Crop anatomy

The crop is a thin walled diverticulum of the oesophagus and retains the basic structural pattern of the digestive tract (Hodges, Reference HODGES1974). Extending from the lumen are non-keratinised stratified squamous epithelium of the mucous membrane, lamina propria (connective tissue), muscularis mucosae, sub-mucosa (a layer of loose connective tissue), muscularis externa and the serosa. The structure of the crop is well innervated and vascularised implying potential for interaction with other gut segments, and possibly a variety of roles related to digestion and gut health. Mucous glands are found in the oesophagus and also at the interface between the oesophagus and crop, but are not found in the crop diverticulum (Fuller and Brooker, Reference FULLER and BROOKER1974). Similarly, structural differences have been noted between the oesophageal area of the crop (slightly convoluted surface with a high density of bacteria) and the apex of the diverticulum (flatter surface, numerous sloughing epithelial cells, and fewer bacteria) (Bayer et al., Reference BAYER, CHAWAN and BIRD1975). The non-secretory nature of the crop suggests a limited capacity for digestion, but moisture and enzymes derived from consumption (water, feed), saliva and microorganisms play a role in initiating the digestive process. Demonstrated bacterial adhesion to starch granules is just one example of how digestion can occur in the crop (Bayer et al., Reference BAYER, CHAWAN and BIRD1975). The importance of the crop in diet digestion will be affected by the proportion of the feed that enters the crop and the amount of time it spends there. Although the nature of the crop epithelium indicates less absorptive capacity than areas such as the small intestine, absorption by diffusion of organic acids such as lactic acid and DL-2-hydroxy-4(methylthio) butanoic acid has been suggested (Cutler et al., Reference CUTLER, RASMUSSEN, HENSLEY, WILHELMS, GRIFFITH and SCANES2005; Richards et al., Reference RICHARDS, ATWELL, VÁZQUEZ-AÑÓN and DIBNER2005). The possibility of transporter mediated absorption has not been evaluated in chickens, but since it occurs in other examples of stratified squamous epithelium (Connor et al., Reference CONNOR, LI, BALDWIN and LI2010), this possibility can't be ruled out in the avian crop. Again, residency time of feed and absorbable molecules in the crop impact this potential.

