ABSTRACT
Hot conditions decrease the difference between ambient temperature (AT) and the average temperature of the body surface. A smaller difference reduces the rate of sensible heat loss of excessive internal heat, elevates the body temperature (BT), and may lead to mortality during heat waves. Under conditions of chronic heat, broilers avoid lethal BT elevation by reducing their feed intake; consequently, growth rate and meat yield are lower. Practices to avoid hot conditions are costly, whereas breeding for heat tolerance offers a sustainable approach. Being featherless was shown to provide heat tolerance; this was reevaluated in experimental broilers with a growth rate similar to that of contemporary commercial broilers. In experiment 1, 26 featherless birds and 49 feathered siblings (sibs) were reared at warm AT and exposed to moderate and acute heat waves. The featherless birds maintained normal BT under a moderate heat wave, with a slight elevation under an acute heat wave, and only 1 bird died. In contrast, the heat waves led to a significant elevation in BT of the feathered sibs, and 34% of them died. In experiment 2, featherless broilers were compared with feathered sibs and commercial broilers at 2 AT treatments: a constant temperature of 25°C (control AT) or a constant temperature of 35°C (hot AT). The birds were reared to 46 or 53 d at the control and hot AT, respectively, and the measured traits included BT, growth, and weight of the whole body and carcass parts (breast meat, legs, wings, and skin). At the hot AT, only the featherless broilers maintained a normal BT; their mean d 46 BW (2,031g) was significantly higher than that of birds maintained at the control AT, and it increased to 2,400 g on d 53, much higher than the corresponding means of all feathered broilers (approximately 1,700 g only). Featherless broilers had significantly higher breast meat yield (approximately 20% in both AT), lower skin weight, and supposedly better wing quality. These results confirmed that being featherless improved the livability and performance of fast-growing broilers in hot conditions and suggests that introduction of the featherless phenotype into commercial broiler stocks would facilitate highly efficient yet low-cost production of broiler meat under hot conditions.
INTRODUCTION
Remarkable genetic progress has been achieved in broiler growth rate (GR) and meat yield since the 1950s (Havenstein et al., 2003a,b). The greater GR of broilers is driven by a greater rate of feed intake and metabolism; consequently, the production of internal heat is elevated (Sandercock et al., 1995). However, the continuously increasing genetic potential for rapid growth, and the consequent desirable reduction in time to marketing, with its contribution to better feed efficiency, cannot be fully expressed under hot conditions [see Cahaner (2008) for a review]. Hot conditions decrease the difference between the ambient temperature (AT) and the average temperature of the body surface. A smaller difference reduces the rate at which the excessively produced internal heat of broilers is dissipated. The lower rate of sensible heat loss leads to an elevation in body temperature (BT), which may lead to mortality during heat waves. Under chronic heat, broilers acclimate by reducing feed intake (Eberhart and Washburn, 1993; Cooper and Washburn, 1998; Deeb and Cahaner, 1999, 2002); consequently, GR is reduced, resulting in lower marketing BW and poorer breast meat yield (Cahaner and Leenstra, 1992; Leenstra and Cahaner, 1992; Settar et al., 1999).
Hot conditions can be avoided in modern broiler houses equipped with efficient cooling systems. However, the global broiler industry continues to expand to developing countries in hot climates, where climatic control of broiler houses is lagging because of high installation and operational costs and an unreliable supply of electricity. The use of cooling systems is also increasing in countries with temperate climates because contemporary commercial broilers (CCB) are continuously selected for greater GR and meat yield and reared to higher BW; consequently, they generate more heat (Sandercock et al., 1995) and thus need lower AT to maintain normal BT and to fully express their genetic potential for rapid growth (Emmans and Kyriazakis, 2000). With the limited availability and rising cost of energy, and the increasing tendency to minimize the total amount of resources used for human food production, artificial cooling of broiler houses is also becoming an economic and political burden in developed countries. Breeding heat-tolerant broilers may offer a sustainable approach to mitigate the negative effects of heat on broiler production.
