Wood products as poultry litter

The WS and SD are commonly used for poultry bedding in many parts of the world. For instance, in France, around 80% of turkeys are raised on WS (Dennery et al., Reference DENNERY, DEZAT and ROUSSET2012). However, other wood products like fresh/dried bark chips/sawdust, ground stumps and hardwood pellets can also be used as litter for poultry, though the performance of birds can be affected depending on the type of wood product used for bedding (Carter et al., Reference CARTER, ALLISON, MILLS and WEST1979; Pearson et al., Reference PEARSON, LEAVENGOOD and REEB1999; Grimes et al., Reference GRIMES, SMITHI and WILLIAMS2002; Bilgili et al., Reference BILGILI, HESS, BLAKE, MACKLIN, SAENMAHAYAK and SIBLEY2009). The WS and SD are by-products of wood milling and sawing process and can be used to make pellets used as fuel for energy production as well as litter (Anonymous, 2018). Pine WS are the most desirable product for poultry bedding, but the cost of this by-product is often high, and hence wood by-products from other, local, hardwood and softwood species can be used (Carter et al., Reference CARTER, ALLISON, MILLS and WEST1979; Malone et al., Reference MALONE, RIDER, COLLIER and JAMES2008; Cressman et al., Reference CRESSMAN, YU, NELSON, MOELLER, LILBURN and ZERBY2010).

Physiochemical properties of wood as bedding material for poultry

Generally, wood products used for bedding have the moisture content of 11 to 14% (oven dried basis) at the time of application (Bilgili et al., Reference BILGILI, HESS, BLAKE, MACKLIN, SAENMAHAYAK and SIBLEY2009), which increases with time depending upon the density, health status and nutrition of the flock, as well as ambient humidity and temperature. The porosity and hygroscopic nature of wood provides a buffering effect against fluctuations of the ambient relative humidity (Kamba, Reference KAMBA1994) by adsorption and desorption of moisture (Pearson et al., Reference PEARSON, LEAVENGOOD and REEB1999; Bilgili et al., Reference BILGILI, HESS, BLAKE, MACKLIN, SAENMAHAYAK and SIBLEY2009) which is considered as an important criteria for an ideal litter material (Garcês et al., Reference GARCÊS, AFONSO, CHILUNDO and JAIROCE2013). Hafeez et al. (Reference HAFEEZ, SUHAIL, DURRANI, JAN, AHMAD, CHAND and REHMAN2009) reported that the water absorbing ability for SD, sand and wheat straw as broiler litter was 246, 152 and 181%, respectively. Meanwhile, SD and WS have a higher water releasing capacity than other commonly used poultry bedding materials, like wheat straw, sugarcane bagasse, sugarcane peat and rice hulls (Farhadi, Reference FARHADI2014). Therefore, in the presence of a proper ventilation system, the wood-based bedding can be a better humidity regulator, if it is relatively dry. The moisture holding and releasing capacity depends on surface area and bulk density of the substrate, therefore, different forms of same material like WS, wood chips and SD can have different moisture regulation abilities (Pearson et al., Reference PEARSON, LEAVENGOOD and REEB1999; Bilgili et al., Reference BILGILI, HESS, BLAKE, MACKLIN, SAENMAHAYAK and SIBLEY2009). Moreover, smaller particle size has a higher surface area and more absorption potential, however, with a decrease in particle size dustiness increases that may result in respiratory concerns for birds and farm workers (Dennery et al., Reference DENNERY, DEZAT and ROUSSET2012; Teixeira et al., Reference TEIXEIRA, DE OLIVEIRA, MENEZES, GOUVEA, TEIXEIRA and GOMES2015; Dorothy et al., Reference DOROTHY, TANUSHA, EDITH, PAYAL, ONNICAH, ROSLYNN and JEEBHAY2018). In addition, the absorbance of moisture is dependent on the structural anatomy of wood varying in various species (Pearson et al., Reference PEARSON, LEAVENGOOD and REEB1999).

