Introduction
During the last few years poultry research has been focused on the use of natural additives. To achieve more favourable terms and less stressful effects on the digestive tract, various types of clay were tested and recommended in animal feed as a natural additive that can improve digestibility, health status, growth performance and product quality (Ouhida et al., Reference OUHIDA, PÉREZ, GASA and PUCHAL2000a; Reference OUHIDA, PEREZ, PIEDRAFITA and GASA2000b; Wester, Reference WESTER2002; Xia et al., Reference XIA, HU and XU2004; Ouachem et al., Reference OUACHEM, SOLTANE, KALKIL, SOUALAH, BERGHOUTI, ABDESSEMED, MEKAOUSSI and YAKHLEF2009; Ouachem, Reference OUACHEM2011). The consumption of soil is a natural phenomenon observed in all animal species. Chaumande (Reference CHAUMANDE2011) has reported that all animals consume kaolinite daily regardless of the amount of diet available. The spontaneous consumption of clay is described in particular situations, especially in cases of digestive disorders or for reducing a state of unrest (Andrews and Horn, Reference ANDREWS and HORN2006). Clay is also recommended for its pro-digestive properties, to increase feed efficiency and promote a healthy digestive tract, as well as for its anti-toxic capacity to many undesirable substances in the gut (e.g. biogenic amines, mycotoxins, endotoxins). This paper reviews the effects of clay when used as a natural additive on growth rate, feed consumption ratio, nutrient digestibility, egg production, meat quality, animal welfare and the environment.
Effects of clay on poultry performance
effects on broiler performance
Clay is usually incorporated as an ingredient in animal feed or as an antimicrobial to improve food retention time, increase efficiency, to promote a healthy digestive tract and to enhance performance. Animal responses depend on the animal species, its health state, performance level, dietary composition, clay treatment (Xia et al., Reference XIA, HU and XU2005), clay type and purity, incorporation rate and rearing period (Prvulović et al., Reference PRVULOVIĆ, JOVANOVIĆ, STANIĆ, POPOVIĆ and GRUBOR2007).
At the startup age, the addition of clays seems favourable for better nutrient utilisation and therefore provides a good start for the chicks. The addition of zeolite (0.5%) at startup promotes an increase in average daily weight of 5.3% (Prvulović et al., Reference PRVULOVIĆ, KOGIĆ, LASIĆ and KOSARCIĆ2008). At the age of two weeks, 3% of marl significantly increased weight gain by 7.2% (Ouachem et al., Reference OUACHEM, SOLTANE, KALKIL, SOUALAH, BERGHOUTI, ABDESSEMED, MEKAOUSSI and YAKHLEF2009). Positive responses have been reported with the inclusion of 3.5% bentonite at six weeks of age (Tauqir et al., Reference TAUQIR, SULTAN and NAWAZ2001), with the inclusion of 1.5% montmorillonite at 56 days of age (Xia et al., Reference XIA, HU and XU2004) and with the inclusion of 3% marl, producing significant increases in slaughter weight (3491g vs. 3222g) and decreases in feed consumption ratio by 5% (Ouachem et al., Reference OUACHEM, BAKROUNE, BENSALEM, HADJAR and ABDESSEMED2011a). Feed conversion ratio was also enhanced with a natural zeolite or bentonite (Pasha et al., Reference PASHA, MAHMOOD, KHATTAK, ABDUL JABBAR and KHAN2008).
During the starter period (1-10d) the use of 3% marl in the starting diet containing 5% oil acid, significantly increased body weight gain by 13.5% and improved feed consumption ratio by 10.3% (Ouachem et al., Reference OUACHEM, SOLTANE, HADJAR, BAKROUNE, KALKIL, BENSALEM, SMAILI, HADDAD and ABDESSEMED2011b). Moreover, in a comparative study on the effects of various clay types, Safaei et al. (Reference SAFAEI, BOLDAJI, DASTAR and KHAN2010) showed that zeolite or bentonite use (3%) was accompanied by a significant increase in starting weight gain and at 42 days, whereas the effect of kaolinite addition was seen more significantly in the feed conversion ratio. Montmorillonite was treated with copper (Cu-MMT) to enhance effectiveness and tested on broilers (Xia et al., Reference XIA, HU and XU2004) and pigs (Xia et al., Reference XIA, HU and XU2005). Treatment increased the body weight of broilers at 42 days by 11.7% and reduced feed conversion ratio by 7.14%, while untreated montmorillonite enhanced weight by only 3.5%. Similarly, sodium bentonite (5%) was tested in broiler chicken diets characterized by their crude protein, major mineral elements and vitamin disabilities. The results indicated that bentonite addition was favourable for growth and feed conversion ratio (Southern et al., Reference SOUTHERN, WARD, BIDNER and HERBERT1994).
