Introduction
Moulting, which is generally defined as the periodic shedding and replacement of feathers, has become focus of attention for poultry physiologists and nutritionists as well as egg producers. Moulting involves reproductive quiescence, and is a natural phenomenon in wild and domestic birds. Each laying bird begins egg production at a slow pace (although modern strains reach peak production rapidly), and gradually reaches maximum production, which then declines. This decline provides the equivalent of a ‘reproductive rest’ to the birds. For commercial egg producers, this represents a period of unprofitably low egg production signifying the end of the productive life of a flock. To avoid this, most of the ‘spent’ hens are marketed at low prices. The low prices of eggs and spent flocks have resulted in interest in replacing the natural moulting process with the artificial one called ‘forced’ moulting. Forced moulting is associated with complete halt of egg production and precipitous loss of feathers. Egg production is resumed and increases rapidly to a profitable rate following this artificial moult. Currently, forced moulting has been coined as the technically correct term while other term such as pause, rest, forced rest and recycling are used synonymously (Berry, Reference BERRY2003).
Conventionally induced moulting is usually involved withdrawal of feed, water or both whilst maintaining the light schedule near to or less than natural lighting patterns. Hens are starved for a particular period of time sufficient to induce complete involution of the reproductive tract. Induced moulting is an important economic tool in spent flocks. In many developing countries like Pakistan, India, Bangladesh, forced moulting involves complete withdrawal of feed, however, this practise has been proscribed in developed countries like Japan, USA and UK. Recently the increasing pressure of the welfare of animals has led to calls for the abolition of induced moulting (Berry, Reference BERRY2003) on the grounds that complete feed withdrawal is cruel. Curtailment of induced moulting may be accompanied by reduced profitability of laying hen enterprises and an increase in the cost of eggs and related products at the consumer level (Berry, Reference BERRY2003).
Understanding these consequences, it has become necessary to rationally address the welfare of the laying hens and balance this with the human need for cheap sources of food. As a result, various induced moulting techniques that do not incorporate complete fasting have been introduced. These alternative methods employ dietary manipulation to create an imbalance of a particular nutrient(s). The most important among these include manipulation of minerals including sodium (Na), calcium (Ca), iodine (I) and zinc (Zn), with full or partially reduced dietary intakes (Martin et al., Reference MARTIN, MORRIS, GEHLE and HARWOOD1973; Whitehead and Shannon, Reference WHITEHEAD and SHANNON1974; Wakeling, Reference WAKELING1978; Hebert and Cerniglia, Reference HEBERT and CERNIGLIA1979; Breeding et al., Reference BREEDING, BERRY and BRAKE1992; Berry, Reference BERRY2003; Sandhu et al., Reference SANDHU, RAHMAN and RAHMAN2006; Reference SANDHU, Z.U., RAHMAN and HASSAN2007). The use of particular techniques to induce moulting varies from region to region, depending upon mainly economic ground and poultry welfare laws.
Zn-induced moulting
Dietary Zn, commonly used in the form of ZnO, has been reported to be an effective agent for halting egg production. When fed at the level of 10,000 to 20,000 ppm caused a complete cessation of egg production in five to six days (Shippee et al., Reference SHIPPEE, STAKE, KOEHN, LAMBERT and SIMMONS1979; Berry and Brake, Reference BERRY and BRAKE1987). A combination of low levels of Ca and Zn (2,800 ppm) induced reproductive involution and moulting similar to seen when feeding 20,000 ppm dietary Zn (Breeding et al., Reference BREEDING, BERRY and BRAKE1992). Sandhu et al. (Reference SANDHU, RAHMAN and RAHMAN2006) and Yousaf et al. (Reference YOUSAF, RAHMAN, SANDHU, BUKHARI and YOUSAF2008) induced moulting with 3,000 ppm Zn and reduced feed intake to 35 g/bird/day. The huge differences between the doses of Zn used by different authors may be due to the period and intensity of moulting.