The pH of wood material is acidic, ranging from 3.5-6. However, when used as poultry bedding, the pH of wood-based litter continuously rises with the accumulation of droppings. Garcês et al. (Reference GARCÊS, AFONSO, CHILUNDO and JAIROCE2013) reported a rise in pH from 6.3 to 8.9 in birds housed from 0 to 35 days of age, while other studies have observed a change from pH 5.3 to 8 (Benabdeljelil and Ayachi, Reference BENABDELJELIL and AYACHI1996; Kaukonen et al., Reference KAUKONEN, NORRING and VALROS2017). As there are other wood species with lower pH (Fengel and Wegener, Reference FENGEL and WEGENER1983; Munir et al., Reference MUNIR, BELLONCLE, PAILHORIES, AVIAT, FEDERIGHI and EVEILLARD2017a), it would be interesting to see how the pH of wood-based bedding, originating from different species, comparatively changes during various stages of poultry production. Moreover, the treatment of wood litter with some chemicals like lime, bentonite, alum, sulphuric acid and sodium bisulphate has the potential to reduce the pH of the substrate and improve health, welfare and production parameters in broiler chicken which are affected from the high pH (Lopes et al., Reference LOPES, ROLL, LEITE, DAI PRÁ, XAVIER, HERES and VALENTE2013; Taherparvar et al., Reference TAHERPARVAR, SEIDAVI, ASADPOUR, PAYAN-CARREIRA, LAUDADIO and TUFARELLI2016; Sahoo et al., Reference SAHOO, KAUR, SETHI, SHARMA, CHANDRA and CHANDRAHAS2017; Sheffield et al., Reference SHEFFIELD, CRIPPEN and BEIER2018; Williams and Macklin, Reference WILLIAMS and MACKLIN2013; Garrido et al., Reference GARRIDO, SKJERVHEIM, OPPEGAARD and SØRUM2004).

The growth of microbes in litter produces compounds (Broucek, Reference BROUCEK2018), including NH₃, which is colourless and highly irritant, and one of the most abundant pollutant gasses of poultry houses which effects bird and human respiration and health (Taherparvar et al., Reference TAHERPARVAR, SEIDAVI, ASADPOUR, PAYAN-CARREIRA, LAUDADIO and TUFARELLI2016; Cabrera et al., Reference CABRERA, KISSEL, HASSAN, REMA and CASSITY-DUFFEY2018; Mushtaq et al., Reference MUSHTAQ, KASUR, KHAN, KHALID and IQBAL2018). The release of NH₃ from the litter is higher with increasing pH, moisture content and temperature (Dunlop et al., Reference DUNLOP, MCAULEY, BLACKALL and STUETZ2016). Studies have shown that pine WS and SD bedding releases less NH₃ as compared to other litter types (Miles et al., Reference MILES, ROWE and CATHCART2011; Garcês et al., Reference GARCÊS, AFONSO, CHILUNDO and JAIROCE2013). An experimental study by Miles et al. (Reference MILES, ROWE and CATHCART2011) reported that other organic litter forms, like rice hulls and WS, release lower amounts of NH₃ compared to inorganic materials like sand and vermiculite. Additionally, at days three and four of the study, the WS released 50% less NH₃ than rice hulls at their original moisture content. This phenomenon was most likely caused by the acidic pH (Teixeira et al., Reference TEIXEIRA, DE OLIVEIRA, MENEZES, GOUVEA, TEIXEIRA and GOMES2015; Mushtaq et al., Reference MUSHTAQ, KASUR, KHAN, KHALID and IQBAL2018) and moisture regulating characteristics of the litter (Benabdeljelil and Ayachi, Reference BENABDELJELIL and AYACHI1996). Certain compounds in the wood, like terpenes, polyphenols and tannins, can inhibit the number of ammonia-producing organisms (Cabrera et al., Reference CABRERA, KISSEL, HASSAN, REMA and CASSITY-DUFFEY2018). It has been reported that the type of litter affects odour emissions from poultry litter (Dunlop et al., Reference DUNLOP, MCAULEY, BLACKALL and STUETZ2016), although research is needed to identify the relationship between wood species used as bedding and NH₃ production.

