Management tools to reduce footpad dermatitis in broiler chickens Ingrid C. de Jong, Teun Veldkamp and Jan van Harn

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1 Management tools to reduce footpad dermatitis in broiler chickens Ingrid C. de Jong, Teun Veldkamp and Jan van Harn Wageningen UR Livestock Research, PO Box 65, 8200 AB Lelystad, The Netherlands; Summary Foot pad dermatitis (FPD) is an important welfare indicator in broiler chickens, not only because it is painful, but also because of it reflects other welfare aspects. A few European countries such as Denmark, Sweden and recently The Netherlands started FPD monitoring programmes. It has been shown in Sweden and Denmark that prevalence of FPD in these countries reduced considerably after the start of such monitoring programmes. The prevalence of FPD is strongly linked to litter quality; wet, sticky and caked litter increases the prevalence of FPD. Factors that affect litter quality are discussed in the current paper. Nutrition has an effect through diet composition (e.g. mineral and protein content). In addition, feed form and feed additives play a role in the prevention of FPD. Other factors that affect litter quality are litter type, litter depth, water management, ventilation and temperature in the house and light intensity, light schedules and light colour. In addition, the breed affect FPD susceptibility. In conclusion, research has shown that there are various tools to reduce the incidence in FPD in broiler houses through management. As the cause of FPD is multifactorial, farm specific solutions should be sought. Introduction Footpad dermatitis (FPD), also called pododermatitis or footpad lesion, is a condition of inflammation and necrotic lesions on the plantar surface of the bird s feet (GREENE ET AL., 1985; MICHEL et al., 2012). It is likely that FPD causes pain and therefore has a negative effect on bird welfare and because of its association with litter quality, it also reflects other welfare aspects (HASLAM et al., 2007). FPD is therefore considered to be an important welfare indicator in broilers (EKSTRAND AND CARPENTER, 1998; MELUZZI et al., 2008). Broilers spend a large part of their life being in contact with litter material. If litter conditions are suboptimal there is a considerable risk that broilers will develop FPD (SHEPHERD AND FAIRCHILD, 2010; KYVSGAARD et al., 2013). FPD might coincide with hock burn and/or breast irritation (DE JONG et al., submitted), but of these types of contact dermatitis footpad dermatitis (FPD) is most commonly observed, and prevalence of hock and breast burn in the same birds is usually much lower (e.g., ALLAIN et al., 2009). FPD not only affects broiler welfare but also technical performance. Broilers with severe lesions are thought to be less active and eat and drink less. In addition to causing pain, the lesions can be a gateway for bacteria, which can cause impaired product quality (increased downgrades and rejections at the processing plant) and secondary infections (Staphylococci spp. and E. coli) (SHEPHERD AND FAIRCHILD, 2010). Flocks with a high incidence of FPD will have a lower growth rate, increased downgrades and reduced profitability (CENGIZ et al., 2011; DE JONG et al., submitted). In Asia, chicken feet are a valuable product for human consumption. The increased demand from the Asian markets has made broiler feet an important export commodity for the EU broiler industry. However, only feet without lesions can be exported (SHEPHERD AND FAIRCHILD, 2010). The economic and welfare significance of FPD for the broiler industry is clear. 1

2 FPD is commonly observed in broilers. Several countries started monitoring programs to reduce FPD in broilers. In Sweden, before the welfare monitoring programme started 32% of all broilers were had mild and 6% of all broilers had severe footpad lesions (EKSTRAND et al, 1997). Before Denmark began monitoring FPD about 40% of all broilers had severe footpad lesions. The incidence of FPD in Denmark has decreased sharply since the mandatory monitoring of FPD was introduced in 2002 (KYVSGAARD et al., 2013). In The Netherlands, DE JONG et al (2012) showed that 38.4% of broilers had severe lesions while 26.1% of broilers displayed mild lesions. The same study also showed a clear seasonal influence on the incidence of FPD. Broilers placed between June and August had lower levels of severe FPD and a lower FPD score than broilers placed in March and December (Figure 1; DE JONG et al., 2012). A similar seasonal pattern in FPD has been observed in Denmark and Sweden (e.g., KYVSGAARD ET AL., 2013). Figure 1. Average percentage of broilers per FPD score 0, 1 or 2 (left Y-axis, white, hatched and grey