use of liquid diets, with or without co_ products, may at the same time increase effluent volume and reduce nutrient load per litre. Therefore, it is very important when making comparisons of environmental loading produced by different feeding systems that these Table 3 Relative variability in composition of liquid and dr y feed components (Brooks and McGill 1995). Dry matter (g/kg) Liquid products Yoghur t Whey Delactosed whey Milk Wheat bottom stills (a) Wheat bottom stills (b) `C' starch Poultr y processing residue Dr y products Biscuit meal Wheatfeed Maize gluten Hi_pro Soya bean meal 22 _ 191 20 _ 58 210 _ 406 126 _ 193 155 _ 193 76 _ 160 133 _ 159 84 _ 239 87 _ 95 87 _ 89 87 _ 90 87 _ 90 Crude protein (g/kg DM) 139 _ 389 115 _ 234 206 _ 293 211 _ 396 207 _ 258 192 _ 367 68 _ 106 211 _ 364 76 _ 126 152 _ 187 203 _ 227 421s _ 514 DE (MJ/kg DM) 17.3 _ 21.8 13.4 _ 15.9 6.8 _ 15.1 14.6 _ 24.3 12.6 _ 17.2 13.9 _ 16.3 15.6 _ 16.2 16.2 _ 23.5 15.4 _ 17.9 12.4 _ 13.1 12.7 _ 13.1 12.9 _ 16.2 Table 4 Perfor mance of growing_finishing pigs (25_112 kg) fed a liquid diet with or without liquid co_products (Scholten et al. 1997). Control diet (meal + water) Daily gain (g) Feed intake (kg/day) Feed conversion ratio Lean meat percentage 740 a a a Co_product diet SEM 768 b b b 4.7 0.01 0.02 0.16 c,d 1.99 2.69 55.3 c 1.98 2.58 54.8 a,b d Data in a row with a different superscript differ significantly P<0.001; P<0.05 52 Brooks et al. are expressed in terms of nutrients voided per kg growth made by the pig or, better still, nutrients voided per kg meat produced. Scholten et al. (1997) considered the outputs of pigs fed diets based on meal and water or on co_ products. They found that ammonia emissions were similar for pigs fed conventional liquid diets and those fed diets that included co_products (Table 8); manure production of pigs fed co_products was 2.4% higher when based on manure per kg growth. However, they did not measure the nutrient content of the manure. In studies with young pigs (7_25 kg), it was found that effluent volume increased when pigs were fed on liquid diets rather than dry pelleted diets (Table 9). Some of the co_products that are used in liquid form could be dried and incorporated into conventional dry compounded feed. However, feeding them in liquid form removes the cost of drying and reduces dependence on non_renewable energy sources. On the debit side, the use of liquid co_products increases transport costs because more water is shipped with the dry matter and, consequently, there is as an increased demand for non_ renewable energy. As a result, disposing of some products by processing them through a pig may only be efficient if pig production units are situated close to the source of supply. However, in Europe many products are transported considerable distances as `back loads' in tankers that would otherwise travel empty. Thus in real terms, there is only a marginal increase in fuel cost to set against the material (i.e. the difference between running the tanker empty and full). Table 5 Effect of inclusion level of wheat bottom stills (Greenwich GoldTM) in the diet on the performance of growing_finisher pigs (2_90 kg) (Brooks and Russell 2001). Greenwich Gold % 0 Daily gain (g) Daily DM intake (g) DM FCR Carcass weight (kg) Backfat P2 (mm) Killing_out % a,b 15 759 a 22.5 746 1523 2.05 62.22 11.20 71.70 b 30 793 1504a 1.91 ab SE D 779 1608 2.08a 63.57 11.00 70.38 15.9 33.0 0.041 0.97 0.68 0.93 1511 2.00 61.83 10.73 70.85 62.42 10.16 69.70 Means with the same superscript differ at P<0.05 or greater Greenwich Gold contained (g/kg fresh mater ial) DM 192; crude protein 6.25; NDF 1.6; ash 1.28 and DE 3.3 MJ/kg. The diets were formulated to provide (at a nominal 87% DM) 13.4 MJ DE/kg and 12 and 9.5 g lysine/kg in the grower and finisher diets respectively. Table 6 Voluntary water use and perfor mance of grow_finish pigs offered liquid diets at different water to meal ratios (Gill et al. 1987). Water to dry ingredients ratio 2:1 Meal intake (kg/d) Voluntary water use ( kg/d) Total water use (kg/d) Daily gain (kg) Dr y matter feed conversion