Skin temperature in broilers is lower than BT by only about 0.5°C (Yahav et al., 1997), but because of the insulation of the feathers, the temperature of the feather-covered body surface is close to AT; hence, this surface hardly contributes to the overall sensible heat loss (Cangar et al., 2008). It has been shown that in genetically fast-growing broilers under hot conditions, the feather coverage negatively affects thermoregulation because it hinders sensible heat loss (Yahav et al., 1998; Deeb and Cahaner, 1999). Several authors have tested the hypothesis that the negative effects of heat can be alleviated by introducing genes that reduce or eliminate feather coverage into the genetic makeup of CCB stocks (e.g., Somes and Johnson, 1982; Hanzl and Somes, 1983; Merat, 1986). Many studies have been conducted with the codominant naked neck (Na) gene, which reduces feather coverage by 20 and 40% in heterozygous (Na/na) and homozygous (Na/Na) chickens, respectively (Crawford, 1976; Cahaner et al., 1993, 2008). Under hot conditions, naked-neck broilers exhibited greater sensible heat loss (Yahav et al., 1998) and better thermoregulation (Deeb and Cahaner, 1999), resulting in greater actual GR and meat yield than their fully feathered counterparts (Cahaner et al., 1993; Yalçin et al., 1997; Deeb and Cahaner, 2001). However, in these studies, the naked-neck broilers raised at 25°C were superior to their counterparts raised under hot conditions, suggesting that the 20 or 40% reduction in feather coverage provides only partial heat tolerance. Therefore, it was hypothesized that complete feather elimination may enhance the heat tolerance of genetically fast-growing broilers (Cahaner et al., 2003; Cahaner, 2008).
Abbott and Asmundson (1957) reported on a recessive mutation called scaleless that blocks feather formation in homozygous (sc/sc) chicken embryos. This spontaneous mutation was found in the New Hampshire breed, which is characterized by a substantially lower GR and BW than those of meat-type chickens; therefore, the featherless mutants were not considered for practical purposes (Somes, 1990). In the late 1970s, experimental featherless broilers were derived from a cross between the scaleless mutant and commercial broilers of that time. Under hot conditions, the GR and carcass composition of these featherless birds were superior to those of their feathered counterparts (Somes and Johnson, 1982), but the effects were small because the GR of the birds in this study was very low, a maximum GR of 30 g/d and an average BW of approximately 1,200 g at 8 wk (compared with about 100 g/d in the broilers of today, which reach the same BW in about 4 wk). In a recent study (Cahaner et al., 2008) with an experimental line segregating for the Na and sc genes, the featherless phenotype was substantially superior to the naked-neck ones at high AT. The experimental birds in that study reached a mean BW of about 1,200 g already at 6 wk, with a maximal GR of about 55 g/d, faster than the birds studied by Somes and Johnson (1982) yet substantially lower than the CCB; hence, the practical relevance of the conclusions regarding the advantages of featherless broilers has remained questionable. Therefore, the genetic potential of the featherless experimental line was enhanced by 2 additional cycles of backcross to CCB stocks. The backcross progeny were used in the present study, with the objective of comparing actual GR and several performance-related traits of featherless broilers vs. their normally feathered siblings (sibs) and also a group of CCB as an industry reference, under hot conditions of a constant 35°C, compared with a constant 25°C (after the brooding period) as a control condition. The response of featherless broilers and their feathered sibs to a heat wave was also studied.
MATERIALS AND METHODS
Experimental Stock
This study consisted of 2 experiments, experiments 1 and 2. The birds used in both experiments were the progeny of intermating among parents that were heterozygous for the fully recessive scaleless gene (sc) so that one-fourth of their progeny were featherless (sc/sc) and three-fourths were normally feathered (+/+ and +/sc), and other than this difference, all birds in the same experiment shared the same average genetic background. The featherless parents of the birds in experiment 1 descended from the original scaleless mutants (found in the New Hampshire egg-type breed; Abbott and Asmundson, 1957) after only 1 cycle of backcross to stocks of CCB; hence, the genetic potential for GR and meat yield of birds in experiment 1 was similar to that of the featherless birds in the study by Cahaner et al. (2008) and was much lower than that of CCB stocks. Between the time of experiment 1 and experiment 2, there were 2 additional cycles of backcross to CCB stocks; hence, the genetic potential for GR and meat yield of the birds in experiment 2 was expected to be higher than that of birds in experiment 1 but was still lower than that of CCB. In experiment 2, a group of CCB (Comm) were also used as an industry reference.
Experiment 1
Experimental Design.
After hatch, the featherless (n = 27) and normally feathered (n = 49) chicks were brooded together on deep litter in a single room. The AT was 35°C during the first 3 d, and then was gradually lowered to 28°C at d 21 and maintained at that level until d 43, when it was slightly elevated to 30°C. The chicks were kept under 23 h/d of light with ad libitum feeding of commercial diets. At 4 wk of age, the chicks were moved to individual cages in the same conditions. A 2-d moderate heat wave was initiated on the morning of d 47 by gradually (within approximately 12 h) elevating AT to 35°C until the evening of d 48, when AT was lowered back to 30°C and remained at this level until the morning of d 53, when an acute heat wave was initiated by rapid (within 3 h) elevation of AT to 36°C for 12 h. Relative humidity was approximately 70% throughout the entire experiment. The Ethics Committee of the Faculty of Agriculture of Hebrew University approved experiment 1 as well as experiment 2.
