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The Endocrine Interface of Environmental and Egg Factors Affecting Chick Quality¹

Laboratory of Livestock Genetics, Immunology and Physiology, Faculty of Bioscience Engineering, Department of Biosystems, Katholieke Universiteit Leuven, 3001 Heverlee, Belgium

² Corresponding author: Eddy.Decuypere{at}biw.kuleuven.be

	ABSTRACT

TOP ABSTRACT DAY-OLD CHICK QUALITY:... FACTORS AFFECTING DAY-OLD CHICK... FACTORS AFFECTING DAY-OLD CHICK... INVOLVEMENT OF THYROID AND... SOME CONSEQUENCES OF... UNSCORED FACTORS AFFECTING... REFERENCES

 
Day-old chicks are the endproduct of the hatchery industry and form important starting material for the broiler farms. The major objective is to obtain a high hatchability of marketable chicks and a low spread of hatch. For the farmers, these chicks have to perform well, which is translated in high viability, high growth rate, high breast meat yield, and low feed conversion. A good-quality 1-d-old chick is hence a crucial hinge between the hatchery and the broiler farm. Moreover, maximal hatchability is not always synonymous with maximal posthatch viability and growth potential of the chick. Quantitative and qualitative scoring of chick quality is assessed. We will briefly discuss some recently developed scoring systems, which will convert differences in qualitative parameters into a quantitative score. Preincubation factors such as egg storage duration and age of broiler breeders, as well as incubation conditions, affect day-old chick quality and subsequent broiler performance. Heat production and metabolism, hormonal balances of thyroid hormones and corticosterone, and gas exchange (O₂, CO₂) are of fundamental importance for embryonic development and survival during incubation. Results from our studies indicated that embryos with higher pCO₂ levels in the air cell and higher triiodothyronine-thyroxine ratios at internal pipping or in the newly hatched chicks had higher hatchability, chick quality, and posthatch chick growth until 7 d of age. Incubation factors such as temperature, turning conditions, or gaseous environment also affect development, change concentrations of hormones related to metabolism and growth of the embryo, and in this way affect 1-d-old chick quality. Moreover, the spread of the hatch process is affected by incubation conditions as well as by the aforementioned preincubation factors. Depending on the spread of the hatching curve together with the place in the sequence of hatching (early or late) and in interaction with quality of the eggs set for incubation, storage duration, and age of breeders, there will be a period between hatch and first feeding of variable length. This may have repercussions on overall growth and many related physiological processes such as yolk uptake, metabolic level, and gastrointestinal development. In its turn, time of first feeding is related to some crucial hormone levels and enzyme activities for growth that are strongly influenced by posthatch food intake such as insulin and p70S6 kinase activity, a key enzyme in the control of protein synthesis. The magnitude of the effect of delayed feeding is dependent on the spread of hatching as well as on the hatching period within the hatching window. This may be related to the different intrinsic quality or characteristics of chicks, e.g., early vs. late hatchers, as is shown by their respective hormonal levels. The latter may be a causal factor for the actual hatching time within the hatching window as well as for the later intrinsic quality of the hatched chick, which is not reflected in any of the actual scoring systems for chick quality. This is largely ignored in previous studies and in hatchery practice so far.

Key Words: chick embryo • chick quality • incubation • thyroid • corticosterone

	DAY-OLD CHICK QUALITY: DEFINITION?

TOP ABSTRACT DAY-OLD CHICK QUALITY:... FACTORS AFFECTING DAY-OLD CHICK... FACTORS AFFECTING DAY-OLD CHICK... INVOLVEMENT OF THYROID AND... SOME CONSEQUENCES OF... UNSCORED FACTORS AFFECTING... REFERENCES

 
Day-old chicks are the endproduct of the hatchery industry and form important starting material for the broiler farmers. The major objective of a hatchery is to obtain a high hatchability of marketable chicks and a low spread of hatch. For the farmers, these chicks have to perform well, which is translated in high viability, high growth rate, high breast meat yield, and low feed conversion. A good-quality 1-d-old chick is thus a crucial hinge between the hatchery and the broiler farm.

