Growth promoters - ALTERNATIVES



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The contributions begin with general description of swine characteristics and the structure of the pig industry. Descriptions of the various classes of feed nutrients follow and how they are metabolized by hogs, factors affecting their utilization, practical aspects of swine feeding from birth, finishing, through gestation and lactation in sows and the feeding of adult boars. The nutritional aspects of the feedstuffs commonly fed to swine are covered in the following section.

There are chapters on Feed Additives for pigs, including enzymes and probiotics, the environmental impact of swine production and Performance-Enhancing Substances (growth promoters, digestive enhancers). The final chapters of the book are devoted to details of the techniques and technology used in swine nutrition research.
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Manipulation of the intestinal microflora for improved health and growth in pigs

Presented to the British Society for Animal Science (BSAS) conference in Scarborough, March 1999
Dr. K. Hillman, Microbiology Section, Animal Biology Division, SAC, Ferguson Building, Craibstone, Aberdeen AB21 9YA, UK


The intestine is a large organ, and its maintenance is expensive in terms of both its protein and energy requirements. It is therefore evident that any dysfunction or inefficiency in the functioning of this organ must result in a reduction in the growth efficiency of the animal as a whole. From the microbiologist's point of view, the area of the monogastric gut most amenable to manipulation is the colon, since this is the main site of microbial fermentation, although the ileum also harbours a significant microbial population. The colon, however, by virtue of the enormous numbers of bacteria it contains, offers the greatest potential for the application of microbial changes which may have a significant effect on the overall health and productivity of the animal. These changes can be effected by alterations in the composition of the diet, by the addition of large numbers of live bacteria (either with the diet or by separate oral administration), or by the inclusion of antibiotics in the diet.


The potential future removal of growth-promoting antibiotics from farm animal feeds has led to renewed interest in the use of live microbial cultures (probiotics) as growth promoting agents, in addition to their current involvement in the control of enteric disease. The intestinal microflora of the pig is, in itself, capable of resisting the establishment of certain intestinal pathogens (Hillman et al, 1994), and it has often been shown that certain lactic acid bacteria within the pig intestinal microflora possess an inhibitory activity towards coliform pathogens, and that the addition of large numbers of these bacteria to the porcine microflora in vitro results in a consistent and reproducible increase in the rate of removal of the pathogen (Hillman et al, 1995). However, the application of these isolates to live animal studies generally produces highly variable results - a preparation which is effective in one herd may be ineffective in another. There are a number of possible reasons for this variation, and among the proposed bacteriological reasons are:

a)The source of the probiotic bacteria
Bacterial taxonomy can be misleading, particularly to the non-specialist. As an example, isolates of the bacterial species Lactobacillus acidophilus can be obtained from many sources, including silage, milk, the rumen of cattle and sheep, and the lower intestine of most animals, including pigs. Although classification of these four isolates would produce the same species name, only the isolate from the pig intestine would, for example, exhibit tolerance to bile acids: the isolates from the other environments would have no need of this facility and so would not have developed it. The isolation of L. acidophilus from the pig intestine does not mean that any isolate of this bacterial species will be capable of growth in the intestine: only that "strain" of the species which is adapted to life in the intestine can be assured of activity in that environment.

b)The age of the bacterial cultures,
or more accurately the number of bacterial generations between isolation and application. Bacteria reproduce quickly, and adapt rapidly to the environment in which they find themselves. An isolate of Lactobacillus spp. from the pig intestine may be found to be effective at inhibiting coliform pathogens on first isolation, but may "lose" its efficacy over time. In the complex microflora of the intestine, these bacteria are in constant competition with other microbial species for nutrients, so that the production of compounds inhibitory to other species confers a survival advantage. However, in isolation, the bacteria are grown in pure culture in a nutrient-rich medium. In this case, the production of an inhibitor active against an absent competitor represents a waste of resources for the bacterial cell. As a result, those members of the population which do not produce the inhibitor may grow a little faster, so that every time the cultures are transferred to new media, the proportion of non-inhibitor-producing cells increases. Eventually, the isolates adapt to their new "environment", the nutrient-rich laboratory medium with no competing species, and the population loses its inhibitory activity against the pathogen.

c)The stability of the intestinal fermentation,
whereby the addition of large numbers of a single species may result in a population which cannot be maintained on the nutrients available. The probiotic bacteria are then subject to rejection by the resident microflora, by the same mechanism as applied to pathogens. The variation in efficacy may be ascribed to the differing microbial populations between animals, which depends on many factors including the intestinal physiology and diet of the animal. Where the probiotic is of a species which is already present in high numbers in the gut, then the "new", pathogen-inhibiting strain may partially replace the "old", non-inhibiting strain. However, where the "new" strain is of a species which is poorly represented within the microflora, then its scope for activity within that environment is limited.

