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The Effect of Broiler Breeder Genetic Strain and Parent Flock Age on Eggshell Conductance and Embryonic Metabolism

J. A. Hamidu^*, G. M. Fasenko^*,1, J. J. R. Feddes^*, E. E. O’Dea^*, C. A. Ouellette^*, M. J. Wineland and V. L. Christensen

^* Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, T6G 2P5 Canada; and Department of Poultry Science, North Carolina State University, Raleigh 27695

¹ Corresponding author: gaylene.fasenko{at}ualberta.ca

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
The effect of genetic strain (Ross 308; Cobb 500) and parent flock age [young (29 wk), peak (Ross = 34 wk; Cobb = 36 wk), postpeak (40 wk), mature (45 wk), old (55 wk), and very old (59 wk)] on eggshell conductance and embryonic metabolism were examined. At each flock age, eggs from each strain were incubated for 21.5 d in individual metabolic chambers to measure embryonic O₂ intake and CO₂ output. From these data, the respiratory quotient (RQ) and metabolic heat production were calculated. Data were analyzed by the GLM procedure of SAS at P 0.05. Neither strain nor flock age influenced conductance. Total embryonic O₂ consumption, CO₂ output, RQ, and metabolic heat production over the entire incubation period were not affected by strain. Daily differences existed between strains for embryonic O₂ intake (1, 7, 16, 17, 19, 20 d of incubation), CO₂ output (1 to 4, 16 to 20 d of incubation), and heat production (4, 7, 16 to 19 d of incubation). Embryos from young, mature, old, and very old flocks produced significantly more total embryonic heat over the entire 21 d (1,712, 1,677, 1,808, and 1,832, respectively) than embryos from peak (1,601) and postpeak (1,693) flocks. Average RQ for the entire incubation period was higher in embryos from mature flocks compared with all other flock ages. Daily differences among embryos from different flock ages were shown for O₂ consumption (all but d 8 of incubation), CO₂ production (all but d 7 and 9 of incubation), and heat output. The results showed that genetic strain and parent flock age influence daily embryonic metabolism, especially during the early and latter days of incubation. These daily differences coincide with the days of incubation having a higher incidence of embryonic mortality; these 2 factors may be related. Further investigation into the relationship between embryonic metabolic heat production and mortality during incubation may lead to the development of specific incubation conditions for different genetic strains and flock ages.

Key Words: breeder strain • flock age • O₂ consumption • CO₂ production • embryonic metabolism

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
In recent years, commercial hatchery employees have noted increases in the environmental temperature surrounding the last few egg racks in multistage incubators. These racks hold eggs that are the next to be transferred to the hatcher at 18 d (these eggs have embryos that are the most developed within the incubator). The speculation is that there is greater embryonic heat production from embryos of modern broiler strains selected for growth. When first developed, multistage incubators were designed to take advantage of the fact that developmentally advanced embryos produce more metabolic heat, and this heat could be used to help incubate eggs with developmentally immature embryos that were recently placed in the incubator.

Recent research has indicated that broiler breeder genetic strains differing in growth potential have different embryonic metabolic rates (Tona et al., 2004). The strains examined were a standard heavy line (selected for broiler growth), an experimental line (heavy line selected for reproductive performance and livability), and a label-type line (selected for a medium size and slow growth). The overall embryonic heat production from 428 h of incubation to hatching was significantly different among all 3 lines (standard > experimental > label).

There is growing concern that the embryonic heat output of modern broiler embryos is much higher than that from strains of several years ago. This may be causing localized overheating in the incubator in the environment surrounding the last few egg racks, and could be influencing embryo survival and hatched chick quality. Recent research in which eggshell surface temperature was used as an indicator of embryonic temperature has examined the effect of high eggshell temperatures on embryonic mortality and chick quality (Lourens et al., 2005). The results showed that embryonic mortality was highest (during incubation wk 3) and chick quality suffered when eggs were incubated during the final week of incubation at high (38.9°C) eggshell temperatures compared with normal (37.8°C) temperatures. To optimize hatchability and chick quality, egg incubation conditions specific to various genetic strains or parent flock ages may have to be developed. In fact, O’Dea et al. (2004) previously reported that embryonic metabolism increases with increasing flock age.