Day-old chick quality at takeoff seems to be an all or none question: marketable or nonmarketable chicks. Moreover, it has been reported that good hatchability does not necessarily positively correlate with a high percentage of good-quality chicks and that maximal hatchability is not always linked with the highest posthatch viability and growth of the chick. The parameters used for quality selection are neither well defined nor standardized and depend on the judgment of individual persons; in the hatchery, the general chick appearance and vitality and alertness is often used as criteria for assessing chick quality.

Quantitative Scoring of Chick Quality
Chick hatching weight is often used as quantitative measurement of chick quality. However, there are also reports stating that 7- or 10-d-old BW may be more related to slaughter weight than 1-d-old chick weights. In literature, there are conflicting results about a clear positive relationship between 1-d-old chick weight and BW at slaughter age. The use of growth potential measured as relative growth up to 7 d of age is also used as an a posteriori quantitative assessment of chick quality, predicting performance at 6 wk of age better than 1-d-old chick weights. There are also reports describing a relationship between 1-d-old chick length and rate of yolk absorption and age of breeders or incubation conditions (Hill, 2001). A study of Wolanski et al. (2003) showed that chick length correlated well with yolk-free body mass at hatch throughout 8 different Hybro pure lines. Moreover, there was a weak but significant correlation (r = 0.20) between chick length at hatch and BW at 6 wk of age, indicating that chick length could be a tool for predicting chick growth potential, although Deeming (2005) doubted the usefulness of chick length as a meaningful indicator of chick quality.

Qualitative Scoring of Chick Quality
It is generally agreed that a 1-d-old chick of good quality must be clean, dry, and free from dirt and contamination, with clear and bright eyes free from deformities and with completely sealed and clean navels and no yolk sacs or dried membranes protruding from the navel area. It should be alert and interested in its environment, responding to sound. It should also have a normal conformation of legs, no hock, no swelling, no skin lesions, and it should have a well-formed beak and no soft but straight toes. However, the individual or collective influences of these qualitative parameters on posthatch performance are not well known. Recently, 2 scoring systems have been developed to convert differences in qualitative parameters into a quantitative score (Boerjan, 2002), thus describing more objectively the quality of the 1-d-old chick by taking into consideration all these morphological (qualitative) criteria. In the Pasgar score, chicks lose points from a maximum score of 10 when abnormalities are observed. The incubation and egg quality research group of Leuven (Tona et al., 2003a) developed a trapped scoring system (differential importance of different parameters) with a total score from 0 to 100 based on a wide range of parameters, each with a hedonic score. Chicks of optimal quality, being free from any abnormalities, had a maximum score of 100. The chicks with a score of 100 showed the highest relative growth up to 7 d of age and the highest BW at 6 wk of age. These results indicate that scoring of 1-d-old chick qualitative aspects is relevant for predicting broiler performance. Interestingly, most parameters were highly correlated with the condition of the navel area, amount of retracted yolk, and chick activity. In addition, the incidence of 1-d-old chicks with subnormal conditions in the navel area was the highest.

From the above, it is clear that 1-d-old chick quality affects broiler performance; chicks of high quality, as scored by their morphological and qualitative characteristics at takeoff, will result in a good-performing broiler. However, chick quality itself can be influenced by preincubation factors and incubation factors such as egg storage duration, age of broiler breeders, and incubation conditions. We will focus on these aspects in the following topics.

	FACTORS AFFECTING DAY-OLD CHICK QUALITY: PREINCUBATION FACTORS

TOP ABSTRACT DAY-OLD CHICK QUALITY:... FACTORS AFFECTING DAY-OLD CHICK... FACTORS AFFECTING DAY-OLD CHICK... INVOLVEMENT OF THYROID AND... SOME CONSEQUENCES OF... UNSCORED FACTORS AFFECTING... REFERENCES

 
Storage Time
Hatching eggs can be successfully stored for up to 7 d with little or no effects on hatchability. However, when stored for more than 1 wk, embryonic abnormalities and mortality increases, which causes a decline in hatchability; moreover, incubation time is delayed when eggs are stored for a longer time.

