Introduction
The selection of broiler breeders for increased growth rate has resulted in an increased appetite by modulating central and peripheral mechanisms of hunger regulation. As a consequence, ad libitum feed intake in broiler breeders causes obesity resulting in lameness and premature death (Mench, Reference MENCH2002).
To ensure health and reproductive capacity of the birds, broiler breeders are feed restricted during the rearing period (Mench, Reference MENCH2002), and, to a lesser extent, during the production period (De Jong and Jones, Reference DE JONG and JONES2006). Although restricted feeding improves health and, hence, bird welfare, there is a problem in that the birds are chronically hungry and suffer from frustration of the feeding motivation which has a negative effect on bird welfare (e.g., Hocking et al., Reference HOCKING, MAXWELL and MITCHELL1993; Savory et al., Reference SAVORY, MAROS and RUTTER1993; De Jong et al., Reference DE JONG, VAN VOORST, EHLHARDT and BLOKHUIS2002; D'eath et al., Reference D'EATH, TOLKAMP, KYRIAZAKIS and LAWRENCE2009).
The restricted feeding of broiler breeders during rearing is generally considered as one of the most important issues with respect to broiler breeder welfare (Mench, Reference MENCH2002; De Jong and Guemene, Reference DE JONG and GUEMENE2011). The aim of the current paper is to give an overview of the relation between feed restriction and broiler breeder welfare and the role of feed (nutritional aspects as well as feeding management) as a possible tool to improve broiler breeder welfare.
Growth potential, feed efficiency and reproduction
Over the past 30 years, the growth potential of commercial broilers increased drastically (Renema et al., Reference RENEMA, RUSTAD and ROBINSON2007).Modern broiler strains grow 4.6 times the rate of a 1957 strain (Havenstein et al., Reference HAVENSTEIN, FERKET and QURESHI2003b). According to Havenstein et al. (Reference HAVENSTEIN, FERKET and QURESHI2003a) the six-fold increase in carcass yield in 2001 birds fed a modern diet compared to 1957 birds fed a 1950's diet is 85-90% due to genetics and 10-15% due to nutritional changes. This extreme increase in carcass yield is the result of increased growth potential of the broiler breeders.
Ad libitum fed standard broiler breeder chicks from 11 to 24 wk of age consume 48 g/d more feed (210 vs. 162 g/d) compared to restricted fed chicks, resulting in a dramatic increase (5.4 vs. 2.2 kg) in bodyweight (Heck et al., Reference HECK, ONAGBESAN, TONA, METAYER, PUTTERFLAM, JEGO, TREVIDY, DECUYPERE, WILLIAMS, PICARD and BRUGGEMAN2004). As a result of this increased growth potential, to prevent serious health problems and to maintain a good production of eggs and chicks, broiler breeders have to be fed restrictedly at a young age.
Over the past 30 years, broiler breeder bodyweight targets have undergone change, although the degree of change is small compared to the large increases in growth potential. As a consequence, the ratio in bodyweight between broiler breeders to broilers at six weeks of age in female Hubbard birds decreased over the period from 1979 to 2005 from 52% to 27% (Renema et al., Reference RENEMA, RUSTAD and ROBINSON2007). To realise target weights, the degree of feed restriction for maintaining broiler breeder bodyweight targets has had to be continuously increased.
Decades of selection for meat production traits have impaired the reproductive abilities of broiler breeders (Siegel and Dunnington, Reference SIEGEL and DUNNINGTON1985). Ad libitum feeding of broiler breeders results in low egg production which has been associated with multiple ovulations caused by the presence of more than one hierarchy of ovarian yellow follicles during early lay (Hocking et al., Reference HOCKING, WADDINGTON, WALKER and GILBERT1989). Feed intake during rearing and body weight at sexual maturity seem directly correlated with the number of yellow follicles in the ovary (Hocking, Reference HOCKING1993). Therefore, feed restriction is necessary to obtain an acceptable reproductive performance in broiler breeders.