Feed residency in the crop

The degree to which feed enters and the time it is resident in the crop is variable and highly dependent on the nature of bird feeding behaviour, feed presentation (e.g., meal vs. ad libitum) and bird management (e.g., periods of darkness). Estimates of crop retention time in birds given ad libitum access to feed and 24 hours of light per day are 7.4 and 24.6 minutes for broiler and White Leghorn chicks, respectively (Shires et al., Reference SHIRES, THOMPSON, TURNER, KENNEDY and GOH1987). In this setting, it appears that feed is primarily directed to and accumulates in the gastric stomach and small intestine. The observation that broiler chickens eat very frequently, approximately every 30 minutes when having ad libitum access to feed, corroborates this (Svihus et al., Reference SVIHUS, LUND, BORJGEN, BEDFORD and BAKKEN2013b). Therefore, not only will crop residency be short, but it is also likely that most feed will not enter the crop (Chaplin et al., Reference CHAPLIN, RAVEN and DUKE1992; Savory, Reference SAVORY1985). An extreme contrast to this situation is broiler breeders fed on an every-other-day feeding schedule during the rearing period, where the crop does not empty until 20 hours after feeding (de Beer et al., Reference DE BEER, MCMURTRY, BROCHT and COON2008). Similarly, the time feed spends in the crop can be affected by lighting programs with extended periods of darkness. In turkeys given a 14L:10D lighting program, digesta remained in the crop for 9 hours after the end of the photophase (Cutler et al., Reference CUTLER, RASMUSSEN, HENSLEY, WILHELMS, GRIFFITH and SCANES2005). Turkeys and chickens are diurnal and eat during the day if not limited by day length or other environmental factors such as high temperature. Feeding behaviour during the photophase has a daily pattern with the nature of the pattern dependent on the length of the dark period (Schwean-Lardner et al., Reference SCHWEAN-LARDNER, FANCHER, LAARVELD and CLASSEN2014). Feed intake may or may not be high immediately after lights come on, and increases again prior to lights going off (Buyse et al., Reference BUYSE, ADELSOHN, DECUYPERE and SCANES1993; Schwean-Lardner et al., Reference SCHWEAN-LARDNER, FANCHER, LAARVELD and CLASSEN2014). Early morning feeding can be rationalised by hunger associated with the extended period of time during darkness, but when not seen may be attributed to the capacity of the crop to store feed for an extended period of time and therefore a lack of hunger (Scanes et al., Reference SCANES, CAMPBELL and GRIMINGER1987). The late day increase in feed intake is anticipatory and takes time for birds to learn after first being exposed to a dark period. The rationale is that birds increase feed intake so that their nutritional needs are met for at least a portion of the dark period. Buyse et al. (Reference BUYSE, ADELSOHN, DECUYPERE and SCANES1993) found that the crop contained only small quantities of ingesta during the photoperiod in a 14L:10D lighting program, in apparent agreement with Shires et al. (Reference SHIRES, THOMPSON, TURNER, KENNEDY and GOH1987). However, the ingesta content of the crop increased dramatically (10.5-fold) at the beginning of the scotophase as a result of late day feeding. The crop content decreased gradually during the scotoperiod and of note feed transit time was longer during the night; this was also found by others (Cutler et al., Reference CUTLER, RASMUSSEN, HENSLEY, WILHELMS, GRIFFITH and SCANES2005; Duve et al., Reference DUVE, STEENFELDT, THODBERG and NIELSEN2011; Scanes et al., Reference SCANES, CAMPBELL and GRIMINGER1987). These authors speculated that the storage of feed in the crop, its gradual release and the increased food transit time at night resulted in the majority of the bird's nocturnal energy needs being met. Anticipatory feeding appears to be affected by the length of the scotoperiod. In a comparison of 16L:8D and 13L:4D:3L:4D lighting programs, Duve et al. (Reference DUVE, STEENFELDT, THODBERG and NIELSEN2011) found that birds on the latter lighting program failed to display anticipatory feeding before either of the dark periods and suggested that its absence was caused by a lack of need to feed, rather than not predicting the dark period. This research plus other work (Schwean-Lardner et al., Reference SCHWEAN-LARDNER, FANCHER, GOMIS, VAN KESSEL, DALAL and CLASSEN2013) suggest that dark periods of greater than four hours are necessary to induce anticipatory feeding behaviour. In conclusion, use of the crop as a feed storage device is minimal when highly nutritious feed is readily available and there are no constraints on feeding behaviour. However, birds utilize crop storage in response to hunger (e.g. food deprivation) or regular periods of darkness (Savory, Reference SAVORY1985).

The control of crop filling and emptying is complex, but the gizzard plays a central role (Chaplin et al., Reference CHAPLIN, RAVEN and DUKE1992; Jackson and Duke, Reference JACKSON and DUKE1995) as it regulates passage of feed to the remainder of the digestive tract. Signalling includes neurological (vagus nerve; Denbow, Reference DENBOW1989) and hormonal (e.g. ghrelin; Kaiya et al., Reference KAIYA, FURUSE, MIYAZOTO and KANGAWA2009; glucagon-like peptide-1, Tachibana et al., Reference TACHIBANA, MATSUMOTO, FURUSE, HASEGAWA, YOSHIZAWA and SUGAHARA2003) mechanisms.

Bacterial populations and role in digestive tract health and pathogen colonisation

