The controversy over direct-fed antimicrobials has fuelled interest in the application of acids for their nutritive and health effects. Digestive disorders in piglets are often caused by the difficulty to adapt to the transition from milk feeding to solid feed. Stress and a low tolerance against pathogens from the feed may lead to bacterial infections of the gut and consequently to an increased incidence of diarrhoea and a low utilisation of feed nutrients.
This situation leads to the need for supporting the digestion of piglets through optimal feed hygiene and the application of suitable additives. The use of organic acids and their salts is widely accepted in animal nutrition, although acids differ in their range of effectiveness. On the one hand, they are preservatives and their efficacy in feed and food hygiene has been studied extensively. Apart from an improved feed stability due to their mould-inhibiting and antibacterial properties, acids may also be used as a mineral and energy source in the animal.
It is also known that different organic acids possess performance enhancing properties. Formic, lactic, acetic and their salts, as well as fumaric and citric acid, belong in this category.
Using organic acids in the feed utilises their antimicrobial properties in the hindgut. This effect is dependent on different factors:
- the actual concentration in the feed;
- adsorption and metabolisation in stomach and small intestine;
- activity of the acid at pH 6; and
specific activity of the acid.Most organic acids are effective in a strongly or moderately acidic environment (Table 1). Because different acids have different pH optima, a blend increases the general efficacy against microorganisms.
Organic acids are also used to reduce the buffering capacity in the feed. The chemical term buffering capacity (B-value) is used to describe the change in the pH value of a defined volume or mass after the addition of a strong acid. Usually, in the animal feed sector, hydrochloric acid is used for this purpose for physiological reasons.
A more practical definition is that theB- value is the amount of 0,1 N HCl solution which needs to be added to a 10 percent slurry of the feed or a feed ingredient in 100 ml of water in order to obtain a pH-value of 5 (4 or 3). This definition is the reason why we find in practical application different values for the same expression. In Europe, the most common B-value is the method developed by Prohaska and Baron (1980). With the B-value, there exists a neutral measure for the buffering capacity of a ration or a single component, which can be determined in the routine laboratory of the mixed feed manufacturer.
The buffering capacity can provide an indication of the digestibility of an individual feed or a ration in piglet rearing. The higher the value, the more critically the component should be evaluated. Lime and protein sources are especially highly buffering components in the diet. In the formulation of piglet diets, care should be taken to optimise for buffering capacity by reducing protein content and using an optimal amino acid profile.
In piglet feeds, a buffering capacity of <700 meq/kg is considered optimal. In practice, the buffering capacity can be lowered very effectively with inorganic acids like phosphoric acid. The zootechnical effects, however, are not equivalent. A low buffering capacity in the feed has a positive effect on gut health and the development of a healthy intestinal mucosa. A healthy mucosa is prerequisite for optimal adsorption of nutrients. This means that one of the parameters that can be positively influenced by the use of feed acids is digestibility.
The application of acids in animal nutrition may, however, pose some problems. While formic acid is one of the most effective acids, it is very corrosive and has an acrid smell.
This has led to the development of special acid and salt blends with a high acid and calcium content. These blends have the advantage of providing a highly available calcium source with a low pH in exchange for the highly buffering lime and enable the feed mill to provide diets with low buffering capacity. Furthermore these blends are not corrosive and are easy to handle in the mills. Commonly used are blends of formic, lactic and citric acid and their salts which combine the nutritive effects of the formic acid with the pleasant taste and low pH of citric and lactic acid.
Nowadays, combinations of organic acids with essential oils have been used to improve the efficacy of the acids. While the antibacterial properties of organic acids are dependent on a low pH, essential oils are able to function at higher pH values. To test the efficacy of the combination of organic acids with essential oils, a piglet trial was conducted at the University of Kiel. Two different acid combinations with essential oils (CaPlus Arome, AntaCid Arome, Dr Eckel GmbH) were tested in their effect on daily gain and feed conversion rate (Figure 1). Both acid formulations were able to improve the daily weight gain of the piglets.
The inclusion of organic acids in piglet and grower-finisher feeds results in an improved feed hygiene which can be attributed to the anti-microbial effect of this acid-mix. Organic acids can also improve digestion as a result of the acid's direct nutritive effect as well as of a reduction of nutrient losses through microbial degradation of the feed.
Further positive effects are gained through a reduction of the feed's buffering capacity which leads to regulation of the pH values in the gastro-intestinal tract of the animal. This effect is especially important in piglets due to their deficient production of hydrochloric acid in the stomach.