Restricting the quantity of feed during rearing, breeding period or both significantly reduces the weight of the ovary and the number of large follicles at sexual maturity, the incidence of erratic ovipositions, defective eggs and multiple ovulations (Yu et al., Reference YU, ROBINSON and ROBBLEE1992; Hocking, Reference HOCKING1993) and results in a lower mortality of the hens (Katanbaf et al., Reference KATANBAF, DUNNINGTON and SIEGEL1989). In addition, restricting the quantity of feed during the rearing period has several repercussions on the different levels of the hypothalamic—pituitary—gonadal axis (Bruggeman et al., Reference BRUGGEMAN, ONAGBESAN, VANMONTFORT, BERGHMAN, VERHOEVEN and DECUYPERE1998).
Heck et al. (Reference HECK, ONAGBESAN, TONA, METAYER, PUTTERFLAM, JEGO, TREVIDY, DECUYPERE, WILLIAMS, PICARD and BRUGGEMAN2004) compared ad libitum and restricted fed broiler breeders. Restricted feeding delayed sexual maturity by six weeks and the peak of lay by seven to eight weeks. The maximal laying rates, however, were 83.3% for restricted fed hens, versus 57% for ad libitum fed hens. The restricted fed birds showed a good persistency, whereas the rate of lay of the ad libitum fed group rapidly decreased to only 20 to 25% after peak production. A significantly higher proportion of eggs were laid with multiple-yolks and soft or broken shells by the ad libitum fed hens than by the restricted fed hens, resulting in a reduced percentage of settable eggs in the former group. Mortality was dramatically increased to 40.4% in the ad libitum fed hens versus 5.6% in the restricted fed hens.
Consequences of feed restriction for bird welfare during rearing and production
The interaction between nutritional and reproductive traits is complex and continually changing with the introduction of new genetics. It is necessary to provide enough, but not excessive, nutrient intake for maximum egg production. Female broiler breeders that are not restricted-fed have decreased total egg output (Robinson et al., Reference ROBINSON, ROBINSON and SCOTT1991; Yu et al., 1992b) and adverse changes in ovarian function leading to multiple ovulation and poor fertility (Hocking, Reference HOCKING2009). However, too severe feed restriction can result in a decrease in egg production as well (Wilson and Harms, Reference WILSON and HARMS1986).
With respect to bird welfare, unrestricted feeding in broiler breeders leads to heavy body weights, associated with severe pathological conditions such as lameness and premature death, which have a negative effect on bird welfare (Mench, Reference MENCH2002). However, despite the positive effect on health and reproductive efficiency when adult, there is substantial evidence that the applied feed restriction, especially during rearing, has negative effects on broiler breeder welfare. In commercially applied restriction programs, feed intake is restricted to about 25-33% of the intake of ad libitum fed birds of the same age during rearing (Savory and Kostal, Reference SAVORY and KOSTAL1996; De Jong et al., Reference DE JONG, VAN VOORST, EHLHARDT and BLOKHUIS2002) and it is restricted to 50-90% of the ad libitum intake of hens at the same age when they are in lay (Bruggeman et al., Reference BRUGGEMAN, ONAGBESAN, D'HONDT, BUYS, SAFI, VANMONTFORT, BERGHMAN, VANDESANDE and DECUYPERE1999). The restricted feeding regime results in a reduction of body weight of adult birds by approximately 40-50% of that of unrestricted fed birds (Katanbaf et al., Reference KATANBAF, DUNNINGTON and SIEGEL1989; Savory et al., Reference SAVORY, HOCKING, MANN and MAXWELL1996).
