Salmonella continues to be an important cause of disease. Every now and then it emerges from its slumbering state and makes newspaper headlines when food has been contaminated and vulnerable groups become ill. Apart from strict hygiene measures, feeding strategies can also contribute to diminishing salmonella incidents. Part 1 of this two part series discusses technical means influencing salmonella proliferation and intestinal health in pigs.
By Heinrich Kleine Klausing and Georg Riewenherm , Deutsche Tiernahrung Cremer, Germany
Nutrition is only one element at the level of primary production in the whole range of activities that can be undertaken to eradicate salmonella, but is essential in terms of support for the intestinal health of pigs. Animal feed by itself – following various official and scientific results – only contributes a small fraction of the potential sources for entry of salmonella in the pork production chain. Statistics show that single feedstuffs are the most likely feeds to bring in salmonellas on the farm, especially oilseed cakes. Therefore, measures in the areas of feed production, transportation and storage, and the feeding (as such) of the animals, are designed to decrease the risk of entry of salmonella to zero. Nutritional concepts are also designed to improve the pig’s ability to enable itself to block the entrance of salmonella from the environment into the gastrointestinal tract, and to avoid the intestinal manifestation of the bug.
The key factor for the targeted support of intestinal health is and remains “digestible feeding”, which can only be established through phase feeding aimed at developing maximum enzyme activity. This applies to the sow, weaner and fattening pig alike. The carbohydrates and their digestibility are mainly of importance, where for example in piglet nutrition primarily the use of “hydrolysed grains” are of importance. With hydrothermical treatment (extrusion) of cereals the structure of the starch granules is changed. This enables digestive enzymes (mainly amylases) to break down the starch easier and more starch can be degraded within a limited time frame. The typical starch granules are to a great extent destroyed by the treatment and are largely changed into a structure that looks like melted plastic. Depending on the portion of degraded starches in the recipe (between 35 to 15%) the starch digestion is significantly supported.
Besides the pure output effect, the salmonella bacteria is also deprived of a vital nutritional factor. However, it is also important to ensure that the fermentation processes in the caecum are specifically developed and triggered. This begins after weaning of the piglets with the use of specific sources of fermentable fibres in the feed. When hydrolysed cereals are used, at least one-fifth of the extruded cereals should consist of barley. The fibre fractions from the cereals are fermented by bacteria in the caecum. In young weaners, fibre fractions from extruded cereals are particularly effective are. This produces, among other things, lactic acid, which increases the regulating effect of the pH value. The establishment of short chain fatty acids also have a growth inhibiting effect on salmonellas. Butyric acid, in particular, has a positive influence on the development of the intestinal villi (length of the microvilli and crypt depth).Different cereals are used as an energy source in feed for fattening pigs and sows. From a nutritional standpoint, extruding of cereals is not a necessity at this age since the concentration of enzymes secreted by the pancreas (among others, amylases) is sufficient. To provide high quality cereal hulls in the supply of bacterial fermentable fibres (esp. wheat bran), sugar beet pulp, maize and wheat gluten, as well as rapeseed and sunflower cakes can be used.
Assessing fibre fractions
Unfortunately, up until now, the assessment of the fibre fractions – or the structure of carbohydrates – in feed for pigs is based on crude fibre. The bacterially fermentable carbohydrates in the caecum and the differences between components are not well enough valued. A much more differentiated analysis and assessment of structural carbohydrates offer the analytical methods and the resulting advanced valuation metrics described by Van Soest (1991). The structures of the carbohydrates are more differentiated and can be divided in the following fractions:
Neutral Detergent Fibre (NDF) is the most common measure of fibre used for animal feed analysis, but it does not represent a unique class of chemical compounds. NDF represents mainly the plant cell walls. Hemicelluloses, pectin and cellulose are the most important structural carbohydrates that are more (hemicelluloses, pectins) or less (cellulose) fermentable.
Acid Detergent Fibre (ADF) includes the cellulose and lignin in the feed. The difference between the recorded levels for analytical NDF and ADF is the content of hemicelluloses. The amount of cellulose is calculated from the difference between ADF and ADL.
Acid Detergent Lignin (ADL) is the fraction of lignin that is found by chemical analysis. Lignin is virtually indigestible for monogastric animals. Gidenne (2003) also defined the group “digestible fibre”, which can especially be used for assessing the capacity of feed fermentation in the large intestine of pigs.
Digestible Fibre (DF) is determined from the sum of the two fractions hemicelluloses (NDF and ADF) and the Water Insoluble Pectins (WIP). Pectins are mostly digested by fermentation in the colon.
Table 1 gives a breakdown of the different fibre fractions of a number of raw materials that are used in pig nutrition. Of course the digestibility of proteins in relation to the digestive capacity of the pig at a specific age or production stage needs to be assessed in relation to reducing the pressure on fermentation in the colon.
Intestinal health and pH
How to assist the improvement of intestinal health is always a question of what the optimum pH range of the digesta for the various gastrointestinal segments needs to be. The conditions and influences are very different between stomach, duodenum and caecum (Figure 1).
Considering the potentially pathogenic intestinal bacteria, the even acidification of the digesta in the stomach and a stable pH value at the transition of the digesta into the duodenum are of great importance in relation to avoiding salmonella and E. coli settling in the gastrointestinal tract. For these gram-negative bacteria to develop, a pH optimum is in the neutral to slightly alkaline range.