Lactobacilli dominate the bacterial community of the crop, but coliforms, streptococci and bifidobacteria have also been found (Abbas Hilmi et al., Reference ABBAS HILMI, SURAKKA, APAJALAHTI and SARIS2007; Fuller, Reference FULLER1973; Fuller and Brooker, Reference FULLER and BROOKER1974; Fuller and Turvey, Reference FULLER and TURVEY1971; Guan et al., Reference GUAN, HAGEN, TANNOCK, KORVER, FASENKO and ALLISON2003; Peinado et al., Reference PEINADO, ECHÁVARRI, RUIZ, SUÁREZ-PEREIRA and MELLET2013; Petr and Rada, Reference PETR and RADA2001). At least some lactobacilli are capable of binding to the crop epithelium to form biofilm layers that are relatively uniquely found on non-secretory stratified squamous epithelium (Edelman et al., Reference EDELMAN, WESTERLUND-WIKSTRÖM, LESKELÄ, KETTUNEN, RAUTONEN, APAJALAHTI and KORHONEN2002; Fuller and Turvey, Reference FULLER and TURVEY1971; Lebeer et al., Reference LEBEER, VERHOEVEN, CLAES, DE HERTOGH, VERMEIRE, BUYSE, VAN IMMERSEEL, VANDERLEYDEN and DE KEERSMAECKER2011). The nature of the adherence is not fully understood, but S-layer proteins may be responsible (Hagen et al., Reference HAGEN, GUAN, TANNOCK, KORVER and ALLISON2005). Hagen et al. (Reference HAGEN, GUAN, TANNOCK, KORVER and ALLISON2005) speculate that the diversity of S-layer proteins among Lactobacillus gallinarum strains may provide an opportunity for different strains to live together without direct competition for attachment sites on the crop epithelium. Adherence is vital for colonisation to prevent washout in intestinal compartments with continuous digesta passage. However, if only a small proportion of feed enters the crop, the low flow-through in the diverticulum may enable reproduction of bacteria that are not capable of resisting washout by adherence. Indeed, the crop could act as a natural fed-batch fermenter (like the caecum), seeding the intestine with metabolically active bacteria. The exact nature of the crop microbiota continues to increase in clarity as newer techniques of identification become available (Cousin et al., Reference COUSIN, GULAT-OKALLA, MOTREFF, GOUYETTE, BOUCHIER, CLERMONT and BIZET2012; Hammons et al., Reference HAMMONS, OH, MARTÍNEZ, CLARK, SCHLEGEL, SITORIUS, SCHEIDELER and WALTER2010).

Bacterial colonisation of the crop is initiated either just prior to or after hatch (Barnes et al., Reference BARNES, IMPEY and COOPER1980). Colonisation is variable in young chicks immediately after hatch, and the speed and nature of crop bacterial colonisation is influenced by a variety of factors in the diet. Grains are naturally rich in lactic acid producing bacteria and unless toasted at extreme temperatures bacteria recover after rehydration (Apajalahti, unpublished). This is an important part of the bird's life as they also are most susceptible to colonisation by non-desirable pathogenic and zoonotic organisms at that time (Gast and Beard, Reference GAST and BEARD1989; Smith and Tucker, Reference SMITH and TUCKER1980). Factors affecting speed of lactobacilli colonisation include dietary introduction of competitive Lactobacillus strains, prebiotics and organic acids affecting their competitiveness, medication (Rada and Marounek, Reference RADA and MAROUNEK1996), and the nutrient content of the feedstuffs themselves (Rubio et al., Reference RUBIO, BRENES, SETIÉN, DE LA ASUNCIÓN, DURÁN and CUTULI1998). It is worth noting that lactobacilli are fastidious bacteria with equally complex nutrient requirements as the host itself; they need to have access to simple sugars, amino acids and vitamins for growth. Lactobacilli can be eliminated by anti-bacterial agents such as penicillin and monensin, and when this occurs, the number of coliforms increases (Apajalahti and Kettunen, Reference APAJALAHTI and KETTUNEN2006; Rada and Marounek, Reference RADA and MAROUNEK1996). Of note, Escherichia coli are capable of binding to crop squamous cells. Their colonisation can be inhibited by adhering strains of lactobacilli (ST1 Lactobacillus crispatus) because of shared adhesion sites (classic competitive exclusion), but not by those that are weakly adhesive (L. crispatus strain 134mi) (Edelman et al., Reference EDELMAN, LESKELÄ, RON, APAJALAHTI and KORHONEN2003). Similarly, lactobacilli are capable of inhibiting Salmonella colonisation of the gut (Gusils et al., Reference GUSILS, GONZÁLEZ and OLIVER1999). This ability to prevent colonisation can be attributed to a number of mechanisms including competing for adherence sites, stimulation of the immune system, antibacterial agents and lactic acid production (direct inhibitory effect of lactate or indirect effect of lowered pH). These studies (and many others) support the concept that a stable and dominant position of lactobacilli in the crop is essential for gut health and the development and maintenance of a balanced crop microbiota (Fuller Reference FULLER1973; Reference FULLER1977).