Several studies have shown that feed restricted broiler breeders show behaviour indicative of frustration, boredom and hunger, including stereotypic object pecking, over-drinking and pacing (De Jong and Jones, Reference DE JONG and JONES2006; D'eath et al., Reference D'EATH, TOLKAMP, KYRIAZAKIS and LAWRENCE2009). Stereotypic pecking is mainly observed after feeding, and may be directed at the litter, drinker, (empty) feeder, the walls of the pen or to other birds (Kostal et al., Reference KOSTAL, SAVORY and HUGHES1992; Savory and Maros, Reference SAVORY and MAROS1993; Savory and Kostal, Reference SAVORY and KOSTAL1996; De Jong et al., Reference DE JONG, VAN VOORST, EHLHARDT and BLOKHUIS2002; Hocking et al., Reference HOCKING, MAXWELL, ROBERTSON and MITCHELL2002). Restricted fed birds spend a considerable time exhibiting stereotypic pecking. For example, at eight weeks of age it has been observed that restricted fed broiler breeders spent about half of the observed time on stereotypic pecking at the empty feeder (De Jong et al., Reference DE JONG, VAN VOORST, EHLHARDT and BLOKHUIS2002).
To prevent over-drinking, water intake is often restricted in practice. Pacing is mainly apparent before the birds are fed (Savory and Maros, Reference SAVORY and MAROS1993). The amounts of stereotypic pecking and pacing are positively correlated with the level of feed restriction (Savory and Maros, Reference SAVORY and MAROS1993; Hocking et al., Reference HOCKING, MAXWELL and MITCHELL1996). It has been reported that aggressive pecking, caused by feeding competition, is increasing in prevalence in commercial flocks of broiler breeders, which is also detrimental for bird welfare (Jones et al., Reference JONES, ZACZEK, MCLEOD and HOCKING2004). In addition, when feed is supplied, birds may run over each other to reach the food which may cause skin damage (scratches, wounds) which has negative consequences for bird welfare (De Jong, unpublished observation).
It has been shown that feed-restricted broiler breeders have a very high feed intake motivation. Using operant conditioning, it has been shown that the motivation to eat in broiler breeders kept under commercial restriction programmes was approximately four times as high as that of unrestricted fed birds that had been subjected to food withdrawal for 72 h (Savory et al., Reference SAVORY, MAROS and RUTTER1993). Compensatory feed intake (where previously restricted animals are allowed unrestricted access to food) of broiler breeder pullets is linearly related to the level of restriction they were subjected to (De Jong et al., Reference DE JONG, VAN VOORST and BLOKHUIS2003). Broiler breeder pullets fed commercial restriction programmes had a three times higher relative compensatory feed intake (feed intake/g metabolic weight) as compared to unrestricted fed birds. In addition, this very high level of feed consumption was maintained for several days and was still two times higher than the intake during the feed restriction period after three weeks (De Jong et al., Reference DE JONG, VAN VOORST and BLOKHUIS2003).
Feed-restricted broiler breeders may show physiological signs of stress. Those fed at commercially applied restriction levels had chronically elevated plasma corticosterone levels (Hocking et al., Reference HOCKING, MAXWELL and MITCHELL1996; Savory and Mann, Reference SAVORY and MANN1997; De Jong et al., Reference DE JONG, VAN VOORST, EHLHARDT and BLOKHUIS2002). However, it is unclear yet whether these elevated plasma corticosterone levels reflect psychological stress, metabolic effects resulting from feed restriction or both (De Jong et al., Reference DE JONG, VAN VOORST and BLOKHUIS2003). Some studies have reported increased H/L ratios, indicative of chronic stress (Maxwell, Reference MAXWELL1993), in restricted fed birds (Hocking et al., Reference HOCKING, MAXWELL and MITCHELL1993; Savory et al., Reference SAVORY, MAROS and RUTTER1993; Hocking et al., Reference HOCKING, MAXWELL and MITCHELL1996). However, other studies could not find increased H/L ratios in restricted fed birds (Savory et al., Reference SAVORY, HOCKING, MANN and MAXWELL1996; Hocking et al., Reference HOCKING, MAXWELL and MITCHELL1999; De Jong et al., Reference DE JONG, VAN VOORST, EHLHARDT and BLOKHUIS2002). Thus, although there is substantial evidence from behavioural studies that feed restriction is a potent stressor, there is poorer agreement between different studies when physiological indicators of hunger are considered (D'eath et al., Reference D'EATH, TOLKAMP, KYRIAZAKIS and LAWRENCE2009).