Romijn and Lokhorst (1960) demonstrated that embryonic heat can be estimated by measuring O₂ consumption and CO₂ production. Because embryo metabolism is almost exclusively fueled by lipids, the O₂ consumption, and thus the metabolism of the embryo, can be estimated if CO₂ production and the respiratory quotient (RQ; ratio of CO₂ produced to O₂ consumed) are known (O’Dea et al., 2004). Rahn et al. (1974) determined that the RQ at the plateau stage of oxygen consumption in the domestic fowl (Gallus gallus) was 0.86.

Segura et al. (2006) developed a metabolic calorimeter system to indirectly measure heat production of domestic avian embryos during incubation. The design of the system was such that it monitored CO₂ output and, by using an RQ of 0.84 (Romanoff, 1967), the O₂ consumption of the embryo was estimated via calculation. Recent upgrades to this system have included the addition of an O₂ analyzer so that both embryonic CO₂ output and O₂ consumption can be directly measured and the RQ calculated.

Through genetic selection, the growth potential of broilers has increased in the past 50 yr. However, few studies have examined the influence of genetic selection on egg characteristics such as eggshell conductance. One of the factors that can influence embryonic gas exchange, and thus metabolism and heat production, is eggshell conductance. In a recent study, Wineland et al. (2006) investigated the effects of different incubator temperatures during late incubation on the intestinal maturation of embryos from eggs from broiler strains known to have either low or high eggshell conductance. The results showed that temperatures greater than 37°C during late incubation were detrimental to intestinal maturation in broiler chicks, and that this negative effect was more pronounced in embryos from eggs with low conductance.

Research has shown that metabolism is almost exclusively fueled by lipids and that every liter of O₂ consumed by a chicken embryo is equivalent to the production of 4.69 kcal of heat (Romanoff, 1967). Vleck and Vleck (1987) reported that typical O₂ consumption of a chicken egg just before internal pipping was approximately 570 mL/d, which is equivalent to 2.67 kcal/d, or 130 mW of heat production. However, these data were obtained more than 19 yr ago. During the latter periods of incubation, when there is high embryonic heat output, a lack of heat removal can be detrimental to the embryo. The quantification of heat production from embryos of modern strains is therefore of great importance.

The objectives of the present study were to examine the effect that broiler genetic strain and parent flock age have on eggshell conductance and embryonic heat production. We hypothesized that Cobb 500 embryos would have higher embryonic metabolism and eggshell conductance than Ross 308 strains, and that these 2 physiological factors would increase with increasing parent flock age. A secondary objective of this research was to continue to modify and improve metabolic equipment, which is capable of directly measuring avian embryonic O₂ consumption and CO₂ production throughout the entire 21-d incubation period.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
All experimental procedures were approved by the Faculty of Agriculture, Forestry and Home Economics Animal Policy and Welfare Committee according to the principles and guidelines outlined by the Canadian Council on Animal Care (1993).

Experimental Design

Eggs were obtained from a commercial hatchery (Maple Leaf Hatchery, Wetaskiwin, Alberta, Canada) from 2 modern broiler breeder genetic strains at 6 flock ages. A Ross 308 (R) strain, genetically selected to meet broad market requirements, was compared with a Cobb 500 (C) strain, known for its high breast meat yield. Eggs were selected from the 2 strains when the flocks were 29 wk [young (Y)], 34 (R strain) or 36 (C strain) wk [peak (P)], 40 wk [postpeak (PP)], 45 wk [mature (M)], 55 wk [old (O)], and 59 wk [very old (VO)]. Each flock age constituted 1 trial. Experimental eggs were selected within a close weight range (64.4 to 64.8 g), but an increase in average egg size as the parent flocks aged required that the average weights of the eggs be adjusted (±0.1 g) between trials.