The absence of feed, such as during withdrawal prior to slaughter or moulting procedures, results in a shift in the bacterial community and predisposition of the crop to Salmonella and Campylobacter spp. colonisation. Feed withdrawal increases the potential for pathogen presence as a result of decreased lactobacilli colonisation and consequent weakened barrier of entry for pathogens. Among other changes, decreased lactobacilli numbers result in decreased production of lactic and acetic acid (the ratio of which is a species specific feature; Hammes and Vogel, Reference HAMMES and VOGEL1995), and increased pH (Durant et al., Reference DURANT, CORRIER, BYRD, STANKER and RICKE1999; Hinton et al., Reference HINTON, BUHR and INGRAM2000a). The presence of Salmonella and Campylobacter spp. in the crop of broilers at slaughter represents a human disease risk because of the higher potential of carcass contamination at slaughter from the crop than caecal rupture (Corrier et al., Reference CORRIER, BYRD, HARGIS, HUME, BAILEY and STANKER1999; Hargis et al., Reference HARGIS, CALDWELL, BREWER, CORRIER and DELOACH1995; van Gerwe et al., Reference VAN GERWE, BOUMA, WAGENAAR, JACOBS-REITSMA and STEGEMAN2010). This emphasizes the need to reduce Salmonella spp. colonisation during grow-out and maintaining this status during the feed withdrawal period. Inclusion of organic acids or fermentable growth substrates for lactobacilli in water can reduce the Salmonella and Campylobacter spp. contamination of crops and broiler carcasses (Byrd et al., Reference BYRD, HARGIS, CALDWELL, BAILEY, HERRON, MCREYNOLDS, BREWER, ANDERSON, BISCHOFF, CALLAWAY and KUBENA2001 (lactic acid); Chaveerach et al., Reference CHAVEERACH, KEUZENKAMP, LIPMAN and VAN KNAPEN2004 (acidified drinking water); Hinton et al., Reference HINTON, BUHR and INGRAM2000b (glucose); 2002 (sucrose)).

Feed withdrawal during moulting in laying hens also markedly increases the survival of Salmonella spp. in the crop (Humphrey et al., Reference HUMPHREY, BASKERVILLE, WHITEHEAD, ROWE and HENLEY1993) and may provide an environment that increases the expression of genes necessary for intestinal invasion (Durant et al., Reference DURANT, CORRIER, BYRD, STANKER and RICKE1999). In comparison to feed withdrawal, providing low calcium, low calcium and zinc, or low energy by-product diets have been shown to reduce S. Enteritidis infection, and maintain lactobacilli in the crop and a lower crop pH (Ricke et al., Reference RICKE, PARK, MOORE, KWON, WOODWARD, BYRD, NISBET and KUBENA2004; Reference RICKE, DUNKLEY and DURANT2013; Seo et al., Reference SEO, HOLT and GAST2001).

The presence of pathogens harboured in the crop and additional organisms that pass through to the lower gut suggest the need for expression of both adaptive and innate immune responses in the crop. Research has confirmed this logic with the demonstration of well-developed lymph nodules in the upper alimentary tract in the form of oesophageal tonsils (Arai et al., Reference ARAI, HASHIMOTO, KITAGAWA, KON and KUDO1988). A further demonstration of immune competency in the crop is the development of lymphoid aggregates in crop walls following challenge with S. enterica ser. Enteritidis (Seo et al., Reference SEO, HOLT, VAUGHN, GAST and STONE2003; Vaughn et al., Reference VAUGHN, HOLT and GAST2008a; Reference VAUGHN, HOLT, MOORE, GAST and ANDERSON2008b) and the presence of secretory immunoglobulins (IgA) that specifically bind to S. enterica ser. Enteritidis antigens (Seo et al., Reference SEO, HOLT, BRAKETT, GAST and STONE2002; Reference SEO, HOLT, VAUGHN, GAST and STONE2003). The high expression of β-defensin gallinacin-6 (Gal-6) in the oesophagus and crop, and the demonstration of its antimicrobial activity against food-borne pathogens, demonstrates that the crop can play a role in chicken innate host defence (Hong et al., Reference HONG, SONG, LEE and LILLEHOJ2012; van Dijk et al., Reference VAN DIJK, KALKHOVE, TJEERDSMA-VAN BOKHOVEN, ROMIJN and HAAGSMAN2007). Another innate immune function is indicated by Thaxton et al. (Reference THAXTON, CUTLER, GRIFFITH and SCANES2006) who found an increase in crop nitrate content after gavaging turkeys with S. Typhimurium. The authors suggest that the results are indicative of the activation of inducible nitric oxide synthase in resident macrophages. Taken together, these findings suggest an influence of the crop in local and total digestive tract health and pathogen colonisation.