Physiology of hunger feeling in broiler breeders
Jungle fowl spend 60% of their time on eating and foraging (Dawkins, Reference DAWKINS1989). Modern laying hens still spend up to 52% of their time on feeding related behaviour(Aerni et al., Reference AERNI, EL-LETHEY and WECHSLER2000), showing that the behaviour of the modern bird is very similar to that of its ancestor (Dawkins, Reference DAWKINS1989). As a consequence of restricted feeding, broiler breeder pullets, however, spend less than 15 min per day on eating (Savory and Maros, Reference SAVORY and MAROS1993; Savory et al., Reference SAVORY, HOCKING, MANN and MAXWELL1996; De Jong et al., Reference DE JONG, VAN VOORST, EHLHARDT and BLOKHUIS2002), indicating that these types of birds are not fulfilling their natural time budgets. The signs of stress and frustration that are observed when broiler breeders are fed restrictedly, indicate that they are willing to consume more feed than they are offered (De Jong et al., Reference DE JONG, VAN VOORST, EHLHARDT and BLOKHUIS2002; Reference DE JONG, VAN VOORST and BLOKHUIS2003). Feed intake and feed intake motivation are the result of a complex of different mechanisms, which involve regulation by the central nervous system, hormones and nutrient levels in blood and tissues (Boorman and Freeman, Reference BOORMAN and FREEMAN1979; Forbes, Reference FORBES1988; Reference FORBES1995; Reference FORBES1999).
The motivation to consume feed can be induced by the emptying of the digestive tract. Information from mechanoreceptors in the wall of the crop, gizzard and intestines, and from specific chemoreceptors in the intestinal walls, is transferred to the central nervous system (Mei, Reference MEI1985; Greenberg et al., Reference GREENBERG, TORRES, SMITH and GIBBS1989; Forbes and Barrio, Reference FORBES and BARRIO1992). The emptying of the digestive tract is mediated by gastro-intestinal hormones, which are produced when feed or specific nutrients are present in the intestinal tract. The release of these hormones reduces feed intake (Kuenzel, Reference KUENZEL1994) and slows down feed passage rate (Cuche and Malbert, Reference CUCHE and MALBERT1999; Cuche et al., Reference CUCHE, CUBER and MALBERT2000). The gizzard plays a regulatory role in feed passage, and it is thought that accumulation of insoluble fibre in the gizzard triggers a temporary satiety (Hetland et al., Reference HETLAND, CHOCT and SVIHUS2004; Van Krimpen et al., Reference VAN KRIMPEN, KWAKKEL, VAN DER PEET-SCHWERING, DEN HARTOG and VERSTEGEN2011). Highly insoluble fibre diets accumulate in the gizzard, probably because the fibre has to be ground to a critical particle size before entering the small intestine (Hetland and Choct, Reference HETLAND and CHOCT2003; Hetland et al., Reference HETLAND, CHOCT and SVIHUS2004).
The filling or emptying of the digestive tract is mediated by homeostatic plasma glucose levels (Edwards et al., Reference EDWARDS, LADENHEIM and RITTER1986; Forbes, Reference FORBES1999) or gastro-intestinal hormones, such as cholecystokinin (CCK) and neuropeptide Y (NPY), which are produced when feed or specific nutrients are present in the intestinal tract. It has already been established that CCK, NPY, glucose (insulin) and corticosterone exert some of their effects on behaviour and appetite through interaction with 5-HT and DA pathways e.g. (Morley and Blundell, Reference MORLEY and BLUNDELL1988).
The mechanisms that determine when the bird starts or stops eating is only partially understood. According to Savory et al. (Reference SAVORY, DUKE and BERTOY1981), maximum feed intake capacity in broiler breeders is more determined by feed intake than nutrient levels in blood after feed consumption. Burkhart et al. (Reference BURKHART, CHERRY, VANKREY and SIEGEL1983)concluded that selection for increased growth rate in broilers was accompanied by a reduction in satiety as affected by nutrient levels in blood. As a consequence, the feed intake levels of broilers is mainly determined by the filling capacity of the digestive tract. Likewise, eating behaviour of modern broilers is more controlled by physical satiety mechanisms than by hunger mechanisms (Bokkers and Koene, Reference BOKKERS and KOENE2003). This indicates that diets that increase gastrointestinal filling, or decrease feed passage rate, might increase satiety levels in broiler chickens and parentstock.