Long-term egg storage is known to affect general egg quality (yolk membranes, yolk, perivitelline layers). The incidence of more abnormal and dead embryos may be related to a higher number of embryonic cells with necrotic nuclei and an increase in number of apoptotic cells (cells programmed to die) as a result of storage. The initiation of embryonic development is delayed, which could be brought into relation with the delay in hatching time. This delay is seen in a later occurrence of the start of internal pipping (IP) and a prolonged IP stage (Tona et al., 2003b). Moreover, the rate of embryonic development is slower due to longer storage, but this phenomenon is not observed for all embryos, indicating that not all embryos are affected by storage in the same way (Fasenko et al., 2002). Not only growth but also metabolism is found to be influenced by storage time. It has been shown that the metabolism of embryos, as measured indirectly by embryonic CO₂ output, proceeds at a slower rate. Interestingly, Fasenko (1996) showed that turkey embryos from 14-d stored eggs relied more upon gluconeogenesis during pipping and hatching than embryos from 4-d stored eggs. Christensen et al. (2001b) showed that embryos from a line that resisted storage mortality maintained higher glycogen concentrations in muscle and heart tissues than those from a line susceptible to storage. The ability of the embryos to metabolize adequate carbohydrate reserves at time of hatching seems to be an advantage for embryonic growth and survival.

Day-old chick quality is the result of events during embryonic development, and from the foregoing, it is clear that storage clearly affects embryonic development in different aspects. Storage for a longer duration results in a greater occurrence of poor-quality chicks (Boerjan, 2002 ; Fasenko et al., 2002 ; Tona et al., 2003b) than from eggs stored for a shorter duration, and this is further exacerbated by the storage of eggs of breeders older in age. However, the poorer growth of a flock hatching from longer-stored eggs cannot be totally ascribed to the greater occurrence of poor-quality chicks (Tona et al., 2004a).

Age of Breeders
It is well known that incubating egg weight and therefore day-old chick weight at hatch depends on the age of the breeder. Similarly, Hill (2001) reported an increase in chick length with increasing age of the breeder. The incidence of chicks of subnormal quality is higher in chicks of hatching eggs of older breeders (Tona et al., 2001; Boerjan, 2002 ; Tona et al., 2004a). Fresh eggs from young breeders have better albumen quality, hatch better, and produce higher percentages of high-quality 1-d-old chicks, although with lower weights at hatch but with a higher posthatch growth rate compared with older breeders (Tona et al., 2004a).

There are several reports indicating differences in hormones and metabolites between chicks originating from a young or old broiler breeder flock, although in these studies, no direct relation has been made with chick quality. Christensen et al. (1996) showed that the physiology of turkey embryos originating from breeders from different ages is different in terms of glycogen concentrations in tissues, blood glucose plasma concentrations, and thyroid hormone concentrations. Results of Noble et al. (1986) and Latour et al. (1998) indicate that, besides glucose, lipids and fatty acid profiles in the developing embryo are also affected by broiler breeder age. Weytjens et al. (1999) reported that the thermoregulatory ability of broiler chickens originating from a young or old flock is also different.

The age effect could be reversed by moulting the breeders, leading to an improvement of hatchability and chick quality in terms of growth performance (Tona et al., 2002). This suggests that the effect of moulting on the 1-d-old chick could be due to an improvement of egg quality.

