Broilers respond to cooler ambient temperatures after temperature acclimation during incubation and early postnatal age

©2013 Poultry Science Association, Inc.

Broilers respond to cooler ambient temperatures after temperature acclimation during incubation and early postnatal age M. Akşit,* S. Yalçın,†1 P. B. Siegel,‡ Ç. Yenisey,§ D. Özdemir,* and S. Özkan† *Department of Animal Science, Adnan Menderes University, 09100 Aydın, Turkey; †Department of Animal Science, Ege University, 35100 Izmir, Turkey; ‡Department of Animal and Poultry Science, Virginia Polytechnic Institute and State University, Blacksburg 24061-0306; and §Faculty of Medicine, Adnan Menderes University, 09100 Aydın, Turkey Primary Audience: Broiler Producers, Hatchery-Breeder Flock Personnel, Service Personnel, Production Managers SUMMARY Temperature acclimation, either during the incubation or the early growing period, has been suggested as an effective method to improve thermotolerance of broilers to cooler rearing temperatures. We compared the response of broilers from younger (Y) and older (O) breeder flocks to cooler rearing temperatures after temperature acclimation during incubation and early posthatch. Eggs were exposed to either a control incubation temperature (37.6°C) or a cooler temperature (36.6°C; ITcool) for 6 h (from1000 to 1600 h) daily from d 10 to 18 of incubation. From d 22 to 42, chicks were reared at a control temperature (RTcont), a cooler temperature (17 ± 2°C; RTcool), or at a cooler temperature after acclimation of chicks to cooler temperatures by exposing them to 17 ± 2°C for 6 h on d 5 posthatch. The ITcool conditions reduced mortality and mortality due to ascites of O broilers reared under cooler temperatures from d 22 to 42. Early age acclimation to cooler temperatures did not support thermotolerance of broilers. Regardless of IT, the RTcool broilers from Y gained more weight but had poorer feed conversion than RTcont. Repeated cooler temperatures applied during incubation and early postnatal stage disturbed homeostasis and energy balance of broilers from O breeders. Key words: incubation temperature, early age acclimation, broiler, ambient temperature 2013 J. Appl. Poult. Res. 22:298–307 http://dx.doi.org/10.3382/japr.2012-00675

DESCRIPTION OF PROBLEM Among environmental factors, ambient temperatures can have a large effect on the growth and performance of broilers. This is due to the fact chicks are not homeothermic at hatch and cannot control their own body temperature, which is between 40 and 40.5°C. Thereafter, 1

Corresponding author: [email protected]

body temperature increases to reach the adult level by a relatively rapid improvement in thermoregulation system during the first 2 wk. The effect of ambient temperature on embryos and young chicks and the mechanisms involved have long been of interest to poultry scientists [e.g., 1–5].The metabolic costs to maintain stable body temperatures discussed in those ear-

Akşit et al.: ACCLIMATION TO COOLER TEMPERATURE lier reports have received renewed interest during the past decade due to the effect of cooler temperatures on growth and feed conversion of broilers. Broilers exposed to cooler temperatures are lighter or of similar BW to controls, but consume more feed, which has a negative influence on feed conversion [e.g., 6–9]. The high metabolic rate of fast-growing broilers increases their oxygen requirements; therefore, low ambient temperatures can trigger an increase in the metabolic requirements for oxygen because the metabolic rate must increase to meet the demands for body heat production. The increased demand for oxygen may exceed lung capacity, causing the broiler to become hypoxic. When cardiac demands exceed pulmonary vascular capacity the ascites syndrome can develop [10–12]. Temperature acclimation of broilers by exposure to cooler temperatures may enhance their resistance to cooler temperatures. Shinder et al. [13] reported that exposing chicks to 15°C for 3 h at d 3 and 4 posthatch improved their thermotolerance to low temperatures, and manipulation of incubation temperatures can generate a long-term adaptation to lower temperatures [13–15]. Embryos exposed to lower incubation temperatures have a reduced metabolic rate, as measured by lower egg shell temperatures and embryo weights [15, 16]. Previously we reported that, although cooler incubation temperatures lowered hatchability (81.8 vs. 76.2% for eggs from cooler and control incubation, respectively), higher body lipid content of chicks from cooler temperatures may enhance their resistance to cold stress during the growing period [16]. Breeder age influences the response of embryos to cooler incubation temperatures; embryos from older breeders, which have numerically higher chick weights at d of hatch and blood triiodothyronine levels, are more resistant to cyclical incubation temperatures than those from younger breeders [16]. In the present study, we measured the response of broilers from younger and older breeder flocks to cooler temperatures after acclimation to cooler temperatures during incubation and on d 5 posthatch.

