Donna Hill, DVM, MAM, Diplomate ACPV
276 Smoketree Lane
Mountain Home, AR 72653
phone: (870) 424-6890
fax: (870) 424-6890
voice mail: 800-298-1056
INCUBATION QUALITY AND YIELD BREEDS: A RESEARCH REVIEW
As the industry changes to yield type birds, it becomes more difficult to reach hatch of fertile and one week
mortality goals. The problem is seasonal, generally it is worse in the winter. The field generally complains
about small, dehydrated birds that die in the first week, especially in the youngest breeder flocks. The
hatchery will see increased late dead, especially in the oldest breeder flocks.
The problem is driven by the fact that the embryo of today has changed just as much as the weight and feed
conversion performance of the broiler has changed. The embryo brings a different physiology to the
incubation equation. The incubation equipment design has not changed to meet the needs of the yield type
embryos. The eggs from the youngest and the oldest breeder flocks are the eggs that bring the most
extreme changes to the incubation equation. Consequently, the losses are most evident in eggs from the
The embryo characteristics that are driving the problems described below. I have tried to give you an
overview of the relevant research that has been done to date. I also recommend that you read “Modeling
Incubation Temperature: The Effects of Incubator Design, Embryonic Development, and Egg Size” by N.
A. French. It is an excellent article that goes into the physics of heat transfer and it’s impact on incubation.
It was published in Poultry Science in 1996.
1. Current incubation technology is based on heat production estimates from the classic broiler of
.11 watt/egg. The heat production of the optimally incubated yield type embryo has not been determined.
Hulet and Meijerhof 1reported in two trials that the maximal heat production was .14 and .16 watt/egg. This
is based on carbon dioxide production as the indicator of optimal growth. In personal communications with
the authors, in later trials to further improve incubation, they calculated .2 to .3 watt/egg.
2. Multistage incubation is not optimum for today’s yield breeds. In turkeys, Christensen 2found that
the embryonic survival was optimum with higher temperatures and increased growth rates (higher
incubation temperatures) in week 1 and 2 and decreased growth rates (lower incubation temperatures) in
week 3 and 4. This is the opposite of the multistage environment where the embryos do not reach 100
degrees till they begin producing heat at approximately 9 days of age. As the embryo heat production
increases with growth, the embryo temperature increases. Embryo temperature can reach 104 degrees
commonly in the hatcher and setter. With multistage incubation, a constant embryo temperature that is
the result of a varying incubation environment cannot be achieved.
Lourens3 showed that with a constant embryo temperature of 100 versus 99.5 in the first week, 101.5 in the
second week, and 101.5 in the third week, that there were more first quality chicks and less cull chicks in
the 100-degree embryo temperature group. He also showed that there was improved feed conversion in
three of four 100-degree groups. As you can see, his field profile is only a very mild change from 100
degrees as optimum. A more typical multistage incubation profile is: 98.2 to 99 degrees from day 1 to day
9, 100 to 100.5 degrees from day 10 to day 15, 100.5 degrees to 103 degrees from day 15 to day 18, and 99
to 104 degrees from day 18 till hatch.
3. Over time the industry has increased egg numbers in the machines. Each egg now is producing
more heat. The incubator and hatcher design is basically a fan system with heating and cooling sources.
The air is expected to be conditioned and distributed uniformly throughout the egg mass. This system will
have variability based on the physics of airflow, resistance (eggs), and heat buildup within the egg mass.
The more heat that the embryos produce, the more that the machine has to work. This creates more
variability within the egg mass; some areas are cold and others hot. The chick quality and the hatchability
suffer in the extreme areas. This is especially true in the hatchers. I routinely find in “good hatchers” that
the range is from 1 to 8% hatch loss per tray. The good areas are predictable based on machine type and
airflow patterns. In problem machines the hatch loss can range from 1 to 35% late deads on a tray by tray
Lourens4 demonstrated the variation in air temperature, air velocity, and the embryo shell temperature. The
areas of low air velocity, high air temperature, and high embryo shell temperature had higher hatch loss and
lower first grade chicks.
4. The embryo rate of growth has changed. In the past, the hatcher was designed to only keep the
eggs from getting cold. The job of incubation was done once the eggs were transferred. Today the yield
breeds have a different growth profile, many grow predominantly on the end of incubation, and (the broiler
grows more at the end of the growout period). This means that the hatcher design does not provide
adequate heat removal for all of the eggs in a crucial developmental stage.
Wineland 5 compared the eggshell properties between genetic lines in chickens and found variations that
influence metabolism and embryo growth. While there are differences between yield lines, the biggest
differences were between the yield breeds and the genetics used before yield selection.
Christensen6 reported that embryos during the plateau stage are sensitive to changes of 0.5º C and genetics
interact with the environment to affect embryo survival. In commercial hatchers, the variation in embryo
temperatures can range from 99 to 106 degrees in the same hatcher.
