Introduction
Healthy day-old chick production is the prime objective of modern hatcheries in the world. For hatcheries, not only is it important to have a high level of hatchability, but the quality of the chicks provided has to be good because broiler farmers are looking for chicks with a high growth potential, resulting in a greater slaughter yield at the end of the rearing period. The freshly hatched chicks can be scored by qualitative and quantitative parameters (Decuypere and Bruggeman, Reference DECUYPERE and BRUGGEMAN2007). There are many visual traits used to measure chick quality in meat type chickens. These traits include chick colour, hatchling length, day old chick weight, navel condition, yolk free body mass, hock colour, Tona or Pasgar score, visual score, abdomen score by abdominal palpation (to evaluate the residual yolk content in live chicks) and shank length (Hill, Reference HILL2001; Meijerhof, Reference MEIJERHOF2005; Wolanski et al., Reference WOLANSKI, RENEMA, ROBINSON, CARNEY and FANCHER2006: Ulmer-Franco et al., Reference ULMER-FRANCO, FASENKO and O'DEA CHRISTOPHER2010). Well developed visual traits are the future picture of high growth rates, greater breast meat yield, better feed conversion ratios and higher viability (Decuypere and Bruggeman, Reference DECUYPERE and BRUGGEMAN2007). Recently, hatchling length and body weight on hatch day have both been suggested as predictors of broiler performance, and used to assess chick quality in the hatchery (Mauldin et al., Reference MAULDIN, MAULDIN and JAVIER2008). Additionally the hatchling length is the best indicator of pre-incubation and incubation conditions and also is an indicator of the rate of yolk absorption after hatching in the body cavity of the chick (Hill, Reference HILL2001).
Meijerhof (Reference MEIJERHOF2005) reported that the most practical way to measure chick development is to determine the length of the chick at hatch, either by measuring the spinal cord length, the shank length or the length of the stretched chicken, measured from tip of the beak to the middle toe or from the crown to the toe. Furthermore, this author stated that the most important goal in measuring chick length at hatch is to evaluate the incubation process, egg handling, and breeder flock performance. Keeping in mind the above facts, the purpose of the present paper is to review and expose the importance of hatchling length in the chick's future performance and its parent past performance.
Why chick length is important
Many visual chick quality parameters including visual scoring, chick weight, Tona or Pasgar Score, and chick length at hatch are used to measure the chick's quality for future production performance. As the Pasgar score is mainly influenced by the conditions in the hatchery, factors of navel closure, yolk uptake and vitality have a large influence on the score, however to date, a strong positive correlation between Tona or Pasgar score and broiler performance has not been demonstrated. Additionally, it has been reported that parameters included in the assessment of chick quality and the scoring system may need to be revised (Tona et al., Reference TONA, ONAGBESAN, De KETELAERE, BRUGGEMAN and DECUYPERE2008). Chick weight is the most widely used indicator for day-old chick quality assessment (Decuypere et al., Reference DECUYPERE, TONA, BAMELIS, CAREGHI, KEMPS, De KETELAERE, De BAERDEMAKER and BRUGGEMAN2002). However, more recent research has shown that the hatchling length is now considered to be the best method of evaluating visual quality in meat type chickens (Joseph et al., Reference JOSEPH, LOURENS and MORAN2006). Generally, day old chick body weight is measured by weighing the whole chick and this weight includes an unknown amount of residual yolk in the chick's abdomen. Some chicks have as little as 2 g of yolk while others can have as much as 8 g. A larger day-old chick has a large residual yolk and a small yolk-free body mass. This indicates that the bird did not efficiently use the yolk and it therefore does not play any role in development and growth during the incubation and post-incubation processes.