The moulting period and subsequent recovery from a Zn-induced moult vary from author to author and procedure to procedure. Hens that received a Zn moulting ration for seven days resumed production one week after returning to their normal diet (Berry, Reference BERRY2003). Hens that received a Zn ration for 12 days returned to production within 25 days (Scott and Creger, Reference SCOTT and CREGER1976). Shippee et al. (Reference SHIPPEE, STAKE, KOEHN, LAMBERT and SIMMONS1979) found that hens moulted with Zn incorporated diet returned to production after two weeks and reached 50% of maximum production five weeks later. Sandhu et al. (Reference SANDHU, RAHMAN and RAHMAN2006) and Yousaf et al. (Reference YOUSAF, RAHMAN, SANDHU, BUKHARI and YOUSAF2008), used similar methods of Zn-induce moulting (3,000 ppm ZnO), and found that birds returned to peak production in three weeks. In our own experiment, it was found that broiler breeder males returned to semen production in five weeks post-moult. Maximum involution of the reproductive organs is essential for optimum post-moult performance. Variability of body weight loss, and differences in reproductive involution has been suggested as an explanation for the conflicting results obtained by using similar moulting procedures (Berry, Reference BERRY2003).
Mechanism of induced moulting by dietary Zinc
Although feeding high levels of Zn is known to induce the regression of the reproductive tissues, the mechanisms of initiation are unknown. Identification of major factors that initiate reproductive tissue regression will be helpful for improving the existing methods of moulting. Bell (Reference BELL2003) concluded that, in chicken, regression of the ovary and oviduct during moulting is achieved through apoptosis that is induced mainly by the withdrawal of oestrogen and progesterone. The exact mechanism of Zn-induced moulting is not understood, however, it has been postulated that high concentrations of dietary Zn (10,000-20,000) induces follicular atresia and subsequent cessation of egg production by interfering with the process of ovulation in hens (Creger and Scot, 1977; Shippee et al., Reference SHIPPEE, STAKE, KOEHN, LAMBERT and SIMMONS1979; Berry and Brake, Reference BERRY and BRAKE1987; Park et al., Reference PARK, BIRKHOLD, KUBENA, NISBET and RICKE2004a). A lower level of dietary Zn in the absence of Ca directly suppresses the reproductive organs because Ca is required for the initiation and secretion of luteinizing hormone (LH) (Luck and Scanes, Reference LUCK and SCANES1980; Park et al., Reference PARK, BIRKHOLD, KUBENA, NISBET and RICKE2004a). Ca plays an important role in gonadotropin secretion and reproduction in avian ovarian cells (Taylor, Reference TAYLOR1965). Zn supplementation interferes with Ca uptake, and causes Ca to fall below the critical level essential for gonadotropin production and release of LH (Berry and Brake, Reference BERRY and BRAKE1987). McCormick and Cunningham (Reference MCCORMICK and CUNNINGHAM1983) reported that feeding high levels of zinc induced moulting due to reduction in feed intake.
Sundaresan et al. (Reference SUNDARESAN, ANISH, SASTRY, SAXENA, MOHAN and SAXEN2007) studied the involvement of a cytokine in reproductive regression in White Leghorns during Zn-induced moulting. A novel transcription factor, named lipopolysacheride (LPS) induced TNF-α factor (LITAF) was found to play an important role in the upregulation of several inflammatory cytokines, including IL-1 α, RANTES, IFN-γ, MCP-2 and IL-10, which were involved in reproductive regression during induced moulting. In another study in chickens, Sundaresan et al. (Reference SUNDARESAN, ANISH, SASTRY, SAXENA, NAGARAJAN, SUBRAMANI, LEO, SHIT, MOHAN and SAXENA2008) concluded that a high dose of Zn induced reproductive regression via upregulation of cytokines and chemokines, the suppression of feed intake and the increase in serum corticosterone which ultimately resulted in the apoptosis of reproductive tissues.