ratio Water to feed ration (w/w) a, b, c 2.5:1 1.49 0.78 4.51 0.74 b b a 3:1 1.46 0.44 4.86 c c b 3.5:1 1.47 0.24 5.60 0.77 1.90 3.68 d d b 1.48 1.26 a 4.23a 0.73a 2.01 2.97 0.75a, 1.95 3.36 2.00 3.12 Means with the same superscript do not differ significantly at P<0.05. Table 7 Effect of water to feed ratio on diet digestibility (Barber et al. 1991). Water to dry ingredient ratio 2:1 Dr y matter digestibility (%) Estimated DE (MJ/kg DM) a, b 2.67:1 77.8 15.0 a 3.33:1 80.3 a, b 4:1 82.9 15.8 b 79.1a 15.1 15.4 Means with the same superscript do not differ significantly at P<0.05. Liquid feeding of pigs 53 Drying and subsequent incorporation into dry diets would not be a viable economic option for co_products with very low dry matter content. These materials would still have to be disposed of in an environmentally acceptable manner. The alternative routes for disposal of these materials would be through a sewerage system (either public or privately owned), land application or through addition to landfill sites. In developed economies both the economic and the environmental cost of such disposal continues to increase. Therefore, when deciding whether to utilize co_ products as feed stocks or make them environmentally non_damaging through waste treatment it is important to audit the alternative systems in their entirety to ensure that they are ultimately beneficial to the environment. `Step' and `Phase' feeding The pork industry naturally wishes to maximise lean growth and consequently diets are designed to maximise lean growth in those animals capable of making a response. The rate of lean growth relative to voluntary energy intake declines continuously with increasing liveweight of the animal. Thus, the requirement for protein and energy can only be matched by a continuous adjustment of the lysine (ideal protein) to digestible energy intake (Gill 1998). In order to maximise protein utilization and minimize the nitrogen content of the ef fluent the energy: lysine ratio should change continuously to reflect the animal's requirement (Figure 1a). However, few units use more than two or three diets throughout the whole wean_finish period (Figure 1b). As a result, grow_finish pigs on most production units are supplied with an excess of protein during most of their growth period. Large `all in, all out' finishing units provide an opportunity to feed a series of diets that closely match the requirement of the pig at each stage of growth (so_called `step_feeding'; Figure 1c). In such piggeries this can be achieved whether the pigs are fed dry or liquid diets. On smaller units, and those which operate multi_age finishing units, it is much more difficult to match the diet to the requirement of individual groups (pens) of pigs. However, modern computerised liquid feeding facilities Improving nutritional management to reduce environmental impact It is beyond the scope of this paper to consider all the environmental benefits that can be obtained through the adoption of liquid feeding. However, two opportunities for reducing environmental loading should be noted namely; reduced nitrogen output in effluent through improved feed management and reduced phosphorus output in effluent through the activation of endogenous phytase in cereal grains. Table 8 Environmental impact of growing_finishing pigs fed a liquid diet with or without liquid co_products (Scholten et al. 1997). Control diet (meal + water) Ammonia emission (kg per pig place/year) Manure production (l/place/year) Manure production (l/kg growth) Dr y matter content of the manure (%) pH of the manure 1.9 1,092 4.1 8.3 7.3 Co_product diet 2.0 1,156 4.2 6.8 7.5 Table 9 Production of effluent by weaner pigs fed dr y and liquid diets (Russell et al. 1996). Trial 1 Dry Daily gain (g) Total water use (ml/pig/d) Effluent production (ml/pig/d) Effluent production (l/kg a Trial 2 SE D Liquid 428 2298 1058 2.47 Dry 397 1499 982 2.47 Liquid 454 2028 1189 2.62 SE D 343 1306 754 2.20 21*** 64*** 46** +12.3% 14*** 84** 31* +6.1% gain)a Note that in Trial 2 trough design for the liquid fed pigs was improved and resulted in a