bars respectively) in relation to month of chick placement, and flock FPD score per month (right Y-axis; black line). (source: DE JONG ET AL., 2012). Causes of footpad dermatitis The prevalence of FPD is strongly linked to litter quality; wet, sticky and caked litter increases the prevalence of FPD (SHEPHERD AND FAIRCHILD, 2010; KYVSGAARD et al., 2013). Footpad lesions are sometimes referred to as ammonia burns but this is not completely correct. Several studies have shown that litter moisture alone can cause or induce FPD (MAYNE et al, 2007; YOUSSEF et al., 2008, 2011). FPD increases in severity as litter moisture increases (DE JONG et al., submitted). DE JONG et al. (2012) showed that farm management had a large effect on the prevalence of FPD. As litter quality is influenced by several factors the cause of FPD may be multifactorial. Large differences between individual farms (Figure 2; DE JONG et al., 2012) indicate that farm specific solutions should be sought to prevent wet litter and thus the development of FPD. In this paper the different factors affecting litter quality will be discussed. 2

3 Figure 2. Distribution of farms over FPD scores (FPD score = 100% * ((0.5 * the total number of feet with score 1) + (2 * the total number of feet with score 2)) / the total number of scored feet). Source: DE JONG et al., Nutrition The moisture content of the litter will be affected by the consistency of the faeces and this in turn will be affected by the diet (EKSTRAND AND CARPENTER, 1998). Factors such as minerals, crude protein level and quality, digestibility of raw materials, fat quality and feed form play a role in the consistency of the faeces. Excess levels of sodium and potassium in the feed increase water intake which leads to wet litter problems. Sodium and potassium levels in broiler rations should be balanced to supply adequate levels for broiler performance without encouraging excessive water intake (CENGIZ et al., 2012a). Some feed ingredients, like soy and tapioca, have high potassium levels which, if not accounted for in feed formulation, may lead to an increased water intake and poor litter (FRANCESCH AND BRUFAU, 2004; CENGIZ et al., 2012b). In a study that examined the effects of diet density, two density levels were examined while keeping the feed formulation isocaloric and isonitrogenous. Broilers raised on the low-density diet had significantly less incidence of FPD compared with the high-density diet due to reduced faecal viscosity from lower soybean meal content in the ration (BILGILI et al., 2006), but, in a recent study we found the opposite effect (VAN HARN AND DE JONG, personal communication). High levels of crude protein are also known to have a negative effect on litter quality. High crude protein levels lead to high levels of uric acid being formed in the liver and excreted by the kidneys. This stimulates water uptake, causing wet droppings and thus increasing the risk of FPD. Feeding diets which are unbalanced in crude protein will have the same impact (SHEPHERD AND FAIRCHILD, 2010). Formulating diets on the basis of digestible amino acids will ensure that the nutrient content of the feed matches the needs of the birds and 3

4 reduces the risk of wet litter. The CP requirement of the broiler decreases with age. This means that by the end of each feeding phase there is usually a surplus of CP (current feeding programmes for broilers do not exactly match the reductions in CP requirement with age). Reducing CP levels on a daily basis by adding whole wheat to the diet can help to better match CP supply to requirements. VAN HARN AND VELDKAMP (2005) showed that whole wheat feeding (CP intake was reduced by 13%) resulted in a better litter quality and less severe FPD. However, growth rate and FCR were negatively affected as compared to the control groups. The use of raw materials that have a low digestibility or are particularly high in fibre should be avoided as these will have a negative effect upon gut integrity, excreta of the broilers, and litter quality. The use of non-starch polysaccharide (NSP) enzymes in wheatbased diets is an important tool for improving gut health and controlling litter quality. These enzymes reduce gut viscosity and will lead to drier litter (BEDFORD, 1995; BEDFORD AND MORGAN, 1996; FRANCESCH AND BRUFAU, 2004; SHEPHERD AND FAIRCHILD, 2010). The benefit to broiler performance in terms of live-weight gain and FCR from feeding a good quality crumb and pellet is well documented. A feed that is of poor physical form with high levels of dust not only leads to problems with broiler performance but could also lead to an increased ratio of water to feed intake, which in turn could lead to poor litter conditions