Egg production and quality
During the period of natural or forced moulting, egg production ceases and is restored gradually after moulting. In a study by Abdullah (Reference ABDULLAH2007), the decline in egg production started almost five weeks later in the 3rd recycling compared to the 2nd recycling, which may be correlated with the age of the birds. This is because increasing age leads to physiological changes and the slower growth of follicles (Palmer and Bahr, Reference PALMER and BAHR1992). Yousaf et al. (Reference YOUSAF, RAHMAN, SANDHU, BUKHARI and YOUSAF2008) concluded that during induced moult, egg production diminishes and then increases gradually to peak production. In the same study, the egg laying process started three to four weeks earlier in Zn-induced moult than a conventional forced moult. Park et al. (Reference PARK, BIRKHOLD, KUBENA, NISBET and RICKE2004b), Sandhu et al. (Reference SANDHU, RAHMAN, HASAN, RAHMAN and YOUSAF2008) and Yousaf et al. (Reference YOUSAF, RAHMAN, SANDHU, BUKHARI and YOUSAF2008) reported that egg production was better in Zn-induced moulting compared to the fasting method. Abdullah (Reference ABDULLAH2007) found that the birds induced to moult by Zn supplementation showed better egg production percentage with less mortality and enhanced feed conversion ratio per dozen eggs. Similar conclusions were reported by Hassan (Reference HASSAN2003). In the study of Onbasilar and Erol (Reference ONBASILAR and EROL2007), hen day egg production was significantly higher in Zn-induced moulting as compared to those fed an ad libitum whole-grain barley diet for 1-10 days, or the California method (no feed for 1-10 days). Baker et al. (Reference BAKER, BRAKE and KRISTA1981a; Reference BAKER, BRAKE and MCDANIEL1981b) found that the improvement in post moulting performance was associated with increase body weight loss of hens up to 31% of their original body weight. Hens that lost the greatest amount of weight exhibited the best improvements in egg production and quality.
Qualitatively, eggs are classified according to weight, size, shell thickness and internal parameters. Increased egg weight has been recorded by Peebles and Brake (Reference PEEBLES and BRAKE1987), Park et al. (Reference PARK, BIRKHOLD, KUBENA, NISBET and RICKE2004b) and Abdullah (Reference ABDULLAH2007) in Zn-induced moulted birds. However, Abdullah (Reference ABDULLAH2007) found that egg weight decreased significantly during 3rd production cycle, which he suggested may be correlated with the slower growth of follicles and decline in uterine gland function needed to generate albumin. Sandhu et al. (Reference SANDHU, RAHMAN, HASAN, RAHMAN and YOUSAF2008) and Yousaf et al. (Reference YOUSAF, RAHMAN, SANDHU, BUKHARI and YOUSAF2008) reported increased egg weight after Zn-induced moulting. Abdullah (Reference ABDULLAH2007) found higher eggshell weight and thickness in eggs from Zn-induce moulted hens. This has been correlated with better Ca metabolism, absorption, transport and deposition (Roland and Brake, Reference ROLAND and BRAKE1982). Due to Zn-induced moulting, extra Zn is deposited in the egg shell, which synergises the effect of Ca and results in better eggshell weight and thickness (Abdullah, Reference ABDULLAH2007; Klecker et al., Reference KLECKER, ZEMAN, JELINEK and BUNESOVA2002; Innocenti et al., Reference INNOCENTI, ZIMMERMAN, FERRY, SCOZZAFAVA and SUPURAN2004).