Hatchling length measurement protocol
Meijerhof (Reference MEIJERHOF2005) reported that the most practical way to measure chick development is to determine the length of the chicken, either by measuring the spinal cord length, the shank length or the length of the stretched chicken, measured from tip of the beak to the middle toe (crown to toe length). Petek et al. (Reference PETEK, ORMAN, DIKMEN and ALPAY2010) reported that hatchling length in meat type birds can be measured by stretching the chick from the tip of the beak to the middle toe using a ruler and recorded in centimetres. Measuring hatchling length at hatch is a very effective and realistic approach to appraise chick quality. For this purpose only simple things are needed; the hatchling length recording form/ register, a simple ruler and a digital chick length analysis program. There are seven simple steps to clearly determine the hatchling length at hatch. Firstly, if you want to determine the parents’ and incubation performance you must select five groups of birds randomly before grading or culling. If you want to determine the chicks own performances select 100 chicks, 20 chicks from each of five groups. Then take the group with 100 chicks for transportation (after culling and selection). Secondly, line up the tip of the beak on the end of the ruler and stretch the chick along the ruler. Thirdly, stretch the chick gently and measure the total length from crown to toe or from the tip of the beak to the end of the middle toe. Fourthly, hatchling length must be rounded to the nearest 0.6 centimetre and recorded. Fifthly, count the total number of chicks per category you have formed. Sixthly, enter the total number of chicks in each category in the digital chick length analysis program, which will ultimately calculate the percentage of chicks in each category, the average hatchling length, and the percentage of small chicks. Lastly, work out the hatchling length data. Based on the body mass index, as used in human medical health care, the ratio between day-old BW and hatchling length squared can be calculated (HatchTech, b).
Different studies have shown that hatchling length is related to yolk free body mass and predicts subsequent performance (Willemsen et al., Reference WILLEMSEN, EVERAERT, WITTERS, DeSMIT, DeBONNE, VERSCHUERE, GARAIN, BERCKMANS, DECUYPERE and BRUGGEMAN2008; Petek et al., Reference PETEK, ORMAN, DIKMEN and ALPAY2010). Hatchling length measurements can vary between technicians collecting the data, but research has shown that it can be standardised by experience. Flock age must be taken into account when data is evaluated. Measuring hatchling length is a fast method to evaluate chick quality, and only 25 birds need to be sampled.
Methods to improve hatchling length
Keeping in mind the importance of hatchling length, as compared to chick weight at hatch, there is a need to improve this quality parameter. Researchers have recommended the following to improve the hatchling length at hatch - maintain the optimum embryo temperature until internal pipping, ensure maximum turning of eggs, minimise egg weight losses at the 18th day of embryonic development, maintain an ample O2 supply and removal of CO2 through gas exchange programs and aim for better health and better fertility of the breeder flock through better management procedures and nutrition. Ultimately, proper egg storage conditions and time are important factors to consider in order to increase chick length in meat type birds. The proper monitoring of the above factors will ensure optimum uniformity in hatchling length, which ultimately results in better field performance. In this regard, instruments like Micro Climer setters and hatchers by Hatch Tech help in providing optimum incubation environments and producing the longest chicks with the optimum performance (Hatch Tech a; b).
Hatchling length and broiler performance
Hatchling length is a good tool for predicting the growth potential of meat type chickens (Decuypere and Bruggeman, Reference DECUYPERE and BRUGGEMAN2007; Molenaar and Reijrink, Reference MOLENAAR and REIJRINK2011). Hatchling length has a positive correlation with broiler performance at six weeks of age, substantially higher than the correlation between day old body weight and broiler performance (Meijerhof, Reference MEIJERHOF2005). Similar findings were reported by Molenaar and Reijrink (Reference MOLENAAR and REIJRINK2011) who found this relationship at seven days of age. They further reported that each extra cm of hatchling length at day of hatch meant an increase of 18 g BW at seven days of age. Chicks with longer hatchling length have better feed efficiency and survival rates as compared to smaller chicks (Petek et al., Reference PETEK, ORMAN, DIKMEN and ALPAY2010). According to Hill (Reference HILL2001), Wolanski et al. (Reference WOLANSKI, LUITEN, MEIJERHOF and VEREIJKEN2003), Meijerhof (Reference MEIJERHOF2005), and Molenaar et al. (Reference MOLENAAR, REIJRINK, MEIJERHOF and VAN DEN BRAND2008), hatchling length is positively correlated with BW at 42 d of age, with an average correlation coefficient throughout these studies of 0.33. Moreover, Wolanski et al. (Reference WOLANSKI, RENEMA, ROBINSON, CARNEY and FANCHER2006) concluded that chick length plus shank length of day-old chicks is a better predictor of growth performance than BW at 1 d of age. However, as Deeming (Reference DEEMING2005) pointed out, substantial scientific evidence of the predictive value and objectivity of hatchling length as a predictor of growth potential has yet to be provided. The measurement of uniformity of chick length at hatching is more reliable than body weight because it gives a positive correlation with body weight (Petek et al., Reference PETEK, ORMAN, DIKMEN and ALPAY2010) and is an important tool in predicting the exact growth performance of broilers (Hill, Reference HILL2001; Wolanski et al., Reference WOLANSKI, RENEMA, ROBINSON, CARNEY and FANCHER2006; Decuypere and Bruggeman, Reference DECUYPERE and BRUGGEMAN2007).