In another study the use of zeolite in aflatoxin contaminated poultry feed was shown to enhance weight gain and feed conversion ratio (Miazzo et al., Reference MIAZZO, ROSA, DE-QUEIROZ, MAGNOLI, CHIACCHIERA, PALACIO, SAENZ, KIKOT, BASALDELLA and DALCEROS2000). In addition to the effect on performance, it has been reported in a limited number of papers that the use of clay improves sensory value, organoleptic quality, cutting yield of chicken meat and meat processing abilities (Grashorn and Brose, 1997; Mallek et al., 2012). Additionally, by adding 0.5% clinoptilolite, a significant response was noted by Prvulović et al. (Reference PRVULOVIĆ, KOGIĆ, LASIĆ and KOSARCIĆ2008) on breast mineral content (23%), drumstick (19%) and breast protein content (21.9%). A significant decrease in abdominal fat of 45% was also observed by these authors and another study using the addition of 3% marl reported a decreased in abdominal fat of 11.8% (Ouachem et al., Reference OUACHEM, BAKROUNE, BENSALEM, HADJAR and ABDESSEMED2011a). According to these authors, marl significantly improves the yield of ready to cook chicken and breast by 15% and 11%, respectively.
effects on laying hen performance
Some studies have provided explanations on the mechanisms of aluminosilicate (zeolites) action. Among the hypotheses explaining the effects of zeolite on egg shell quality is that the high affinity of this clay to calcium and its great exchange capacity are responsible for these effects (Roland, Reference ROLAND1988). According to Roland (Reference ROLAND1990) the sodium aluminosilicate exchange capacity towards calcium is greater than 7 mg/g and the phosphorus excess tends to affect egg shell quality (Elliot and Edwards, Reference ELLIOT and EDWARDS1991) and the aluminium zeolite would be able to make a complex with phosphorus that improves shell quality (Roland, Reference ROLAND1990; Elliot and Edwards, Reference ELLIOT and EDWARDS1991).
In laying hens, it has been observed that bentonite addition (5%) increases egg size, improves feed conversion ratio and internal egg qualities (Quisenberry and Bradley, Reference QUISENBERRY and BRADLEY1964). Furthermore, Rizzi et al. (Reference RIZZI, SIMIOLI, RONCADA and ZAGHINI2003) have shown that clinoptilolite acts on egg weight, albumen quality, yolk pigmentation and shell strength. According to the same authors, clinoptilolite helps to protect the liver against the negative effects of mycotoxins, improves egg weight and yolk pigmentation. In recent studies, the addition of 2% diatomaceous soil (soil associated with algae and fossilized skeletal marine organisms) enhances spawning performance (weight and number of eggs) and increases albumen and yolk weight (Bennett et al., Reference BENNETT, YEE, RHEE and CHENG2011). According to these authors, in organic farming the use of diatomaceous soil as a deworming product, significantly decreased shell contamination by Capillaria and Heterakis infestation and limited the frequency of occurrence of Eimeria. Parallel to this it has increased organic egg production and improved feed efficiency. Outdoors, the scattering of this clay reduced the hen mite infestation level (Bennett et al., Reference BENNETT, YEE, RHEE and CHENG2011). According to Wu et al. (Reference WU, POWERS, ANGEL, HALE and APPLEGATET2007) the use of zeolite in laying hens from 38 to 59 weeks of age, increases egg weight and decreases the levels of NH3 by 39% and CH4 by 17%.
Effects on intestinal viscosity and moisture of droppings
Broilers and turkeys are in permanent contact with the litter and its quality directly affects the poultry environment, health status, growth performance and carcass quality. Many issues and methods are available to improve litter state. Among them, the respect of hygiene rules, standard animal husbandry (density, heat), diet composition and water quality are the most important factors. According to Çabuk et al. (Reference ÇABUK, ALÇIÇEK, BOZKURT and AKKAN2004) phylosilicates increased dropping dry matter by 8.6%. Moreover, Safaei et al. (Reference SAFAEI, BOLDAJI, DASTAR and KHAN2010) reported that the addition of bentonite, kaolinite or zeolite may improve the state of droppings by increasing the dry matter level of 20.9% toward rates situated in the interval 24%-27%. Ouachem et al. (Reference OUACHEM, SOLTANE, KALKIL, SOUALAH, BERGHOUTI, ABDESSEMED, MEKAOUSSI and YAKHLEF2009) reported that marl addition (3%) to the broiler chicken diet significantly increased dropping dry matter content by 10.8% and the same effect of marl was observed by Ouachem et al. (Reference OUACHEM, SOLTANE, HADJAR, BAKROUNE, KALKIL, BENSALEM, SMAILI, HADDAD and ABDESSEMED2011b) in broilers fed a diet containing 5% oil acid (22.2% vs. 16.6%).