Effect of Zn on Salmonella enteritidis infection during moulting
The feed withdrawal method of moulting has been associated with increased susceptibility of birds to S. enteritidis colonisation and invasion (Thiagarajan et al., Reference THIAGARAJAN, SAEED and ASEM1994; Holt et al., Reference HOLT, MACRI and PORTER1995; Park et al., Reference PARK, BIRKHOLD, KUBENA, NISBET and RICKE2004a). The microenvironment produced as a result of emptying of the gastrointestinal tract of birds during fasting provides a chance for S. enteritidis not only to survive, but enhances the pathogenic expression of such bacteria. Moreover, the immune system of the birds is compromised as a consequence of complete feed withdrawal which augments the chances of pathogenic bacteria to colonise the gut of the birds. More recent approaches to moulting are based on strategies which do not compromise the gastrointestinal tract function, indigenous gut microflora and bird's physiology (Holt, Reference HOLT2003; Park et al., Reference PARK, BIRKHOLD, KUBENA, NISBET and RICKE2004a). Zn-amended diets have been suggested as a superior method to induce moult which limits S. enteritidis colonisation. In an experiment, Zn was fed to the laying hens with or without Ca and it was concluded that Zn-fed birds had lower level of S. enteritidis symptoms compared to birds on the feed withdrawal method (Moore et al., Reference MOORE, PARK, KUBENA, BYRD, MCREYNOLDS, BURNHAM, HUME, BIRKHOLD, NISBET and RICKE2004). One possible explanation of the drop in infection is that the feed withdrawal method may reduce the colonisation of beneficial microflora, such as Acidophilus and Bifidobacterium, which have been reported to limit the establishment of S. enteritidis bacteria (Khan, Reference KHAN2010). Park et al. (Reference PARK, BIRKHOLD, KUBENA, NISBET and RICKE2004a) suggested various mechanisms through which Zn limits the infection of S. enteritidis, viz., directly or indirectly decreasing the pH of the intestines, physical detachment of Salmonella bacteria and beneficial effects of Zn on immune system of the birds.
The most important aspect of Zn in the gastrointestinal tract may lie with the beneficial interaction of Zn with the immune system. It is hypothetically possible that increased intake of Zn during moulting stimulates the immune system, and this leads to higher resistance to Salmonella establishment, even if colonisation of the gastrointestinal tract occurs (Park et al., Reference PARK, BIRKHOLD, KUBENA, NISBET and RICKE2004a). Kidd et al. (Reference KIDD, QURESHI, FERKET and THOMAS1994) observed that that zinc–methionine supplementation enhanced the reduction of intravenously introduced S. arizona from the spleen of young turkeys and increased in vitro phagocytosis. Park et al. (Reference PARK, BIRKHOLD, KUBENA, NISBET and RICKE2004a) suggested that, if synergism occurs between the direct limitation of Salmonella growth by dietary zinc in the gastrointestinal tract and a more responsive immune system, moulting birds under a zinc-feeding regime should be more resistant to sustained Salmonella infections and horizontal transmission among susceptible birds in the flock.
Liveability
Liveability in a layer flock is defined as the number of hens remaining in the layer house, and is expressed as a percentage of the number of hens originally placed (Berry, Reference BERRY2003). Shippee et al. (Reference SHIPPEE, STAKE, KOEHN, LAMBERT and SIMMONS1979) reported that Zn moulting had no effect on liveability as compared to a non-moulted control. Abdullah (Reference ABDULLAH2007) found that birds induced to moult with Zn showed less mortality as compared to fast induced moulted hens, and concluded that the lower mortality in the Zn-induced moulted group was due to continuous feeding of the birds. Sandhu et al. (Reference SANDHU, RAHMAN and RAHMAN2006) reported overall 99% liveability in laying hens during Zn-induced moulting.
Feed consumption
Little data is available on feed consumption and the efficiency of Zn-induced moulting. In two separate studies, Zn-induce moulting did not affect post-moult feed consumption (Shippee et al., Reference SHIPPEE, STAKE, KOEHN, LAMBERT and SIMMONS1979; Palafox and Ho-A, Reference PALAFOX and HO-A1980). Abdullah (Reference ABDULLAH2007) concluded that feed consumption per dozen eggs was better during Zn-induce moulting compared to birds subjected to the fasting procedure. Hassan (Reference HASSAN2003) observed reduce feed consumption in laying hens fed on 5,000 to 20,000 ppm ZnO. If feed intake is not affected and egg production is increased, then feed efficiency will be improved (Berry, Reference BERRY2003).