There are almost no indications that digestible nutrient levels in isocaloric and isonitrogenious diets with balanced amino acid profiles can affect satiety. Hocking et al. (Reference HOCKING, ZACZEK, JONES and MACLEOD2004) suggested that diets with sugar beet pulp could improve satiety and welfare due to a higher water content in the digestive tract, as a consequence of an increased bulk capacity of the diet. In poultry, soluble fibres seem to be more effective in increasing satiety, due to an increased digesta viscosity and reduced feed passage rate, compared to insoluble fibres (Van Der Klis et al., Reference VAN DER KLIS, VAN VOORST and VAN CRUYNINGEN1993; Smits et al., Reference SMITS, VELDMAN, VERSTEGEN and BEYNEN1997).
In most species, including poultry, the neurotransmitters serotonin (5-HT) and dopamine (DA) play a role in the regulation of hunger, satiety, and eating (Denbow, Reference DENBOW1999; Richards, Reference RICHARDS2003; Van Hierden et al., Reference VAN HIERDEN, KOOLHAAS, DE BOER and KORTE2004a). Potential nutritional factors that influence feed intake behaviour are tryptophan (TRP) and tyrosine (TYR) (Van Hierden et al., Reference VAN HIERDEN, KORTE and KOOLHAAS2004b) that serve as biosynthetic precursors for 5-HT and DA, respectively (Groff and Gropper, Reference GROFF and GROPPER1999). Van Hierden et al. (Reference VAN HIERDEN, KOOLHAAS, DE BOER and KORTE2004a; Reference VAN HIERDEN, KOOLHAAS, KOŠT’ÁL, VÝBOH, SEDLACKOVÁ, RAJMAN, JURÁNI and KORTE2005) showed that high levels of dietary TRP increased 5-HT and DA in the brains and consequently increased the time birds spent eating. The impact of specific dietary nutrients on the signals that determine feed intake regulation and satiety in the brains of laying hens, however, is still poorly understood.
Recently, it has been shown that dietary energy reduction affected serotonin turnover and dopamine turnover, whereas an increase of coarsely ground insoluble NSP's in the diet affected the levels of plasma glucose, corticosterone, and dopamine turnover (Van Hierden et al., Reference VAN HIERDEN, KORTE, RUESINK, VAN REENEN, ENGEL, KORTE-BOUWS, KOOLHAAS and BLOKHUIS2002; Van Krimpen, Reference VAN KRIMPEN2008). Furthermore, significant effects of these nutritional factors were found on mean retention time of the digesta in the foregut (Van Krimpen et al., Reference VAN KRIMPEN, KWAKKEL, VAN DER PEET-SCHWERING, DEN HARTOG and VERSTEGEN2011). More research, however, is needed to understand the role of CCK, NPY, glucose, and corticosterone in initiating hunger, satiety, and feed intake behaviour.
How to improve broiler breeder welfare?
During the past 20 years, research has focused on practical applied management strategies to reduce the negative effects of feed restriction while maintaining the desired growth rate. Generally applied measures included changed feeding strategy or diet composition, which are discussed below.
diet composition
Several studies have evaluated the use of low-density diets (qualitative restriction) as a possible method to reduce stress and hunger due to quantitative feed restriction in broiler breeders. Savory et al. (Reference SAVORY, HOCKING, MANN and MAXWELL1996) tested diets diluted with unmolassed sugar beet pulp, two concentrations of oat hulls or sawdust up to ten weeks of age. Although the incidence of stereotypic pecking significantly reduced, activity levels were not affected and the use of sugar beet pulp and sawdust increased H/L ratios and plasma corticosterone concentrations. In a subsequent study, neither feeding motivation nor activity levels were affected by feeding diets with 15% oat hulls (Savory and Lariviere, Reference SAVORY and LARIVIERE2000).