In laying hens, Öztürk et al. (Reference ÖZTÜRK, ERENER and SARICA1998) reported that clinoptilolite promotes a significant increase in dropping dry matter level by 19.4%. This effect was also observed by Miles and Henry (Reference MILES and HENRY2007) with sodium aluminosilicates or calcium hydrated aluminosilicates. Accordingly, the clay can contribute to improve ambient conditions and welfare in poultry farms. Indeed, it was reported by Öztürk et al. (Reference ÖZTÜRK, ERENER and SARICA1998) that the use of 0.6% and 0.8% clinoptilolite promotes an atmosphere containing less odour and reduces fly problems on farms with abundant wet litter. The zeolite (Çabuk et al., Reference ÇABUK, ALÇIÇEK, BOZKURT and AKKAN2004) can also significantly reduce ammonia concentration (17.25 vs. 21.25 ppm).
Antimicrobial and detoxifying effects
The antibacterial and detoxifying effects of clay are abundant in the literature. It is well known that clay can selectively adhere to bacteria and adsorption is the main interaction between clay and bacteria (Girardeau, Reference GIRARDEAU1987). Montmorillonite was more efficient than antibiotic treatment against the pathogens of diarrhoea (Wu et al., Reference WU, GONG and JIN1999), the magnesium sepiolite richness increases its adsorbent capacity, reduces pathogenic flora and therefore protects the intestinal mucosa (Wolter et al., Reference WOLTER, DUNOYER and SEEGMULLER1990). In vitro, Hu et al. (Reference HU, XU and XIA2005) have shown that montmorillonite adsorbs some bacterial species such as Escherichia coli and Streptococcus aureus.
Moreover, non-metallic minerals naturally present in some clay types (silver, copper, etc.) or introduced by treatment provide a high antimicrobial activity. In pigs (Xia et al., Reference XIA, HU and XU2005) and chickens (Xia et al., Reference XIA, HU and XU2004) montmorillonite enriched with copper was more efficient against pathogenic flora and eliminated Escherichia coli at a rate of 97%. According to Herrera et al. (Reference HERRERA, BURGHARDT and PHILLIPS2000) the treatment of montmorillonite with cetylpyridinium creates a surplus of positive charges on the surface and using scanning electron microscopy, they found that a large amount of Salmonella enteritidis accumulated on the cetylpyridinium exchanged montmorillonite surface. Surplus positive charges of cetylpyridinium-exchanged montmorillonite were probably the most important factor for antibacterial ability. Studies conducted in vivo or in vitro confirmed the ability of calcium and sodium aluminosilicates and natural zeolite to eliminate mycotoxins. According to Miazzo et al. (Reference MIAZZO, ROSA, DE-QUEIROZ, MAGNOLI, CHIACCHIERA, PALACIO, SAENZ, KIKOT, BASALDELLA and DALCEROS2000) the use of sodium zeolite in poultry feed almost completely protects diseased broilers due to the high affinity that exists between aflatoxins and zeolite. Moreover clinoptilolite promotes a hygienic digestive tract by adsorption of aflatoxins (Prvulović et al., Reference PRVULOVIĆ, JOVANOVIĆ, STANIĆ, POPOVIĆ and GRUBOR2007) and improves immunity by an increase (10%) of bursal Fabricius relative weight (Prvulović et al., Reference PRVULOVIĆ, KOGIĆ, LASIĆ and KOSARCIĆ2008).
Conclusions
A great deal of new information is available about the digestive properties, antibacterial, detoxifying and technological effects of clay. This shows that the use of clay insures a healthy gut and promotes growth rate, feed consumption ratio, the digestibility of nutrients, egg production and meat quality. It also appears to provide practical solutions to the problems usually encountered in poultry farms caused by wet droppings and constitute a great food alternative that can substitute drug substances. Finally the positive responses reported should encourage the producers of poultry foods to take into consideration natural additives in order to incorporate them as a component in their various poultry food formulations.
