the association between broiler potential growth rate … · seasons, or using the naked-neck...
TRANSCRIPT
The Association Between Broiler Potential Growth Rateand Sensitivity to Heat Stress
Avigdor Cahaner, Nader Deeb
The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot 76100, ISRAEL
Petek Settar
The Aegean University, Faculty of Agriculture, Bornova-lzmir 35100, TURKEY
Broiler performance is sigTtificantly reduced by heat stress. This phenomenon isbecoming more
marked as the genetic potential of commercial stocks improves. The problem cannot be
completely eliminated by management practices, particulary in developing countries, because
they are too costly. Better genetic adaptation to heat stress can facilitate efficient broiler
production in hot climates. Improved adaptation to heat stress can be achieved by selection
under the stress conditions, or by introducing stress-resistance genes into commercial stocks.
Several examples are described, such as selecting for weight gain during hot vs. temperate
seasons, or using the naked-neck (?Ca)gene to reduce feather coverage and alleviate heat stress.
Introduction
Heat stress can be either acute or chronic. Acute stress occurs when ambient temperature
increases drastically for a short time (no more than a few days). Due to the sporadic and short-
term nature of acute heat stress, chicken survival should rely on management practices, such as
cooling or feed deprivation. The present paper deals with chronic heat stress, which depresses
broiler growth, especially when it comes with extremely low or high relative humidity.
The strong negative effect of heat stress on broiler growth, feed efficiency and meat yield
is well documented (Adams and Rogler, 1968; Washburn and Eberhart, 1988; Chwalibog and
Eggum, 1989; Howlider and Rose, 1989; Osman et al., 1989; Cahaner and Leenstra, 1992;
Leenstra and Cahaner, 1992b). Deeb (unpublished data) calculated that a l-degree centigrade
(°C) increase in ambient temperature reduces 4-wk to 6-wk weight gain (WG) and 6-wk body
weight (BW) by about 4% of the performance at 24°C, i.e. BW of broilers at 32°C is only 2/3
of that exhibited by their counterparts at 24°C. Resistance (or tolerance) to chronic heat stress is
therefore expressed by lower heat-induced depression of broiler performance.
With the rapid development of the poultry industry worldwide, importation of high-
performance broiler stocks to hot regions is on the rise. However, primary breeding programs
of the international poultry breeding companies are located in North America and Western
Europe, in temperate climates and optimally controlled facilities, without any selection for
adaptation to extreme climates. To date, none of the large companies has developed a broiler
29
stock with improved adaptation to hot climates. Farmers are advised to use expensive
management practices to control ambient temperature in their facilities. However, the
depression of broiler growth due to high temperature cannot be completely eliminated by such
management practices. Moreover, practices aimed at alleviating heat stress are for the most part
quite expensive and hence not economically feasible, especially in developing countries.
Breeding chickens for stress resistance, once successful, is a more cost-effective
approach to mitigating the stress. Breeding for improved adaptation to a particular stressful
environment should be the strategy of choice when genotype X environment (GxE) interaction
significantly affects economically important traits (Cahaner, 1990; Hartmann, 1990). Such a
breeding may take place in a particular stressful location ("localized breeding") or under
arti.ficiaUy induced stress. Localized breeding of heat-tolerant chickens has been suggested for
the Tropics (Horst, 1982; Hot'st and Mathur, 1990; Mukherjee, 1992; Singh, 1992). In theory,
whatever stress affects the individuals in the primary breeding populations, selection during the
course of localized breeding will improve, or at least maintain resistance to that stress. The
major poultry breeding companies have been doing just the opposite, by applying optimal
environmental conditions for their selection flocks. This approach goes against the concept of
selection for adaptation and may lead to a further increase in sensitivity to stresses, including
heat stress. Localized breeding is not necessary once genes which provide stress resistance arebred into commercial stocks.
The effect of reduced feathering on sensitivity to heat stress
Heat stress negatively affects chickens because their feather coverage hinders the
dissipation of internal heat, leading to elevated body temperatures. To avoid a dangerous
increase in body temperature, broilers minimize endogenous heat production by reducing feed
intake, and consequently exhibit decreased growth and meat yield. Decreasing the feather
coverage should enhance heat dissipation and consequently alleviate the heat stress on chickens
reared in hot climates. In addition, reduced feathering saves valuable protein which is turned
instead into meat tissues (Cahaner et al., 1987; Ajang et al., 1993).
