160 Handbook of herbs and spices
conversion ratios in 17-day-old poults fed a diet supplemented with a plant extract
containing capsaicin, cinnamaldehyde and carvacrol at 300 ppm. Water-soluble
extract from rosemary, containing rosmarinic acid, flavones and monoterpenes,
enhanced hepatic metabolism and increased relative liver weight in rats (Debersac et
al., 2001).
An essential oil combination derived from herbs growing wild in Turkey, was
found to have a beneficial effect on body weight, feed intake, feed conversion ratio
and carcass yield when used as a feed additive of broiler chickens (Alcicek et al.,
2003, 2004). The incorporation of carvacrol, cinnamaldehyde and capsicum oleoresin
promotes changes in the digestive function and microbial ecology (Manzanilla et al.,
- while herbal extract containing cinnamon, thyme and oregano extract reduced
the proliferation of coliform bacteria of weaned pigs (Namkung et al., 2004). It has
been reported also that blends of essential oil components can control Clostridium
perfringens (causative agent of necrotic enteritis) colonization and proliferation in
the gut of broilers (Mitsch et al., 2004).
On the other hand there are some contradictory results about the effectiveness of
certain herbs and extracts as growth promoters. It has been reported by Botsoglou et
al., (2002) that oregano oil exerted no growth-promoting effect when administered at
50 or 100 mg/kg of feed. Others have found that an essential oil mixture and thymol
and cinnamaldehyde did not stimulate growth performance in broiler chickens. They
attributed this to the composition of the basal diet (highly digestible) and/or the
environmental conditions (Lee et al., 2003a). Dietary thymol and its isomer carvacrol
did not affect growth performance and did not show hypocholesterolemic activity
when used as alternatives to antibiotic feed additives in broiler chickens (Lee et al.,
2003b).
10.4.1 Experimental assays
The experimental assay usually includes modification of the feeding programme of
broilers for some days by supplementation of their basal diet with essential oil extract
or their constituents. For feed intake, the feed:gain ratio per pen is measured throughout
the experiment (Hernandez et al., 2004). At the end of the experiment, the weights of
the proventriculus, gizzard, small and large intestines without content, pancreas and
liver without gall bladder are measured individually. Diet, excreta and ileal digesta
are analyzed for nitrogen, dry matter and acid insoluble ash. Diet and excreta are
analyzed for lipid and diets and ileal digesta are analyzed for starch. The effects of
additives on performance, digestibility and organ size are analyzed statistically
(Hernandez et al., 2004).
10.4.2 Mechanisms of action – active compounds
Plant extracts contain different molecules that have intrinsic bioactivities on animal
physiology and metabolism. The mechanisms by which these products influence the
gut microflora and growth performance of animals are not elucidated. As antibiotics,
plant extracts could control and limit the growth and colonization of numerous
pathogenic and nonpathogenic species of bacteria in the gut as described in the
previous part of this chapter. Their possible mechanisms of action are also discussed
above. There is evidence to suggest that herbs, spices and various plant extracts have
appetite- and digestion-stimulating properties, as is also discussed in this chapter