The crop forms part of the acidic barrier formed by the crop and gizzard, which reduces passage of bacteria including pathogenic Clostridium spp. and representatives of zoonotic genera such as Salmonella and Campylobacter spp. to the distal gut (Sekelja et al., Reference SEKELJA, RUD, KNUTSEN, DENSTADLI, NÆS and RUDI2012). The modulation of crop bacteria may affect caecal microbiota, as demonstrated by experiments using dietary butyric acid in an unprotected and partially protected form to study S. enterica ser. Enteritidis colonisation in the crop and caeca (Fernández-Rubio et al., Reference FERNÁNDEZ-RUBIO, ORDÓÑEZ, ABAD-GONZÁLEZ, GARCIA-GALLEGO, HONRUBIA, MALLO and BALAÑA-FOUCE2009). Both sources of butyric acid reduced crop and caecal colonisation, but the unprotected butyric acid was more effective in the crop and less effective in the caeca as expected. Unprotected butyric acid in the diet is taken up by the small intestinal epithelium and hardly reaches the caeca. However, the still important effect of the diet amended with unprotected butyric acid in the caeca suggests that controlling colonisation in the crop benefits the entire digestive tract.

Crop pH and fermentation

The acidity of the chicken crop can vary with values ranging from well below pH 5 to greater than pH 6 (Bowen and Waldroup, Reference BOWEN and WALDROUP1969; Hinton et al., Reference HINTON, BUHR and INGRAM2000a; Józefiak et al., Reference JÓZEFIAK, RUTKOWSKI, JENSEN and ENBERG2006; Rynsburger, Reference RYNSBURGER2009; Svihus et al., Reference SVIHUS, LUND, BORJGEN, BEDFORD and BAKKEN2013b). The pH varies with the degree of crop fermentation primarily by lactobacilli and by the production of lactic acid together with weaker acetic acid (Cutler et al., Reference CUTLER, RASMUSSEN, HENSLEY, WILHELMS, GRIFFITH and SCANES2005; Józefiak et al., Reference JÓZEFIAK, RUTKOWSKI, JENSEN and ENBERG2006). In turn, fermentation is influenced by factors such as the presence of substrate (feed or prebiotic) and again colonisation by lactobacilli (Barnes et al., Reference BARNES, IMPEY and COOPER1980; Fonseca et al., Reference FONSECA, BELETTI, DA SILVA, DA SILVA, DUARTE and ROSSI2010). The pH of the crop can also be impacted by other non-fermented components of the diet. Feed ingredients have been found to have differing acid binding (buffering) capacity that can affect crop pH with mineral ingredients having the highest capacity followed by protein ingredients and then energy sources (Lawlor et al., Reference LAWLOR, LYNCH, CAFFREY, O'REILLY and O'CONNELL2005). Considering the high calcium levels in laying hen diets, one would expect that crop pH values would be high. Although this has been reported to be the case (Bolton, Reference BOLTON1965), an examination of pH values for hens fed on an ad libitum basis from 17 research trials averaged pH 4.91 with a range of 4.29 to 6.00 (Durant et al., Reference DURANT, CORRIER, BYRD, STANKER and RICKE1999, Reference DURANT, CORRIER, STANKER and RICKE2000; Gordon and Roland, Reference GORDON and ROLAND1997; Kubena et al., Reference KUBENA, BYRD, MOORE, RICKE and NISBET2005; Moore et al., Reference MOORE, PARK, KUBENA, BYRD, MCREYNOLDS, BURNHAM, HUME, BIRKHOLD, NISBET and RICKE2004; Nahashon et al., Reference NAHASHON, NAKAUE and MIROSH1994; Ricke et al., Reference RICKE, PARK, MOORE, KWON, WOODWARD, BYRD, NISBET and KUBENA2004). Of interest, the values for hens being moulted by feed withdrawal averaged pH 5.91 with a range of 5.42 to 6.96. These values suggest that considerable fermentation occurs in the crop of laying hens and in turn the reduction in pH can affect its microbial community.

