On-line version ISSN 2221-4062
S. Afr. j. anim. sci. vol.38 n.1 Pretoria Jan. 2008
E. SengorI; M. YardimciI; N. OkurII; U. CanIII
IFaculty of Veterinary Medicine, Department of Animal Husbandry and Nutrition, Afyon Kocatepe University, 03200 Afyonkarahisar-Turkey
IIBeypi Inc. Research & Development Manager, Bolu-Turkey
IIIBeypi Inc. Research & Development Department, Bolu-Turkey
The aim of this study was to investigate the effect of thermal conditioning of broiler chickens during embryonic development on subsequent performance under standard rearing conditions. During incubation eggs from 32-, 45- and 56-week old Ross 308 broiler parent stock were subjected to a 2 h heat shock of 39 °C on days 14 and 15 of incubation. Eggs in the control were incubated throughout incubation at 37 °C. Chicks were feather sexed and equal numbers of each sex were placed in each pen per treatment, and reared for 42 days. Live weight, mortality and feed conversion ratio (FCR) were used as measures of performance. The final (six-week) live weights of broilers from young, mid and older parents for the treatment and control groups were 2113 ± 13.8 vs. 2159 ± 20.0, 2084 ± 29.2 vs. 2139 ± 20.0 and 2096 ± 17.6 vs. 2131 ± 24.3 g, respectively. The six-week live weight of the heat-treated group (2098 ± 12.0 g) was significantly lower than that of the control (2143 ± 12.2 g). The 1-6 week mortality figure was significantly lower in the heat-treated group of chickens from the young (83) and mid parent (77) groups compared to their controls (130 and 119), respectively. However, in the treatment group the incidence of mortality in broilers from the older parent group was significantly higher (105) than that of the control (79). The overall mortality without considering the parent age group was significantly lower in the treatment group (265) than in the control group (328). Mean FCR (g feed/g gain) of the chickens of the three parent groups was 1.79 ± 0.02 vs. 1.75 ± 0.03, 1.85 ± 0.03 vs. 1.77 ± 0.02 and 1.80 ± 0.03 vs. 1.77 ± 0.03 for the treatment vs. control groups, respectively, but the difference was significant only in the mid age parent group. These results suggest that prenatal thermal conditioning is not detrimental to broiler growth under standard rearing conditions in the absence of thermal stress. However, survival rate was improved but live weight and FCR were in some cases significantly poorer.
Keywords: Heat stress, incubation, parent age, live weight, mortality
The physiology of poultry under heat stress has been investigated extensively (McCormick et al., 1979; 1980 a; b; Edens et al., 1992; Ait-Boulahsen et al., 1989; 1993; 1995), and consequently many approaches have been developed to increase thermotolerance, leading to the minimizing of heat-related mortality while maintaining productivity.
In many regions of the world heat stress, usually in summer months, is one of the major problems in poultry production. Heat stress is an abnormal condition for birds and is caused by hot and humid weather, but is usually minimized through the control of environmental factors such as temperature and ventilation in poultry houses. However, sophisticated housing designs are not always available because of economic and other constraints.
Epigenetic thermal conditioning prior to hatching involves changes in some physiological characteristics and probably serves as a method for epigenetic temperature adaptation since the same mechanisms are employed for coping with heat stress during post-embryonic growth. Previous research suggested that days 14-15 of incubation might be the optimal and most sensitive time for evoking this mechanism during embryonic development (Moraes et al., 2004).