Embryo Physiology and Chick Quality
The hatched 1-d-old chick is the result of 21 d of development in which several physiological and endocrine changes take place in the embryo in controlled incubation conditions. Heat production and metabolism, hormonal balances of thyroid hormones and corticosterone, and gas exchange (O₂, CO₂) are of fundamental importance for embryonic development and survival during incubation. Results from our studies indicated that embryos with higher pCO₂ levels in the air cell, and higher triiodothyronine- (T₃) thyroxine (T₄) ratios at IP or in newly hatched chicks had better hatchability, chick quality, and post-hatch chick growth up to 7 d. In addition, the work of Christensen, 2001b showed that adequate carbohydrate metabolism in the embryo is important for development and survival.

	FACTORS AFFECTING DAY-OLD CHICK QUALITY: INCUBATION CONDITIONS

TOP ABSTRACT DAY-OLD CHICK QUALITY:... FACTORS AFFECTING DAY-OLD CHICK... FACTORS AFFECTING DAY-OLD CHICK... INVOLVEMENT OF THYROID AND... SOME CONSEQUENCES OF... UNSCORED FACTORS AFFECTING... REFERENCES

 
Humidity
Incubator humidity is a controlling factor for the evaporation of water from the egg and must be properly controlled for proper embryo growth to proceed. The mass loss by the egg is dependent on the incubator humidity and the eggshell conductance, and it is known that a high natural variation of conductance exists in eggs. This causes a variation in the mass loss of eggs from 5 to 20% (optimal 11 to 13%) in standard conditions. One well-known factor affecting the eggshell conductance is the age of the breeder, with increasing conductance with age (eggs from older flocks should need a higher humidity during incubation). Due to the high variation in eggshell conductance among incubating eggs, the optimal humidity level in the incubator for maximal hatchability may be variable as well (40 to 60% RH in literature). To optimize the mass loss of the eggs, it is therefore advisable to match the incubator humidity with the eggshell conductance to decrease embryonic mortality and increase hatchability.

Only 1 report of Meir and Ar (1987) showed that adaptation of RH to eggshell conductance resulted in higher hatchability of turkey poults with improved poult quality. In contrast, the work of F. Bamelis (Katholieke Universiteit Leuven, unpublished data) showed that when RH was matched to eggshell conductance, no significant improvement of chick quality was observed.

Temperature
The optimum operating temperature for poultry species during incubation appears to be from 37 to 38°C. Incubation temperature is not only important for normal embryonic development and hatching success but also affects posthatch performance. It is recommended that incubation temperature is adapted to the natural heat production pattern of the incubating egg to obtain the highest hatchability with good-quality chicks. A recent publication of Lourens et al. (2005) showed that the highest hatchability, embryo development (higher embryo length and yolk-free embryo weight), and posthatch performance was found when eggshell temperature was maintained at 37.8°C constantly throughout incubation.

Too high of temperatures in the hatcher must be avoided to avoid late mortality and slower posthatch growth. In addition, constant exposure to high temperatures will adversely affect normal maturation and hatch, producing many stunted chicks with poorly closed navels and red hocks.

Boerjan (2002), using the Pasgar score, reported an improvement in quality score (9.1 vs. 8.6) when eggs from hens of 45 wk of age were incubated at higher temperatures (0.12°C from d 10 to 12 and 0.55°C at d 18 of incubation ) compared with standard incubation conditions.

Increased incubation temperature during the initial stages of development of eggs stored 15 d improves livability of turkey embryos by accelerating embryonic growth and development early in the incubation period (37.8 vs. 37.5°C; interaction storage time x incubation temperature). A possible mechanism for the improved livability might be due to a better utilization of carbohydrates (Christensen et al., 2003a,b). However, it remains to be investigated if incubation under higher temperatures during the first period of incubation also favors quality from these chicks hatched from eggs stored for a longer period and improves posthatch performance.

Additionally, changes in incubation temperature during the late stages of incubation can induce physiological changes in avian embryos. Results of Christensen et al. (2001a) showed that growth of turkey embryos can be altered by increasing incubator temperatures during the last 3 d of incubation. This increased incubator temperature elevated embryonic plasma glucose concentrations of the embryos and altered insulin-like growth factor concentrations. These findings indicate that incubation conditions can change concentrations of hormones related to metabolism and growth of the embryo and in this way could affect 1-d-old chick quality.