As noted above, extended dark periods result in anticipatory eating and crop storage (Cutler et al., Reference CUTLER, RASMUSSEN, HENSLEY, WILHELMS, GRIFFITH and SCANES2005). In turkeys provided with a 14:10D lighting program, feed gradually emptied from the crop for 9 hours after the beginning of darkness and during that time pH decreased from 5.9 to 5.0; at the same time levels of lactic acid and other short chain fatty acids were increasing. This demonstrates that the presence of feed in the crop for an extended time enhances fermentation. In a second experiment, Cutler et al. (Reference CUTLER, RASMUSSEN, HENSLEY, WILHELMS, GRIFFITH and SCANES2005) found that the number of S. enterica ser. Typhimurium in crop digesta decreased from 7.1 to 4.9 (log10)/gram of crop digesta over an eight hour period starting after the end of the photophase. This research confirms the beneficial effect of fermentation on crop pH and subsequently bacterial colonisation.

The low pH has direct inhibition effects against a variety of pathogenic and zoonotic organisms and also enhances the impact of butyric acid when it is used in water or broiler feed. The effectiveness against these organisms increases at lower pH because butyric acid in its undissociated form can cross bacterial membranes and acidify the bacterial cell cytoplasm. Therefore, unprotected free sodium butyrate should be more effective against bacteria when converted to protonated butyric acid at the acidic pH of the upper portion of the digestive tract (Van Immerseel et al., Reference VAN IMMERSEEL, RUSSELL, FLYTHE, GANTOIS, TIMBERMONT, PASMANS, HAESEBROUCK and DUCATELLE2006).

Pendulous crop

Loss of tone in muscle can result in pendulous crop. The condition is more prevalent in turkeys, but can be found in broiler chickens and occasionally occurs at relatively high levels (Ebling et al., Reference EBLING, PONTALTI, VILLANUEVA and RIBEIRO2015). Despite the usually low incidence, research on pendulous crop may provide clues on management and nutrition required to produce a ‘healthy’ crop. The etiology of pendulous crop is not clear, but incidence has been linked to genetic, environmental and nutritional influences (Asmundson and Hinshaw, Reference ASMUNDSON and HINSHAW1937; Wheeler et al., Reference WHEELER, REID, FERGUSON and COUCH1960). In a comparison of feeding glucose monohydrate or starch to turkeys, a high incidence of pendulous crop was found for the birds fed glucose monohydrate and none were found for birds fed starch. In addition, abnormal fermentation was found as a result of yeast and fungi colonisation of the pendulous crop birds. Evidence from turkey lighting research (Vermette et al., Reference VERMETTE, SCHWEAN-LARDNER, GOMIS, CROWE and CLASSEN2016) has found a higher incidence of pendulous crop in hens given 23 in contrast to those receiving 14, 17 and 20 hours per day. It may be that crop feed storage is required to maintain a scheduled flow of feed through the crop and maintenance of a desired lactobacilli biofilm on crop epithelium, thereby preventing abnormal colonisation by other microbiota.