MATERIALS AND METHODS Experimental procedures were approved by the Adnan Menderes University Animal Care

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and Ethics Committee and complied with the Turkish Code of Practice for the Care and Use of Animals for Scientific Purposes (License number 2009–63). Birds and Experimental Design The details of the incubation conditions have been previously reported [16]. Briefly, eggs obtained from 32-wk-old (younger; Y) and 48-wkold (older; O) Ross 308 broiler breeders were randomly divided into 2 groups and incubated at 37.6°C (ITcont) or 36.6°C (ITcool) for 6 h (from 1000 to 1600 h) daily from d 10 to 18 of incubation. On the day of hatch, 1,224 mixed sex chicks from Y and O breeders (612 chicks from each breeder age) from ITcont and ITcool incubation (306 chicks from each IT) were wing-banded and weighed individually. Body weights of chicks at the day of hatch [16] were 41.63 and 41.46 g for broilers for Y and 47.18 and 48.95 g for O exposed to ITcont and ITcool, respectively. Chicks from each breeder age and IT were randomly distributed to 36 environmentally controlled pens (1.3 × 1.8 m; 34 chicks/pen) and assigned to 1 of 3 treatments. (1) Standard brooding and rearing temperatures (RTcont), in which brooding temperatures were gradually reduced from 32 to 24°C from the day of hatch to d 21, then, from d 22 to 42, room temperature was 21 ± 1°C. (2) Standard brooding temperatures, except chicks were exposed to 17 ± 2°C for 6 h on d 5, then, from d 22 to 42, chicks were reared at 17 ± 2°C (RT5d+cool). (3) Standard brooding temperatures and, from d 22 to 42 of the rearing period, chicks were exposed to 17 ± 2°C (RTcool). Humidity was maintained between 50 and 60% in all treatments. Feed and water were provided ad libitum. From the day of hatch to d 10, the crumbled diets consisted of 23% CP and 3,100 kcal/kg ME; from d 11 to 21, 22% CP and 3,150 kcal/ kg ME; and from d 22 to 42, 20% CP and 3,200 kcal/kg ME. The lighting schedule was 23L:1D throughout. Individual BW (g) of chicks were obtained on d 7, 21 (1 d before cooling ambient temperature), 28 (7 d after cooling ambient temperature), and 42. Mortality was recorded daily. Chicks that died were necropsied to assess the