5. The conductance of the eggshell decreases with genetic selection for yield. Conductance of the
shell also decreases as the age of the breeder flock increases 7 Egg shell conductance theory implies that a
hen optimizes the survival rate of her offspring by creating a shell that is precisely matched to the
metabolism of the embryo. Because the timing of the plateau stage in oxygen consumption is also
determined by the ratio of the embryonic metabolism to the porosity of the shell, the length of the
incubation period is also determined by the functional characteristics of the egg. Christensen 8
demonstrated that turkeys selected for growth require longer incubation periods to optimize survival.
The combination of egg shell conductance changes, increased heat production, and the changing metabolic
needs of the embryo as the growth pattern changes is the primary problem in reduced hatch and chick
livability in yield type breeds. These factors come together to create the following scenario. With the
decreased conductance moisture loss and gas transfer is more difficult. Overheating of the embryo at the
end of incubation increases the metabolism of the embryo and increases the demand for oxygen to meet the
increased metabolism. When this occurs with decreased eggshell conductance, the embryo cannot supply
his oxygen needs for yolk utilization. The embryo must use anaerobic metabolism to survive. Anaerobic
metabolism does not require oxygen for energy. The sources of anaerobic energy are the heart, liver and
kidney. The plateau period in the hatch process is naturally an anaerobic state, but with the combined
problems of decreased eggs shell conductance, increased muscle mass, and increased metabolic rate, the
problem anaerobic state is occurs earlier. Those that run out of energy are pips. Those that survive are
weakened and compromised in the first days in the house. Since they have utilized the heart muscle to
survive, they can also be more prone to ascites.
6. In the winter, the airflow through the hatchery is deceased and the heat transfer capacity of the
air is diminished. Both factors are important for consistent incubations within the egg mass. The embryo
temperature is a result of the heat production of the embryo, the airflow over the egg, and the ability of the
air to transfer heat (relative humidity of the air). With decreased incoming air, the air through the egg mass
is decreased and heat removal is less consistent than with increased airflow.
Humidity is also a very key factor in the winter time problem. Humidity is the heat transfer agent, both to
transfer heat to the embryos early in incubation and to remove it from the embryos late in incubation. The
quality of the air from a heat transfer standpoint is extremely important. Unless steam humidification is
used, artificially humidified air is very inferior to naturally humidified air. In the winter, the humidity
problem cannot be easily resolved. If you are using fossil fuels to heat the air, when you increase the air
temperature to promote airflow through the machines, you dry the air out. If you humidify the air with
traditional means you cool the air and cause the machine to heat and decrease airflow to counteract the cold
air. The droplet size in conventional humidifiers is 2-100 microns, with steam it is 0.0006 microns. Steam
improves the cooling quality of the air without cooling the room. One pound of water absorbs 1050 BTU’s
of heat, one pound of steam has 1106 BTU’s of available heat.
7. Hatchers are not designed to consistently remove the heat given off by the yield chick after hatch.
The normal chick rectal temperature in the first 48 hours, whether in the hatcher, chick processing, chick
delivery, or in brooding is 104 to 105 degrees. During this time period, the thyroid and intestinal tract are
still developing and it is crucial to maintain the chick’s body temperature. With the airflow variation
within the hatcher, the rectals of dry chicks will commonly range from 103 to 108 degrees. The chicks
with rectals above 106 degrees F are panting. The down collects on the moisture at the corners of the
beaks. These birds are indicators of heat stress in the hatcher after hatch.
The energy reserves in the yolk will supply the chick for 3 days. If the chick is comfortable, the water
reserves will last for 3 days. When the temperature surrounding the chick is 104 degrees, the water
reserves are depleted in 8-10 hours. If the chicks were comfortable in the hatcher, it would not be
detrimental to leave them in the hatcher. The problem is that the air temperature can easily reach 104
degrees in areas of low airflow within the hatcher. A tray of 100 40-gram chicks (at 90degrees F) produces
165 BTU’s per hour. If the air circulation stops, the temperature will increase 1.5 degrees F per minute. At
this rate, the air temperature around the chicks will exceed 104 degrees in less than 10 minutes 9.
Since the airflow through the egg mass in the hatcher is variable, the heat buildup in areas of low airflow
results in overheated chicks.
In an unpublished study done in the Netherlands, the chicks were left in a hatcher for 4 days. The rectals of
the chicks in this hatcher are 104 to 105 degrees. The one week mortality was 0.35% versus 2% in those
placed in cold brooding conditions on day one. If the chicks are comfortable, leaving them in the hatcher is
This problem also has performance implications. Ernst 10 in studies on day old chicks found that heat stress
at 104 degrees F for periods as short as one hour significantly reduced growth rate to 16 days. This weight
difference persisted with no compensatory gain. Several companies that I have worked with have shown
significant improvement in feed conversion by managing the hatcher environment just prior to pull.
1. Incubation affects embryo development and therefore performance and mortality in the field.
Gladys11 reported that embryo temperature the last five days impacted 44-day body weights and feed
conversion. Embryo temperatures of 101.5 in the young breeder flock had 3.2 points adjusted feed
conversion advantage over 103.5-degree embryo temperature the last five days. In the older breeder flock,
the 101.5 embryo temperatures had 7.4 points adjusted feed conversion advantage over the 103.5 embryo
Christensen12 showed the conductance constant (k) which is determined by egg weight, egg shell
conductance, and incubation period influenced body weight and intestinal maturation as measured by
maltase activity at 7 days post hatch. As I discussed earlier, the eggshell conductance is a major influence
on the quality of the chick that is hatched in multistage incubation. The objective in the current research is
to find physiological measurements that define a quality chick. In this case, maltase activity correlated
with the conductance constant. This also shows that the incubation quality of the embryo impacts the chick
physiology after hatch, and therefore performance and livability.