Plasma Zn concentration
Yousaf et al. (Reference YOUSAF, RAHMAN, SANDHU, BUKHARI and YOUSAF2008) concluded that plasma Zn concentration did not increase after Zn-induced moulting and suggested that any decrease may be due to excessive mobilisation of Zn for albumin and yolk. Similar conclusions were drawn by Abdullah (Reference ABDULLAH2007). Johnson and Greger (Reference JOHNSON and GREGER1984) suggested that the brush border of the intestinal membrane becomes less sensitive to high dietary Zn, resulting in decreased absorption and increased excretion via faeces. Similarly Stahl et al. (Reference STAHL, GREGER and COOK1989) and Kaya et al. (Reference KAYA, UMUCALILAR, HALILOGLU and IPEK2001) reported a high amount of dietary Zn being excreted in faeces, thereby decreasing the concentration of Zn in plasma.
Immunity
The nutritional–immunological relationship between Zn deficiency and immune responses has been extensively examined in poultry (Sahin et al., Reference SAHIN, SAHIN, KUCUK, HAYIRLI and PRASAD2009). Zn is an important element for all aspects of immunity and is critical for the integrity of the cells involved in the immune system. Deficiency of Zn in the chickens causes a decrease in cellular immunity and adversely affects the thymus, spleen and interleukin production (Sahin et al., Reference SAHIN, SAHIN, KUCUK, HAYIRLI and PRASAD2009). Moreover, Zn deficiencies cause imbalances in the functions of T helper-1 and T helper-2 cells (Shankar and Prasad, Reference SHANKAR and PRASAD1998).
Moulting is a process that contributes towards the process of stress, impairs cellular immune function and promotes systemic diseases (Holt and Porter, 1992). Burns (Reference BURNS1983) reported that feeding diets supplemented with Zn improved the ability of birds to produce antibodies. Poultry diets that are deficient in Zn or diets supplemented with Zn have been extensively studied, however, the immunomodulating effect in the laying birds after Zn-induced moulting has not been studied until recently. Sandhu et al. (Reference SANDHU, Z.U., RAHMAN and HASSAN2007) compared some immunological parameters in fast and Zn-induced moulting. They reported that macrophage engulfment against RBC (red blood cells) and nitric oxide (NO), a potent oxide radical, were significantly higher in Zn-induced moulted birds compared to fast-induced moulting. In the same experiment, antibody production (IgG and IgM) was significantly higher in the Zn-induced moulted flock. This elevated level of immune response was attributed to the immune-potentiating effect of Zn (Sandhu et al., Reference SANDHU, Z.U., RAHMAN and HASSAN2007). Previously Sandhu et al. (Reference SANDHU, RAHMAN and RAHMAN2006) also demonstrated that, during Zn-induced moulting, antibody response against Newcastle disease (ND) virus and egg drop syndrome (EDS) improved significantly.