Experiment 1—Eggshell Conductance

Equipment and Procedures for Measuring Eggshell Conductance. In each trial 15 eggs per strain were weighed and placed vertically in a 300-mL plastic container as described by O’Dea et al. (2004). The eggs were completely covered with desiccant (Drierite, W. A. Hammond Drierite Company Ltd., Xenia, OH) and placed in a desiccator. The desiccator was constructed from a metal box frame with glass panels and measured 30 x 30 x 26 cm. To equalize the air pressure between the desiccator and the external environment, a 1-cm hole was drilled into the metal frame of the desiccator and a 30-cm-long tube (0.79 cm in diameter) was packed with desiccant and inserted into the hole. The edges of the tube inserted into the desiccator were sealed with silicone caulking to prevent air leakage. A thermometer was placed inside the desiccator to record daily ambient temperature, and the daily temperatures were used to calculate saturated vapor pressure of the air. To determine the amount of moisture loss from the eggs, the eggs were removed and weighed daily for 9 d (at the same time each day). The first 2 d that the eggs were in the desiccator were considered to be an equilibrium period; therefore, temperature and egg weight loss data collected on these days were not used to calculate conductance.

Calculation of Eggshell Conductance. The water vapor conductance of the egg, or eggshell conductance (G) was calculated by using the formula provided by Ar et al. (1974):

([1])

where G_H2O is water vapor conductance or eggshell conductance (mg/d per mmHg); M_H2O is the rate of water loss from egg (mg/d); P_H2O is P_H2O – P_O (where P_H2O is water vapor pressure of the egg contents and P_O is water vapor pressure in the environment surrounding the outside of the egg; if the egg is covered in dessicant, this is equal to zero); and P_H2O is the water vapor pressure difference across the shell (mmHg).

Egg Composition Characteristics. After 9 d in the desiccator, all the eggs were weighed one final time, broken open, and the yolk separated from the albumen. The yolk and the shell were weighed; the wet albumen weight was calculated by subtracting the yolk and eggshell weight from the final egg weight. The eggshells and yolks were placed in a drying oven (V Series model VRC2-26-1E, Despatch, Minneapolis, MN) at 65°C for 4 d to determine dry weights. All the egg components were expressed as a percentage of the initial egg weight.

Experiment 2—Embryonic Metabolism

Incubation of Eggs. In each of the 6 trials, 10 eggs from each of the R and C strains were randomly placed in 1 of 24 identical 1-L airtight metabolic chambers housed inside a Jamesway AVN single-stage incubator (Jamesway Incubator Company Inc., Cambridge, Ontario, Canada) set at 37.5°C and 56% RH. In each trial, 5 spare eggs per strain were placed in another AVN incubator under the same temperature and humidity conditions. After 7 d of incubation, the O₂ consumption and CO₂ production of all the eggs were reviewed to determine whether the eggs contained viable embryos. Any eggs that were suspected of having nonviable embryos were removed and replaced with a spare egg.

The incubator configuration consisted of 3 rows and 2 columns of metal racks; each row within a column held 4 metabolic chambers (Figure 1). A piece of plastic egg incubator flat was cut and placed on the bottom of each metabolic chamber to hold the eggs in place during turning once per hour. At 18 d of incubation, the pieces of egg flats were removed to allow the eggs to lie on their sides, and egg turning was stopped for the remainder of incubation.

View larger version (135K): [in this window] [in a new window]   Figure 1. Egg metabolism equipment showing the incubator, metabolic chambers, CO2-H2O analyzer, differential O2 analyzer, computers for recording data, and accessory parts.

Embryonic O₂ Consumption and CO₂ Production

Twenty of the 24 metabolic chambers containing eggs were used to monitor embryonic O₂ consumption and CO₂ output. Three of the 4 empty metabolic chambers (1 per each row of the incubator) were connected together to form a single unit to monitor ambient CO₂. The entire inside of the incubator was used to monitor ambient O₂.