ITcont ITcool SE RTcont RT5d+cool RTcool SE

21 d

756 753 6 737b 773a — 6

0.722 <0.001 0.019

7d

149a 140b 1 141b 148a — 1

<0.001 <0.001 <0.001

0.643 <0.001 0.018

1,225 1,232 11 1,170c 1,298a 1,220b 13

28 d

0.282 0.044 0.170

2,204 2,239 18 2,186b 2,264a 2,216ab 18

42 d 69.50 70.20 0.92 59.05b 76.44a 74.07a 0.94

0.419 0.593 <0.001 <0.001 <0.001 0.260

34.82 32.99 0.31 32.99b 34.82a — 0.30 0.194 0.212 0.012

70.03 71.88 1.00 69.96 72.62 70.29 1.26

0–21 d 21–28 d 28–42 d

BW gain, g/d

<0.001 0.522 <0.001

141b 151a 1 146 145 — 1

7d

0.647 0.001 <0.001

762 757 8 778a 741b — 7

21 d

0.589 <0.001 <0.001

1,233 1,243 13 1,224b 1,198b 1,293a 15

28 d

BW, g

0.0833 0.016 0.008

2,335 2,280 22 2,338a 2,223b 2,360a 24

42 d

Older breeder

0.519 <0.001 <0.001

34.06 33.72 0.37 34.80a 32.98b — 0.37

0–21 d

0.367 <0.001 0.015

69.50 67.96 1.21 62.24c 67.44b 76.49a 1.48

0.002 0.001 0.066

78.40a 73.94b 1.00 79.46a 73.21b 75.87ab 1.22

21–28 d 28–42 d

BW gain, g/d

1

Means in the same column for both IT and RT with no common superscript differ significantly (P < 0.05). Pooled SE. 2 ITcont = temperature of 37.6°C; ITcool = eggs exposed to 36.6°C for 6 h from 1000 to 1600 h each day from d 10 to 18 of incubation. 3 RTcont = chicks reared at standard brooding and rearing temperatures; RT5d+cool = chicks acclimated by exposing to 17 ± 2°C for 6 h on d 5 of the posthatch period and were reared at 17 ± 2°C from d 22 to 42; RTcool = broilers exposed to 17 ± 2°C from d 22 to 42.

a–c

P-value  IT  RT   IT × RT

Rearing temperature (RT)

Incubation temperature (IT)

Treatment

BW, g

Younger breeder

Table 1. Least squares means and SE1 for BW and BW gain from 0 to 42 d of broilers from younger and older breeders by incubation temperature2 and rearing temperature3

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Akşit et al.: ACCLIMATION TO COOLER TEMPERATURE cause of death and those with fluid in the abdominal cavity were classified as having ascites [17]. Feed consumption was recorded on a pen basis to 21 and 42 d and a mortality-adjusted FCR was calculated. At d 21, 12 chicks (6 males and 6 females) were randomly selected from each group and a blood sample was obtained from the brachial vein to measure triiodothyronine (T3), thyroxine (T4), and triglycerides. Serum was obtained after centrifugation at 4°C and 4,500 × g for 10 min and stored at −20°C. Values for T3 and T4 were determined using Immulite 2000 automated chemiluminescent assays [18] and triglycerides using Abbott Diagnostic [19]. After blood was obtained, chicks were euthanized by cervical dislocation. Livers and hearts were immediately removed and weighed (0.01 g) and relative organ weights were calculated on the basis of BW. At d 28, the same procedure was repeated except samples were not taken from RTcont. Statistical Procedures Based on preliminary ANOVA, age of breeder was significant for all traits measured.

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Therefore, data were analyzed separately for each breeder age by ANOVA using JMP [20]. For the data obtained at d 7 and 21, a 2 × 2 factorial arrangement of treatments for IT and RT and the interaction between the two were the main effects. For data obtained at d 28 and 42, the arrangement was a 2 × 3 factorial. When the interaction was significant, comparisons were performed by Tukey’s comparisons test. A χ2 was used to statistically evaluate changes in mortalities due to IT and RT within each breeder age. Combined data were also used to analyze effects of IT and RT on mortalities. Significance implies P < 0.05 unless otherwise stated.

RESULTS AND DISCUSSION There was a significant interaction between IT and RT treatments for BW of Y broilers at d 7 and 21 (Table 1). This interaction resulted from ITcont-RT5d+cool broilers being heavier than ITcont-RTcont, whereas the weights of ITcool broilers were similar for both treatments (Table 2). The Y broilers responded to RT5d+cool by increasing their feed intake to regulate their energy balance from d 0 to 21 regardless of IT

Table 2. Interactions between incubation temperature1 (IT) and rearing temperature2 (RT) for BW, BW gain, feed intake, and FCR from 0 to 42 d ITcont

Breeder age

Trait

Younger

BW, g

Older

a,b

Age, d

7 21 28 BW gain, g/d 0–21 28–42 BW, g 7 21 28 42 BW gain, g/d 0–21 21–28 28–42 Feed intake, g/chick 0–21 0–42 FCR, kg/kg 0–42

ITcool

RTcont

RT5d+cool

RTcool

142b 727b 1,167b 32.41b 74.17a 138a 753a 1,193a 2,327a 33.64a 61.00b 77.38a 960a 4,073a 1.77a

157a 785a 1,322a 35.75a 66.52b 144a 771a 1,250a 2,324a 34.47a 71.69a 79.63a 965a 4,390a 1.92b