Improved thermotolerance by acclimation could be a valuable tool to maintain the productivity of the broiler industry in tropical and subtropical regions during hot seasons. In broiler chickens, exposure to heat stress at an early post-hatch age could precondition the chickens to improved thermotolerance later in life. Many studies have shown that heat tolerance can be improved by acclimation. Several reports covering several species have documented the use of postnatal heat treatment to induce thermotolerance later in life, including poultry. The effect of early-age thermal conditioning on the performance of poultry under heat stress later in life is well documented in the literature (Arjona et al., 1988; Yahav & Hurwitz, 1996; Yahav, 2000). On the other hand, not much research has been conducted to improve thermotolerance at later ages using prenatal but not postnatal heat conditioning (Moraes et al., 2003; 2004; Yalçin et al., 2005). In fact, the consequences of prenatal heat conditioning without secondary environmental heat stress conditions in later life have not been studied. Additionally, postnatal heat treatment is a burdensome process for farmers to apply under field conditions and would be almost impossible to implement.
The purpose of the present study was to investigate the effect of thermal conditioning of broiler chicks originating from breeder flocks of three different ages, using heat stress during embryonic development on the performance of broilers reared under standard conditions without heat stress.
Materials and Methods
Eggs collected from 32-, 45- and 56-week old Ross-308 broiler parent stocks were used in this study. These eggs were classified as young, middle aged and older parents, and further classified as control and treatment. Eggs in the control groups were subjected to the normal incubation temperature (37 °C) while those in the treatment groups were incubated at 37 °C but were subjected to two hours of heat treatment of 39 °C on days 14 and 15 of incubation.
Once hatched 2 200 chickens subjected to the heat treatment during incubation and 2 200 from the control were selected for each parent group. Consequently the study consisted of six treatments and a total of 13 200 chickens. The experimental house contained 60 identical pens, and 10 pens were allocated to each experimental treatment. Each pen was stocked with 220 birds at 17 chicks per m2, and contained equal numbers of males and females. Heating, humidity and ventilation in the house were applied according to commercial standards. In the hatchery the chicks were vaccinated against New Castle disease (ND) and infectious bronchitis-H120, and again on days 12 and 26 against ND. On days 14 and 20 they were vaccinated with the Gumboro vaccine via their drinking water. The chicks were fed a standard broiler feed prepared according to the Ross 308 management guide.
Live weight gain, feed consumption and mortality were recorded daily during the 42 days of the experimental period, using an automatic in-house built weighing device that recorded daily average live weight of a pen at the end of the each day. Therefore the means and standard error of means were the means within a pen. Birds were weighed at the slaughterhouse before slaughter.
Data for live weight and feed conversion ratio (FCR, g feed/g gain) for the heat-treated and control groups were analyzed using the Student's t-test. The comparisons between the three parent age groups for live weight and FCR were made by analysis of variance (ANOVA) in a completely random design. A least significant difference (LSD) test was carried out to determine the significance of the differences among the groups at the 5% level of significance. Data on the incidences of mortality in the heat-treated and control groups were analyzed using the Chi-Square test. A SPSS-10 (1998) Package Program for Windows was used for the statistical analysis. Since the sex ratios of the pens were equal, no statistical analyses were done on a sex basis.
Results and Discussion
The live weights of the six-week old broilers originating from young, middle aged and older parents for the treatment and control groups are presented in Table 1. The live weight of the broilers in the heat treatments was not affected by the age of the parents that produced the eggs.
The six-week live weights of birds for the treatment and control groups without considering parent age are shown in Table 2. Regardless of parent age, weekly live weights of broilers were almost identical for the control and treatment groups except at week 6 when the control group was heavier (P <0.05) than the heat-treated group.
Yalcin et al. (2005) studied the effects of pre- and postnatal conditioning to induce thermotolerance in broilers obtained from eggs originating from younger and older breeder flocks. As prenatal treatment eggs were subjected to 39.6 °C for six hours per day from days 10 to 18 of incubation. The broilers were reared under standard (control), conditioned (treatment-1) and high ambient (treatment-2) temperatures. Contrary to the present findings Yalcin et al. (2005) found differences (P <0.05) in body weight at seven and 28 days between chicks from the different parent age groups. The 49-day body weight was higher (P <0.05) in the standard incubated group compared to the high temperature incubated group for both parent groups. This result is in accordance with our finding for final (42 day) body weight without considering parent age.