Metabolism Equipment. Each of the 24 metabolic chambers was connected to a 3-way electric solenoid valve (ASCO Valve Canada, Brantford, Ontario, Canada; located on top of the incubator) that controlled airflow through the metabolic chambers. When an electric solenoid valve was on, airflow from one metabolic chamber was first directed into a CO₂-H₂O analyzer (model LI-6262, LI-COR Inc., Lincoln, NE) and then a differential O₂ analyzer (DOX; Qubit Systems Inc., Kingston, Ontario, Canada). Airflow through the CO₂-H₂O analyzer was accomplished through the use of a sample pump (Qubit Systems Inc.) situated between the CO₂-H₂O analyzer and the DOX sample inlet. For the first 4 trials, this pump was set daily at a constant flow rate of 300 mL/min. In the final 2 trials, the pump was set at 150 mL/min for the first 7 d of incubation, and then changed to 300 mL/ min for the remainder of incubation in the last 2 trials. This change was made to improve the accuracy of the CO₂ and O₂ readings during the first third of incubation, when the embryonic CO₂ output and O₂ consumption are very low.

A second pump set up between the DOX and the incubator pulled air directly from the incubator into the DOX reference inlet. The airflow through the 2 DOX inlets was controlled by the DOX controller pumps (Qubit Systems Inc.) situated right after the DOX. One pump pulled air from the metabolic chambers through the CO₂-H₂O analyzer and then on to the DOX sample port. The other pump pulled air directly from the incubator through the DOX reference port. The pumps were set to draw only 25 mL/min of air through each of the DOX inlets ports. In this way, a zero differential pressure between the sample and reference cells of the DOX could be attained to prevent damage to the DOX analyzing cells.

Airflow through the metabolic chambers was adjusted daily by flow meters connected to the solenoid valves. These flow meters were calibrated at the beginning of each experiment by a standardized external flow meter (Dry Cal DC Lite-ML, Bois International Corporation, Butler, NJ). While the CO₂-H₂O analyzer recorded the concentration in the metabolic chamber, the DOX recorded the differential O₂ concentration between the chamber and the inlet air to the chamber. Daily calibration of the CO₂-H₂O analyzer and the DOX were done with certified N₂ gas and certified span gas (CO₂ = 3,038 µ_L/ L; O₂ = 21.02%).

At the start of every trial, both the CO₂-H₂O analyzer and DOX were calibrated to standard settings. The CO₂- H₂O analyzer was calibrated according to the procedures outlined by O’Dea et al. (2004). The DOX sample and reference O₂, differential O₂, atmospheric pressure, and differential pressure zero sensors were also calibrated following the principles and procedures provided by Qubit Systems Inc. To test the accuracy of the O₂ and CO₂ analyzers, the O₂ and CO₂ were measured in a small chamber in which propane gas was combusted. Propane has a standard RQ of 0.6.

Data Recording. Both the CO₂-H₂O analyzer and DOX were connected to a data acquisition card (National Instruments Inc., Austin, TX). LabVIEW software (Qubit Systems Inc.) running in Microsoft Word (Redmond, WA) enabled monitoring of different metabolic chambers every 5 min. The data were recorded into Microsoft Excel as differential O₂ and CO₂ in microliters per liter. The O₂ and ^CO2 data from each metabolic chamber were recorded 6 times per day.

Calculation of Embryonic Heat Production and RQ

The O₂ and CO₂ exchange rates, in liters per second, were calculated by using the following formula:

The airflow rate was corrected for environmental room temperature and pressure. The equation was based on the sample airflow rate of 0.15 L/min (first 7 d in last 2 trials) or 0.3 L/min in all other trials. The O₂ and CO₂ exchange rates were then used to calculate embryonic heat production by using the formula provided by Kleiber (1987):

The RQ was calculated as the average amount of CO₂ produced divided by the average amount of O₂ consumed.