— — 1,189b — 69.41ab — — 1,256a 2,354a — 75.80a 78.19a — 4,230a 1.83a

RTcont 140a 745a 1,174b 33.57a 71.08a 155a 804a 1,255ab 2,349a 35.95a 63.52b 79.31a 1,066a 4,206ab 1.82a

RT5d+cool

RTcool

140a 761a 1,275a 33.88a 73.40a 146b 710b 1,146b 2,124b 31.49b 63.20b 69.04b 903b 3,902b 1.85ab

— — 1,250a — 71.16a — — 1,330a 2,367a

SE3

18

2 8

0.45 1.60 2 10 23 38 0.50 77.17a 2.10 73.48ab 1.90 — 20 4,446a 133 1.91b 0.03

Means in the same row within an IT with no common superscript differ significantly (P < 0.05). ITcont = temperature of 37.6°C; ITcool = eggs exposed to 36.6°C for 6 h from 1000 to 1600 h each day from d 10 to 18 of incubation. 2 RTcont = chicks reared at standard brooding and rearing temperatures; RT5d+cool = chicks acclimated by exposing to 17 ± 2°C for 6 h on d 5 of the brooding period and reared at 17 ± 2°C from d 22 to 42; RTcool = broilers exposed to 17 ± 2°C from d 22 to 42. 3 SE = pooled SE. 1

0.137 0.023 0.424

0.218 0.002 0.435

3,986 4,048 33 3,867b 4,132a 4,052a 38

0–42 d

0.095 0.814 0.572

1.33 1.39 0.02 1.36 1.37 — 0.02

0–21 d

0.782 0.021 0.195

1.84 1.85 0.01 1.80a 1.86b 1.88b 0.01

0–42 d

FCR, kg/kg

0.621 0.190 —

8.17 9.80 1.51 7.35 11.76 8.84 1.70

Total

0.784 0.552 —

3.59 4.57 1.00 3.92 5.39 2.94 1.45

Ascites

Mortality, %

0.289 0.001 0.001

962 984 13 933b 1,013a — 14

0–21 d

0.657 0.220 0.037

4,231 4,185 7 4,139 4,146 4,338 84

0–42 d

Feed intake, g

0.036 0.115 0.216

0.263 0.004 0.015

1.84 1.86 0.01 1.80a 1.88b 1.86b 0.01

a

1.35 1.40b 0.02 1.36 1.39 — 0.02

0–42 d

0–21 d

FCR, kg/kg

Older breeder

0.345 0.038 —

9.19 7.84 1.20 6.37 11.27 7.64 1.18

Total

0.871 0.154 —

3.67 3.92 1.23 2.45 5.39 3.52 1.18

Ascites

Mortality, %

1

Means in the same column for both IT and RT with no common superscript differ significantly (P < 0.05). Pooled SE. 2 Grams of feed intake per gram of BW gain. 3 P-values were obtained by χ2 test. 4 ITcont = temperature of 37.6°C; ITcool = eggs exposed to 36.6°C for 6 h from 1000 to 1600 h each day from d 10 to 18 of incubation. 5 RTcont = chicks reared at standard brooding and rearing temperatures; RT5d+cool = chicks acclimated by exposing to 17 ± 2°C for 6 h on d 5 of the brooding period and were reared at 17 ± 2°C from d 22 to 42; RTcool = broilers were exposed to 17 ± 2°C from d 22 to 42.

a,b

P-value  IT  RT   IT × RT

Rearing temperature (RT)

953 987 15 943b 997a — 15

Incubation temperature (IT)

ITcont ITcool SE RTcont RT5d+cool RTcool SE

0–21 d

Treatment

Feed intake, g

Younger breeder

Table 3. Least squares means and SE1 for feed intake, FCR,2 and mortality3 of broilers from younger and older breeders by incubation temperature4 and rearing temperature5

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Table 4. Mortality for incubation temperature1 (IT) and rearing temperature2 (RT) treatments ITcont Item Total mortality, % Ascites, %