Hocking et al. (Reference HOCKING, ZACZEK, JONES and MACLEOD2004) used diets diluted with 50 g/kg, 100 g/kg or 200 g/kg sunflower meal, ground unmolassed sugar beet pulp or ground oat hulls during rearing and measured behaviour as well as H/L ratios. Body weights at 15 weeks of age decreased with increasing fibre concentrations. Results showed that diets with 50 g/kg sugar beet pulp or the highest concentration (200 g/kg) of oat hulls reduced stereotypic object pecking, and that H/L ratios were higher in the standard control diet as well as with the lowest (50 g/kg) concentration of fibres. In addition, birds fed the diets with sugar beet pulp showed a decreased prevalence of damaged pecking and cannibalism. Thus, the results indicate that welfare of birds fed the diets with 200 g/kg oat hulls or sugar beet pulp may be improved. The authors suggested that diets with sugar beet pulp were associated with higher water content in the gastrointestinal tract, which might improve satiety and thus bird welfare.
De Jong et al. (Reference DE JONG, ENTING, VAN VOORST, RUESINK and BLOKHUIS2005a) tested four different diets during the rearing and laying period. The standard diet was either diluted with sunflower seed meal, wheat bran, lucerne, wheat gluten feed, and palm kernel meal in two concentrations or with sugar beet pulp and oats. The diet with the lowest density (8.4 MJ/kg) appeared to reduce hunger and frustration in the first half of the rearing period, which was indicated by reduced stereotypic pecking behaviour. There were no differences between the diet diluted with sugar beet pulp and oats or the other diluted diet with the same energy content. From this study it was concluded that for substantial improvement of broiler breeder welfare, more extreme diet modifications using higher fibre concentrations or combinations with appetite suppressants were required. Others did not find any positive effects at all of high fibre contents in broiler breeding diets (up to 6.6 MJ/kg) on behavioural and physiological indicators of welfare (Zuidhof et al., Reference ZUIDHOF, ROBINSON, FEDDES, HARDIN, WILSON, MCKAY and NEWCOMBE1995; Jones et al., Reference JONES, ZACZEK, MCLEOD and HOCKING2004; Hocking, Reference HOCKING2006).
In contrast, a combination of calcium propionate (an appetite suppressant) and 400 g/kg oat hulls appeared to be a viable alternative to commercial quantitative feed restriction, which was indicated by a change in parameters indicative of hunger (Sandilands et al., Reference SANDILANDS, TOLKAMP, SAVORY and KYRIAZAKIS2006). Besides that stereotypic pecking was virtually absent in this treatment group, the time spent sitting significantly increased and feeding motivation (by measuring rate of eating) was reduced as compared to broiler breeders fed a quantitative restricted diet, suggesting improved welfare due to reduced hunger in these birds (Sandilands et al., Reference SANDILANDS, TOLKAMP and KYRIAZAKIS2005; Tolkamp et al., Reference TOLKAMP, SANDILANDS and KYRIAZAKIS2005; Sandilands et al., Reference SANDILANDS, TOLKAMP, SAVORY and KYRIAZAKIS2006).
However, appetite suppressants may cause the bird to feel ill and, as a consequence, reduce appetite (Hocking and Bernard, Reference HOCKING and BERNARD1993). It must be questioned if bird welfare is indeed improved when these suppressants are used. Savory et al. (Reference SAVORY, HOCKING, MANN and MAXWELL1996) only found a significant effect on stereotypic pecking but not on other welfare indicators when using appetite suppressants.
Nielsen et al. (Reference NIELSEN, THODBERG, MALMKVIST and STEENFELDT2011) tested diets with high levels of soluble fibre or insoluble fibre in the rearing period, using scattered feeding. Levels of tail pecking were highest in birds fed control diets, lowest in birds fed the diet with soluble fibre and intermediate in birds fed diets with insoluble fibre. In addition, in a hunger test, birds fed the control diet ate significantly faster and showed a higher compensatory feed intake as compared to the two experimental diets. Stereotypic pecking was never seen in birds fed the diet with insoluble fibre and frequently seen in birds fed the control diet. Birds fed the diet with insoluble fibre also showed more dust bathing, comfort behaviour and foraging, which was interpreted as positive welfare indicators. In contrast, birds fed the soluble fibre diet seemed to be scruffier in their plumage, probably due to the higher moisture content of their litter, affecting their thermoregulation and additionally showed behavioural signs of discomfort. It was concluded that a high ratio of insoluble fibre may improve the welfare of broiler breeders.