Potential effects on bird performance

Exposing birds to darkness and meal feeding both promote crop storage of feed, and can improve feed efficiency (Schwean-Lardner et al., Reference SCHWEAN-LARDNER, FANCHER and CLASSEN2012; Su et al., Reference SU, SØRENSEN and KESTIN1999; Svihus et al., Reference SVIHUS, SACRANIE, DENSTADLI and CHOCT2010; Reference SVIHUS, LUND, BORJGEN, BEDFORD and BAKKEN2013b). The improvement in feed efficiency in these cases has been attributed to factors such as altered metabolism during darkness (Apeldoorn et al., Reference APELDOORN, SCHRAMA, MASHALY and PARMENTIER1999), a more concave growth curve (Buyse et al., Reference BUYSE, SIMONS, BOSHOUWERS and DECUYPERE1996b), and improved nutrient retention (Buyse et al., Reference BUYSE, KÜHN and DECUYPERE1996a; Svihus et al., Reference SVIHUS, LUND, BORJGEN, BEDFORD and BAKKEN2013b). An argument can be made that increased food softening and acid exposure, the initiation of the digestive process due to endogenous and exogenous enzymes, and slower digesta transit (at least during darkness) as a result of feed presence in the crop may result in improved nutrient digestibility and feed efficiency. Svihus et al. (Reference SVIHUS, LUND, BORJGEN, BEDFORD and BAKKEN2013b) speculated that improved gizzard digestion as indicated by reduced gizzard pH contributed to the improvement in feed efficiency seen with intermittent feeding. Of interest, in a comparison of graded levels daylength (14, 17, 20, 23 hours), gizzard size increased and jejunum and ileum weights decreased with shorter days, which result in improved feed efficiency (Classen, Schwean-Lardner and Fancher, unpublished).

As mentioned above, lactobacilli require simple sugars and amino acids in their growth. Why then does the utilisation of dietary substrates and nutrients by the crop lactobacilli not reduce the feed conversion efficiency of the host? The effect of this apparent competition for nutrients between the host and the upper intestinal microbiota (in the crop and small intestine) on bird performance depends on i) the extent to which the host can benefit from the produced microbes and their metabolic products, ii) other nutritional and health benefits provided by the growing intestinal lactobacilli, and iii) the proportion of feed entering the crop and becoming utilised by its microbes. Lactobacilli have anaerobic metabolism, and therefore are able to extract only little energy from carbohydrates such as glucose. In their metabolism, homofermentative species produce mainly lactic acid, but heterofermentative species also to a varying extent produce acetic acid and ethanol (Hammes and Vogel, Reference HAMMES and VOGEL1995). The acids and ethanol are central metabolic intermediates and excellent substrates for the host and, provided that they are readily taken up from the digestive tract, cellular dehydrogenases convert them into intermediates that enter the citric acid cycle and subsequent ATP-producing electron transport chain. If lactobacilli converted 100% of glucose to lactate, acetate, or ethanol and the metabolites would be completely absorbed, the energy deficit for the bird would be only 5% (calculation based on ATP yield from glycolysis, citric acid cycle and aerobic respiration).

In a crop with lower pH as a result of fermentation, conditions are optimised for some enzymes, both endogenous and exogenous (Zeller et al., Reference ZELLER, KÜHN and RODEHUTSCORD2016). In particular, it is well established that phytate hydrolysis in the crop can be extensive (Lan et al., Reference LAN, ABDULLAH, JALALUDIN and HO2010; Onyango et al., Reference ONYANGO, BEDFORD and ADEOLA2005; Svihus et al., Reference SVIHUS, SACRANIE, DENSTADLI and CHOCT2010; Zeller et al., Reference ZELLER, KÜHN and RODEHUTSCORD2015; Reference ZELLER, KÜHN and RODEHUTSCORD2016). Increased crop hydrolysis does not necessarily lead to differences in terminal ileum phytate disappearance (Zeller et al., Reference ZELLER, KÜHN and RODEHUTSCORD2015). However, reducing phytate's ability to bind minerals and protein at this early stage of the digestive process should reduce its negative effects on nutrient digestibility (Yu et al., Reference YU, COWIESON, GILBERT, PLUMSTEAD and DALSGAARD2012). Of interest is the finding that β-glucanase supplementation of broiler diets containing oats or barley increased the lactic acid concentration and lowered the pH in the crop (Józefiak et al., Reference JÓZEFIAK, RUTKOWSKI, JENSEN and ENBERG2006). This demonstrates that exogenous enzymes are active in the crop and as a consequence influence conditions in the lower gut that beneficially affect production characteristics. In the latter research, enzyme use improved performance criteria, but it is not possible to relate this improvement to effects in the crop. Unfortunately, the lighting program used was not identified so it is also not possible to know whether increased crop storage would enhance the β-glucanase effect. The potential for enzyme activity in the crop will be affected by the match between the crop environment and enzyme activity requirements, as well the time of substrate exposure.