considerable reduction in effluent production per kg gain. 54 Brooks et al. make this possible as they enable a series of diets to be prepared and distributed to different pens of pigs via a single line. Thus, effective `step_feeding' can be undertaken. However, the limitation in this is that the pigs generally need to be fed in discrete meals, and ad libitum feeding is difficult. Additional investment in a two pipeline system makes it possible to `phase_feed' pigs either on a rationed or an ad libitum basis. Two different diets can be prepared having different energy: lysine ratios and these can be given in different proportions in order to match precisely the nutrient requirement of the individual pen of pigs. Phase feeding undoubtedly improves the biological efficiency of pigs. However, the equipment to do this is expensive, and before undertaking such an investment producers need to consider whether it is necessary in their particular situation. In Europe new regulations on ammonia emissions, and limits on the quantity of effluent that can be applied to land, have made such an investment a necessity for some producers. In other situations, a very different approach may be needed. Animal excreta is a valuable source of plant nutrients and if we are committed to the concept of sustainability, we should look first for opportunities to recycle these nutrients. Once again, a full systems analysis should be undertaken before committing to a particular approach. (a) Relationship between live weight and required lysine:energy ratio (after Gill 1998). 1.00 Lysine (g/MJ DE) 0.90 0.80 0.70 0.60 0.50 30 40 50 60 70 80 90 Lysine:energy ratio declines continuously throughout the growing period. Live weig h t (kg) (b) Excess protein provided by a two diet system in the grow_finish period. 1.00 Lysine (g/MJ DE) 0.90 0.80 0.70 0.60 0.50 30 40 50 60 70 80 90 When only two diets are fed the pigs receive an excess of protein Protein supplied in excess of requirement has to be deaminated and voided as nitrogen in the urine Live weig h t (kg) (c) Excess protein provided by `step feeding' in the grow_finish period. 1.00 Lysine (g/MJ DE) 0.90 0.80 0.70 0.60 0.50 30 40 50 60 70 80 90 Using more diets, with different energy:lysine ratio, reduces the amount of excess protein fed and this in turn reduces nitrogen content of effluent. Liv e weight (kg) Figure 1 Phase feeding reduces the wastage of protein and reduces the nitrogen content of effluent. Liquid feeding of pigs 55 Pre_treatment of diet components and diets Another benefit of liquid feeding is that it provides the opportunity for modifying raw materials prior to feeding. Simply steeping materials in water for a period can activate naturally occurring enzymes. A liquid medium creates many more opportunities for using exogenous enzymes and, if there is sufficient cost_benefit, enzyme activity can be improved by adjusting temperature, pH and dwell time. Two examples illustrate the point. Phytases, which occur naturally in the pericarp of some cereal grains and seeds, are activated when the raw materials are soaked; thus soaking wheat and barley 120 100 % phytate hydrolysis 80 60 40 20 0 0 2 4 6 8 Time (h) 10 12 14 Bar le y Wh e at Soya Rapesee d Figure 2 Effect of soaking on phytate hydrolysis in cereals and oilseeds. increased phytate hydrolysis whereas rapeseed and soyabean were little affected (Figure 2). The addition of exogenous phytase to the material being steeped is much more beneficial in the case of soyabean than in the case of wheat (Figure 3) but even with wheat there is some increase in phosphorus availability. Phytic acid binds other minerals as well as phosphorus. Consequently, steeping and/or the addition of phytase increase the bio_availability of other minerals as well (Table 10). We have investigated the effects of other enzymes. Increasing protein digestibility would enable nutritionists to reduce N excretion by pigs. Even materials that are considered to have good protein digestibility, like soyabean meal, can be improved by enzyme treatment. Steeping soyabean meal with protease increased the in vitro digestibility of protein (Table 11), and an interesting aspect of this study was that the magnitude of this effect depended upon the processing treatment to which the meal had been subjected. We also considered the possibility that enzyme treatment might substitute for heat treatment (Beal 1999). Steeping with a protease enzyme for 24 hours prior to feeding improved the daily gain and FCR in both raw and micronized soya (Beal et al. 1999). However, the improvement was only statistically significant in the case of micronized soya in the finisher phase (Table 12). 