The pH in the stomach is normally between 2 and 4, but due to varying feed intake and poor distribution in the stomach leading to inadequate dissolving of the feed, pH may rise partly to values above 7. All feeding measures must therefore first seek to keep the pH in the stomach uniformly low to ensure a smooth transition with pH of digesta into the duodenum and then securing it in the duodenum with the desired pH range near 7.
Fluctuating pH levels in the digesta from the stomach to the duodenum induce a fluctuating secretion of bicarbonates (buffering agent) from the pancreas into the duodenum. This may cause the pH in the duodenum to increase temporarily higher than pH 8 – the best conditions for rapid multiplication of Gram-negative bacteria. In addition, it should be noted that a stable, low pH below 4 in the stomach is of major importance for protein digestion. For only in this pH range an effective conversion of pepsinogen into the active form “pepsin” will take place. This enzyme together with trypsin (secreted from the pancreas into the duodenum) is a key factor in protein digestion.
The neutral to slightly alkaline pH value range is physiologically normal, and important for optimal digestion. In particular, amylase has its activity optimum at these values. From a feed perspective, the systematic manipulation of the pH of the digesta is a matter of the right combination of solubility of the feed in the liquid medium (smooth, quick acidification of the total diet), acid-binding capacity of the feed and the addition of acids.
Impact of feed structure
The structure of the feed is of great importance with regard to supporting intestinal health. This feature has been intensively studied in pigs in Denmark and Germany.
When comparing meal and pelleted feed, thermal treatment by using pressure and temperature for sanitising the feed is preferred. Up to 99% of the natural microbial flora is eliminated this way. In Denmark, for many years it was advised to heat feed for fatteners at least to 81°C during pelleting to be sure all salmonellas were destroyed. However, it can be questioned if pelleted feed only has advantages. It speaks for itself that there is hardly any segregation during transport and storage. Also, feeding losses with dry feeding are minimal. At the animal level, however, a pig in a time frame of five minutes, for example, consumes more pelleted feed than mash feed. The pelleted feed in the stomach requires significantly more time to go into solution and thus the risk of inadequate acidification in the stomach rises. In contrast, feed in meal form is consumed in smaller portions during the day, it is better salivated, and is faster acidified in the stomach. According to Hansen (2004), the pH of a coarsely ground feed mash reached a pH 2 in two hours, it took four hours to reach this level with pellets (finely ground meal). The influence of grinding (fine versus coarse) of mash and pelleted feed on salmonella presence in pigs was studied in Denmark. When coarsely ground mash was fed, sal-monella detection was significantly reduced. Also, the use of coarse meals for pellets reduced salmonella presence in the pig, but the level of detection was not as low as with the pigs fed the coarse meal (Table 2).
At the Institute of Animal Nutrition of the University of Veterinary Medicine Hannover (TIHO) in Germany, several dissertations were devoted to the level of the influence of feed structure (fine or coarse ground) and the use of acids in the diet on the frequency of salmonella detection in weaners and fatteners. It was demonstrated under experimental conditions and through field studies that coarse grinding of feed in combination with the use of organic acids (or their salts) forms an adequate means for reducing salmonella levels in pigs.When feed is coarsely ground, some effects take place. Firstly it reduces the passage rate in the stomach, the DM content of the stomach contents increases, the growth of lactic acid bacteria is stimulated, the level of dissociated lactic acid increases and the pH value is reduced. This leads to a significantly reduced survival rate of orally ingested salmonella. Jorgensen et al. (2000) also identified in experiments with pigs that with mash feeds the concentration of acetic, butyric, and propionic acid in the stomach was significantly higher than with pellet feeding – an additional “barrier effect” against salmonella and other gram-negative bacteria. This study also showed that in the mash fed group of pigs compared with the pellet fed group the concentration of coliform bacteria in the stomach contents was significantly reduced. Secondly, with coarse ground feeds a larger part of the starch is not digested in the duodenum and thus reaches the caecum, which favours the caecal microflora to break down the starch into primarily propionic and butyric acid. These short-chain fatty acids inhibit proliferation of salmonella, which can lead to an overall decrease in salmonella prevalence.
Offenberg et al. (2007) report from their experiments on performance parameters in piglet rearing and fattening that the assumed decrease in performance when feeding coarse ground compound feeds could not be confirmed. No differences were noted in daily gain and FCR between piglets fed coarse or fine ground feeds. In daily practice it is important to know how “fine” and “coarse” can be assessed. A basis for this is a sieve analysis of mash feed. At TIHO Hannover “coarse ground” meal on average contains 22-38% particles that are >1.4 mm (pelleted, crumbled and mash feeds). Figure 2 shows the particle distribution of a well structured mash feed for fatteners (13.0 MJ ME/kg, 17% CP, 0.9% lysine), which is composed of cereals, cereal by-products, soybean meal and rapeseed meal as major components. This structure is from a practical point of view classified as “well structured” and “perfect” and meets the requirements in the studies on feed structure and pig performance. With crumbled or pelleted feed is important to note that particles will be smaller than those of the mash before pelleting. As the mash is pushed through 3-5 mm holes in the pelleting die another crushing step takes place.
Translated and adapted from Agriculture and Veterinary Academy “Practical pig farming”, Horstmar, Germany. Website: www.ava1.de (German)
Source: AllAboutFeed vol 1 nr 1, 2010