Depth of litter substrate

The thickness of bedding affects the health and welfare parameters for birds and the housing environment (Cohuo-Colli et al., Reference COHUO-COLLI, SALINAS-RUÍZ, HERNÁNDEZ-CÁZARES, HIDALGO-CONTRERAS, BRITO-DAMIÁN and VELASCO-VELASCO2018). The optimal level of bedding thickness depends on the production system, climatic conditions and characteristics of the material. Monira et al. (Reference MONIRA, ISLAM, ALAM and WAHID2003) reported that there was no significant difference in broiler performance when they were raised on SD bedding with different thickness (2, 3, 4 and 5 cm) in Bangladesh, when average ambient temperature and humidity during the study were 27.8°C and 76.7% respectively. Contrarily, it has been reported that the welfare and production issues were ameliorated by increasing the thickness (4, 8, 12 and 16 cm) of SD bedding for yellow broilers in China (Shao et al., Reference SHAO, HE, LU, WANG, CHANG, SHI and BING2015) and 2.5, 7.6 and 12.7 cm using pine WS for broilers in US (Shepherd et al., Reference SHEPHERD, FAIRCHILD and RITZ2017). Similar results have been reported when three different broiler strains were grown on WS bedding with a depth of 3 and 6 cm in Nigeria (Sogunle et al., Reference SOGUNLE, OGUNJIMI and FANIMO2006). Obviously, thicker bedding requires more litter material which means a higher economic investment. Therefore, if the requirements of welfare and optimum production are already met, it is of no use to increase the quantity of material. For instance, the depth of 2 cm vs. 20 cm of WS bedding showed no a great difference in terms of dustbathing behaviour in laying hens (Moesta et al., Reference MOESTA, KNIERIM, BRIESE and HARTUNG2008). Even shallower WS or straw bedding (≤5 cm) has been reported to have a positive effect on footpad lesion scores from broilers in Sweden (Ekstrand et al., Reference EKSTRAND, ALGERS and SVEDBERG1997).

In general, 5-7 cm thickness for WS and SD litter is used for broilers in experimental designs (Nowaczewski et al., Reference NOWACZEWSKI, ROSIŃSKI, MARKIEWICZ and KONTECKA2011; Ramadan et al., Reference RAMADAN, MAHBOUB, HELAL and GAAFAR2013; Garcês et al., Reference GARCÊS, AFONSO, CHILUNDO and JAIROCE2013). However, it should be noted that, with the passage of time, litter absorbs moisture from poultry faeces and becomes packed down, ultimately decreasing bedding thickness (Kaukonen et al., Reference KAUKONEN, NORRING and VALROS2017). During poultry production, maintaining the thickness and moisture of bedding may require the addition of fresh material. Martrenchar et al. (Reference MARTRENCHAR, BOILLETOT, HUONNIC and POL2002) found the addition of extra bedding material for female turkeys was a risk factor for footpad lesions. However, it was not clear from this study if this occurs for all wood-based bedding. Contrarily, a previous study reported that footpad lesion scores of turkey poults was better on WS than straw, and improved with further addition of bedding material during the production period (Ekstrand and Algers, Reference EKSTRAND and ALGERS1997).

Welfare and behaviour of poultry rearing on wood-based litter

Poultry have a preference for some bedding materials to perform their natural behaviour. For instance, broiler chicks prefer resting on WS, standing on straw, dustbathing on sand, walking, pecking and scratching on rice hulls and paper (Toghyani et al., Reference TOGHYANI, GHEISARI, MODARESI, TABEIDIAN and TOGHYANI2010; Ramadan et al., Reference RAMADAN, MAHBOUB, HELAL and GAAFAR2013; Villagrá et al., Reference VILLAGRÁ, OLIVAS, ALTHAUS, GÓMEZ, LAINEZ and TORRES2014) and foraging on food particles in the litter (Scholz et al., Reference SCHOLZ, URSELMANS, KJAER and SCHRADER2010). On the other hand, if the birds are not given the choice of bedding materials, they perform their natural behaviour on wood substrates without compromising production (Shields et al., Reference SHIELDS, GARNER and MENCH2005; Toghyani et al., Reference TOGHYANI, GHEISARI, MODARESI, TABEIDIAN and TOGHYANI2010). In terms of dustbathing, birds prefer smaller particle size and the depth of litter seems to have a smaller role (Moesta et al., Reference MOESTA, KNIERIM, BRIESE and HARTUNG2008). The frequency and time of application of litter and density of birds influences behaviour. For example, laying hens reared in enriched cages seemed to do more dustbathing when the density of birds is lower and litter (e.g. food particles on a synthetic mat) was added twice per day around mid-day (Lee et al., Reference LEE, LOUTON, SCHWARZER, RAUCH, PROBST, SHAO, SCHMIDT, ERHARD and BERGMANN2016). Such behaviours seem to decrease with increasing the age of broiler birds on a wood based litter as compared to sand bedding (Shields et al., Reference SHIELDS, GARNER and MENCH2005).

The prevalence of locomotion problems in meat-type birds has become an important welfare and economic issue. Although leg problems are multifactorial in origin, it is likely that bedding material has a significant role in the extent of damage. Almeida et al. (2010) reported that the broilers raised on clean WS had fewer leg problems as compared to those raised on rice husks. In general, the most important wet litter related leg problem is foot pad dermatitis (FPD), otherwise known as pododermatitis and contact dermatitis, which is characterised by inflammation and necrotic lesions, ranging from superficial to deep, on the plantar surface of the footpads and toes of birds (Shepherd and Fairchild, Reference SHEPHERD and FAIRCHILD2010). Litter wetness can aggravate foot problems, and this factor alone has been seen to cause FPD in growing turkeys (Mayne et al., Reference MAYNE, ELSE and HOCKING2007). Materials like chopped straw have greater moisture retention than wood products (Bilgili et al., Reference BILGILI, HESS, BLAKE, MACKLIN, SAENMAHAYAK and SIBLEY2009), however, birds grown on WS show 80% better foot health than those who are kept on whole or chopped wheat straw (Nowaczewski et al., Reference NOWACZEWSKI, ROSIŃSKI, MARKIEWICZ and KONTECKA2011). Moreover, shredded paper is a good absorbant, although broilers raised on WS showed comparatively better FPD health and breast and thigh feathering (Terči et al., 2015). The lower severity of FPD lesions on wood can be attributed its ability to stop the growth of Staphylococcus spp. bacteria (Pailhoriès et al., Reference PAILHORIÈS, MUNIR, AVIAT, FEDERIGHI, BELLONCLE and EVEILLARD2017), which is known to aggravate lesions (Shepherd and Fairchild, Reference SHEPHERD and FAIRCHILD2010). However, Kaukonen et al. (Reference KAUKONEN, NORRING and VALROS2017) reported that broiler chickens had better foot lesion scores when raised on peat compared to WS, even though the moisture content, pH and NH₃ generation was similar in both types of litter.

Performance of poultry housed on wood-based litter

The type of bedding material can influence the performance of birds (Dennery et al., Reference DENNERY, DEZAT and ROUSSET2012). Various scientific reports have shown that the birds have better weight gain, FCR and carcass quality when they are kept on wood-based beddings (Benabdeljelil and Ayachi, Reference BENABDELJELIL and AYACHI1996; Ramadan et al., Reference RAMADAN, MAHBOUB, HELAL and GAAFAR2013; Taherparvar et al., Reference TAHERPARVAR, SEIDAVI, ASADPOUR, PAYAN-CARREIRA, LAUDADIO and TUFARELLI2016). A study conducted in Bangladesh reported that the broilers raised on SD had 5-6% higher survivability, live weight gain and feed consumption as compared to those kept on rice husk, sugarcane bagasse and wheat straw beddings (Monira et al., Reference MONIRA, ISLAM, ALAM and WAHID2003). Another study from Poland demonstrated that, under optimum temperature and moisture conditions, broilers kept on WS had better EPEF index (European Production Efficiency Factor), weight gain (5%), live body weight (6%), foot health (80%), FCR (7%), haemoglobin saturation and lower mortality rates, compared to those kept on the whole or chopped wheat straw (Nowaczewski et al., Reference NOWACZEWSKI, ROSIŃSKI, MARKIEWICZ and KONTECKA2011). Other studies have shown that wood litter performed equally well as other bedding types. For example, a study from Morocco observed no differences in body weight gain, FCR, feed consumption or water intake in cockerels. However, the moisture content, pH and litter cleanliness were better for WS and SD than whole/ground wheat straw and rice hulls/straw, except for dustiness (Benabdeljelil and Ayachi, Reference BENABDELJELIL and AYACHI1996). Similarly, other studies from Turkey, Iran, Pakistan and India have observed that bedding types including WS, SD, sand, paper, rice husk and wheat straw, had no effect on feed consumption, weight gain, live weight, FCR or production index of broilers (Hafeez et al., Reference HAFEEZ, SUHAIL, DURRANI, JAN, AHMAD, CHAND and REHMAN2009; Taherparvar et al., Reference TAHERPARVAR, SEIDAVI, ASADPOUR, PAYAN-CARREIRA, LAUDADIO and TUFARELLI2016; Sigroha et al., Reference SIGROHA, BIDHAN, YADAV, SIHAG and MALIK2017). Contrarily, different broiler strains had better body weight gain, feed consumption and FCR on sand, wheat straw, rice husk and corn cobs than on WS litter (Mahmoud et al., Reference MAHMOUD, SOLIMAN, EL-DEEN and SEBAI2014; Irfan et al., Reference IRFAN, MEHMOOD, HUSSAIN, SAIMA, SHAHEEN, AHMAD and ZIA2017).