350 Phosphorus concentrati on (� g/ l) 300 250 200 150 100 50 0 0 10 20 30 Soa ki ng tim e (h) 40 50 Whe at Whe at + Ph yt as e 350 Phosphorus concentration (� g/ l) 300 250 200 150 100 50 0 0 10 20 30 Soa ki ng ti me (h) 40 50 Soya Soya + Ph yt as e Figure 3 Effect of soaking and phytase addition on free phosphorus concentration in wheat and soya (Gear y and Brooks unpublished data). Table 10 Mineral Effect of soaking and phytase addition to a wheat (75%) based diet on mineral bio_availability (%) in the pig (Seguier and Orr unpublished data). Control Feed soaked in water for 12 h 56 41 69 24 25 94 b ac Feed soaked in water + phytase for 12 h 63 50 76 45 39 94 ab bc a a Phosphorus (total) Phosporus (plant origin) Calcium Magnesium Copper Zinc a, b, c 52 37 a a 66 13 15 92 a ab Means with the same superscript differ at P<0.05 56 Brooks et al. Table 11 In vitro nitrogen digestibility (%) of soyabean meals steeped for 24 h with three different proteases (Beal et al. 1998). Control Protease 2 85.8 a1 Protease 2 88.9 84.9 82.7 74.4 78.1 a a1 1 a Protease 4 85.8 87.8 77.8 74.8 82.3 a1 a1 a a a Raw soya bean Steam pressure cooked Micronized Toasted and milled Autoclaved a 1,2 75.8 80.3 74.7 67.81 70.1 1 84.0a1 79.1 73.5 81.1 a2 a a2 Values in the same row with the same letter are not significantly different P<0.05 Values in the same column with the same number are not significantly different P<0.05 Table 12 Performance of pigs fed raw or micronized soya with, or without protease pre_treatment (Beal et al. 1999). Raw soya Micronized soya + 1.45 499 3.24 _ 1.63 762 2.12 + 1.65 804 2.05 Enzyme addition Feed intake (kg/pig Daily gain (g) FCR ) _ 1.48 467 3.24 The important message from these results is that liquid feeding presents an opportunity for the use of exogenous enzymes that can be targeted on specific raw materials. In this way, the nutritional value of ingredients could be increased. If the improved availability of nutrients is reflected in raw material specifications when formulating diets, the pollutant potential of effluent can be reduced. A caution must be added: once materials are in a liquid medium, the natural microflora will be activated and can produce changes that can be either beneficial or harmful; malfermentation can negate any benefits that may be gained from pre_treatment of raw materials (see later). Health benefits of liquid feeding systems There is considerable concern about the incidence of zoonoses in animal feeds and in particular the transmission of Salmonellas through the food chain. In Europe, the animal feed industry has reduced the incidence of Salmonella in feed by stringent quality control and the use of high temperature treatments to kill any residual Salmonella in raw materials. Despite this, there is growing evidence that this approach has been unsuccessful in reducing the incidence of Salmonella in pigs in production units. Tw o hypotheses can be advanced to explain this: changes to the non _ starch polysaccharide fraction of the diet resulting from heat treatment may produce a gastrointestinal environment that is more favourable to the colonisation by Salmonella; secondly, non _ pathogenic Salmonellas may exclude pathogenic strains. Elimination of non_pathogenic Salmonellas from feed may create ecological niches that are subsequently colonised by pathogenic strains. A study of Salmonella incidence on German farms (von Altrock et al. 2000) identified the use of pelleted feed as a common risk factor. A study cited by the United States Animal Health