The efficacy of live bacterial cultures as feed additives, whether intended for use in disease control or as growth promoters, is therefore dependent on the conditions prevalent in the intestine, which in turn depend on intestinal physiology, on the dietary materials reaching that part of the intestine, and on the current microbial population of the intestine. It may be possible to improve the reproducibility of probiotic preparations by the provision of a diet suitable for the activity of the microorganism. Alternatively, probiotics could be matched to diet, where for any given diet a list of those probiotics likely to prove successful would be available.

Dietary manipulation

This provides an alternative to probiotics as a means of altering the composition of the intestinal microflora, and does not involve the use of feed additives in the accepted sense of the word. Rather, the composition of the diet is adjusted in order to provide the microflora of the lower gut with a particular balance of nutrients, resulting in the formation of a particular bacterial population.

The success of this approach depends on the composition of the original microflora, and on the variability of the feed components. It is nonetheless possible to produce positive effects on both the composition of the microflora and on the growth of piglets simply by changing the type of starch in the diet. Using a high-amylopectin maize starch (waxy maize, approx 99% amylopectin), processed by heating in water followed by cooling to produce a gel prior to inclusion in the diet, the proportions of Lactobacillus spp. to coliform bacteria can be increased to approximately 100 times that observed in pigs fed diets with a normal maize starch (approx 75% amylopectin: Reid and Hillman, in press). This effect has been replicated in vitro, using porcine intestinal microflora, but with unprocessed starches. Although the differences observed were not so marked as in the pig, they did reach statistical significance (P<0.01: Table 1).

Table 1: Ratio of lactobacilli:coliform bacteria in an in vitro colon simulation, using three maize starches as substrate.
Starch type lac:coli
Waxy maize90.6 (1.87)a
High amylose maize 10.4 (0.99)b
Maize7.9 (0.84)b

Note: Data represent the means of triplicate determinations. Values in brackets are the means of the log10 transformed data used in the statistical analysis. Values having different superscript letters differ significantly (P<0.01).

The retrograded starches produced a very wet feed, which was difficult to manage even on the small scale of a feeding trial. It was considered that, although this feed could result in a suppression of coliform activity within the intestine, it would be unlikely to prove useful in practical application. A further feeding trial, using only unprocessed starches, has demonstrated that this effect can also be acheived using raw potato starch. The various raw starches produced neither a significant improvement in liveweight gain, nor any detrimental effects, over the three weeks of this trial.

It must be remembered that, since starches are natural products, the characteristics of a starch type will vary with harvesting time, growing conditions for that year, type of soil, and numerous other factors. It is therefore difficult to derive more than comparative conclusions for the findings observed in any one year.


Dietary manipulation and the use of probiotic preparations are two apparently disparate techniques, both providing potential for improvements in health and (possibly) performance in growing pigs. Both techniques have innate variabilities in response when used in pigs. Could a combination of these techniques prove successful in future studies, or would the combination simply prove more variable than the individual approaches? Although the use of starches to increase the lactobacillus:coliform ratio in the intestine suffers from the variability implicit in the use of this natural product, comparative data seems valid, so that a starch which produces the best response in one year would be expected to produce the best response in any year, although the degree of the response may vary from year to year. With this in mind, it should be possible to select a diet which provides an enhanced population of Lactobacillus spp. within the gut. This does not necessarily imply that the species enhanced will be those which are active against pathogens, but it does mean that the gut environment has been adjusted to provide conditions suitable for the activity of Lactobacillus-based probiotics.

If this approach is to replace the use of antibiotics in animal feeds, then it must be cost-effective. It must provide consistency in activity, at least to the same extent as observed in antibiotic feed additives. The principal advantage of such an approach is political: as it makes use of entirely natural products, it is unlikely ever to be controversial. The challenge with any approach of this kind is to bring the efficacy and reproducibility to a level comparable with antibiotics, while keeping costs to a minimum.


The work discussed has derived from a variety of funding sources, including SOAEFD, and from the efforts of several PhD and MSc students, including Carol-Ann Reid, Rania Khaddour and Paul MacFarland.


Hillman, K., Murdoch, T.A., Spencer, R.J. and Stewart, C.S. (1994) Inhibition of enterotoxigenic Escherichia coli by the microflora of the porcine ileum, in an in vitro semicontinuous culture system. Journal of Applied Bacteriology 76; 294-300.

Hillman, K, Spencer, R.J., Murdoch, T.A. and Stewart, C.S. (1995) The effect of mixtures of Lactobacillus spp. on the survival of enterotoxigenic Escherichia coli in in vitro continuous culture of porcine intestinal bacteria. Letters in Applied Microbiology, 20: 130-133.

Reid, C-A. and Hillman, K. The effects of retrogradation and amylose/amylopectin ratio of starches on carbohydrate fermentation and microbial populations in the porcine colon. Animal Science, in the press.


*Antibacterial Growth Promoters - Legislation & Control in European Union & U.S.: report

*Swine Feeding & Nutrition new books



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