Side effects of dietary dilution and fibre addition
Besides the positive effects of dietary fibre addition on welfare aspects of broiler breeders, there is evidence that it improves digestibility of the feed itself. Poultry require a minimal amount of fibre in the diet for proper functioning of the digestive organs (Hetland et al., Reference HETLAND, CHOCT and SVIHUS2004; Mateos et al., Reference MATEOS, JIMÉNEZ-MORENO, SERRANO and LÁZARO2012). The response to fibre inclusion depends on the source and level of dietary fibre and the characteristics of the diet as well as on the physiological status and health of the bird. In particular, the inclusion in the diet of moderate amounts of coarse, insoluble fibre sources, such as oat hulls, at dietary levels between 2 and 3% usually improves nutrient utilisation. These findings were confirmed in an experiment of Zuidhof et al. (Reference ZUIDHOF, ROBINSON, FEDDES, HARDIN, WILSON, MCKAY and NEWCOMBE1995), who observed an improved feed and crude protein conversion efficiency in broiler breeder layers that were fed a diet diluted with 15% ground oat hulls compared to hens fed an undiluted diet. Moreover, because of a higher level of satiety and a lower level of feed competition, hens that were fed a 15% diluted diet had a better flock uniformity, higher egg and chick production as compared to hens fed the standard diet. These positive results, however, were only found in the 15% diluted diet and not in a 30% diluted diet (Zuidhof et al., Reference ZUIDHOF, ROBINSON, FEDDES, HARDIN, WILSON, MCKAY and NEWCOMBE1995). Nutrient digestibility, however, decreased in broiler breeder pullets that were fed 12% and 23% diluted diets in which palm kernel meal, wheat bran, wheat gluten feed and sunflower seed meal were used as dilution sources. Despite this, birds fed the 23% diluted diet had a faster ovarium and oviduct development and heavier jejunum and ileum weights (Enting et al., Reference ENTING, VELDMAN, VERSTEGEN and VAN DER AAR2007c). Broiler breeders that were fed 11% or 21% diluted diets during the laying period, had a higher rate of lay and heavier eggs compared with hens fed undiluted diets (Enting et al., Reference ENTING, KRUIP, VERSTEGEN and VAN DER AAR2007b). Percentage of fertile eggs did not differ among treatments. Embryos of young hens, that were fed the 21% diluted diet, had a better development of the area vitellinaexterna and increased heart and embryo weights. Low-density broiler breeder diets were found to improve offspring growth rates, reduce mortality, and reduce or increase immune responses, depending on breeder age and egg weight (Enting et al., Reference ENTING, BOERSMA, CORNELISSEN, VAN WINDEN, VERSTEGEN and VAN DER AAR2007a). Addition of soluble fibres (sugar beet pulp) to the diet was found to increase water intake, which was reflected in a reduced friability of the litter and less comfort behaviour of broiler breeders (Nielsen et al., Reference NIELSEN, THODBERG, MALMKVIST and STEENFELDT2011).
Feeding management
Besides evaluating the effects of diet dilution, the effects of changing feeding management have been studied. De Jong et al. (Reference DE JONG, FILLERUP and BLOKHUIS2005b) investigated whether scattering the feed in the litter or feeding twice a day might have a positive effect on broiler breeder welfare. However, both feeding strategies did not have any positive effects on behavioural and physiological indicators of stress and hunger. In practice, skip-a-day feeding programmes are often used in the rearing period. It has been shown that skip-a-day feeding did not impose more stress in the birds as compared to every day feeding, measured by behavioural observations and H/L ratios (Skinner-Noble and Teeter, Reference SKINNER-NOBLE2009 in EFSA, 2010).