Statistical Analysis

Although O₂ consumption, CO₂ production, and calculated heat production were recorded as repeated measures (6 readings taken per day), they were analyzed in a single dependent data file with the generalized linear model procedure of SAS (SAS Institute, 2005), and means are expressed as least squares means. Eggshell conductance and egg composition, time taken to externally pip and hatch, wet and dry yolk sac, and chick carcass weights were analyzed as a 2 x 6 factorial design of the generalized linear model of SAS, and means are expressed as least squares means. Where significant differences were observed, least squares means were separated by the PDIFF procedure of SAS at P < 0.05.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Experiment 1: Egg Weight and Eggshell Conductance

Breeder Strain. Genetic strain did not influence initial egg weight or final egg weight (Table 1). Because egg weights were selected within a very tight weight range, this result was expected. Eggshell conductance was also not influenced by strain, thus showing that in these 2 strains when egg weights are the same, there was no effect of genetic selection on conductance. This finding is in agreement with the study of O’Dea et al. (2004), who found no differences in eggshell conductance among modern high-yielding strains, a modern strain selected for the whole bird market, and a broiler strain that has remained unselected since 1978.

Breeder Strain and Flock Age Interaction. There was no effect of the interaction of strain and age on initial and final weights or eggshell conductance (data not shown).

Percentage of Egg Components

Breeder Strain. Genetic strain did not significantly affect the wet or dry yolk, wet albumen, or wet or dry shell weights as a percentage of the initial egg weight (Table 2). This was a result of maintaining egg weights within a tight weight range for both strains.

In contrast, the percentage of wet albumen content decreased with increasing breeder flock age (Table 2). This was previously demonstrated by Ahn et al. (1997), who showed that the lowest yolk:albumen ratio was in eggs produced by 28-wk-old hens and the highest ratio was in eggs produced by 55- and 78-wk-old hens. Peebles et al. (2000) also found significantly higher yolk:albumen ratios as the parent flock aged.

There was no clear pattern with respect to percentage of wet shell weight, but O and VO flocks had lower percentages of dry shell weights than all other breeder flock ages (Table 2). This was an anticipated result based on previous research indicating that eggshell weight decreases as flock age increases (Fletcher et al., 1981).

Breeder Strain and Flock Age Interaction. There was no effect of the interaction of strain and age on the percentage of egg components (data not shown).

Experiment 2: Egg Set and Transfer Weights, and External Pipping and Hatching Times

Breeder Strain. Average egg weight at setting for the eggs placed into metabolic chambers was significantly higher in the R strain than the C strain (Table 3). This was unexpected because eggs were specifically selected to be the same in weight range. By the time of transfer, the weight difference between the 2 strains was erased. Because there was an interaction effect on external pipping and hatching times, the main effect of strain is not discussed for these variables.

Breeder Strain and Flock Age Interaction. Both egg weight at setting and weight at transfer were not affected by the interaction of strain and flock age (Table 3). For the interaction, there was a trend of increasing time taken to reach external pipping as the flocks aged. This result was exclusive of the O flocks in the R strain and the Y flocks in the C strain, which did not follow this pattern. Eggshell temperatures, as well as incubator temperatures, were measured on all the eggs in the metabolic chambers for each flock age (data not shown). Neither shell nor incubator temperatures were significantly different between the strains or among the flock ages, indicating that the different pipping patterns shown by Y flocks in the C strain and O flocks in the R strain were not due to a variation in incubation temperatures. An explanation for the differences in pipping time in these 2 strain and flock age interaction groups cannot be provided by the data obtained.

Regardless, these results are very interesting because egg weights at transfer for all the strain by age interactions were equal. The data provide strong evidence that the interaction effect on external pipping time is independent of egg weight in these 2 strains. One might expect that pipping time would be related to chick weight; however, there was no relationship between hatched chick weight (Table 4) and external pipping time.