However, in an investigation at North Carolina State University (NCSU) it was found that seven- and 28-day body weights of broilers incubated under high and standard incubation temperatures did not differ (P >0.05) (Leksrisompong, 2005). This finding is in accordance with our results up to five weeks, but contrary to the body weights at 42 days of age, when body weights in the present study differed significantly (P <0.05) between treatments.
Mean body weights of broilers at all ages, except at six weeks, originating from the old breeder flocks, were higher (P >0.05) compared to those from the younger flocks. At six weeks of age broilers from the younger parents performed better compared to the older and middle aged parents (P >0.05). Weytjens et al. (1999) studied thermoregulation in chicks from breeder flocks of two different ages and reported that chicks from young breeders had lower (P <0.05) body weights than those from older breeders. This report was only contrary to our findings for the six-week body weights.
Mortality figures up to six weeks from the three parent groups are presented in Table 3. Mortality of broilers in the treatment groups during the first week and for the 1-6 week period from young parents and during the second, third and 1-6 weeks from middle aged parents was lower (P <0.05) than that of the birds in the control. Conversely, over the six week period mortality among chicks from the older parents in the treatment group was higher (P <0.05) than in those from the other heat-treated groups.
Incidence of mortality in the treatment and control groups, without considering parent age, is shown in Table 4. Mortality in the first (P <0.05), third and over the 1-6 week period was lower in the treatment than in the control group (Table 4). Birds in the heat-treated group therefore appeared to be less prone to postnatal mortality under normal environmental rearing conditions that those incubated under standard temperature conditions.
Moraes et al. (2003) reported thermotolerance of broilers from conditioned embryos, compared with broilers from untreated embryos, and suggested that these birds might have less difficulty in coping with adverse environmental conditions than the controls. The authors additionally suggested that thermal conditioning during embryonic life might offer an additional practical method for easing heat stress in broilers especially under tropical conditions. In our experiment the prenatally heat-treated birds survived better than the control birds during the rearing period, although they were not exposed to extra postnatal heat stress. Leksrisompong (2005) conducted a study to determine the effect of temperature during incubation and rearing on broiler performance and found no significant differences between the mortality of broilers incubated under high and standard incubation conditions for the ages of 0 - 14, 0 - 21 and 0 - 28 days. This result is contrary to our findings where high incubation temperature appeared to reduce the postnatal mortality compared to control (P <0.05).
The FCR of six-week old broiler birds obtained from the three parent groups for the treatment and control groups is presented in Table 5. The FCR of birds for the treatment and control groups without considering parent age is presented in Table 6.
Overall FCR of birds from the three parent groups in the heat-treated groups was higher (poorer) than that of the control groups (Table 5) but the FCR of only the middle aged parent group differed significantly (P <0.05). Without considering parent age, FCR of broilers was numerically higher (poorer) in the treatment compared to the control (Table 6); this difference being significant (P <0.05) in week 6. Leksrisompong (2005) found that the difference in FCR for birds hatched under high incubation temperatures was lower (P <0.05) than that under standard incubation temperatures for the periods 0 - 7, 0 - 14 and 0 - 21 days but differences over the 0 - 28 days were not significant (P >0.05).
Feed intake, weight gain and FCR for broilers from the three parent age groups for the heat-treated and control groups are shown in Table 7.
In terms of feed intake, weight gain and FCR, no differences were observed (P >0.05) between the parent age groups.
The results of this study suggest that prenatal thermal conditioning is not detrimental to broiler growth under standard rearing conditions in the absence of postnatal heat stress.
This study was supported by an integrated poultry company (Bey-Pi Inc.) in Bolu Province of Turkey and carried out between 24 March to 4 May of 2006 in the Research & Development farm of the same company. We wish to thank Sait Koca the General Manager of the company for the support. We also thank Peter Hunton for suggestions regarding the manuscript.
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