and ultimately increase the risk of FPD. Fine meal diets will also increase the risk of FPD, because of the higher water intake and a faster passage of the meal through the digestive tract (VAN HARN, personal communication). Some feed additives may help to reduce the occurrence of FPD in broilers. Clay mineral binders, zinc and biotin are examples of such additives. The addition of clay minerals such as bentonites and zeolites into the diet may improve the gut health since they have a water binding capacity which will result in drier droppings, a better litter quality, and thus decrease the risk of FPD developing (KARAMANLIS et al., 2008). Zinc plays an important role in cell regeneration and is involved in protecting the skin. Zinc deficiency will increase the risk of FPD. Nutrients such as biotin, riboflavin, pantothenic acid, and sulfur amino acids have been shown to affect the structural components of the skin. Biotin improves the formation and development of the upper skin and trials have shown that deficiencies can result in FPD. Therefore it is important to ensure that dietary levels of zinc (min. 100 mg/kg) and biotin (between 0.1 and 0.2 mg/kg) are adequate and that deficiency is avoided. Deficiency of B vitamins should also be avoided for similar reasons. If intestinal problems (e.g. dysbacteriosis) and wet litter problems occur, diluting the diet by feeding extra whole wheat may be benificial (SHEPHERD AND FAIRCHILD, 2010). Litter In Northern European countries (white) wood shavings and chopped wheat straw are the most commonly used litter materials for broilers but other materials such as peat, lignocellulose, rapeseed straw, and maize silage are also used. Litter materials have been compared in various studies. Lignocellulose Pelletino Strohstreugranulat reduced FPD when compared with wood shavings and chopped straw. This reduction in FPD is thought to be due to the higher water binding capacity and the faster water release of lignocellulose (BERK, 2009). FPD was found to be higher on straw as compared to wood shavings (KYVSGAARD et al., 2013) although VAN HARN et al (2009) did not find a difference in severity of FPD between the two litter materials in an experimental setting. In Scandinavian countries, peat is often used as a litter material for broilers. Comparisons of wheat straw, wood shavings and peat have demonstrated that most severe FPD occurs on (chopped) wheat straw, followed by wood 4

5 shavings, with FPD being least severe on peat (PEDERSEN, personal communication). Belgian research has also shown that the use of peat as a litter material for broilers reduces the severity of FPD (DE BAERE, personal communication). The influence of litter material depth on FPD is not always clear which may be due to confounding with litter material and floor type (SHEPHERD AND FAIRCHILD, 2010). Researchers found no difference in FPD incidence between using 1.0 kg or 1.5 kg/m 2 wood shavings or chopped wheat straw. However, using more than 1.5 kg/m 2 of chopped wheat straw was found to increase the severity of FPD (DE BAERE AND ZOONS, personal communication). In contrast, in a study by VAN HARN et al (2009) comparing the use of 1.0 and 2.0 kg/m 2 wood shavings and 1.25 and 2.5 kg/m 2 chopped wheat straw it was concluded that amount of litter material had no influence on broiler performance or incidence of FPD. However, for control of FPD, using a thinner layer of litter material ( kg/m 2 ) may be beneficial as a rough guide 1 kg/m2 equates to a depth of 1 cm for wood shavings, for chopped wheat straw a depth of 1 cm will be achieved with 1.5 kg/m2. Chickens are more likely to peck, scratch and turn the litter if it is not too deep; working of the litter in this way increases aeration and helps to keep the litter drier. A thin layer of litter may also be able to be kept more dry and friable by the air stream from the fans than a thick layer of litter. If a thinner layer of litter material is used, pre-warming of the floor prior to flock placement is essential. Floor temperature prior to placement should be at least 30 C. This will prevent condensation from building up on the concrete floor which will help to maintain litter quality. The litter material should not be spread out until after the target floor temperature has been reached. Spreading the litter prior to pre-warming the house means the concrete floor will warm up more slowly and the risk of condensation forming will be increased (VAN HARN, personal communication). Water management The type of drinking system used has a clear influence on the prevalence of FPD. However, performance is not always optimised with drinking systems that result in a low incidence of FPD (PEDERSEN, personal