Developing and maintaining a healthy crop

The crop, though often relegated to being just a storage organ, can play a role in broiler health and nutrient retention, as well as colonisation by zoonotic organisms. Positive effects of the crop on broiler health, nutrient retention, as well as colonisation by zoonotic organisms appear to relate to regular utilisation of the crop for feed storage. Because of the susceptibility to colonisation by undesirable organisms in newly hatched birds, early promotion of crop lactobacilli colonisation is highly desirable.

Providing birds with darkness is an easy method of stimulating crop utilisation. However, the use of continuous or near-continuous light is predominant in the early part of a broiler's life with introduction of more substantial periods of darkness no earlier than four days and most often seven or more days of age. The degree of crop utilisation during this period is not known, but it is common to judge crop fill on the day after placement, with a recommendation that ~90% of birds will have feed in their crop. This degree of fill may relate to it being the first meal, and as feeding behaviour acclimatizes to continuous light the degree of crop storage may be decreased. It is possible that earlier provision of darkness would increase crop storage of feed. Intermittent lighting post-hatch has been recommended for broilers to synchronize flocks and provide young chicks with rest (Malleau et al., Reference MALLEAU, DUNCAN, WIDOWSKI and ATKINSON2007). Similarly, day-of-placement introduction of darkness (4 x 4L:2D) is also recommended for laying hen pullets (Lohmann Tierzucht, 2016).

During the rearing period of broilers, turkeys and laying hens, a longer dark period could be initiated to encourage anticipatory feeding prior to the scotophase. A longer dark phase would appear to be appropriate because of the evidence that four hours of darkness is required to develop anticipatory feeding behaviour. Alternately, a longer dark period could be combined with intermittent lighting during the ‘photophase’. Simulating dawn and dusk would be recommended for both intermittent and diurnal phases of the broiler life cycle to provide a stronger signal of the upcoming dark period.

Near slaughter age, it is not uncommon for broilers to be returned to near continuous light to increase ease of bird handling and preparation for feed withdrawal. This may reduce crop lactobacilli numbers and make the crop more susceptible to Salmonella and Campylobacter spp. colonisation, a situation that will worsen during feed withdrawal. Maintaining a longer dark period until marketing and then planned feeding prior to broiler harvesting may reduce this problem by maintaining a healthy crop, and provide feed longer in the crop (with fermentation and low pH) during withdrawal. Concerns regarding digesta contamination at slaughter because of feed in the crop may not be warranted if the feed withdrawal process is properly managed.

Feed withdrawal stimulates enhanced crop use, and this can readily be accomplished by planned meal timing or simply allowing feeders to empty on one or more occasions during a day (Svihus, Reference SVIHUS2015). This type of feeding could also be used to enhance crop use in broilers exposed to a diurnal lighting program (one light and one dark period per day). Meal feeding during the mid portion of the photophase would ensure crop use for a longer portion of the total day period, and complement crop use after lights come on and during anticipatory feeding before lights go off.

Enhancing the maturation of the crop microbiota after hatch may provide a less susceptible environment to pathogenic load, and appropriate probiotics (Beasley et al., Reference BEASLEY, TAKALA, REUNANEN, APAJALAHTI and SARIS2004; Abbas Hilmi et al., Reference ABBAS HILMI, SURAKKA, APAJALAHTI and SARIS2007), and carbohydrates and other nutrients targeted for lactobacilli may be useful to accomplish that goal (Lebeer et al., Reference LEBEER, VERHOEVEN, CLAES, DE HERTOGH, VERMEIRE, BUYSE, VAN IMMERSEEL, VANDERLEYDEN and DE KEERSMAECKER2011). Although in ovo application of probiotics would seem appropriate to ensure earliest possible bacterial colonisation (Edens et al., Reference EDENS, PARKHURST, CASAS and DOBROGOSZ1997), the authors are unaware of successful applications.