A similar, but less evident, trend also occurred for hatching times for the C-strain embryos when the Y flock was excluded (Table 3). In the Y flock of the C strain, significant differences existed between the length of time required to reach external pipping and hatching, which were strain dependent. Excluding the VO flock, embryos from the Y flock of the C strain took longer to reach external pipping and hatching compared with the other flock ages. In offspring from Y parents of the C strain, different embryonic growth strategies may have developed because of genetic selection (i.e., embryos from these young flocks may have physiological differences, especially during the final days of incubation, which extend the length of time taken to hatch).

Chick Weight, Chick Length, and Shank Length

Breeder Strain. Because there was an interaction effect on chick weight at hatching, the main effect of breeder strain is not discussed for this parameter. Chick length and shank length were not influenced by strain (Table 4).

Breeder Flock Age. Because there was an interaction effect on chick weight at hatching, the main effect of breeder flock age is not discussed for this parameter. The VO flock had longer chicks compared with the P and PP flocks. There may be an expectation that a longer chick will have a longer shank (metatarsal bone); however, chicks from P flocks, which were 1 of the 2 flock ages that had the shortest chick length, had a significantly longer shank length compared with all other flock ages. The data showed that shank and chick length were not positively correlated when eggs of similar weight were chosen.

Breeder Strain and Flock Age Interaction. There was an interaction effect of strain and flock age on chick weight (Table 4). For the R chicks, there was a significant trend of decreasing weight when moving from the Y to the O flock. This pattern was not repeated in the weight of the C chicks. Chicks from the VO flocks were the heaviest when compared within the R (significantly heavier than all flocks but the Y flock) and C strains (VO flock significantly heavier than the O and Y flocks, but not different from the other flock ages). This result was independent of egg set weight, which did not differ because of the interaction (Table 3). Especially for the C strain, there seemed to be a relationship between the hatching time (Table 3) and chick weight (Table 4); excluding the Y flock, chicks that took the longest to hatch were the heaviest. This result was not due to dehydration of chicks hatching earlier, because the chicks were removed from the hatcher as soon as the down was dry (i.e., they did not stay in the hatcher until all the chicks had hatched). This result was also not due to yolk sac weight (Table 5), because there was no effect of the interaction on wet or dry yolk sac weights. There was no effect of the interaction on chick or shank length.

Breeder Strain. Strain had no significant effect on wet carcass, wet yolk sac, dry carcass, and dry yolk sac as a percentage of the hatched chick weight (Table 5).

Breeder Flock Age. The percentages of yolk free body mass (wet chick carcass) were significantly higher in the Y and P flocks compared with the O flocks, with no difference among the other flock ages (Table 5). When the yolk-free carcasses were dried, the percentages of dry weight for the PP flocks were greater than for all other flock ages, which did not differ from one another. The percentages of wet and dry yolk sacs generally increased as the flocks aged (Table 5). These results agree with previous research. Peebles et al. (2001) found that as broiler flock age increased from 26 to 31 wk and from 31 to 35 wk, yolk weight also increased. Suarez et al. (1997) reported a higher percentage of yolk in 59-wk-old flocks compared with 29-wk-old flocks.

Breeder Strain and Flock Age Interaction. The interaction had no significant effect on percentages of carcass and yolk sac (data not shown).

Total Embryonic O₂ Consumption, CO₂ Production, Average RQ, and Total Heat Production

Breeder Strain. Total embryonic O₂ consumption, total CO₂ production, average RQ, and total heat production over the entire 21-d incubation period were not significantly different between R and C embryos (Table 6). This was somewhat unexpected, because we hypothesized that embryos from the high-meat-yielding C strain would have a higher metabolic rate (heat production) throughout incubation compared with embryos from the R strain.

Embryos from the M flock also had a higher average RQ than embryos from all the other flock ages, indicating that a greater proportion of carbohydrates may have been used in these embryos compared with embryos from other flock ages. The embryos of Y and O flocks had the lowest RQ, indicating that these embryos were likely using more lipids for metabolism, and thus would have a greater O₂ consumption (as is shown in the data in Table 6) to metabolize the fat via β-oxidation.