ITcool

RTcont

RT5d+cool

RTcool

χ2

RTcont

RT5d+cool

RTcool

χ2

7.35 3.43

9.80 3.92

8.82 3.52

2.08 (0.353)3 0.199 (0.905)3

6.37 2.45

13.23 7.35

6.86 2.94

7.02 (0.029)3 5.725 (0.057)3

1

ITcont = temperature of 37.6°C; ITcool = eggs exposed to 36.6°C for 6 h from 1000 to 1600 h each day from d 10 to 18 of incubation. 2 RTcont = chicks reared at standard brooding and rearing temperatures; RT5d+cool = chicks acclimated by exposing to 17 ± 2°C for 6 h on d 5 of the brooding period and were reared at 17 ± 2°C from d 22 to 42; RTcool = broilers were exposed to 17 ± 2°C from d 22 to 42. 3 P-values obtained by χ2 test are given in parentheses.

(Table 3). This increased feed intake was converted to BW at d 21. Although it was reported [15] that early age cold acclimation had no effect on the BW of broilers at d 21, based on our results, the response of broilers to early age acclimation was dependent on the age of the breeder flock because, in contrast to Y broilers, treatments did not influence BW of ITCont, O broilers at d 7 and 21. The O broilers exposed to ITcool-RT5d+cool were lighter in weight than ITcool-RTcont (Table 1) and their feed intake to d 21 was lower (Table 2). Therefore, repeated cooler temperatures applied during incubation and early in the postnatal period disturbed homeostasis and the energy balance of broilers from older breeders. During the week of exposure to the cooler temperature (d 22 to 28), Y broilers reared under cooler temperatures (RT5d+cool and RTcool) gained more weight than RTcont regardless of IT (Table 1). Among ITcont and Y broilers, the RT5d+cool were heavier than the RTcont and RTcool at d 28 (Table 2). Although ITcont-RT5d+cool broilers gained less weight than the RTcont, at d 42 the heaviest broilers were from IT5d+cool, which were followed by RTcool and RTcont (Table 1). These results were associated with higher feed consumption by Y broilers from RT5d+cool and RTcool at d 42; however, they had poorer feed conversion than RTcont (Table 3). There were significant IT and RT interactions for BW at d 28 and 42, BW gain from d 21 to 28 and d 28 to 42 (P = 0.066), and feed conversion from d 0 to 42 in O broilers. Rearing temperature treatments did not affect BW of O broilers from ITcont at either d 28 or 42. In other words, under ITcont, cooler temperatures from d 21 to 42 did not influence BW, BW gain, feed

intake, or FCR of O broilers. The ITcont-RT5d+cool broilers had poorer feed conversion from d 0 to 42 (Table 2). The RT5d+cool O broilers from ITcool could not compensate for their early growth retardation by d 42, although their feed conversion was similar to that for ITcool-RTcont broilers. Body weights of broilers from embryos exposed to an incubation temperature of 15°C for 30 or 60 min at d 18 and 19 of incubation and raised under control conditions were similar or greater than those of broilers from control incubation [21]. Similarly, from d 28 to 42, BW gains of ITcool broilers under RTcont were similar to ITcont-RTcont broilers regardless of breeder age (Table 2). Mortality is one of the better indicators of the effect of rearing temperature on performance [22]. There was no effect of IT on total mortality or mortality due to ascites for Y and O broilers (Table 3). Although there was no effect of treatment on total mortality for Y broilers, the higher mortality of O broilers subjected to ITcool and RT5d+cool can be interpreted to mean that repetitive cooler incubation and postnatal temperature compromised their ability to cope with cooler temperatures. However, mortality was not significantly affected by breeder age and combined data are presented in Table 4. For ITcont broilers, mortality was not changed by RT, whereas, for ITcool broilers, total mortality (P = 0.029) and mortality due to ascites (P = 0.057) was significantly affected by RT. Therefore, the cooler temperature used at an early age in this experiment did not support thermotolerance of broilers. That these results were not in agreement with those of Shinder et al. [13] was probably due to the differences in the severity of the acclimation process.