The naked-neck (Na) gene reduces feather coverage in chickens by 20 and 40% in the
heterozygous (Na/na) and homozygous (Na/Na) states, respectively. The effects of this gene
on growth rate and egg production have been reviewed by Merat (1986, 1990). The potential
usefulness of heterozygous naked-neck broilers under heat stress was studied in the early ' 80s
(Hanzl and Somes, 1983), but this genotype's effect has been larger in the fast-=crowing
broilers of the '90s (Lou et al., 1992; Cahaner et al., 1993; Eberhart and Washburn, 1993a,b;
Deeb and Cahaner, 1994). These studies, all conducted in controlled-environment facilities and
mostly under constant ambient temperature, showed that naked-neck broilers are superior to
their normally feathered counterparts when reared at ambient temperatures above 25°C, more so
above 30°C. This advantage was associated with the naked-neck broilers' higher rate of heat
30
dissipation, as measured by infrared thermal imaging radiometer (Yahav et al., 1996). The
increase in body temperature under high ambient temperature was higher in normally feathered
broilers than in their naked-neck counterparts (Eberhart and Washburn, 1993a; Deeb and
Cahaner, 1994). In the natural climates of Israel and Turkey (Cahaner et al., 1994a), as well
as Egypt (Saleh, Horst and Cahaner, unpublished results), heterozygous naked-neck broilers
were superior to normal ones in the summer, but inferior during the cold winter, mainly
because brooding conditions were not adjusted accordingly. Naked-neck broilers are expected
to exhibit a larger, year-round advantage in tropical climates.
A higher reduction in feathering, up to 40%, is obtained in homozygous naked-neck
broilers, which exhibit higher performance than their heterozygous counterparts at a constant
32°C (Cahaner et aL, 1993). Reduced feather coverage may result from selection for polygenes
affecting the rate of feather development (Ajang et al., 1993). The frizzle gene (F) reduces the
density of feather coverage and provides some heat tolerance to egg-type layers (Merat, 1990;
van Haaren-Kiso et al., 1992) and broilers (Yunis and Cahaner, 1994). Feather reduction
accumulates in combinations of naked neck with slow feathering (Lou et al., 1992) or with
frizzled feathers (¥urtis and Cahaner, 1994). The concept of relieving heat stress via reduced
feather coverage was reviewed by Cahaner et al. (1994b), who suggested that even the sc gene
for complete nakedness (Somes and Johnson, 1982) may be useful for extremely fast-growingfuture broilers under severe heat stress.
Potential Growth Rate and Sensitivity to Heat
Rural breeds in hot regions, which exhibit resistance to heat stress, are characterized by
very low BW and productivi_ (e.g. Sinai Bedouin fowl, Arad and Marder, 1982). The heat-
stress effect is more pronounced in fast-growing commercial broilers than in non-selected meat-
b'pe lines (Adams and Rogler, 1968; Washburn et al., 1992; Eberhart and Washburn, 1993b),
or in broiler lines with somewhat higher growth rates (Cahaner and Leenstra, 1992; Leenstra
and Cahaner, 1992b; Washburn et aL, 1992; Cahaner et al., 1995b). The association between
heat-stress effect and growth potential was more evident when birds were fed high-protein diets
(Cahaner et al., 1995b), and more evident in males than in females (Cahaner and Leenstra,
1992). In the latter study, males and females of two fast-growing lines exhibited an advantage
of about 15% in potential growth over their counterparts from three other lines, when reared at
low or normal ambient temperatures. At high ambient temperature (constant 32°C), means of 6-
to 8-wk WG and 8-wk BW of the five lines were similar for females, whereas for males, line
means ranked in reverse order relative tO their genetic potential. However, the higher heat-
induced _owth depression of the high-potential lines, found in that study (Cahaner and
Leenstra, 1992) could have resulted from other genetic differences between those two lines andthe three others.
31
To isolate the genetic effect of growth potential on heat tolerance from other genetic
factors, a study was initiated within a commercial sire line. One group of breeders was selected
from the heaviest males and females, and another group was taken from those near the line's
average (Figure 1). Pedigree matings were conducted among three males and 15 females within
each of the two groups. Offspring of each dam were equally divided into heat-stressed and
temperate environments and their BW was determined weekly. In the temperate environment,
offspring of the "Fast" parents reached an average BW of 2365 g at 6 wk of age, 238 g more
than the offspring of the "Average" parents, whereas at higJa ambient temperature, the
difference in 6-wk BW between the two groups was only 127 g (Figxtre 2). The interaction
between the potential growth rate and ambient temperature was more pronounced when weekly
WGs were compared (Figure 3). In the temperate environment, the expected advantage of the
"Fast" offspring was similar from 14 to 42 d of age (about 7 g/d). In the hot environment, the
advantage of "Fast" offspring was exhibited only up to 21 d of age, i.e. before the chicks were
actually stressed by the heat. During the 4th week, the advantage of the "Fast" broilers was
reduced to about 3g/d, and during the last 2 weeks, the two groups had similar growth rates
(Figure 3). These results indicate that the genetic advantage of the "Fast" group, obtained by a
within-line selection similar to the breeding procedure used by commercial breeders, could not
be expressed under heat-stress conditions. In other words, the experimental selection on BW
was very successful for temperate environments, but less effective for hot environments.
Figure 1.Construction of experimental groups
BWSwk ×crz 91s x cyz
Ambient temperature 240C 320C 24°C 32°C
32
Figure 2. The effect of ambient temperature and potential growth rate onbody weight
2500i 236s Fast 240C2127 Aver.
2000Io_ 1500 lS3O Fast 32"C l
_" 10o0
°o;I5
I I I I
0 3 14 21 28 35 42
Age (day)
Figure 3. The effect of ambient temperature and potential growth rate onweight gain
80 _ 74.5 Fast 240C
70 ""'°" ......... 66.9 Aver.
60
= 50on 40
"_ 22.2 Fast20 22.0 Aver. 32"C
10
Age (day)
33
A more detailed experiment was conducted with a commercial broiler parent stock at the
experimental station "Erbeyli" in Turkey, in 1995. In this study, temperate and heat-stressed
environments were established by the natural climatic differences between spring and summer
(Figure 4). The full-pedigree, randomly-assigned mating scheme consisted of 29 sires and
about five dams per sire. Their offspring were produced in two hatches. The spring broilers
hatched in April, the summer broilers in July. All the chicks were weighed at hatch (BW 0),
4-wk and 7-wk, and their WG from 4- to 7-wk was calculated.
The climatic differences between the seasons had a substantial effect on the growth of the
broilers, indicating that they indeed represented two different environments (Table 1). Hatch
weight was lower in summer by 8% (possibly due to a heat-induced decrease in egg weight),
and BW was lower by about 30% at 7 wk of age. In agreement with previous findings, the
heat-induced depression of broiler growth was larger in males than in females, but only after 4
wk of age. Summer WG 4--7wk was lower than in the spring, by 29% (20.3 g/d) in males and
only 22% (12.6 g/d) in females (Table 1)_
Table 1. Means (andSD) of body weight (BW) and body weight gain (WG) by sex and season(Erbeyli, Turkey, 1995)
Sex Season n BW 0 BW 4wk BW 7wk WG 4-7wk
.............................. O" ................................
Male (M') Spring 316 52.5 (3.7) 895 (117) 2381 (248) 70.9 (9.6)Summer 225 48.1 (3.5) 568 (103) 1623 (222) 50.6 (7.7)
Spring 296 51.9 (4.o) 851 (106) 2074 (237) 57.6 (9.5)Female (F) Summer 289 47.9 (3.3) 542 (87) 1493 (189) 45.0 (7.2)
Spring 612 52.3 873 2228 64.3M + F Summer 514 47.9 555 1558 47.8
The data of the four traits were subjected to a mixed-model ANOVA, in which the Gx.E
interaction was expressed by the Season*Sire factor. This factor was sigrtificant for all growth
traits (Table 2), indicating a substantial interaction between the genotypes in the tested
population and the seasonal differences in ambient temperature. The GxE interaction resulted
from a variation in the magnitude of heat-induced growth depression among sire families, i.e.
in the sensitivity to heat stress. The GxE interaction completely masked the genetic differences
between sires, as apparent from the non-significant Sire effect in the ANOVA of their
offspring's performance data from both seasons (Table 2). Therefore, breeding values of each
sire were estimated from data of each season separately.
34
Table 2. A.NOVA of body weight (BW) and body weight gain (WG) in the spring andsummer (Erbeyli, Turkey, 1995)
Source DF BW 0 BW 4wk BW 7wk WG 4-7wk
............................ P(F3 ...............................