Hatchling length and body weight ratio
Hatchling length and body weight on hatch day have both been suggested as predictors of broiler performance, and used to assess chick quality in the hatchery. It has been suggested that longer chicks may be the result of excess heat during the later stages of incubation and that longer chicks do not necessarily result in better quality chicks. Other proponents of hatch day body weight measuring argue that chick length and body weight are highly correlated, and it is easier to measure body weight. This relationship was further highlighted in research that showed body length uniformity in day old chicks was more important than body weight uniformity (Petek et al., Reference PETEK, ORMAN, DIKMEN and ALPAY2010). These workers concluded that chicks having a larger length and deep yellow colour are preferred because they provide optimal growth performance without major problems. The length of the chicken is related to body weight, but to a lesser extent, as reported by Meijerhof (Reference MEIJERHOF2005). In meat type broiler breeders, longer day-old chicks were shown to have a better growth potential throughout the experiment (Petek et al., Reference PETEK, ORMAN, DIKMEN and ALPAY2010). Research has shown that a 1 cm chick length advantage at day of hatch can result in 264 g more body weight with 45 g more breast meat yield at 38 d of age (Molenaar et al., Reference MOLENAAR, REIJRINK, MEIJERHOF and VAN DEN BRAND2008).
Hatchling length, egg weight and parent age
The length of the chicken is related more to the size of the egg than to its body weight (Geidam et al., Reference GEIDAM, BUKAR and AMBALI2007), and egg size is related to the age of the breeder (Ulmer-Franco et al., Reference ULMER-FRANCO, FASENKO and O'DEA CHRISTOPHER2010; Nangsuay et al., Reference NANGSUAY, RUANGPANIT, MEIJERHOF and ATTAMANGKUNE2011). There appears to be a positive correlation between hatchling length and breeder age (Hill, Reference HILL2001). In one study, at hatch the chicks of the large eggs had longer chick length (Nangsuay et al., Reference NANGSUAY, RUANGPANIT, MEIJERHOF and ATTAMANGKUNE2011). The total hatchling length increased about 5% (from 19 to 20 cm) for breeders from 25 to 60 weeks of age. The large hatchling length was due to bigger egg size and breeder age, due to hormone levels, blood metabolites levels (Decuypere and Bruggeman, Reference DECUYPERE and BRUGGEMAN2007), better glucose, thyroid and Corticosterone concentration (Christensen et al., Reference CHRISTENSEN and DONALDSON1996), higher rates of metabolism, gas exchange and good thermoregulatory ability (Weytjens et al., Reference WEYTJENS, MEIJERHOF, BUYSE and DECUYPERE1999). On the whole, higher pCO2 concentration in the air cell, better T3: T4 hormone concentration (Decuypere and Bruggeman, Reference DECUYPERE and BRUGGEMAN2007) and higher rates of carbohydrate metabolism (Christensen et al., Reference CHRISTENSEN, WINELAND, FASENKO and DONALDSON2001) are good indicators of improved chick quality that can be related with greater chick length. On the other hand, hatchling length at 18 days can be used as an indicator for the efficiency of an in-ovo injection process, as it is correlated with the place (allantios, amnion, breast muscle or neck) where the vaccine is administrated (Meijerhof, Reference MEIJERHOF2005). Additionally, Deeming (Reference DEEMING2005) doubted that the hatchling length could not be a meaningful indicator in broiler production.