Turning Conditions
Egg turning has been reported to reduce malpositions, to prevent abnormal adhesion of the embryo or embryonic membranes to the shell membrane, to encourage the complete and timely closure of chorioallantois at the small end of the egg, and, most importantly, is needed to achieve an optimal albumen utilization by the embryo (Deeming, 2002; Tona et al., 2005). Insufficient turning during incubation leads to a delay of hatching and adversely affects day-old chick qualitative aspects. Tona et al. (2003c) found that hatchability and percentage of high-quality chicks were lower for eggs turned for 15 d compared with those turned for 12 or 18 d (d 15 is in the period of an increasing functional hypothalamus-pituitary-adrenal axis). Tona et al. (2005) hypothesized that discontinuation of turning at this time may be an additional stressor that can lead to physiological imprinting and altered responsiveness, leading to lower hatchability and chick quality. Other reports, however, do not show any effect of stopping egg turning at d 15 on hatchability (Deeming, 2002).

Gaseous Environment
Recently, a lot of emphasis on research as well as on practical implementation is put on the use of increased CO₂ levels in early incubation (Hogg, 1997; De Smit et al., 2006; Tona et al., 2006). Moreover, not only hypercapnia but also hypoxia is known to affect different aspects of early development, but this is beyond the scope of this paper.

Not only early gaseous environment but also during late incubation, higher concentrations of CO₂ stimulate hatching in eggs, and this may interact with the genetic line of broilers used for later performance (Buys et al., 1998), as well as with endocrine factors involved in the process of pipping and hatching.

	INVOLVEMENT OF THYROID AND ADRENAL HORMONES IN PERINATAL PROCESSES

TOP ABSTRACT DAY-OLD CHICK QUALITY:... FACTORS AFFECTING DAY-OLD CHICK... FACTORS AFFECTING DAY-OLD CHICK... INVOLVEMENT OF THYROID AND... SOME CONSEQUENCES OF... UNSCORED FACTORS AFFECTING... REFERENCES

 
In late embryonic development, the plasma concentration of T₃ rises dramatically. This increase in plasma T₃ occurs considerably later than that of T₄, hence the ratio to T₃:T₄ increases. This is due partly to a shift from 5 to 5'-monodeiodinase activity in ovo, but it is also, and even mainly, due to an inhibition of the T₃-degrading activity.

Indeed, in the chick embryo, glucocorticoids, which are also increasing at the end of incubation, effectively increase plasma T₃ concentration by reducing the hepatic T₃-degrading activity. The interaction of the thyroid and adrenal axis also extends to the hypothalamo-pituitary axis as corticotrophin-releasing hormone induces an elevation of glucocorticoids but also of thyrotropin and hence raises T₄ concentrations, which are a substrate for T₃ production. This is illustrated by in ovo injections of the corticotrophin-releasing hormones adrenocorticotropin and dexamethasone. This mutual interaction initiates and enhances many important physiological processes in the perinatal period.

There is evidence for a role of thyroid hormones in hatching. Not only do plasma concentrations of both T₃ and T₄ peak at the time of hatching, but also the later-hatching chicks in a batch have lower levels of both thyroid hormones (Decuypere et al., 1990). Storage of eggs before incubation results in an overall longer incubation as well as in a prolonged IP stage, but it also results in a decreased corticosterone and decreased T₃ level at the IP stage, both of which are interdependent (Tona et al., 2003b). Additionally, when comparing different genetic lines of broiler chicks, this interdependency of levels of corticosterone, T₃, and pCO₂ at the stage of IP is obvious, all of them being higher in the earlier hatching S-line compared with the other lines (Tona et al., 2004b).

Although administration of thyroid hormones enhances hatching of turkey embryos, the inhibition of the peripheral conversion of T₄ to T₃ in chick embryos in ovo is increasing the length of incubation (Decuypere et al., 1982). Additionally, goitrogen treatment in ovo during the last incubation week has been shown to delay pipping and abolish hatching (Balaban and Hill, 1971; Iqbal et al., 1987).