0.340 0.750 0.697

0.848 0.669 0.076

0.318 0.087 0.641

58.39 63.32 4.00 56.13 65.57 — 3.80

Triglyceride

0.687 0.027 0.019

14.65 14.83 0.78 15.87a 13.63b — 0.72

T3

0.235 0.670 0.730

0.57 0.64 0.05 0.58 0.62 — 0.05

T4

0.436 0.044 0.351

1.030 0.982 0.04 0.943b 1.069a — 0.04

Heart

0.718 0.714 0.118

3.55 3.62 0.13 3.55 3.62 — 0.13

Liver

0.015 0.736 0.999

48.48 59.04a 3.19 55.03 52.50 — 3.25

b

Triglyceride

Older breeder

0.387 0.286 0.149

15.31 16.66 0.83 15.43 16.55 — 0.84

T3

0.974 0.661 0.408

0.60 0.60 0.05 0.61 0.59 — 0.05

T4

1

Means in the same column for both IT and RT with no common superscript differ significantly (P < 0.05). Pooled SE. 2 ITcont = temperature of 37.6°C; ITcool = eggs exposed to 36.6°C for 6 h from 1000 to 1600 h each day from d 10 to 18 of incubation. 3 RTcont = chicks reared at standard brooding and rearing temperatures; RT5d+cool = chicks were acclimated by exposing to 17 ± 2°C for 6 h on d 5 of the brooding period and were reared at 17 ± 2°C from d 22 to 42; RTcool = broilers were exposed to 17 ± 2°C from d 22 to 42.

a,b

P-value  IT  RT   IT × RT

Rearing temperature (RT)

3.43 3.60 0.12 3.54 3.48 — 0.12

0.937 0.927 0.03 0.943 0.921 — 0.03

ITcont ITcool SE RTcont RT5d+cool RTcool SE

Incubation temperature (IT)

Liver

Heart

Treatment

Younger breeder

Table 5. Least squares means and SE1 for the relative weights of heart and liver, blood triglycerides, free triiodothyronine (T3) and thyroxine (T4) levels of broilers from younger and older breeders by incubation temperature2 and rearing temperature3 on d 21 (before cold exposure)

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0.053 0.011 0.010

0.188 0.544 0.031

0.268 0.741 0.209

44.28 38.87 3.20 — 42.52 40.63 3.25

Triglycerides

0.388 0.461 0.700

13.22 13.77 1.25 — 14.49 12.50 1.34

T3

0.766 0.325 0.067

0.880 0.837 0.08 — 0.833 0.885 0.083

T4

0.587 0.453 0.022

0.919 0.896 0.030 — 0.923 0.891 0.030

Heart

0.959 0.304 0.107

3.18 3.19 0.08 — 3.24 3.12 0.08

Liver

0.949 0.876 0.716

41.28 41.44 3.22 — 41.94 40.77 3.22

Triglycerides

Older breeder

0.236 0.357 0.441

15.89 13.91 1.13 — 14.05 15.75 1.13

T3

0.049 0.234 0.406

0.685b 0.928a 0.076 — 0.733 0.880 0.076

T4

1

Means in the same column for both IT and RT with no common superscript differ significantly (P < 0.05). Pooled SE. 2 ITcont = temperature of 37.6°C; ITcool = eggs exposed to 36.6°C for 6 h from 1000 to 1600 h each day from d 10 to 18 of incubation. 3 RTcont = chicks reared at standard brooding and rearing temperatures; RT5d+cool = chicks were acclimated by exposing to 17 ± 2°C for 6 h on d 5 of the brooding period and were reared at 17 ± 2°C from d 22 to 42; RTcool = broilers exposed to 17 ± 2°C from d 22 to 42.

a,b

P-value  IT  RT   IT × RT

Rearing temperature (RT)

3.19 3.52 0.11 — 3.57a 3.12b 0.12

0.880 0.949 0.036 — 0.930 0.899 0.036

ITcont ITcool SE RTcont RT5d+cool RTcool SE

Incubation temperature (IT)

Liver

Heart

Treatment

Younger breeder

Table 6. Least squares means and SE1 for the relative weights of heart, liver, blood triglycerides, free triiodothyronine (T3), and thyroxine (T4) levels of broilers from younger and older breeders by incubation temperature2 and rearing temperature3on d 28 (after cold exposure)

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Table 7. Interactions between incubation temperature1 (IT) and rearing temperature2 (RT) for blood free triiodothyronine (T3) and relative weights of heart and liver at d 21 and 28 Breeder age Younger (Y) Older (O)