Hatchling length and yolk residue in the body
Residual yolk mass is extremely variable at hatch, and various estimates by researchers have ranged from 8 g or 20 to 25% (Noy et al., Reference NOY, UNI and SKLAN1996), 7.9 g or 16.8% (Chamblee et al., Reference CHAMBLEE, BRAKE, SCHULTZ and THAXTON1992) and 6.6 g or 15.5% (Murakami et al., Reference MURAKAMI, AKIBA and HORIGUCHI1992) of a chick's BW. Wolanski et al. (Reference WOLANSKI, LUITEN, MEIJERHOF and VEREIJKEN2003) found a positive correlation between hatchling length with yolk free body mass at hatch in eight Hybro pure lines. Molenaar and Reijrink (2011) reported that there was no yolk residual found after seven days of age in broilers, whereby weights ranged between 0.03 and 0.32 g. Forty-two percent of the chickens in the large chick length (20-20.6 cm) group, 59% of the middle chick length group (19.2-19.8 cm) and 20% of the small chick length group (18.2-18.8) had some residual yolk in the body cavity at seven days of age. Nangsuay et al. (Reference NANGSUAY, RUANGPANIT, MEIJERHOF and ATTAMANGKUNE2011) reported that yolk availability and rate of absorption did not influence hatchling length and that only egg size was a major factor.
Hatchling length and body organ development
The interpretation of the results of Molenaar and Reijrink (2011) indicate that large length (20-20.6 cm) at day of hatch resulted in significantly heavier heart weights which were 7.9% higher compared to those with small hatchling length (19.2-19.8) and 3% higher than medium hatchling length (18.2-18.8). Large heart weight is the best indicator of blood supply, delivering oxygen and nutrients to all the main organs. Higher heart weight contributes to better development and growth of chicks, which ultimately is expressed as maximum genetic potential (Druyan et al., Reference DRUYAN, SHLOSBERG and CAHANER2007). Similarly, liver and spleen weights were significantly higher in large hatchling length as compared to medium and small hatchling length (Molenaar and Reijrink, 2011). The liver in the chicken is the largest glandular organ and maintains homeostatic functions in the body (Diot and Douaire, Reference DIOT and DOUAIRE1999). Hence, larger liver size is the best indicator of maximum metabolic activities in the chicken (Zhao et al., Reference ZHAO, CHEN, ZHAO, ZHENG, JIANG and WEN2009).
The bursa of Fabricius was 13% higher in large hatchling length birds than small hatchlings, and 6.6% higher than medium hatchling length (Molenaar and Reijrink, 2011). The bursa of Fabricius in chicken attains its maximum size a few weeks after hatching and then gradually decreases (Romppanen, Reference ROMPPANEN1982) and produces mature B-cells to all other body parts of the chicken (Paramithiotis and Ratcliffe, Reference PARAMITHIOTIS and RATCLIFFE1997; Chrząstek et al., Reference CHRZĄSTEK, MADEJ, MYTNIK and WIELICZKO2011). The intestine length showed no difference between the hatchling length groups (Molenaar and Reijrink, 2011). The results of this study indicate that hatchling length is a good indicator of various organ functions in the body of the chicken.
Hatchling length and genetic selection
Hatchling length is an important criterion for selection of higher quality chicks in broiler breeders (Petek et al., Reference PETEK, ORMAN, DIKMEN and ALPAY2010). Therefore, hatchling length can be improved by genetic selection in breeding lines. The relationship between day-old chick quality and pre-storage incubation, length of egg storage, age of breeders and other incubation conditions indicates that considering these parameter in practices will give better hatchling lengths (Machal and Simeonovova, Reference MACHAL and SIMEONOVOVA2002; Petek and Dikmen, Reference PETEK and DIKMEN2006). These findings can be correlated with egg size, which ultimately has a positive correlation with the selected breeding lines. In the near future, selection will be carried out on the basis of better length at hatch.
Conclusions
Hatchling length is an important economic trait to predict chick development because it is positively related to yolk-free body mass at hatch and it is found to be beneficial to subsequent performance. Hatchling length is a fast, repeatable and non-destructive method to evaluate chick quality. The authors recommend that hatchling length be used in a Standard Operating Procedure to evaluate chick quality on a regular basis for better poultry production. Routine hatchling length evaluation is an important tool to monitor chick quality and the results of the incubation process. Generally, ongoing hatchling length evaluation ensures that chicks consistently have potential for optimum future performance. Genetic selection may also be used to improve hatchling length.