Breeder Strain and Flock Age Interaction. Total embryonic O₂ consumption, total CO₂ production, average RQ, and total heat production over the entire 21-d incubation period were not significantly affected by an interaction (data not shown).

Average Daily Embryonic O₂ Consumption, CO₂ Production, RQ, and Heat Production

Breeder Strain. The daily average RQ for the 2 strains are listed in Table 7. At approximately 10 d of age, the RQ declined, likely because of exponential growth of the embryo and higher utilization of lipids for maintenance and growth. To our knowledge, these data have not been previously reported for the strains examined. Daily differences in average O₂ consumption, and CO₂ output are also provided (Table 7). Significant daily gas exchange differences between the 2 strains were noted, especially during the latter days of incubation, as was the case for heat production. Although the differences in O₂ consumption between the R and C strains at 18 d were large, there was no statistical difference because of the large variation in the data during this time point. We hypothesized that the C embryos would have a higher metabolic rate, because this strain has been genetically selected for higher growth compared with the R strain. In addition, anecdotal observations from a commercial hatchery have indicated that embryos from the C strain produce a considerable amount of heat during the latter periods of incubation. This statement was somewhat validated by the data showing that the C strain had higher heat production on d 19 of incubation; however, on d 7, 16, 17, and 18, the R embryos were those that produced more metabolic heat. Previous research by Tona et al. (2004) indicated that from 428 h of incubation to hatching, lines selected for growth had higher embryonic heat production. In the case of the C and R commercial strains, genetic selection for broiler growth may not translate into higher embryonic metabolism on every day of incubation, but seems to result in higher heat production on particular days of incubation. What is very interesting is that the latter days of incubation, in which significant differences in metabolism between the 2 strains occurred, is the period during which massive organ and tissue growth occurs and is also the period during which late embryonic mortality is high. This period of high mortality also coincides with the plateau stage of O₂ consumption (Wineland et al., 2006), in which the embryonic O₂ demand exceeds the O₂ that can be supplied by simple diffusion through the eggshell pores. The data provide evidence that daily embryonic metabolic differences occur between these 2 strains independent of egg weight. The relationship between higher average daily embryonic heat production and embryonic mortality cannot be discerned from the data obtained, but should be the focus of future research.

In conclusion, when the entire 21-d incubation period was examined, differences in embryonic metabolism between the 2 modern broiler strains did not exist. However, significant daily metabolic differences did occur during the early and late periods of incubation. This is an important finding, because these are the periods during which most embryonic mortality occurs and there may be a relationship between metabolism and mortality.

Results on the effect of broiler parent flock age showed that embryonic metabolism was highest in embryos from O and VO parent flocks. These data support the hypothesis that as flock age increases, embryonic metabolism also increases.

The results from this research showed that strain and flock age do influence daily embryonic metabolism when egg weights are selected to be the same. We acknowledge that this procedure for egg selection means that the normal egg weight averages for each strain and parent age are not being compared. To account for differences in egg size, and thus embryo tissue weight, future experiments should collect eggs based on egg weight averages for that particular strain and flock age, and then adjust O₂ consumption and CO₂ production based on tissue mass at that embryo age. Future research should also examine different temperature profiles throughout incubation to determine the influence on embryo survival and hatchling quality for eggs from different strains and flock ages. This knowledge would assist hatchery managers when deciding which eggs to place together in an incubator.

	ACKNOWLEDGMENTS

 
We would like to acknowledge the financial support provided by the following organizations: the Alberta Livestock Industry Development Fund (Edmonton, Alberta, Canada), Aviagen (Huntsville, AL), Maple Leaf Hatchery (Wetaskiwin, Alberta, Canada), the Natural Sciences and Engineering Research Council of Canada (Ottawa, Ontario, Canada), the Poultry Industry Council (Guelph, Ontario, Canada), and The Poultry Research Center at the University of Alberta (Edmonton, Alberta, Canada). We are indebted to A. Franco and M. Mackenzie for their assistance with data collection.

Received for publication June 25, 2007. Accepted for publication July 31, 2007.

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