communication). It is well known that nipple drinkers reduce water spillage compared to drinking systems with drinking cups; this decreases the risk of wet litter and therefore FPD. The effect of drinking systems on the incidence of FPD has been clearly demonstrated by a field study completed in Sweden (EKSTRAND et al, 1997). The use of nipple drinkers with drip cups can improve things even further. Comparison tests of drinking systems in Denmark showed that the use of nipple drinkers with drip cups not only reduces the severity of FPD, but also improves broiler performance compared to the use of nipples without drip cups (PEDERSEN, personal communication). Dutch research (VAN HARN et al, 2009) supports these findings (Table 1). 5

6 Table 1. Effects of drip cups underneath the nipples on some performance indicators, FPD and dry matter percentage of the litter (VAN HARN et al., 2009). Parameter Drinking nipples without cups Drip cups underneath nipple Dry matter litter (%) 48.7 a 52.8 b Feed conversion ratio a b Water: Feed ratio 1.81 a 1.78 b Footpad dermatitis: % no lesions (score 0) 0 4 % mild lesions (score 1) 4 a 18 b % severe lesions (score 2) ab P<0.05. Lowering water pressure may help to reduce the severity of FPD by reducing litter moisture content (VAN HARN et al, 2009). However, any reduction in water pressure must be managed with extreme care, if water pressure becomes too low, broiler performance and welfare will be negatively affected. Recent studies in The Netherlands found evidence that the use of an acidified water additive (which removes and prevents the build up of biofilm) may help to reduce the severity of FPD. However, acidifying water was also found to result in a lower growth rate. Similar results were found when water restriction was implemented, suggesting that the acidification of drinking water at least works via reducing the intake of the birds (Van Harn and De Jong, 2012a). Optimal height of the drinking line will minimise water spillage and thus litter moisture. The drinking line is at the optimal height when the birds (when standing) have to reach, not stretch, for the nipple. Water will then flow from the nipple straight into the birds beak. Other important aspects of water management are regularly checks for leakage and prevention of contamination in the drinking system (VAN HARN, personal communication). Light There is evidence that intermittent lighting schedules are beneficial for reducing the occurrence of FPD. During the light period because birds are more active and scratch more which keeps the litter more friable. An intermittent light schedule has also been shown to improve feed conversion ratio. This leads to dryer and more friable litter, and a potential decrease in FPD (DE BAERE, personal communication; VAN HARN, 2009; Table 2). 6

7 Table 2. Day/night lighting scheme versus intermittent light: effect of light scheme on some performance results, dry matter content of the litter, and footpad lesions (VAN HARN, 2009) Parameter 18 h light:6 h dark schedule Intermittent 1 Feed conversion ratio Dry matter litter (%) 58.6 a 51.2 b Foot pad dermatitis: % no lesions (score 0) % mild lesions (score 1) 42.3 a 35.8 b % severe lesions (score 2) L:6D; 24L:4D:3L:1D:3L:1D:3L:1D:3L:1D ab P<0.05. Low light intensities increase the risk for foot pad dermatitis (VAN HARN, 2009), however, according to the Broiler Directive broilers should be housed in light intensities of at least 20 lux. Although there are no scientific studies on the effect of light distribution, according to farmers practice an even distribution of light in the house is essential to maintain the litter in a good condition. This effect is said to be caused by the even distribution of birds in the house instead of birds clumping together in dark areas when resting (VAN HARN, personal communication). There is little information available on the effect of light colour on the severity of FPD. Studies in Belgium have found evidence that the use of incandescent lighting instead of TL fluorescent lighting decreases the severity of footpad lesions in broiler flocks (DE BAERE AND ZOONS, personal communication). However, incandescent lighting is not allowed according to EU regulation and although new light sources are available (LED) their effect on FPD is not clear yet. When fluorescent light was compared with sodium high pressure light and green/blue PL lighting it was found that light colour has a limited effect on FPD, although there was a tendency for FPD to be lower with green/blue light (RODENBURG et al, 2004). Ventilation and Temperature Control Sufficient ventilation is important from the first days of placement in the broiler house onwards, as FPD may already develop in young broilers. If ventilation rates are too much reduced to save energy costs, humidity in the house will increase which will increase the risk for wet litter and thus FPD (VAN HARN, personal communication). During growth environmental temperature in the broiler house is gradually reduced. Recent research showed that a more rapid decline in temperature may increase the risk to develop FPD (VAN HARN AND DE JONG, 2012; Table 3). 7