Such variations can be ascribed to the fact that there are multiple factors linked to birds (type and age), litter (type, moisture, pH, and particle size) and climatic conditions, which may determine the effects of litter on production and performance. Another possibility can be attributed to the consumption of litter by birds. In general, the birds eat litter up to 4% of their feed (Musa et al., Reference MUSA, SA`IDU, KALTUNGO, ABUBAKAR and WAKAWA2012). It has been reported that, in comparison to the sand and straw, the ingestion of wood substrates improves the gizzard weight, slaughter weight and the physiology of birds (Ramadan et al., Reference RAMADAN, MAHBOUB, HELAL and GAAFAR2013; Skånseng et al., Reference SKÅNSENG, SVIHUS, RUDI, TROSVIK and MOEN2013). On the other hand, intake of contaminated litter can alter the microbiological profile of intestine and accordingly, the bird's health (Skånseng et al., Reference SKÅNSENG, SVIHUS, RUDI, TROSVIK and MOEN2013).

The microbiological perspective of wood-based bedding

Wood has antimicrobial properties due to the chemical compounds it contains, its complex physical structure and hygroscopic nature (Milling et al., Reference MILLING, SMALLA, KEHR and WULF2005b). Factors determining the survival of microbes in interaction with wood products is related to wood type (species, part, granulometry, processing method, initial moisture level, biochemical composition and age) microbes (type, inoculation dose and presence of other competitive microbes) and environment (humidity, temperature, time of contact) (Laireiter et al., Reference LAIREITER, SCHNABEL, KÖCK, STALZER, PETUTSCHNIGG, OOSTINGH and HELL2013; Pailhoriès et al., Reference PAILHORIÈS, MUNIR, AVIAT, FEDERIGHI, BELLONCLE and EVEILLARD2017; Munir et al., Reference MUNIR, BELLONCLE, PAILHORIES, AVIAT, FEDERIGHI and EVEILLARD2017a).

The chemical metabolites present in wood have an antimicrobial effect and many have been isolated and used against different pathogens, either in form of medicines or antiseptics (Aviat et al., Reference AVIAT, GERHARDS, RODRIGUEZ-JEREZ, MICHEL, BAYON, ISMAIL and FEDERIGHI2016; Vainio-Kaila et al., Reference VAINIO-KAILA, ZHANG, HÄNNINEN, KYYHKYNEN, JOHANSSON, WILLFÖR, ÖSTERBERG, SIITONEN and RAUTKARI2017). In any case, these extractives can diffuse out from small wood particles and kill the microbes (Munir et al., Reference MUNIR, BELLONCLE, PAILHORIES, AVIAT, FEDERIGHI and EVEILLARD2017a; Reference MUNIR, BELLONCLE, PAILHORIES, AVIAT, FEDERIGHI and EVEILLARD2017b) which ultimately reduces the burden of many infections in poultry (Lopes et al., Reference LOPES, ROLL, LEITE, DAI PRÁ, XAVIER, HERES and VALENTE2013; Umar et al., Reference UMAR, NAWAZ, SHAHZAD, MUNIR and ALI ABDULLAH SHAH2015b). A study on the survival of turkey Coryza spp. on faeces, glass, dust, aluminium and wood, at varying temperatures (10, 20, 30 and 40^○C) and relative humidities (32, 58 and 78%), found that the bacteria showed the shortest survival periods on wood, probably due to the presence of bactericidal chemicals (Cimiotti et al., Reference CIMIOTTI, GLUNDER and HINZ1982).