ITcont

ITcool

Trait

Age, d

RTcont

RT5d+cool

RTcool

RTcont

RT5d+cool

RTcool

SE3

T3 Heart Liver Heart

21 28 28 28

17.05a — — —

12.25b 0.840a 3.18a 0.884a

— 0.922a 3.19a 0.953a

14.67a — — —

15.00a 1.023a 3.96a 0.962a

— 0.875b 3.07b 0.829b

1.09 0.051 0.17 0.042

a,b

Means in the same row within a trait with no common superscript differ significantly (P < 0.05). ITcont = temperature of 37.6°C; ITcool = eggs were exposed to 36.6°C for 6 h from 1000 to 1600 h each day from d 10 to 18 of incubation. 2 RTcont = chicks reared at standard brooding and rearing temperature; RT5d+cool = chicks acclimated by exposing to 17 ± 2°C for 6 h on d 5 of the brooding period and were reared at 17 ± 2°C from d 22 to 42; RTcool = broilers were exposed to 17 ± 2°C from d 22 to 42. 3 SE = pooled SE. 1

Previously, researchers have reported an increase in heart weight by cooler incubation and ambient temperatures [15, 23], which was related to an increased workload in the cardiopulmonary system [24]. In the present experiment, the IT and RT treatments did not affect relative weights of hearts and livers of Y broilers at d 21 (Table 5).The RT5d+cool treatment significantly increased relative weights of the heart in O broilers on d 21. On d 28, there was a significant IT by treatment interaction (Table 6). This interaction occurred because the increase in heart weight under ITcool was highly significant, whereas, for RT5d+cool, relative heart weights of Y and O broilers did not differ under ITcont (Table 7). The relatively heavier heart weights of Y and O broilers at d 28, observed by repetitive cooler temperatures during incubation and early in the postnatal period, could be consistently associated with ascites sensitivity of ITcool-RT5d+cool broilers, as mortalities due to ascites increased in this group. The heavier relative liver weights of ITcool-RT5d+cool broilers was in agreement with Blahova et al. [6], and was probably related to increased lipid accumulation [25] to contribute to the improved tolerance to cold. On d 21, blood triglycerides of Y broilers increased slightly, but not significantly (P = 0.087), for RT5d+cool (Table 4). A significant interaction for T3 resulted from ITCont-RT5d+cool, which had lower levels of T3 than those in ITccont-RTcont; whereas T3 was similar for ITcoolRTcont and ITcool-RT5d+cool (Table 6). Previously, researchers have reported increased plasma T3

under low environmental temperature [26]. The lower T3 observed for ITcont-RT5d+cool indicated an increased plasma clearance rate of T3 in those broilers [27]. There was no effect of treatments on T4 of Y broilers on d 21.The higher total mortality for ITcont-RT5d+cool broilers (Table 3) corroborates the suggestion [28] that low T3 and unchanged T4 would reduce oxygen consumption leading to hypoxia, which links with ascites. Similarly, ascitic broilers had lower T3 values when ascites developed but had not yet resulted in mortality [29]. Blood triglycerides of O broilers were increased by ITcool on d 21 (Table 5) and was probably related to energy balance, which, in turn, affects feed intake [30]. Neither IT nor RT influenced T3 and T4 of broilers from older breeders on d 21, however, T4 was higher for ITcool than ITcont. This suggests a long-term effect of ITcool on the dynamics of conversion of T4 to T3 of broilers from older breeders.

CONCLUSIONS AND APPLICATIONS 1. Younger broilers exposed to RTcool gained more weight than RTcont; however, regardless of IT, RTcool feed conversion was inferior to that of RTcont broilers at d 42. 2. Repetitive cooler temperatures applied during incubation and early posthatch reduced BW and increased total mortality and mortality due to ascites of O broilers at d 42. The BW and total mortality

Akşit et al.: ACCLIMATION TO COOLER TEMPERATURE of O ITcool-RTcool broilers indicated ITcool could be used to increase the tolerance of O broilers to cooler growing temperatures.

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Acknowledgments

We gratefully acknowledge the financial support of the Adnan Menderes University Scientific Research Fund (project number: ZRF 10008).