8 Table 3. Effect of temperature schedule on FPD in broiler chickens (VAN HARN AND DE JONG, 2012). Parameter Control temperature decline Slow temperature decline Dry matter litter (%) 65.5 a 67.0 b Foot pad dermatitis: No lesions (score 0) Mild lesions (score 1) 23.5 a 11.2 b Severe lesions (score 2) ab P<0.05. The type of heat source used can also affect the occurrence of footpad lesions. Since the start of the footpad monitoring programme in Denmark, the number of broiler houses with open combustion heating has decreased rapidly. Today, almost all houses are equipped with central heating systems (PEDERSEN, personal communication). Houses with central heating systems tend to have better quality litter than those with open combustion heating systems, like air heaters, because central heating systems produce less moisture. The use of heaters, which blow hot air over the litter floor, may also result in drier litter, and thus potentially lead to fewer footpad lesions. Under floor heating may also be beneficial for FPD (VAN HARN AND ELLEN, 2009) because control of floor temperature and, therefore, moisture content of the litter is easier. In addition, it is essential to control the relative humidity in the broiler house. A too high humidity may increase the risk for litter moisture and thus FPD. Relative humidity should be lower than 70%. In cold and/or wet seasons ventilation rates are often reduced to preserve heat. This clear seasonal effect of season on the incidence of FPD has been shown in various studies (e.g., DE JONG et al., 2012, KYVSGAARD et al., 2013; Figure 1). Other factors High stocking densities may increase the risk for FPD, however, negative effects of a high stocking density can be compensated for by improvements in housing and management (e.g. insulation, ventilation, feeder and drinking space) (DAWKINS et al., 2004). In the range of stocking densities fast growing broiler strains are usually housed in Europe, the effect of stocking density on FPD is usually limited (KYVSGAARD et al., 2013). It has been shown that there are breed differences in the susceptibility to develop FPD Several previous studies also indicated effects of breed (EKSTRAND AND KARPENTER, 1998; ALLAIN et al., 2009; DE JONG et al., 2012; KAPELL et al., 2012; VAN HARN, 2009; VAN HARN AND DE JONG, 2012). DE JONG et al. (2012) showed in an epidemiological study that hatchery had a significant effect on the incidence of FPD. It is however unclear yet how this effect is mediated. Factors such as parent stock feed, parent stock age, brooding and hatching conditions may play a role. DE JONG et al. (2012) showed that severity of FPD decreased with slaughter age whereas KYVSGAARD et al. (2013) showed an increase with high age at slaughter. The decrease in FPD with slaughter age as found in DE JONG et al. (2012) might been due to thinning of flocks. In that study it was shown that birds sent to slaughter when thinning a flock displayed less severe FPD than birds from completely depopulated flocks (DE JONG et al., 2012). Conclusions 8

9 Footpad dermatitis is commonly observed in broiler chickens and has a negative effect on bird welfare, performance and thus farmer income. Wet and sticky litter is the major cause of footpad dermatitis. By maintaining a good litter quality broiler producers can not only reduce losses, but also improve bird welfare. Solutions to food pad dermatitis should be farm specific and can for example be sought in nutritional factors, temperature and ventilation in the house, litter type and depth, light schedules and breed. References ALLAIN, V., L. MIRABITO, C. ARNOULD, M. COLAS, S. LE BOUQUIN, C. LUPO, AND V. MICHEL, 2009: Skin lesions in broiler chickens measured at the slaughterhouse: relationships between lesions and between their prevalence and rearing factors. Br. Poult. Sci. 50: BEDFORD, M.R., 1995: Mechanism of action and potential environmental benefits from the use of feed enzymes. Animal Feed Science and Technology BEDFORD, M.R., J.A. MORGAN, 1996: The use of enzymes in poultry diets. World s Poulry Science Journal 52: BERG, C., 1998: Foot-pad dermatitis in broilers and turkeys. Doctoral diss. Dept. of Animal Environment and Health, SLU. Acta Universitatis Agriculturae Sueciae, Sweden. BERK, J., 2009: Effect of litter type on prevalence and severity of pododermatitis in male broilers. Berl Munch Tierarztl Wochenschr., 122: BILGILI, S. F., M. A. ALLEY, J. B. HESS, M. NAGARAJ: 2006: Influence of age and sex on footpad quality and yield in broiler chickens reared on low and high density diets. J. Appl. Poult. Res. 15: CENGIZ, O., J.B. HESS, S.F. BILGILI, 2011: Effect of bedding type and transient wetness on footpad dermatitis in broiler chickens. Journal of Applied Poultry Research, 20, CENGIZ, O., J.B. HESS, J. B., S.F.BILGILI, 2012a: Influence of graded levels of dietary sodium on the development of footpad dermatitis in broiler chickens. Journal of Applied Poultry Research, 21, CENGIZ, O., J.B. HESS, J. B., S.F.BILGILI, 2012b: Feed enzyme supplementation does not ameliorate foot pad dermatitis in broiler chickens fed on a corn-soyabean diet. British Poultry Science, 53, DAWKINS, M. S., C.A. DONNELLY, T.A. JONES, 2004: Chicken welfare is influenced more by housing conditions than by stocking density. Nature, 427, DE JONG, I. C., J. VAN HARN, H. GUNNINK, V.A. HINDLE, V. A., A. LOURENS, 2012: Footpad dermatitis in Dutch broiler flocks: Prevalence and factors of influence. Poultry Science, 91, DE JONG, I.C., H. GUNNINK, J. VAN HARN, submitted: Severe footpad dermatitis in broiler chickens is accompanied by a reduction in overall welfare, technical performance and carcass yield. EKSTRAND, C., B. ALGERS, S. SVEDBERG, 1997: Rearing conditions and foot-pad dermatitis in Swedish broiler chickens. Preventive Veterinary Medicine, 31: EKSTRAND, C., T.E. CARPENTER, 1998: Spatial aspects of foot-pad dermatitis in Swedish broilers. Acta Veterinaria Scandinavica 39: FRANCESCH, M., J. BRUFAU, 2004: Nutritional factors affecting excreta/litter moisture and quality. World s Poultry Science Journal, 60, HASLAM, S. M., T.G. KNOWLES, S.N. BROWN, L.J. WILKINS, S.C. KESTIN, P.D. WARRISS, C.J. NICOL, 2007: Factors affecting the prevalence of foot pad dermatitis, hock burn and breast burn in broiler chicken. British Poultry Science, 48,

10 KAPELL, D. N. R. G., HILL, W. G., NEETESON, A. M., MCADAM, J., KOERHUIS, A. N. M., AVENDANO, S: Genetic parameters of foot-pad dermatitis and body weight in purebred broiler lines in 2 contrasting environments. Poultry Science, 91, KARAMANLIS, X., FORTOMARIS, P., ARSENOS, G., DOSIS, I., PAPAIOANNOU, D., BATZIOS, C., KAMARIANOS, A : The Effect of a Natural Zeolite (Clinoptilolite) on the Performance of Broiler Chickens and the Quality of Their Litter. Asian-Australasian Journal of Animal Sciences, 21, MAYNE, R.K., ELSE, R.W., P.M HOCKING, 2007: High litter moisture alone is sufficient to cause footpad dermatitis in growing turkeys. British Poultry Sience 48, MELUZZI, A., C. FABBRI, E. FOLEGATTI, F. SIRRI. 2008: Survey of chicken rearing conditions in Italy: effects of litter quality and stocking density on productivity, foot dermatitis and carcase injuries. Br. Poult. Sci. 49: MICHEL, V., E. PRAMPART, L. MIRABITO, V. ALLAIN, C. ARNOULD, D. HUONNIC, D., S. LE BOUQUIN, O. ALBARIC, 2012: Histologically-validated footpad dermatitis scoring system for use in chicken processing plants. British Poultry Science, 53, RODENBURG, T.B., J. VAN HARN, J.H. VAN MIDDELKOOP, 2004: Effect van gekleurde verlichting op technischeresultaten en welzijn van vleeskuikens. PraktijkRapport Pluimvee SHEPHERD, E.M., B. D. FAIRCHILD, 2010: Footpad dermatitis in poultry. Poultry Science 89 : VAN HARN, J., 2009: Comparison of four light schedules according to EU directive 2007/43 for broilers. ASG-Report 172, VAN HARN, J., H.H. ELLEN, 2009: Use of floor heating in broilers. ASG Rapport VAN HARN, J., I. DE JONG, 2012a. Zuur is zoet voor zolen. 15dc87b3b582_Zuur%20is%20zoet%20voor%20zolen%201.pdf VAN HARN, J., I. DE JONG, 2012b. Goede maar dure warmte. Pluimveehouderij 42/3. 2 maart 2012, VAN HARN, J., I. DE JONG, 2012c. Ander merk in de stal. Pluimveehouderij 42, 5, VAN HARN, J., I.C. DE JONG EN T. VELDKAMP, 2009: Influence of bedding material, bedding amount, drip cup and reduced water pressure on broiler performance. ASG Rapport VAN HARN, J., T. VELDKAMP, T., Dynamisch voeren belooft veel goed, maar... er valt nog wat te sleutelen! De Pluimveehouderij 35 (8). - p YOUSSEF, I.M.I., C. WESTFAHL, A. BEINEKE AND J.KAMPHUES, 2008: Experimental studies in turkeys on effects of litter quality and feeding on development and intensity of footpad dermatitis. In: Proceedings of the 12th Congress of the European Society of Veterinary and Comparative Nutrition, , Vienna, Austria, 138. YOUSSEF, I.M.I., A. BEINEKE, K. ROHN, J. KAMPHUES, 2011: Effects of Litter Quality (Moisture, Ammonia, Uric Acid) on Development and Severity of Foot Pad Dermatitis in Growing Turkeys. Avian Diseases 55: