Performance of Naked Neck and Normal Broilers in Hot, Warm, and Temperate Climates

Transcription

1 Performance of Naked Neck and Normal Broilers in Hot, Warm, and Temperate Climates S. YALÇIN,* A. TESTIK, S. OZKAN,* P. SETTAR,* F. ÇELEN, and A. CAHANER,1 *The Aegean University, Faculty of Agriculture, Izmir 35100, Turkey, University of Çukurova, Faculty of Agriculture, Adana 01330, Turkey, and The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot 76100, Israel ABSTRACT Chickens suffer at high ambient temperatures because their feather coverage hinders internal heat dissipation. Naked neck broilers (Na/na) and their normally feathered sibs (na/na) were evaluated in three natural climates. Three experiments were conducted in Turkey, during the summer in the extremely hot region of Adana (Ad-Sm), and in the moderate region of Izmir during the spring (Iz-Sp) and summer (Iz-Sm), always following the same experimental protocol. Ambient temperatures averaged 19, 28, and 32 C in Iz-Sp, Iz-Sm, and Ad-Sm, respectively. About 300 birds per genotype were included in each experiment. Feather weight was lower by about 20% in Na/na broilers than in na/na ones, independent of climate, sex, and age (6 or 7 wk). The Na/ na broilers exhibited higher breast weight in all cases, from 2.5 to 10.9% higher than their na/na counterparts. Body weight gain from 4 to 7 wk (BWG4-7) clearly reflected the differences in ambient temperature among climates. The effect of the Na/na genotype on BWG4-7 interacted with climate and sex. In the hottest climate Received for publication August 19, Accepted for publication February 17, To whom correspondence should be addressed. (Ad-Sm), both male and female Na/na broilers exhibited a highly significant advantage over their na/na counterparts. In the more moderate climate (Iz-Sm), the Na/na genotype exhibited superior growth only among males, and the magnitude of this advantage was lower than in Ad-Sm. In the cool temperate climate (Iz-Sp), BWG4-7 and BW7 (BW at 7 wk) means were similar for both genotypes. In Iz-Sp, feed efficiency (FE) of the Na/na birds was lower by about 4%, but in the two summer climates (Iz-Sm and Ad-Sm), FE of the Na/na birds was about 9% higher than that of their na/na counterparts. Body temperature was lower in the Na/na broilers than in their na/na counterparts; in all cases, the difference increasing with ambient temperature. The results indicate that the reduction in feather coverage provided relative heat tolerance, and therefore, under hot climates the Na/na broilers were superior to their normally feathered counterparts. It is concluded that naked neck broilers should be preferred in hot climates. (Key words: naked neck, ambient temperature, hot climate, body temperature, growth) 1997 Poultry Science 76: INTRODUCTION The strong negative effect of high ambient temperatures on poultry production, especially broiler growth and meat yield, has been intensively studied and reviewed (Adams and Rogler, 1968; Chwalibog and Eggum, 1989; Howlider and Rose, 1989; Osman et al., 1989; Cahaner and Leenstra, 1992; Leenstra and Cahaner, 1992). Moreover, broiler lines with a higher potential for growth rate have been found to be more sensitive to constant high ambient temperatures (Cahaner and Leenstra, 1992; Leenstra and Cahaner, 1992; Cahaner et al., 1995). Natural heat stress, induced by the summer season in Izmir (Turkey), reduced 7-wk BW and 4- to 7-wk weight gain (BWG) of commercial broilers by about 23 and 33%, respectively, compared to their counterparts reared under the same management during the temperate fall season (Yalçin et al., 1996). It appears that the broiler stocks bred for high growth rate and meat yield in optimal environments, are not able to fully express their genetic potential when reared in hot climates, unless their selection programs include breeding for heat tolerance. Whereas studies on alleviation of heat stress have focused on costly management adjustments, genetic improvement of heat tolerance may provide a low-cost solution, particularly attractive to developing countries with hot climates. Chickens suffer under high ambient temperature because their feather coverage hinders internal heat dissipation, leading to elevated body temperature (BT) (Yahav et al., 1996). To avoid a lethal increase in BT, chickens minimize endogenous heat production by reducing feed intake, resulting in decreased growth and meat yield in broilers (Yahav et al., 1996). Reduced feather coverage should improve and enhance heat dissipation and consequently alleviate the effects of heat on chickens reared in hot climates. In addition, reduced feathering saves on feather proteins, which may be used for meat tissues (Cahaner et al., 1987; Ajang et al., 1993). 930

2 NAKED NECK AND NORMAL BROILERS IN HOT, WARM, AND TEMPERATE CLIMATES 931 The naked neck (Na) gene reduces feather coverage in chickens by about 20 and 40% in the heterozygous (Na/ na) and homozygous (Na/Na) states, respectively. The effects of this gene have been reviewed comprehensively, especially with regard to egg-type chickens (Merat 1986, 1990). The potential usefulness of naked neck broilers at high ambient temperature was studied in the early 1980s (Hanzl and Somes, 1983), but its importance became more apparent in the 1990s (Lou et al., 1992; Cahaner et al., 1993, 1994; Eberhart and Washburn 1993a,b). These studies demonstrated the advantage of naked neck broilers over their normally feathered counterparts, when reared at constant high ambient temperatures (above 30 C). The increase in BT in high ambient temperatures was higher in normally feathered than in naked neck broilers, due to the reduced feathering of the latter. Consequently, the naked neck broilers exhibited higher feed intake, growth rate, and meat yield than their normally feathered counterparts (Deeb and Cahaner, 1994). However, all of these studies were conducted in artificially controlled climates, mostly with constant temperature. Natural hot climates are characterized by a diurnal cycle of ambient temperature, which varies among geographical locations and seasons. Broilers, whose appetite and growth are depressed by the higher mid-day temperatures, may exhibit compensatory feed intake and weight gain during the cooler night hours. In this case, the advantage exhibited by naked neck broilers in cyclic hot climates could be much lower than that observed under a constant ambient temperature that equals the cycle s maximum or even average temperature. To simulate natural climates, variations in relative humidity and air velocity need to be integrated with various diurnal cycles of ambient temperature, substantially increasing the number of possible combinations and the complexity of such a simulation. A more practical approach consists of conducting experiments under the relevant natural climates. A preliminary experiment with Na/na and na/na broilers was conducted during the spring and summer in Israel (Cahaner et al., 1992). However, ambient summer temperatures during the research period were not high enough due to effective cooling systems, which are rarely found in broiler houses in most hot regions. The purpose of the present study was to evaluate the performance of naked neck broilers (Na/na) relative to their normally feathered sibs (na/na), in natural temperate and hot climates, in different seasons and locations. MATERIALS AND METHODS Climate in Poultry Houses Three experiments were conducted in Turkey, two in the moderate region of Izmir and one in the extremely hot region of Adana. The experiments were performed in the 2ANAK Breeders Ltd., Natanya, Israel. spring (April May) and early summer (June July) in Izmir (Iz-Sp and Iz-Sm, respectively) and in June July in Adana (Ad-Sm). The poultry houses at both sites had only windows with curtains in the walls, and minimal effects of outside winds. Ambient temperature within the broiler houses was not controlled, but it was continuously monitored during the experiments, from the 2nd wk onward. Mean temperature after brooding (3 to 7 wk of age) was 18.7, 27.8, and 31.5 C in Iz-Sp, Iz-Sm, and Ad-Sm, respectively. Temperature fluctuations were summarized as weekly averages of six daily measurements, taken at 4-h intervals (Figure 1). Genetic Stock and Management Naked neck (Na/na) chicks and their normally feathered (na/na) sibs were sired by Na/na males from an experimental meat-type stock obtained from P. Horst (Humboldt University, Berlin, Germany) in 1989, which has since then been continuously backcrossed to an Israeli commercial sire-line (Cahaner et al., 1993). For both seasons, Na/na males were mated to the same normally feathered (na/na) ANAK hens.2 Day-old chicks were shipped by air to the experimental sites in Turkey. In each experiment, an imported commercial broiler strain was also included, to verify that the genetic growth potential of the experimental stock was at least as good as that of standard commercial broilers. Chicks were sexed at the hatchery, wing-banded, and randomly assigned to 12 electrically heated floor pens with 3 pens per genotype and sex. Pen size was 5 and 4 m2 in Iz and Ad, respectively. Aiming at densities of 10 to 12 birds per square meter, 60, 50, and 40 chicks were housed in each pen in Iz-Sp, Iz-Sm, and Ad-Sm, respectively. Starter (22% protein and 3,000 ME kcal/kg) and grower (21% protein and 3,100 kcal ME/kg) corn-based standard mash diets were given from 0 to 3 wk and 4 to 7 wk of age, respectively. Birds were provided ad libitum access to feed and water. Incandescent lighting supplemented natural daylight to provide 23 h light and 1 h dark. Measurements Chicks were weighed individually upon arrival at the experimental sites (BW0), and at 4 (BW4) and 7 wk (BW7) of age. Body weight gain from 4 to 7 wk (BWG4-7) was calculated for each chick. Total feed consumption (FC) was measured for each pen, and corrected for mortality. Feed efficiency (FE) was calculated as the ratio between total BW gain to total FC of all birds in each pen. During the afternoon of Day 41, about 12, 18, and 6 birds were randomly selected from each genotype and sex in Iz-Sp, Iz-Sm, and Ad-Sm, respectively, equally representing all pens. The same sampling procedure was repeated on Day 48. Rectal temperature, as a measure of body temperature (BT), was recorded for each of these birds when sampled (BT1), using a thermocouple thermometer inserted approximately 3 cm into the colon. These birds were then deprived of feed overnight for 10 h and their rectal temperature was measured again in the

3 932 YALÇIN ET AL. FIGURE 1. Broiler house ambient temperatures in three different climates in Turkey: Spring in Izmir (Iz-Sp; ÿ); Summer in Izmir (Iz-Sm; ); Summer in Adana (Ad-Sm; π). Daily fluctuations are summarized and presented as weekly averages of six daily measurements, taken daily at 4-h intervals. morning (BT2). Whereas BT1 was measured at the highest daily ambient temperatures, BT2 was measured when daily ambient temperatures were lower by about 4 to 6 C (Figure 1), after 10 h of feed withdrawal. All the birds sampled for BT measurements were killed by cervical dislocation, weighed, scalded for 2 min at 60 C, plucked mechanically, and reweighed to calculate feather weight by subtraction. The birds were manually eviscerated. Neck, head, and shanks were removed, and readyto-cook carcasses were individually weighed. Breasts, including meat, bone and skin, were weighed for each bird. Feather mass, carcass yield, and breast weight were expressed as percentages of BW after 10 h without feed (%F, %C and %B, respectively). Statistical Analysis Several general linear models were fitted to the data by JMP (SAS Institute, 1995). A three-way factorial model with a covariate was used to analyze individual BW and BWG data. The model included three climates, two genotypes (Na/na and na/na), and two sexes as main effects, and all their interactions. Although Na genotypes did not differ in BW0, it was included as a covariate in the analysis of BW4, to correct for variation in initial weight due to age of dams (2 mo older in summer than spring) and differences in conditions during transportation. For BWG4-7 and BW7, the model included BW4 as a covariate. The data for BW and BWG were also analyzed separately within each sex, using similar models but excluding the effects of sex and interactions with sex. The same models were used to analyze pen data on FC and FE. Data on BT, %F, %C, and %B were analyzed according to a four-way model that included climate, genotype, sex, age, and their second- and third-degree interactions. Data were also analyzed for each age separately using a similar model, without age or interactions with age.

4 NAKED NECK AND NORMAL BROILERS IN HOT, WARM, AND TEMPERATE CLIMATES 933 TABLE 1. Least squares means of BW at 4 wk (BW4) and 7 wk (BW7), and BW gain from 4 to 7 wk (BWG4 7) for male (M) and female (F) heterozygous naked neck (Na/na) and normally feathered (na/na) broilers reared in spring (Sp) and summer (Sm) climates at two locations in Turkey (Iz = Izmir, Ad = Adana) Na/na na/na 100 (Na/na-na/na):(na/na) Sex Climate n BW4 BWG4 7 BW7 n BW4 BWG4 7 BW7 BW4 BWG4 7 BW7 (g) (%) M Iz-Sp b 1,314 a 2,264 a 163 1,011 a 1,329 a 2,279 a 4.9*** Iz-Sm c 1,133 b 2,082 b b 1,070 b 2,019 b *** +3.1*** Ad-Sm 87 1,001 a 1,125 b 2,074 b a 1,033 c 1,983 b *** +4.6*** F Iz-Sp a 1,055 a 1,916 a a 1,033 a 1,894 a * +1.2* Iz-Sm b 944 b 1,804 b b 960 b 1,821 b +3.9** Ad-Sm a 919 c 1,780 b a 799 c 1,660 c *** +7.2*** M + F Iz-Sp b 1,185 a 2,088 a a 1,181 a 2,085 a 3.5*** Iz-Sm c 1,038 b 1,941 b b 1,014 b 1,918 b +3.0*** +2.4* +1.2 Ad-Sm a 1,022 b 1,925 b a 916 c 1,820 c *** +5.8*** Source of variation Males + females Males Females (P(F)) Genotype (G) NS <0.001 <0.001 NS <0.001 < <0.001 <0.001 Climate (C) <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 G C < < <0.001 <0.001 Sex (S) <0.001 <0.001 <0.001 C S <0.001 <0.001 G S NS NS G C S NS BW0/BW4 1 < <0.001 <0.001 NS <0.001 < <0.001 a cclimate means within sex and genotype with no common superscript differ significantly (P < 0.05). 1BW0 as a covariate in the ANOVA of BW4, and BW4 as a covariate in the ANOVA of BWG4 7 and BW7. *Differences between genotypes were significant at P < **Differences between genotypes were significant at P < ***Differences between genotypes were significant at P < RESULTS Ambient Temperatures The seasonal and geographical differences between the three experimental sites substantially affected ambient space temperatures in the broiler houses. Figure 1 presents the daily fluctuations of ambient temperature, averaged weekly from Weeks 2 to 7. Spot-heating was provided for the chicks in each pen for the first 2 wk, and hence during this period the chicks actual ambient temperature could be either equal to or higher than the space temperature in the broiler house. Mean BW4 of the Iz-Sm chicks was substantially lower than their Iz-Sp and Ad-Sm counterparts, reflecting insufficient spot heating in the Iz-Sm experiment, independent of climate. This undesired effect was statistically corrected for by including BW4 as a covariate in the analyses of BWG4-7 and BW7 (Table 1). Body Weight and Weight Gain as the level of significance of these main effects and their interactions. A significant three-way interaction was found for BWG4-7 and BW7; therefore, the effects of climate (C) and genotype (G) were tested separately for each sex (S), and their interaction (G C) was significant in all cases (Table 1). Relative to the normally feathered (na/ na) broilers, BW4 was lower in the naked neck (Na/na) males and females in the spring (Iz-Sp) and higher in the two summer climates (Iz-Sm and Ad-Sm). The BWG4 7 of the naked neck birds was higher than that of their counterparts in Ad-Sm (males and females), in Iz-Sm (males) and Iz-Sp (females). These interactions in BWG4 7 may reflect some compensatory growth, which should not affect BW7. For the latter trait, the naked neck males had a 3.1 and 4.6% advantage in Iz-Sm and Ad-Sm, respectively. The naked neck females were either slightly heavier or similar to their fully feathered counterparts in both Iz climates, but exhibited a 7.2% advantage in the hot Ad-Sm climate. In all cases, the relative advantage of the naked neck broilers was about twice as large for BWG4 7 than for BW7 (Table 1). The normally feathered na/na broilers exhibited a 4% higher BW7 than the standard commercial broilers in each of the three climates (data not shown). Table 1 presents mean BW4, BWG4-7, and BW7 of the two genotypes by climate (i.e., experimental site and season) and sex, as well Feed Consumption and Feed Efficiency Feed consumption was significantly affected by climate it decreased as ambient temperatures increased but not by genotype. The G C S interaction

5 934 YALÇIN ET AL. TABLE 2. Least squares means of feed consumption 1 (FC) and feed efficiency (FE = WG/FC) from hatch to 7 wk of age for male (M) and female (F) heterozygous naked neck (Na/na) and normally feathered (na/na) broilers reared in spring (Sp) and summer (Sm) climates at two locations in Turkey (Iz = Izmir, Ad = Adana) Na/na na/na 100 (Na/na na/na):(na/na) Sex Climate FC FE FC FE FC FE (g) (g:g) (g) (g:g) (%) M Iz-Sp 5,380 a c 5,149 a a * Iz-Sm 4,689 b b 4,881 ab b ** Ad-Sm 4,449 b a 4,535 b a * F Iz-Sp 4,930 a c 4,721 a Iz-Sm 4,235 b b 4,460 ab b * Ad-Sm 3,963 b a 4,131 b ** M + F Iz-Sp 5,155 a c 4,935 a a * Iz-Sm 4,462 b b 4,670 b b ** Ad-Sm 4,206 c a 4,333 c a *** Source of variation Males + females Males Females (P(F)) Genotype (G) NS NS NS Climate (C) < < < G C NS <0.001 < NS Sex (S) <0.001 <0.001 C S NS NS G S NS NS G C S NS a cclimate means within sex and genotype with no common superscript differ significantly (P < 0.05). 1FC and FE were calculated for each pen (three pens per genotype per sex). *Differences between genotypes were significant at P < **Differences between genotypes were significant at P < ***Differences between genotypes were significant at P < was significant for FE, and, therefore, the effects of G and C were tested separately for each sex (Table 2). The G C interaction affected FE in both sexes. In males, the naked neck significantly reduced FE in Iz-Sp and increased it in Iz-Sm and Ad-Sm. In females, the effect of the naked neck on FE changed gradually with ambient temperature, from 3.2% in Iz-Sp, to +6.5% in Iz-Sm and +12.7% in Ad-Sm (Table 2). Averaged over sexes, FE of the naked neck birds, as compared to their fully feathered counterparts, was 4.3% lower in the cool climate (Iz-Sp), and 7.7 and 10% higher in the hot climates (Iz-Sm and Ad-Sm, respectively). Body Temperature Sex and interactions with sex had no significant effect on BT and therefore means of males + females are presented in Table 3, by G, C and age (A). The G C A interaction was highly significant (P = 0.002) for BT2, and almost significant (P = 0.090) for BT1; hence, G C interaction was tested using a separate ANOVA for each age. At 6 wk of age, BT under the relatively higher heat effect (BT1) increased with ambient temperature (i.e., Iz- Sp < Iz-Sm < Ad-Sm) in both genotypes (Table 3). The genotype effect also increased with ambient temperature: relative to the fully feathered birds, BT1 of the naked neck birds was similar in Iz-Sp, and lower by 0.35 C in Iz-Sm and by 0.43 C in Ad-Sm (Table 3). At 7 wk of age, BT1 of the naked neck broilers was also lower than that of their counterparts in all three climates; the difference was twice as large in Ad-Sm (0.83 C) and not significant in Iz-Sm, leading to a significant G C interaction at that age. Body temperature under the relatively lower heat load (BT2) was affected by G C interaction, especially at 7 wk of age (Table 3). Birds reared at higher ambient temperatures exhibited higher BT2, but climate effects were larger in the fully feathered broilers than in the naked neck ones. The increase in age, from 6 to 7 wk, increased BT1 in both genotypes, apparently due to the increase in BW and daily FC. In contrast, BT after feed withdrawal (BT2) increased from 6 to 7 wk only in the normally feathered broilers, but not in the naked neck broilers. Relative Weight of Feathers, Carcass, and Breast The relative (to BW) weights of feathers, carcass, and breast were more affected by interactions with age than sex; therefore, means of males + females are presented by genotype, climate, and age (Table 4). The effects of genotype, climate, age, and their interactions were tested by a separate three-way ANOVA for each age. The interactions were insignificant in most cases, but the main effect of genotype with climate or sex was highly significant in all cases.

6 NAKED NECK AND NORMAL BROILERS IN HOT, WARM, AND TEMPERATE CLIMATES 935 TABLE 3. Least squares means (males + females) of on-feed (BT1) and off-feed (BT2) body temperatures of 6- and 7-wk-old heterozygous naked neck (Na/na) and normally feathered (na/na) broilers, reared in spring (Sp) and summer (Sm) climates at two locations in Turkey (Iz = Izmir, Ad = Adana) Na/na broilers na/na broilers Na/na na/na Age Climate n BT1 BT2 n BT1 BT2 BT1 BT2 (C) (C) (C) 6 wk Iz-Sp c c c Iz-Sm b b b 0.35*** 0.15 Ad-Sm a a a 0.43*** 0.25* x *** wk Iz-Sp b b b 0.33** 0.10 Iz-Sm c c b Ad-Sm a a a 0.83*** 1.04*** x *** 0.38*** Source of variation wk of age 6 wk of age 7 wk of age Genotype <0.001 < NS <0.001 <0.001 Climate (C) <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 Sex (S) NS NS NS NS NS NS G C <0.001 NS <0.001 C S NS NS NS NS NS NS G S NS NS NS NS NS NS G C S NS NS NS NS NS NS Age (A) < G A NS C A <0.001 <0.001 S A NS NS C S A NS NS G C A G S A NS NS a cclimate means within age with no common superscript differ significantly (P < 0.05). *Differences between genotypes were significant at P < **Differences between genotypes were significant at P < ***Differences between genotypes were significant at P < The age effect was significant for %F, which increased from 6 to 7 wk of age. However, at both ages %F was lower in the naked neck birds than in their normally feathered counterparts within each climate, with the relative reduction ranging from 18.5 to 23.1% (Table 4). Relative to their normally feathered counterparts, the naked neck broilers exhibited higher %C and %B in the three climates at both ages. However, the genotype effect was more pronounced and significant for %B: averaged over climates, it was higher in the Na/na broilers by 6.5 and 3.7% at 6 and 7 wk of age, respectively (Table 4). Only in Ad-Sm at 7 wk was the %B advantage of the Na/na birds not significant. However, absolute breast weight (%B multiplied by BW7) in Ad-Sm was significantly higher in the Na/na vs na/na birds, by 8.5% (383 vs 353 g). DISCUSSION The purpose of the present study was to compare naked neck broilers with their normally feathered counterparts in temperate, warm, and hot natural climates. To this end, the experiments were conducted in Izmir during the spring (Iz-Sp) and summer (Iz-Sm), and in Adana during the summer (Ad-Sm). Due to (P(F)) seasonal and geographical differences, the three experiments were characterized by temperate, warm, and hot climates (Iz-Sp, Iz-Sm, and Ad-Sm, respectively), which resulted in substantially different temperatures in the broiler houses (Figure 1). In the Iz-Sp experiment, broiler house ambient temperatures were lower than the standard broiler recommendation (Yahav et al., 1996) most of the time, quite typical for a cool season in temperate regions. Ambient temperatures in Iz-Sm were typical of a warm summer climate, whereas those in the hot-climate Ad-Sm experiment were higher by about 5 and 2 C during the day and at night, respectively. Averaged over Weeks 4 to 7, ambient temperature exceeded 30 C for 16 h daily in Ad-Sm and for only 5 h in Iz-Sm. Each of the three experiments was therefore conducted under clearly different climates. The same experimental protocol was applied in all three cases, so that climatic conditions would have the most substantial effect on the differences in broiler performance between experiments. However, this could not be positively verified, and therefore emphasis was placed on the genotype effect and its consistency across experiments, i.e., interactions between Na genotypes and climates (G C).

7 936 YALÇIN ET AL. TABLE 4. Least squares means (males + females) of feathers (%F), carcass (%C), and breast meat + bone + skin (%B), as percentage of BW at slaughter, of 6- and 7-wk-old heterozygous naked neck (Na/na) and normally feathered (na/na) broilers, reared in spring (Sp) and summer (Sm) climates at two locations in Turkey (Iz = Izmir, Ad = Adana) Na/na broilers na/na broilers 100 (Na/na na/na):(na/na) Age Climate n %F %C %B n %F %C %B %F %C %B 6 wk Iz-Sp a 73.8 a 19.4 b a 72.0 c 17.5 b 21.7*** +2.5*** +10.9*** Iz-Sm a 74.9 b 18.8 b a 74.4 b 17.7 b 18.5*** ** Ad-Sm b 76.2 b 20.6 a b 75.9 a 19.9 a 19.0*** * x *** +1.1** +6.5*** 7 wk Iz-Sp a 72.9 b 18.3 b a 72.4 b 17.4 b 19.0*** ** Iz-Sm a 75.4 a 20.5 a a 73.1 b 19.8 a 23.4*** +3.1*** +3.5* Ad-Sm b 76.3 a 19.9 a b 74.4 a 19.4 a 23.1*** +2.6* +2.5 x *** +2.2*** +3.7* Source of variation wk of age 6 wk of age 7 wk of age Genotype (G) <0.001 <0.001 <0.001 < <0.001 <0.001 < Climate (C) <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 Sex (S) <0.001 NS <0.001 <0.001 NS <0.001 NS <0.001 G C NS NS NS NS NS NS NS NS G S NS NS NS NS NS NS C S NS NS NS NS NS NS NS NS NS G C S NS NS NS NS NS NS NS NS Age (A) <0.001 NS NS G A NS NS C A <0.001 NS <0.001 S A NS NS NS G C A NS NS G S A NS NS C S A NS NS NS a cclimate means within age with no common superscript differ significantly (P < 0.05). *Differences between genotypes were significant at P < **Differences between genotypes were significant at P < ***Differences between genotypes were significant at P < As in all previous studies with naked neck chickens (Merat, 1986, 1990; Cahaner et al., 1993), the Na/na genotype reduced %F by about 20%, independent of climate, sex or age (6 or 7 wk). The naked neck broilers exhibited higher %B in all cases, ranging from 2.5 to 10.9% more than their normally feathered counterparts. A similar advantage has been reported previously (Cahaner et al., 1993) for breast meat. In the present study, breast weight included breast skin, and carcass included the entire skin. Because skin weight is reduced in naked neck broilers (Cahaner et al., 1993), the actual advantage in meat yield of the naked neck birds was probably larger than that obtained for %B and %C in the present study. The effect of the naked neck genotype on BWG4 7 and BW7 clearly interacted with climate and sex. In the hottest climate (Ad-Sm), both male and female naked neck broilers exhibited a highly significant advantage over their normally feathered counterparts, in both BWG4 7 and BW7. In the more moderate, warm climate (Iz-Sm), the Na/na genotype exhibited superior growth only among males, and the magnitude of its advantage was lower than in Ad-Sm. In the cool temperate climate (%) P(F) (Iz-Sp), BWG4 7 and BW7 means were similar for both genotypes. The higher FC of naked neck birds under the cool conditions of Iz-Sp could be expected, due to their higher rate of heat dissipation, and resulted in about 4% lower FE than the na/na birds under these cool conditions. However, in both summer climates, the naked neck broilers exhibited superior FE in both sexes. In several temperature-controlled studies in cages (Hanzl and Somes, 1983; Merat, 1986; Cahaner et al., 1993), heterozygous naked neck birds exhibited a nonsignificant advantage in FE at ambient temperatures of 31 to 38 C. The significantly higher FE of the Na/na birds in the present study could be attributed to the higher growth potential of the broilers used, the rearing system, or the natural climatic high temperature. Body temperature was measured at the highest daily ambient temperatures (BT1), and again (BT2) when ambient temperatures were lower by about 4 to 6 C (Figure 1) and internal heat production was reduced due to 10 h feed withdrawal (Teeter et al., 1987). Thus, extreme conditions of heat load were obtained by combinations of high levels (BT1) or low levels (BT2) of

8 NAKED NECK AND NORMAL BROILERS IN HOT, WARM, AND TEMPERATE CLIMATES 937 ambient temperatures and internal metabolism. Climates affected BT1 in both genotypes, BT2 in the normally feathered broilers only. Apparently, the reduced feather coverage of the naked neck birds fully overcame the lower heat load (BT2), but not the higher one (BT1). Nevertheless, the increase in BT1 due to climate was more pronounced in the fully feathered broilers than in their naked neck counterparts, especially at 7 wk of age, when BW and FC were certainly higher than at 6 wk of age. Mean BT1 of the naked neck broilers was lower than that of their normally feathered counterparts in all cases, clearly indicating that the 20% reduction in feather coverage allowed for a higher rate of heat dissipation, which is probably the inherent mechanism responsible for the relative heat tolerance of naked neck broilers. In summary, although ambient temperatures were not controlled in the present study, the information presented on their level and daily fluctuations may facilitate a cautious extrapolation of the results to an array of similar natural climates. In contrast to some previous studies with naked neck broilers, this study s findings are relevant to modern industrial broiler production, because advanced commercial stocks were included as reference in each experiment, and they were outperformed by both Na/na and na/na genotypes in all cases. These data were not presented because different reference stocks were used at each location. It can be concluded, therefore, that naked neck broilers should be preferred for industrial production in hot climates. The magnitude of their relative advantage in BW, FE, and meat yield increased as ambient temperatures inside the broiler house rose. Although as expected, the naked neck genotype had no advantage in Iz-Sp, this cool temperate climate had no negative effect on the growth of naked neck birds. REFERENCES Adams, R. L., and J. C. Rogler, The effect of environmental temperature on the protein requirements and responses to energy in slow and fast growing chicks. Poultry Sci. 47: Ajang, O. A., S. Prijono, and W. K. Smith, The effect of dietary protein level on growth and body composition of fast and slow feathering broiler chickens. Br. Poult. Sci. 34: Cahaner, A., N. Deeb, and M. Gutman, Improving broilers growth at high temperature by the naked neck gene. Pages in: Proceedings 19th World s Poultry Congress. Vol. 2. Amsterdam, The Netherlands. Cahaner, A., N. Deeb, and M. Gutman, Effects of the plumage-reducing naked neck (Na) gene on the performance of fast growing broilers at normal and high ambient temperatures. Poultry Sci. 72: Cahaner, A., E. A. Dunnington, D. E. Jones, J. A. Cherry, and P. B. 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