Immuno-histochemical studies of the pituitary gland during Zn-induced moulting
During the process of moulting, levels of corticosteroids increase significantly and concentrations of oestrogen, progesterone and luteinizing hormone (LH) decrease sharply (Sandhu et al., Reference SANDHU, RAHMAN, HASAN, RAHMAN and YOUSAF2008). These findings suggest that the pituitary gland plays a central role (as these hormones are regulated by pituitary gland) in the control of the many endocrine functions which are altered during moulting. Recently, Sandhu et al. (Reference SANDHU, RAHMAN, HASAN, RAHMAN and YOUSAF2008; Reference SANDHU, RAHMAN, RIAZ, RAHMAN, JAVED and ULLAH2010) studied the variations of different cells in the pituitary gland of laying hens in response to Zn-induced moulting through immuno-histochemical techniques. Sandhu et al. (Reference SANDHU, RAHMAN, HASAN, RAHMAN and YOUSAF2008) found that LH gonadotroph cell size was significantly higher in Zn-induced moulted birds compared to those using the fasting method. Plasma Zn levels increase after Zn-induced moulting (Sandhu et al., Reference SANDHU, RAHMAN and RAHMAN2006) which may cause increases in LH gonadotrophs and nucleus size because of their greater accumulation in mRNA or ribosomes. This suggests a direct role of Zn on LH cells, which may result in increased plasma LH concentration and could ultimately trigger ovulation and enhance egg production. Sandhu et al. (Reference SANDHU, RAHMAN, RIAZ, RAHMAN, JAVED and ULLAH2010) demonstrated the effect of induced moulting on pituitary somatotrophs and latotrophs in laying hens, and concluded that Zn-induced moulting decreased the proportion of latotrophs without any prominent effects on the somatotrophs population, suggesting the suppressive role of corticosterone on prolactin gene transcription.
Using immuno-histochemical techniques, Abdullah (Reference ABDULLAH2007) found that growth hormone (GH) cell nucleus size was significantly lower after Zn induced moulting, but that this increased at the end of 2nd production cycle. This change in nucleus size may be due to the presence of a metalloenzyme in the nucleus which takes part in DNA and RNA synthesis (Glen, Reference GLEN2001) - Zn is one of the basic constituents of metalloenzymes. The avian pituitary gland is unique among vertebrates in that the pars distalis can be divided into two distinct lobes; the cephalic and caudal lobes. In mammals, prolactin releasing hormone (PRL) is present in the central and dorso-caudal zones of pars distalis and increases during pregnancy and lactation. Abdullah (Reference ABDULLAH2007) found PRL in the cephalic and caudal lobes of Zn moulted hens. The presence of PRL in both lobes may be due to intense physiological changes occurring in the whole body and in the pituitary gland of moulted hens. Abdullah (Reference ABDULLAH2007) found that PRL cells increased towards the end of 2nd production cycle and decreased at 5% and peak of 3rd production cycle which may be due to initiation of the hyperprolactinemia stage (Ramesh et al., Reference RAMESH, SOLOW, PRODUMAN and KUENZEL1996). Hyperprolactinemia is a normal physiological mechanism that develops at the end of production cycle in avian species and during lactation in mammals.
Haematopoietic system
Karimi et al. (Reference KARIMI, KHAJALI and RAHMANI2009) reported that starvation during moulting increases the packed cell volume (PCV) parameters in blood. Berry (Reference BERRY2003) showed that PCV increases during the period of reproductive involution and decreases upon the regeneration of reproductive tract, which is associated with removal of oestrogen inhibition of RBC formation, reduced vascular volume and presumably due to reduced fluid intake leading to haem concentration. However, this phenomenon is not observed in Zn-induced moulting. During recovery from a Zn moult, reticulocytosis is extreme and PCV may show a transient increase at the time when PCV is actually declining in fasting moulted hens (Berry, Reference BERRY2003).
Conclusions
From this review, it is clear that Zn-induced moulting is far more appropriate in many aspects compared to fast moulting practises. Zn-induced moulting results in increased egg production and quality, enhanced immunity and a decline in potential food borne disease. The concerns of those concerned with poultry welfare are satisfied with this method of moulting, as feed is not fully restricted. After reviewing the Zn-induce moulting of many workers, one basic question arises, which is whether the decline in weight loss at the end of moulting was due to the presence of Zn in the feed or due to restricted feed.
Acknowledgement
The study is a part of research conducted under 5000 indigenous PhD fellowship (Batch IV) scheme funded by Higher Education Commission (HEC) of Pakistan.