Season 1 <.001 <.001 <.001 <.001
Sex 1 .125 <.001 <.001 <.001
Season*Sex 1 .326 .203 <.001 <.001
Sire 28 .922 .569 .859 .504
Dam:Sire 104 <.001 <.001 .158 .215
Sire*Sex 28 .430 .794 .618 .783
Season*Sire 28 <.001 <.001 <.001 .028
R_ .62 .78 .78 .66
The nature of the genotype by climate interaction was evaluated by regressing sires'
breeding values under heat stress vs. the breeding values estimated from their spring offspring.
The latter values "areindicators of the genetic potential growth rate of each sire. The regression
for BW at 4 wk is presented in Figxtre 5. The slope (b) of the regression calculated from all 29
sires is 0.48, sigxdficanfly lower than 13=1, an additional indication of GxE interaction, and not
different from _--0. When three sires (marked with *) were removed from the calculation, the
regression coefficient (b=.69) was si_maificantlyhigher than 0 but lower than 1. The b estimates
indicate that only about 50% of the genetic differences in spring BW 4wk, were expressed
under the summer conditions. The heritability of BW 4wk in the summer was twice as high as
that found in the spring (Fi_m_re5), indicating that the low b values resulted from a genetic
variation in growth under heat. A similar interaction between sires and tropical vs. temperate
climates was found for egg-production traits (Mukherjee et al., 1980; Mathur and Horst,
1994).
Similar regressions for BW at 7 wk are given in Figure 6. The b calculated from all 29
sires is negative (-0.21) and when four sires (marked with *) were excluded, b=-0.15 was
calculated, significantly lower than _=0. The negative regression estimates indicate that on
average, sires with higher potential (spring breeding value) tend to have lower breeding values
under heat stress. This negative association was more evident for WG 4-7wk (Figure 7). A
slope of b=-0.21 was calculated from all 29 sires, and when only three of them were removed,
a b=-0.45 was obtained, significantly lower than _=0. The results in Figures 6 and 7 clearly
indicate that selection for BW 7wk or WG 4-7wk under optimal conditions will reduce the
stock's performance under hot conditions.
35
Figure 4. Broiler-house ambient temperature in two seasons(Erbeyli, Turkey, 199:5)
Figure 5. Body Weight 4wk - Erbeyli, Turkey, 1995
600
•...U.J
_=_,x=551..........i-_, --------_-------------------------..................• •A i • AqLr- • •
•-- • &"0¢)
P •
"4
if) •S
5OO9
840 x = 875 920
Sire breeding value in SPRING
b P(0=0) P(II=I) r Heritability CV%
All : 0.48 0.15 0.01 0.27 Spring .22_+.09 11"Excluded : 0.69 0.01 0.01 0.46 Summer .47 +.12 17
36
Figure 6. Body Weight 7wk - Erbeyli, Turkey, t9951600
LLI-
5
(/) _ • • • • •
"_ • '_"> _ : 1544 .......... "- ........
o • •
d2 •
t
1500
2100 x = 2227 2400
Sire breeding value in SPRING
b P(I_0) P(8=I) r Heritability CV%
All -0.08 0.46 <.001 0.14 Spring .18 _+.08 11"Excluded -0.15 0.03 <.001 0.43 Summer .18 4-.09 13
Figure 7. Body Weight Gain 4-7wk - Erbeyli, Turkey, 1995
_0 At
A',re l '
w i
_ • .
A.A.__ •_= 47 ................ A- --'...........
_=
• &* • • & _ &C:0
_= &I=Q
.:2 l
=
44
60 X=64 "_
Sire breeding value in SPRING
b P(I_=0) P(_=I) r Heritability CV%
All -0.21 0.22 <.001 0.23 Spring .16 ± .08 14*Excluded -0.45 <.001 <.001 0.50 Summer .13 ±.08 15
37
Summary
The results presented here clearly indicate the negative association between potential
growth rate of broilers and their tolerance to heat stress. This higher sensitivity results from the
higher basal metabolic rate of broilers with more rapid growth rates (_titcheU and Sandercock,
1995; Sandercock et al., 1995). Already today, the increase in genetic potential of the advanced
meat-type stocks is hardly expressed under heat stress. Future broilers will grow even faster,
due to continuous selection for rapid growth, hence they will generate more heat which will
need to be dissipated at a hi_er rate. Therefore the losses due to heat stress in broiler
operations in hot climates are expected to increase, urtless breeding for heat-stress tolerance is
incorporated into their selection programs.
