Trials in Europe are demonstrating the value of research-based feed strategies for the protection of gut integrity in young pigs. This information is timely, because the market has turned against antibacterial agents that have been used for decades to achieve a better control of pathogens and improved growth and viability.

In the European Union, in-feed antibiotics were banned from pig diets in the early part of 2006. The growth and hygiene benefits found from feeding antibiotics had been achieved through many different effects on the pig's gut. They were seen particularly during the delicate period of weaning, when the young pig's primitive gut tries to cope with the switch from a simple, milk-based formulation to more complex, solid, vegetable-based diets.

Subsequently the interest turned to zinc oxide since it represented a direct 1-to-1 replacement for antibiotics in feed formulations for young animals. However, this now is attracting concern over its impact on the environment.

Zinc oxide is known to have an antibacterial effect (it has been used for many years in topical antiseptic creams). Since it is insoluble, it passes through the stomach and essentially "galvanises" the lower gut of the pig with a protective lining before being excreted in an unchanged form. The compound itself is non-toxic to animals, but zinc could accumulate in the environment when present in pig manure that is spread on cropland.

Scientific investigation has therefore focused recently on trying to find feeding concepts that might be helpful environmentally by reducing the use of zinc oxide in young pig diets. The results of this ongoing project have shown that a concerted, multi-level strategy can work. In addition to appropriate housing and husbandry measures, dietary components that are complementary for mode of action and stability can lead to a more sustainable, environmentally-friendly production of pork.

Specifically, the strategy to have emerged already from the investigation (see Figure 1 ) is based on nutraceuticals the term now widely accepted to cover in-feed supplements having specific physiological and microbiological functions in the gastro-intestinal tract, beyond any other standard nutritional contribution that they may impart to the animal. The category includes ingredients ranging from organic acids to prebiotics, plant extracts and probiotics.

A sustainable strategy based on nutraceutical combinations is only as strong as its component parts. Although the ones in our project have been used together, it is easier to discuss them separately in order to show how each piece fits in the nutritional puzzle before describing their synergistic benefits as a replacement for zinc oxide.

In trying to replace zinc oxide, remember that the primary objective is protection of the lower gut. This means that any ingredient must be preferentially targeted. Those for possible use have to be capable of passing the stomach and reaching the lower gut where their effect is needed. Research step by step has earmarked the following candidates, which together may lead to a 3-step approach to zinc oxide replacement.

Step 1: Acidifiers

Our work shows that attempts at replacement are effective only when they include acidification products as one of the 3 Steps of the strategy. Acidifiers have been demonstrated by numerous studies to have a positive influence on pigs' growth performance. Their effects are most pronounced in weaned pigs, where digestive disturbances often result in diarrhoea related to bacterial infections.

Problems at weaning are triggered by a number of factors, such as the piglet's insufficient production of hydrochloric acid and digestive enzymes. These are magnified by feeding a pre-starter diet with a high protein content and other nutrients that have a high acid-binding capacity.

Dietary acidification increases gastric proteolysis and the digestibility of protein and amino acids. An improvement in mineral digestibility arises from the formation of complexes involving anions of the acidifier with calcium, phosphorus, manganese and zinc. Acidifiers based on formic, lactic and propionic acids and their blends seem to yield the best results when applied at levels of 0.3-0.5%.

Step 2: Prebiotics

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Members of the category known as prebiotics give the next step. The ingredients based on mannan or fructo-oligosaccharides are widely used in post-weaning pig diets. The mannan-oligosaccharides tend to be abbreviated as MOS and the fructo-oligosacchardes as FOS.

The FOS family of prebiotics selectively ferment beneficial bacteria, one form of which has been shown to be helpful by converting lactic acid to butyric acid. The MOS types are derived from specific strains of the yeast Saccharomyces cerevisiae. For many years the focus was on mannan residues, but improved chemical methods have broken open and exposed the glucan moieties which appear to act more on the humoral arm of the animal's immune system.

Studies in laboratory rats at Slovenia's University of Ljubljana have extended previous American work from the University of Davis in California, showing that extremely small amounts of beta-glucan (as little as a few milligrams in a ton of feed) can lead to higher circulating levels of Immunoglobulin A in intestinal sections (see Figure 2 ).

Step 3: Plant extracts

Plant extracts based on thymol and carvacrol have received considerable attention at a research level, with over 30 published articles in the scientific literature referring to their possible application in protecting against pathogens associated with bouts of gut upset and diarrhoea. However, there have been even more cases where their use has failed.

What we have learned over the years is that the stability of their formulation directly affects the magnitude and consistency of their efficacy. Several university studies have described a so-called bodyguard effect' from the equal partnering of phytomolecules leading to more stability in their chemical association. Follow-up work by the University of Leeds in the UK and other institutions found significantly better responses from animals under a stress challenge if given a 1:1 blend of thymol and carvacrol, compared with using either of these alone. This was true particularly when these ingredients were included in a feed mixture that had been through the processes of pelleting and expansion.

As noted above, more lactobacilli are found in the gut of pigs fed on the oregano active ingredients. However, species of lactobacilli are also found in the stomach compartment of the weaned pig. With organic acids and acid salts used commonly and effectively in post-weaning diets, the lactobacilli-stimulating action of thymol and carvacrol would seem to be superfluous especially since the animal is taking in feed and water which could only dilute the efficacy of the extracts at the site in the lower gut where an increase in lactobacilli was wanted.

The reality is that much can still be achieved, by protecting the extracts to secure their controlled release. One way to observe the release of thymol and carvacrol uses a laboratory system which simulates the stomach and intestinal compartments of the post-weaning pig. In an experiment using such a system, protection gave a more targeted release of thymol and carvacrol closer to the site of interest.

In this case the active ingredients were protected by a patent-pending method of encapsulation using mono and di-glycerides. It completely changed the kinetics of release of thymol and carvacrol, by-passing the stomach and releasing most in the lower gut. This targeting in release has been shown to increase lactobacilli numbers significantly and to produce more lactic acid, which leads to a reduced pH and the exclusion of potential pathogens.

Furthermore, at the University of Ljubljana have shown the combination of protected thymol and carvacrol with FOS to change the profile of volatile fatty acids towards the production of more lactic acid and butyric acid. This in turn led to a significantly increased height of the intestinal villi and lower E. coli counts in faecal samples when compared the application of FOS alone.

The action of acidifiers in the stomach and proximal intestine, in combination with protected thymol and carvacrol together with MOS and/or FOS reaching the lower gut, provides a total intestinal tract protection system for the young animal. A battery of consecutive trials with weaned pigs has been performed at a private European test farm with a history using an acidifier with zinc oxide to reduce post-weaning gut disturbances and even mortality in piglets. Adopting the 3-step strategy has benefited performance and gut flora compared to feeding zinc oxide at 3000ppm.

Comparable results were measured with regard to feed intake, liveweight gain and feed conversion, with tendencies for improvement from using the 3-step strategy instead of a conventional acidifier plus zinc oxide. Moreover, a deeper look into the raw data indicated that the experimental groups on the 3-step strategy had more homogeneity, with fewer outlying animals. It would be a valuable effect for units practising all-in/all-out production and being penalised if groups sent for slaughter lack uniformity.