	SOME CONSEQUENCES OF DIFFERENTIAL HATCHING

TOP ABSTRACT DAY-OLD CHICK QUALITY:... FACTORS AFFECTING DAY-OLD CHICK... FACTORS AFFECTING DAY-OLD CHICK... INVOLVEMENT OF THYROID AND... SOME CONSEQUENCES OF... UNSCORED FACTORS AFFECTING... REFERENCES

 
Depending on the spread of the hatching curve, together with the place in the sequence of hatching (early or late) and in interaction with the quality of incubating eggs, storage, and age of breeders, there will be a period between hatch and first feeding of variable length. This may have repercussion on yolk uptake and utilization, development of the gastrointestinal tract, metabolic level, immune system development, and IgG uptake and overall growth, because all these parameters may be influenced by the time between hatch and first food uptake.

This is related to some crucial hormone levels and enzyme activities for growth that are strongly influenced by posthatch food intake, such as insulin and p70S6 kinase activity, a rate-limiting step for protein synthesis.

Several reports have demonstrated that delay in feed intake after hatch adversely affects posthatch performance of chicks, especially growth (Pinchasov and Noy, 1993; Bigot et al., 2003; Gonzales et al., 2003).

However, additional factors may aggravate this effect of delay in feed access. Recent data from our research group conclusively showed that delay in feed access after hatch depresses the relative growth rate of chicks, but synergic effects can be seen when one compares chicks from eggs stored for short vs. long durations before incubation or early-hatched chicks vs. late hatchers. The magnitude of the effect of delayed feeding is therefore dependent on the spread of hatching, as well as on the hatching period within the hatching window. This may be related to the different intrinsic qualities or characteristics of chicks hatching from short or longer-stored eggs or early vs. late hatchers, as is shown also by their respective hormonal levels. The latter may be a causative factor for the actual hatching time within the hatching window, as well as for the later intrinsic quality of the hatched chick. This is a factor that has so far been ignored in previous studies and in hatchery practice.

	UNSCORED FACTORS AFFECTING POSTHATCH PERFORMANCE

TOP ABSTRACT DAY-OLD CHICK QUALITY:... FACTORS AFFECTING DAY-OLD CHICK... FACTORS AFFECTING DAY-OLD CHICK... INVOLVEMENT OF THYROID AND... SOME CONSEQUENCES OF... UNSCORED FACTORS AFFECTING... REFERENCES

 
It is clear that the growth rate of chicks is affected by age of breeders, egg storage duration, and turning duration. Between chicks of the same high quality and of the same genetic line, there are still some differences in growth potential, meaning that other intrinsic factors which cannot yet be identified are involved in growth performance. The mechanisms by which these factors affect performance are still not clear. One can speculate that these factors may have modified the physiology of the embryo through changes in gene expression or changes to downstream transcription, leading to changes in relevant modulators of genes involved in growth.

Moreover, although this remains also to be investigated, it is possible that incubation under differential temperature, humidity, and ventilation conditions also leads to a 1-d-old chick with a different intrinsic "luggage" of factors beyond their genetic potential, which cannot be scored and may be involved in differential posthatch growth processes among chicks.

	ACKNOWLEDGMENTS

 
Veerle Bruggeman is a postdoctoral fellow from the Fund for Scientific Research-Flanders (FWO; Vlaanderen, Belgium). Part of this work was supported by the FWO, grant no. G.0286.04 and by the Institute for the Promotion of Innovation by Science and Technology in Flanders (Brussels, Belgium), grant no. 040510.

	FOOTNOTES

 
¹ Presented as part of the Embryo Symposium: Managing the Embryo for Performance, July 19, 2006, at the Poultry Science Association Annual Meeting, Edmonton, Alberta, Canada.

Received for publication November 27, 2006. Accepted for publication November 27, 2006.

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