One approach would be to introduce genes which may alleviate heat stress, possibly
through reduced feather coverage, such as the naked-neck gene (Cahaner, 1994). In the future,
further reduction in feather coverage is expected to be advantageous, even at temperate ambient
temperatures. Incorporation of other genes, for frizzled feathers, slow feathering, or even the sc
gene for complete nakedness ( (Somes and .)'ohnson, 1982), should be tested as potential
broiler growth increases.
Breeding for nonspecific heat tolerance should be feasible if a sigrtificant GxE interaction
is identified, with E being temperate vs. high ambient temperature, and G the relevant
genotypes (families or individuals within a primary breeding population). Such a selection may
take place under simulated climatic stress, if the stress is of a simple, mono-factorial nature.
This approach would allow the breeding companies to keep their standard selection programs,
and run a segment of them (usually sib- or offspring-testing) in facilities where heat stress is
_ficially created. However, superimposing artificial heat stress is complicated because hot
climates may differ in diurnal temperature cycle and relative humidity, as well as in their
interaction with other specific environmental factors (housing, feed, sanitation, etc.). The other
alternative is "localized breeding", i.e. selection under natural climaticstress, which solves the
problem of simulating a complex environment. Such a selection has already proved successful:
as compared to a US-bred broiler stock, its counterpart which had been selected in India for 10
generations exhibited better adaptation to the local environment (Singh, 1992). Primary broiler
breeders should consider a methodological application of this approach, in order to maintain, or
even improve, the heat tolerance of their fast-growing broiler stocks.
38
REFERENCES
Adams, R.L., and J.C. Rogler, 1968. The effect of environmental temperature on proteinrequirement and responses to energy in slow and fast growing chicks. Poultry Science47:579-586.
Ajang, O.A., S. Prijono, and W.K. Smith, 1993. The effect of dietary protein level on growthand body composition of fast and slow feathering broiler chickens. British Poultry Science
• 34:73-91.
Arad, Z., and I. Marder, 1982. Comparison of the productive performance of the SinaiBedouin fowl, the White Leghorn and their crossbreds: study under natural desertconditions. British Poultry Science 23:333-338.
Cahaner, A., 1990. Genotype by environment interactions in poultry. Vol. 16, pages 13-20 in:Proc. 4th World Confess on Genetics Applied to Livestock Production, Edinburgh, UK.
Cahaner, A., 1994. Poultry improvement: Integration of present and new genetic approachesfor broilers. Vol. 20, pages 25-32 in: Proceedings 5th World Congress on GeneticsApplied to Livestock Production, Guelph, Canada.
Cahaner, A., N. Deeb, and M. Gutrnan, 1993. Effect of the plumage-reducing Naked-neck(Na) gene on the performance of fast growing broilers at normal and high ambienttemperatures. Poultry Science 72:767-775.
Cahaner, A., N. Deeb, M. Ya'acobi, O. Yariv, I. Ptichi, S. Yalcin, and A. Testik, 1994a.Performance of naked-neck broilers under different clLmatic conditions in Israel andTurkey. Page 58 in: Proceedings Israel's WPSA 32nd Annual Convention, ZichronYaacov, Israel (Abstr).
Cahaner, A., E.A. Dunnington, D.E. Jones, .I.A. Cherry, and P.B. Siegel, 1987. Evaluationof two commercial broiler male lines differing in feed efficiency. Poultry Science 66:1101-1110.
Cahaner, A., and F.R. Leenstra, 1992. Effects of high temperature on _owth and efficiency ofmale and female broilers from tines selected for high weight gain, favorable feedconversion, and high or low fat content. Poultry, Science 7 I: 1237-1250.
Cahaner, A., Y. Pinchasov, I. Ni.r, and Z. Nitsan, 1995b. Effects of dietary protein under highambient temperature on bcdy weight, breast meat yield and abdominal fat deposition ofbroiler stocks differing in _owth rate and fatness. Poultry. Science 74:968-975.
Cahaner, A., R. Yunis, and N. Deeb, 1994b. Genetics of feathering and heat tolerance inbroilers. Vol. 2, pages 67-70 in: Proceedings 9th European Poultry Conference, Glasgow,Scotland.
