Introduction
Feed additives have received much attention from the feed industry over the last few decades, especially after the ban of in-feed antibiotics as growth promoters in Europe in 2006. It has stimulated the search for alternatives for these in-feed antibiotics, with a focus on feeds for young animals like piglets and broilers. Initially this search was focused on alternative antibacterial effects, but in time nutritionists also appreciated other effects of these in-feed antibiotics like improving intestinal integrity and their anti-inflammatory responses. The majority of experiments conducted in swine and poultry, however, were performance experiments in which the additive was evaluated against a non-supplemented control for registration purposes. In several cases, feed additives have a reasonable track record for improvement in feed conversion. Information on their physiological, immunological, and microbiological modes of action is limited.
In the literature various modes of actions of feed additives on the gastro-intestinal tract (GIT) processes have been described (Turner et al., 2001).
a.
Palatability enhancers
b.
Improved supply of nutrients
c.
Reduced endogenous protein production and losses
d.
Reduced microbial growth
e.
Reduced inflammation and gut permeability
f.
Increased binding of toxins
g.
Enhanced intestinal recovery and function
h.
Increased colonization and improved microbiome
Positive responses of feed additives on production performance are often associated with elevated feed intake levels which in turn will result in a better performance of the animals. However in other situations, the improved nutrient digestibility or reduction of microbial growth can result also in improvedperformance of the animals without increased feed intake. Various factors affect gastro-intestinal health, like management and micro-climate, pathogenic pressure and feed composition. Via a cascade of effects, inflammatory responses are stimulated resulting in a lower feed intake. Feed additives have in common that they exert an influence on gastro-intestinal health. In this review we discuss the mechanisms how gastro-intestinal health reduces feed intake and the effect of different of additives may improve intestinal health and therefore alleviate the negative effect on feed intake. However, the question remains whether feed additives have a direct effect on stimulating feed intake and therefore growth performance, and/or an indirect effect due to improve gut health or gut function.