Wineland13 reported that the incubation environment impacts the development of the chick. With high
setter and high hatcher temperatures, the heart is smaller and the chick without the yolk is also smaller. He
also demonstrated that there was greater residual yolk with higher setter and hatcher temperatures. The
more optimally incubated birds are able to direct resources to organ development and growth. Those in
less than optimum incubation conditions must utilize resources to survive not develop and grow.
Unfortunately, there is no research done to date that investigates the impact of low temperatures in the first
9 days of incubation. The field experience indicates that they restrict growth to survive and utilize the yolk.
Dehydration in the field, can be a small chick that wasn’t incubated properly, whether the problem was cold
in the first 10 days or hot in the last 5 days, they restrict growth and organ development to survive but do
not live or thrive in the field. The chicks from the youngest breeder flocks seem to be more impacted by
early setter problems than those from older breeder flocks, simply because the eggs are smaller and there is
less margin for error.
In the older breeder flocks, the heat per egg is increased due to size, therefore when airflow is diminished
the heat buildup is faster. Therefore older breeder flocks tend to be impacted more from incubation
problems in the last five days of incubation.
2. The age of the breeder flock impacts the thermoregulatory abilities of the chicks. Weytjens14
showed that chicks from young breeder flocks had less ability to maintain rectal temperatures with cold
stress than chicks from older breeder flocks.
It is common to find low rectal temperatures in chicks from young breeder flocks especially in the first 48
hours in the house. The normal comfortable chick rectal appears to be 104 to 105 degrees in the first 48
hours. I have measured chicks from young and old breeder flocks in the same environment in the chick
holding room and found the rectals to be 104 in the chicks from the old flock and 101.5 in the chicks from
the young flock. This is one reason that the embryo mortality in young flocks in the winter increases. Our
handling and brooding conditions are not designed to meet the needs of the chick from the young breeder
flock in this crucial time period when the thyroid and the intestinal tract are still developing.
Hulet, R. M. and R. Meijerhof, 2001. Multi- or single-stage incubation for high-meat yielding broiler
strains. Proceedings of Southern Poultry Science and Southern Conference of Avian Diseases, Page 35.
Christensen, V. L., J. L. Grimes, G. Campbell, and D. Rives, 1997. The relationship of temporal
embryonic growth of turkeys to survival and hatchability. Proceedings of Southern Poultry Science and
Southern Conference of Avian Diseases. Page 115
Lourens, S and J. H. van Middelkoop, 2000. Embryo temperature affects hatchability and grow-out
performance of broilers. Proceedings of the Incubation and Fertility Research Group, Pages 15-17.
Lourens, S., 2001. Matching incubation conditions to hatchability. To be published in World Poultry.
Wineland, M. J., B. D. Fairchild, V. L. Christensen, and K. M. Mann, 1999. Comparison of ten breed
crosses of broilers for eggshell properties and embryo growth. Proceedings of the Poultry Science
Association, Pages 8-9.
Christensen, V. L., W. E. Donaldson, and K. E. Nestor, Dept of Poultry Science, 1996. Interaction of
genetics and temperature during the plateau stage of incubation affects physiology and survival of turkeys.
Proceedings of the Poultry Science Association. Page 77.
Christensen, V. L., 1997. Applied embryology and physiology of avian development. Proceedings from
the Hatchery Workshop, Western Poultry Disease Conference.
Christensen, V. L., W. E. Donaldson, and K. E. Nestor, 1998. Proceedings of the Poultry Science
Association. Page 78.
Savage, S., The University of Georgia Cooperative Extension Broiler Tip, March 1991.
Ernst, R. A., W. W. Weathers, and Jean Smith. 1984. Effect of heat stress on day-old broiler chicks.
Poultry Sci. 63:1719-1721.
Gladys, G. E., D. Hill, R. Meijerhof, T. M. Saleh, and R. M. Hulet, 2000. Embryonic temperature effects
on post-hatch performance in broilers. Incubation and Fertility Research Group, Pages 14-15.
Christensen, V. L., D. T. Ort, S. Suvarna, B. D, Fairchild, and W. J. Croom, 2000. The relationship of
egg conductance constants to neonatal poult growth and quality. Proceedings of the Poultry Science
Association, Page 79.
Wineland, M. J., K. M. Mann, B. D. Fairchild, and V. L. Christensen, 2000. Effect of different setter and
hatcher temperature upon the broiler embryo. Proceedings of the Southern Poultry Science and Southern
Conference on Avian Disease. Page 123.
Weytjens, S., R. Meijerhof, J. Buyse, and E. Decuypere. Thermoregulation in chicks originating from
breeder flocks of two different ages. Journal of Applied Poultry Research, 1999, 8:139-145.