Chwahbog, A., and B.O. Eggum, 1989. Effect of temperature on performance, heatproduction, evaporative heat loss and body composition in chickens. Archive Gefl/.igelkd.53:179-184.
Deeb, N., and A. Cahaner, 1994. Genotype-environment interaction and heat tolerance ofnaked-neck broilers. Vol. 20, pages 65-68 in: Proceedings 5th World Congress onGenetics Applied to Livestock Production, Guelph, Canada.
Eberhart, D.E., and K.W. Washburn, 1993a. Variation in body temperature response of nakedneck and normally feathered chickens to heat stress. Poultry Science 72:1385-1390.
Eberhart, D.E. and K.W. Washburn, 1993b. Assessing the effects of the naked neck gene onchronic heat stress resistance in two genetic populations. Poultry Science 72:1391-1399.
Hanzl, C.J., and R.G. Somes, 1983. The effect of the naked neck gene, Na, on growth andcarcass composition of broilers raised in two temperatures. Poultry Science 62:934-941.
Hartmann, W. 1990. Implications of genotype-environment interactions in animal breeding:Genotype-location interactions in poultry. World's Poultry Science Journal 46:197-210.
Horst, P., 1982. Genetical perspectives for poultry breeding on improved productive ability totropical conditions. Vol. 8, pages 887-892 in: Proceedings 2nd World Congress onGenetics Applied to Livestock Production, Madrid, Spain.
39
Horst, P., and P.K. Mathur, 1990. Genetic aspects of adaptation to heat stress. Vol. 14, pages286-296 in: Proceedings 4th World Congress on Genetics Applied to LivestockProduction, Edinburgh, UK.
Howlider, M.A.R., and S.P. Rose, 1989. Rearing temperature and the meat yield of broilers.British Poultry Science 30:61-67.
Leenstra, F.R., 1993. Future prospects in poultry meat genetics: choice of breeding goal. Pages23-28 in: Proceedings 10th International Symposium on Current Problems in AvianGenetics "Aviagen", Nitra, Slovakia.
Lou, M.L., O.K. Quoi, and W.tC Smith, 1992. Effects of naked neck gene and feather growthrate on broilers in two temperatures. Vol. 2, page 62 in: Proceedings 19th World's PoultryCongress, Amsterdam, The Netherlands.
Mathur, P.K., and P. Horst, 1994. Genotype by environment interaction in laying hens basedon relationship between breeding values of sires in temperate and tropical environments.Poultry Science 73:1777-1784.
Merat, P., 1986. Potential usefulness of the Na (naked neck) gene in poultry production.World's Poulu'y Science Journal 42:124-142.
Merat, P., 1990. Pleiotropic and associated effects of major genes. Pages 429-467 in: PoultryBreeding and Genetics. R.D. Crawford, ed., Elsevier Scientific Publishers, Amsterdam,The Netherlands.
MitcheU, M.A., and D.A. Sandercock, 1995. Increased hyperthermia induced skeletal muscledamage in fast growing broiler chickens? Poultry Science 74 (Suppl. 1):74 (Abstr).
Mukherjee, T.K., 1992. Usefulness of indigenous breeds and imported stocks for poultryproduction in hot climate. Vol. 2, pages 31-37 in: Proceedings 19th World's PoultryCongress, Amsterdam, The Netherlands.
Mukherjee, T.K., P. Horst, D.K. Flock, and J. Peterson, 1980. Sire-location interactions fromprogeny test in different countries. British Poultry Science 21:123-129.
Osman, A.M.A., E.S. Tawfik, F.W. Klein, and W. Hebeler, 1989. Effect of environmentaltemperature on growth, carcass traits and meat quality of broilers of both sexes anddifferent ages. Arch.iv Gefltigelkd. 53:158-175.
Sandercock, D.A., M.A. _Litchell, and M.G. MacLeod, 1995. Metabolic heat production in fastand slow growing broiler chickens during acute heat stress. British Poultry Science 36:868(Abstr).
Singh, H., 1992. Selection for adaptability to sub-optimal conditions. Vol. 2, pages 597-599in: Proceedings 19th World's Poultry Congress, Amsterdam, The Netherlands.