Other strategies to reduce the negative effects of feed restriction
Environmental enrichment during rearing may alleviate frustration of the feeding motivation. Hocking and Jones (Reference HOCKING and JONES2006) provided bunches of strings and bales of wood shavings from hatch or from 8 weeks of age onwards in commercial houses with broiler breeder females. Behaviour and the feather and skin condition were observed throughout rearing. Birds used the bales of wood shavings, but the string was not extensively used. However, providing the environmental enrichment did not reduce aggression due to competition for food and no positive effects of enrichment on feather and skin damage were found.
With respect to bird welfare the use of other genotypes of broilers, i.e. slower growth rate and dwarf breeds, may be alternatives to reduce the negative effects of feed restriction (Jones et al., Reference JONES, ZACZEK, MCLEOD and HOCKING2004; De Jong et al., Reference DE JONG, ENTING, VAN VOORST, RUESINK and BLOKHUIS2005a; Decuypere et al., Reference DECUYPERE, ONAGBESAN, SWENNEN, BUYSE and BRUGGEMAN2007). The dwarf genotypes only concern one sex (females), but numerically the problem of feed restriction concerns more females than males, and it thus may be a solution for a large part of the broiler breeder population. However, they will correspond only to a specific market demand, because their use is approximately 20% in Europe, but is around 85% in France (EFSA, 2010), and the use of dwarf or other slower growing genotypes is unacceptable for economic reasons in many cases.
Interpretation of the results
Assessing the effects of changed feeding strategies or changed diet compositions involves the use of indirect indicators of animal welfare, such as stereotypic and redirected pecking behaviour, increased activity levels, or physiological indicators of stress like increased corticosterone levels or H/L ratios. However, the meaning of these in terms of hunger can be interpreted differently as extensively discussed by D'eath et al. (Reference D'EATH, TOLKAMP, KYRIAZAKIS and LAWRENCE2009). To overcome these problems, recent studies aimed at developing behavioural tests to determine the preference of broiler breeders for different diets, but thus far no appropriate method could be developed (Buckley et al., Reference BUCKLEY, MCMILLAN, SANDILANDS, TOLKAMP, HOCKING and D'EATH2011a; Reference BUCKLEY, SANDILANDS, TOLKAMP and D'EATH2011b). Therefore, more research will be necessary to develop methods to measure a subjective state, such as hunger.
Conclusions
The paradox in broiler breeders is that they are selected for fast growth and efficient meat production, supported by a virtually voluntary feed intake, whereas fast growth is negatively correlated to reproduction effectiveness. To prevent negative effects on reproduction, broiler breeders are restricted-fed. This induces a second paradox; acceptable reproduction and health versus hunger, stress and impaired welfare.
The main reason for this paradox is related to breeding goals that are focussed on improving feed conversion and increasing breast meat percentage. Nutritional strategies cannot solve this paradox, but can be helpful in reducing hunger stress in broiler breeders. The strategy that seemed to have a positive effect on broiler breeder welfare is dietary dilution, by reducing the energy content and/or increasing the NSP content, where diet dilution using insoluble fibres seems to have the most positive effect on bird welfare (Nielsen et al., Reference NIELSEN, THODBERG, MALMKVIST and STEENFELDT2011). Qualitative feed restriction allows the birds to consume more feed which might increase mechanical satiety. It remains unclear whether certain feeding strategies will increase chemostatic satiety. In addition, there is still a need for a reliable indicator of hunger.
Using dwarf broiler breeder hens is another alternative to overcome the dilemma, because dwarf hens combine relatively good reproductive fitness with ad libitum feeding. Future genetic selection might attempt to uncouple control of ovarian follicular growth factors from selection for rapid growth, which seems a structural solution for solving these paradoxes. This view is supported by Dawkins and Layton (Reference DAWKINS and LAYTON2012), who argued that the conflict between good welfare and productivity can be reduced by making use of all available genetic variation from existing breeds and other sources. Another possibility is to accept lower broiler productivity by assigning economic values to welfare and including integrity traits in an extended breeding goal.
