**3. General characteristics of the studies included in the systematic review**

**Table 1** summarizes the information that was extracted from the final selected studies. The studies in the systematic review were published between 2015 and 2021, and 31 papers were finally included. Intermittent TM profile (3–18 h/d) was used in most studies (24 papers, 77%) compared with continuous (24 h/d) (6 papers, 19%),



*Thermal Manipulation: Embryonic Development, Hatchability, and Hatching Quality… DOI: http://dx.doi.org/10.5772/intechopen.101894*



*Thermal Manipulation: Embryonic Development, Hatchability, and Hatching Quality… DOI: http://dx.doi.org/10.5772/intechopen.101894*

#### **Table 1.**

*Overview of thermal manipulation studies during incubation of broiler chicken eggs.*

and a combination of both intermittent and continuous in only one study [17], in which no depressing effect of intermittent (12 h/d) TM between E7 and E16 on embryogenesis was reported, however, continuous (24 h/d) reduced embryo weight from E16 to E21.

Among the 24 intermittent TM studies, 20 articles reported the TM effect on hatchability, of which 65% found no significant effect, 30% being reduced, and a comparative study by Dalab and Ali [46] reported increased and decreased hatchability at different embryonic age. Meanwhile, embryo and/or hatch weight and chick quality were reported in 18 articles, of which 12 (67%) found no significant effect, three studies reported increased [2, 38, 50], and three reported reduced effect of TM on embryo and/or hatch weight and chick quality [42, 45, 46].

Embryo mortality was reported in four of 24 intermittent TM studies, of which three studies observed no significant difference in embryo mortality between intermittent TM and control groups [48, 49, 52]. However, Brannan et al. [54] identified increased embryo mortality in intermittent TM groups compared with control group.

Hatch or chick body temperature was reported in eight of 24 intermittent TM studies, which included no significant effect [48, 52], increased [30, 42, 51], and reduced [18, 49, 50].

From the six continuous TM studies, the application increased [39], and had no significant effect on hatchability [26, 35, 36] compared with control groups. Moreover, continuous TM increased [32], decreased [33] and had no significant effect on embryo, chick weight, or chick quality [26, 35, 39]. Furthermore, Elmehdawi et al. [35] reported no negative effect of continuous TM on hatch body temperature compared with control treatment.

In all the studies, the set standard incubation temperature was the control treatment, which was compared to TM treatments. 37.8°C was used as the standard incubation temperature in most studies (20 papers, 65%), followed by 37.5°C (8 papers,

26%). Only Aminoroaya et al. [34], Nyuiadzi et al. [51], and Elmehdawi et al. [35] used 37.6 and 37.4°C, respectively.

While most studies (23 papers, 74%) only used high-temperature TM (i.e., 1–3°C above the set standard incubation temperature), 7 papers were comparative studies of low and high-temperature TM, and only one study used low-temperature TM at 37.6°C but on 18 and 19 d of incubation, embryos were subjected to short cold exposure of 15°C for 30 minutes [51]. The above authors found no significant effect of TM from E18 to E19 on hatchability and hatch weight, but hatch body temperature was elevated compared with control group (37.6°C). In addition, studies involving lowtemperature TM, the adjustments varied between 1 and 4.5°C below the set standard incubation temperature.

Three studies compared various high-temperature TM; both 38.5 and 40°C did not have any effect on hatchability [40]. Meanwhile, Al-Zghoul and El-Bahr [44] compared 38.5, 39, 39.5, and 40°C for 18 h/d from E12 to E18 and observed that 38.5 and 39°C did not impact hatchability however, 39.5 and 40°C reduced hatchability compared with control group (37.8°C). The latter TM setup was used by Al-Zghoul [42], who reported increased 1-d-old chick body temperature at 38.5°C compared with other TM and control treatments.

Three studies compared TM duration, which Al-Zghoul et al. [19] and Al-Zghoul et al. [37] identified depressed hatchability and increased embryo body weight on E18, respectively, at 39°C regardless of TM duration (9, 12, or 18 h/d) from E12 to E18. Using similar TM profile to Al-Zghoul et al. [19], Al-Zghoul et al. [37], and Al-Zghoul et al. [45] reported elevated body temperature in 1-d-old chicks at 9 h/d TM duration compared with other TM durations and control (37.8°C) treatment.

Among the seven comparative studies, low-temperature TM resulted in increased hatch weight [2, 32], reduced embryo weights [33], and both low and high-temperatures were not significantly different in hatch weight, embryo mortality, and deformed embryos [52]. Meanwhile, high-temperature TM resulted in higher hatchability [36, 39], and both low and high-temperature treatments did not impact and significantly differ in hatchability [52, 53]. Furthermore, continuous TM was used in most comparative studies [32, 33, 36, 39] compared with intermittent [2, 52, 53].

The main RH used in control and low-temperature TM studies was 56% and, 65% in high-temperature TM studies. In **Table 1**, we only reported RH of four studies (four papers, 14%) that used the different RH protocol from the above.

The embryonic age at the time of TM varied between E7 and E18 in most studies (24 papers, 77%), followed by E18–E20 and E0–E8 (three papers each) and one study by Morita et al. [39] reported the timing of TM from E13 to E21, which resulted in no significant influence on hatch weight but higher hatchability in high-temperature TM than low-temperature treatments.

Only seven studies compared embryo age at the timing of TM; hatchability and hatch weight were not affected by high-temperature TM at E0–E5 and E5–E18 [26], and E12–E14 and E15–E17 [34]. Also, Al-Rukibat et al. [40] showed no effect on hatchability after TM at 38.5 or 40°C for 6 h at E16, 9 h at E17 and 12 h at E18. However, TM at 39°C for 18 h/d at early embryonic age (E7–E11) significantly enhanced hatchability but in late (E11–E18) and long-term (E7–E18) negatively affected hatchability and chick quality compared with control (37.8°C) [46]. Janisch et al. [32] reported significant increase in hatch weight with low-temperature TM between (E7–E10) and (E10–E13) and Krischek et al. [33] found a significant decrease in embryo weights at low-temperature TM at E7–E10 and E10–E13 compared with high-temperature and control treatments (37.8°C). Narinç et al. [38] identified

significantly depressed hatchability and chick quality in late (E10–E18) TM compared with early (E0–E8) and control (37.8°C) treatments however, all treatments were not statistically different in hatch weight.