Somes, R.G., and S. Johnson, 1982. The effect of the scaleless gene, sc, on growthperformance and carcass composition of broilers. Poultry Science 61:414-423.
van Haaren-Kiso, A., P. Horst, and A. Valle-Zarate, 1992. Genetic and economic relevance ofautosomal incompletely dominant frizzle gene F. Vol. 2, page 66 in: Proceedings 19thWorld's Poultry Congress, Amsterdam, The Netherlands.
Washburn, K.W., and D.E. Eberhart, 1988. The effect of environmental temperature onfamess and efficiency of feed utilization. Pages 1166-I 167 in: Proceedings 18th World'sPoultry Congress, Nagoya, Japan.
Washburn, K.W., E. E1-Gendy, and D.E. Eberhart, 1992. Influence of body weight onresponse to a heat stress environment. Vol. 2, pages 53-56 in: Proceedings 19th World'sPoultry Congress, Amsterdam, The Netherlands.
Yahav, S., D. Lugar, A. Cahaner, M. Dotan, M. Rusal, and S. Hurwitz, 1996. Heat loss byradiation and hemodynamic changes in naked neck and normally feathered broilers.(Submitted to Poultry Science)
Yunis, R., and A. Cahaner, 1994. The effect of the frizzle (F) and naked-neck (Na) genes andtheir interaction on broilers at normal and high ambient temperature. Vol. 20, pages 69-72in: Proceedings 5th World Congress on Genetics Applied to Livestock Production,Guelph, Canada.
4O
Questions and Answers
G.L. Jaln: Did you measure feed efficiency? If so, was there any difference in this traitbetween naked-neck and normal broilers, in spring and in summer?
Answer: The reduction in feather coverage clearly improves feed efficiency of broilers reared athigh ambient temperature. This advantage, which increases with temperature, is evident whenthe two genotypes are kept to the same age, and more so to the same BW. It is only at ambienttemperatures below 20°C that naked-neck broilers may have somewhat inferior feed efficiency.
G.L. Jain: Was there any difference in mortality between normal and naked neck birds indifferent seasons?
Answer: The heat stress in our experiments is chronic in nature and hardly causes heat-relatedmortality. Therefore no consistent differences in such mortality, nor for any other reason,where observed in either season. However, in a few cases involved in an accidental acute heatwave, we observed better survival of naked-neck broilers and breeders.
A, Ern_ley: Feed conversion and protein need: Na vs. na?
Answer: Indeed the better feed conversion of naked-neck vs. normal broiler may also resultfrom the lower feather mass, which may have a special effect on protein needs. This topic wasrecently studied by Pesti et al. (Poultry Science 75:375-380), under temperate ambienttemperature (constant 22°C). In our experiments we found that high-protein diets increase theadvantage of naked-neck broilers at high ambient temperatures.
A. Ern$1ey: Asymmetry of response, whether selected in optimal or chronic heat conditions?
Answer: Since the best genotypes for optimal conditions are inferior under heat stress, I assumethat genotypes selected as best under heat won't have the same ranking at optimal conditions.
B. Gowe: Would you comment on the K gene in reducing feather mass and in increasingheat tolerance. Is there any interaction with the Na gene or the F gene?
,Answer: A couple of studies have indicated a positive effect of the slow-feathering K gene onbroiler performance, which could result from their reduced feathering, although these studiesweren't conducted at high ambient temperature. In our studies, all birds were fast-featheringand hence I can't comment on the effect of the K gene on heat-stressed broilers, nor on its
• interaction with the Na and F genes.
M. Sadjadi: What was the effect of naked-neck gene on breast blister and processing quality?
Answer: Although not determined methodologically, processing quality of naked neck broilersappear to be superior to that of normal ones. Being leaner and without feather follicles, thenaked skin is more resistant to blisters, infections and tears. However, if some broilers step on
others during live haul, their naked skin may get scratched.
R.P. Reddv: Are there any pleiotropic effects on Na and F genes, especially on reproduction?Answer: We observed the well-documented reduction in hatchability of Na/Na embryos, and
therefore the production of homozygous naked-neck broilers is not feasible. However, ourresults indicate normal reproductivity of Na/Na or Na/na male and female breeders, andnormal hatchability of heterozygous naked-neck chicks, and we therefore expect that theproduction of Na/na broilers will be as feasible as that of normal ones. As for the F gene,frizzle breeders tend to have poor feather coverage due to feather breakage, and hence theysuffer at temperate or low ambient temperature. Consequently, they may exhibit poorreproduction.
41