The role of the physical properties of wood as an antimicrobial can be justified by the complex interaction with microbes. First of all, the physical arrangement of wood does not allow many types of microbes to move freely because they are stuck in the porous structure of this dead organic material (Carpentier, Reference CARPENTIER1997; Ismail et al., Reference ISMAIL, AVIAT, GAY-PERRET, LE BAYON, FEDERIGHI and MICHEL2017). Correspondingly, less microbes are recovered from wood materials, indicating a lower risk of cross-contamination and infection (Ismail et al., Reference ISMAIL, AVIAT, MICHEL, LE BAYON, GAY-PERRET, KUTNIK and FEDERIGHI2013). Another reason, as mentioned above, is that wood-based litter absorbs moisture, causing the microbes to dry out and die off (Aviat et al., Reference AVIAT, GERHARDS, RODRIGUEZ-JEREZ, MICHEL, BAYON, ISMAIL and FEDERIGHI2016). This can be useful in early stages of poultry production when moisture of wood-based bedding is low. However, more research is needed to understand the relationship between water activity of different bedding materials and microbial presence at later stages of production. Milling et al. (Reference MILLING, KEHR, WULF and SMALLA2005a) studied the survival of Enterococcus faecium and Escherichia coli on SD of pine, oak, spruce, maple, poplar, beech and larch and plastic chips. Agar incubation and molecular methods were employed in parallel and microbial quantities (microbe number and the amount of DNA) decreased fastest on pine and oak as compared to other tested wood species and the polyethylene. The authors believed that the antimicrobial effect was induced by the hygroscopic properties of wood. In another study, chicken manure was incubated with plastic pieces and SD to evaluate microbial survival when in contact with these materials and demonstrated that wood SD had antimicrobial properties (Milling et al., Reference MILLING, SMALLA, KEHR and WULF2005b). Even avian influenza virus and avian metapneumovirus have been reported to survive for a shorter time on wood surfaces when compared to other fomites surfaces, including plastic, steel, latex, rubber, polyester and fabric (Tiwari et al., Reference TIWARI, PATNAYAK, CHANDER, PARSAD and GOYAL2006).

The low pH of wood (ranging from pH 3.5 to 6) may be considered as an additional factor limiting microbial growth (Munir et al., Reference MUNIR, BELLONCLE, PAILHORIES, AVIAT, FEDERIGHI and EVEILLARD2017a). For example, oak has very acidic pH of 3.5 - 4 and shows a strong antimicrobial activity against many microbes, even against some antibiotic-resistant strains (Pailhoriès et al., Reference PAILHORIÈS, MUNIR, AVIAT, FEDERIGHI, BELLONCLE and EVEILLARD2017). Likewise, NH₃ production is lower in acidic environments because the secreting organisms are not freely working in such conditions, especially when acidophilic organisms are encouraged and create a competitive environment (Taherparvar et al., Reference TAHERPARVAR, SEIDAVI, ASADPOUR, PAYAN-CARREIRA, LAUDADIO and TUFARELLI2016; Garcês et al., Reference GARCÊS, AFONSO, CHILUNDO and JAIROCE2017; Ismail et al., Reference ISMAIL, AVIAT, GAY-PERRET, LE BAYON, FEDERIGHI and MICHEL2017). However, with the passage of time, pH of wood-based litter rises and acidic treatments of litter with alum, sulphuric or other acids seem to reduce microbial load, even in later stages of production (Sheffield et al., Reference SHEFFIELD, CRIPPEN and BEIER2018; Sahoo et al., Reference SAHOO, KAUR, SETHI, SHARMA, CHANDRA and CHANDRAHAS2017; Williams and Macklin, Reference WILLIAMS and MACKLIN2013; Garrido et al., Reference GARRIDO, SKJERVHEIM, OPPEGAARD and SØRUM2004).