Association (1999) found that operations feeding a pelleted finisher diet had a 26 times greater risk of being Salmonella positive than those that fed a meal diet. It suggested that pelleted diets either influenced the gut environment such that pigs are more susceptible to Salmonella, or that pigs shed Salmonella that were already already present. The same review (United States Animal Health Association 1999) also cited a study on herd _ level risk factors for the introduction and spread of Salmonella in Danish, German, Greek, Swedish and Dutch pig herds; the incidence of Salmonella was 8.2% in herds given pelleted dry feed, 4.2% in herds given non_pelleted and dry feed, but only 1% in herds given non_pelleted and wet feed. It was also observed that the odds of a herd using whey being seropositive was 1% compared with 5.6% in herds not using whey. Danish studies have also shown a reduction in Salmonella when pigs are fed meal rather than pellets, and fewer Salmonella positive pigs when using coarse ground rather than finely ground meal (J�rgensen et al. 1999; Sloth et al. 1998). It is clear from surveillance data that liquid feeding has a positive effect on gut health and reduces the incidence of Salmonella. In a survey of 320 farms in Holland, the incidence of sub_clinical Salmonella infection was found to be ten times lower on farms with liquid feeding than on farms feeding dry compound diets. The incidence was particularly low on farms that fed acidified cheese whey (Tielen et al. 1997). A more recent study (van der Wolf et al. 1999), found that automated liquid feeding of food industry co_products was associated with a decreased risk of infection. It is important to note that most of the studies in which reductions in Salmonella incidence have been Liquid feeding of pigs 57 associated with liquid feeding have come from Denmark and the Netherlands. In both these countries there is a tradition of using food industry co _ products. As Scholten et al. (1999) have pointed out, the majority of these products have been fermented by lactic acid bacteria and as a result have a low pH and contain significant quantities of lactic acid. This high lactic acid concentration inhibits Salmonella in the feed and hence eliminates it at the start of the food chain. Consequently, the inclusion of fermented co_products in liquid diets for pigs makes a significant contribution to food safety. This has led workers in Europe to look more closely at the microbiology of liquid feed and to develop controlled fermentation of feed. Microbial activity in liquid feed Liquid feeding alters both the physico _ chemical properties of the diet and its microbiology. Both of these factors are important in terms of pig health and performance. As noted above, there are benefits from including fermented co_products with high levels of lactic acid in diets for pigs. However, not all producers have access to liquid co_products. Nevertheless, a similar benefit can be obtained even when traditional dry diets are fed in liquid form. Twenty_five years ago Smith (1976) showed that Lactobacillus spp., which occur naturally on cereal grains, proliferate in a wet feed and reduce the pH. In his study, adding water to the meal at feeding time produced a pH of 5.8. Soaking the mixture for 24 h resulted in a massive proliferation of Lactobacilli which produced lactic acid and reduced the pH to 4.1. Virtually any combination of feed ingredients will ferment if left to steep in water. Almost all raw materials have a natural flora (mainly lactic acid bacteria and yeasts). Many may also have an undesirable microflora (Coliforms, Salmonellas and moulds). Generally, the dominant microflora that develops in liquid feed is lactic acid bacteria (LAB). However, at low operating temperatures and particularly with some feed ingredients (e.g. by _ products from brewing and ethanol production), yeasts will dominate. LAB fermentation is beneficial as it produces organic acids, primarily lactic acid which in dry diets has beneficial effects on the feed intake, daily gain and FCR of piglets (Table 13). It seems likely t