The type of wood alters its interaction with microbes, hence, the same microbe may have different survival rates on wood chips, WS or SD (Carter et al., Reference CARTER, ALLISON, MILLS and WEST1979). This might be correlated with the different properties of wood, such as moisture content, surface area, cutting dimensions, species, quality and grade of wood product (Milling et al., Reference MILLING, KEHR, WULF and SMALLA2005a; Laireiter et al., Reference LAIREITER, SCHNABEL, KÖCK, STALZER, PETUTSCHNIGG, OOSTINGH and HELL2013; Pailhoriès et al., Reference PAILHORIÈS, MUNIR, AVIAT, FEDERIGHI, BELLONCLE and EVEILLARD2017; Munir et al., Reference MUNIR, BELLONCLE, PAILHORIES, AVIAT, FEDERIGHI and EVEILLARD2017b). An in vitro study by Pailhoriès et al. (Reference PAILHORIÈS, MUNIR, AVIAT, FEDERIGHI, BELLONCLE and EVEILLARD2017) showed that wooden discs (comparable to the size of chips used as poultry litter) cut in transversal direction were more efficient against bacteria than those cut in the tangential plane. In a similar study, wood discs obtained from bark had more antibacterial activity than those made from sapwood and heartwood of a tree (Laireiter et al., Reference LAIREITER, SCHNABEL, KÖCK, STALZER, PETUTSCHNIGG, OOSTINGH and HELL2013).

It has been seen that using a different type of litter system can influence the colonisation of bacteria in the intestine of birds (Taherparvar et al., Reference TAHERPARVAR, SEIDAVI, ASADPOUR, PAYAN-CARREIRA, LAUDADIO and TUFARELLI2016). Using fresh pine WS as litter was shown to be more favourable for beneficial bacteria than that of reused litter (Cressman et al., Reference CRESSMAN, YU, NELSON, MOELLER, LILBURN and ZERBY2010). Both softwoods and hardwoods showed the effect on intestinal microflora when used as bedding material (Torok et al., Reference TOROK, HUGHES, OPHEL-KELLER, ALI and MACALPINE2009). Such positive fluctuations in intestinal microbial populations can be attributed to changes in immune status and physiology of birds.

On the risky side of microbial interaction with wood-based litter, the survival of microbes may pose a health risk to birds and public health (Dorothy et al., Reference DOROTHY, TANUSHA, EDITH, PAYAL, ONNICAH, ROSLYNN and JEEBHAY2018; Sheffield et al., Reference SHEFFIELD, CRIPPEN and BEIER2018). Macklin et al. (Reference MACKLIN, HESS, BILGILI and NORTON2005) observed that microbial counts were slightly higher on WS than sand. However, this study was based on CFU/g of litter sample, which is not a good representation of figures because of the bulk density of WS is much lower than sand and so the weight of litter required of each material in a given situation was very different. Viegas et al. (Reference VIEGAS, CAROLINO, MALTA-VACAS, SABINO, VIEGAS and VERÍSSIMO2012) observed that pine WS had lower fungal contamination compared to straw bedding, whereas the hardwood WS used were most contaminated. These authors believed that the high level of fungal contamination of hardwood WS may have occurred due to improper storage or because of contamination from the original source.

Salmonella spp. is an important zoonotic agent (Sheffield et al., Reference SHEFFIELD, CRIPPEN and BEIER2018) which is more prevalent on WS compared to peat, chopped straw and silage bedding for poultry (Völkel et al., Reference VÖLKEL, SCHMITZ, MOORS, GAULY and CZERNY2011). In this scenario, the practice of pre-harvest feed withdrawal encourages the birds to consume more litter contaminated with Salmonella and Campylobacter spp., leading to infection of the gastrointestinal tract (Skånseng et al., Reference SKÅNSENG, SVIHUS, RUDI, TROSVIK and MOEN2013; Sheffield et al., Reference SHEFFIELD, CRIPPEN and BEIER2018). However, raising the birds on clean WS or chemical treatment of bedding can reduce Salmonella and Campylobacter spp. on broiler carcasses and litter (Li et al., Reference LI, LEMONAKIS, GLOVER, MORITZ and SHEN2017; Sheffield et al., Reference SHEFFIELD, CRIPPEN and BEIER2018). Moreover, it remains to be seen if the survival of these microbes is affected by the type of wood species and form of wood product as bedding.