Measurements.
Individual BW of all birds were measured weekly and on d 47 and 53, at the onset of the moderate and acute heat waves, respectively. The BT was measured in all birds on the morning of d 36, 43, and 47 at the end of the first and second day of the moderate heat wave (the evening of d 47 and 48, respectively), and at the onset and by the end of the acute heat wave (d 53 morning and evening, respectively). The BT at all ages was measured by inserting the probe of a digital thermometer about 2 cm into the cloaca of the chick for 10 to 15 s until the reading became stable. Obviously, the 16 birds that succumbed to the acute heat wave were not alive on the evening of d 53; hence, their BT at this time could not be measured. All mortality, which occurred only during the heat waves, was recorded.
Experiment 2
Experimental Design.
After hatch, straight-run male and female chicks from each of the 3 groups, featherless (n = 126), their feathered sibs (n = 152), and Comm (n = 92), were equally divided into 2 AT treatments, control (temperate) and hot. Chicks from the 3 groups were reared intermingled in each treatment. This ensured uniform exposure to the experimental conditions within treatment, but feed and water consumption by group could not be measured. In the control treatment, the chicks were brooded under a standard temperature regimen, beginning with an AT of 35°C on the first 3 d after hatch, followed by a gradual reduction to 25°C on d 21, and it was kept constantly (no daily fluctuations) at that level to d 46, when the birds in the control AT treatment were slaughtered. In the hot treatment, the chicks were reared under a constant 35°C from the day of hatch until d 46, when the control treatment was terminated. At this age, BW of the majority of birds under the hot AT was too low for mechanical slaughtering; therefore, these birds were reared for 1 more week, and AT in this treatment was lowered to 30°C on d 49 for the last 4 d before they were slaughtered on d 53. In each AT treatment, the birds from the 3 groups were reared together (hence feed consumption by group could not be measured) on deep litter at a stocking density of 10 birds/m2, with 23 h/d of light and ad libitum feeding of commercial diets; RH was about 70% in both treatments.
Measurements.
The BW of each bird was measured weekly until d 42 and on d 46; the average daily BW gain (DBWG) of each bird was calculated for each of the first 6 wk as well as for the d 42 to 46 period. At 2 wk of age, about 14 birds per phenotype from the segregating line and 9 from the Comm group were randomly selected and marked, and their BT was measured weekly (as in experiment 1).
At the end of the trial (d 46 and 53 in the control and hot AT treatments, respectively), about one-half of the birds from each group within the AT treatment were randomly sampled for carcass measurements. Feed was removed for 10 h before the birds were moved to a commercial slaughterhouse. All carcasses, including the featherless ones, were free of skin scratches and other downgrades. Heads and shanks were removed, birds were eviscerated, and the sex of each bird was confirmed by the presence of ovaries or testicles. The skin was removed from the entire carcass (except wings), the breast meat (pectoralis major and pectoralis minor) was deboned, and the legs (drumsticks with bones and thighs without bones) and wings were cut off; the weights of these parts were recorded for each bird.
Statistical Analysis
In experiment 1, BT and BW data were subjected to 2-way ANOVA with phenotype (featherless vs. feathered) and sex (males vs. females) as main effects. A chi-squared test was performed to test differences in the incidence of mortality during the second heat wave. In experiment 2, the data on BT, BW, and DBWG at each age, and the weights of the carcasses and their parts, were subjected to 2-way ANOVA, with the 3 genetic groups and 2 sexes as main effects. In both experiments, there were no significant interactions with sex; hence, the least squares means of the groups over sex (males + females) are presented in all tables and graphs. All statistical analyses were conducted using JMP software (SAS Institute, 2008).
RESULTS AND DISCUSSION
Experiment 1
Mortality.
In an earlier study (Cahaner et al., 2008), experimental featherless birds were compared with their fully feathered counterparts under 2 distinctive treatments of postbrooding AT: a constant 25°C or a constant 35°C. In the latter treatment, the birds were acclimated to the heat and thus could not be tested for their response to the prevalent phenomenon of heat waves (i.e., a sudden elevation in AT). Experiment 1 in the present study was designed to meet this objective by exposing featherless birds and their fully feathered sibs to moderate and acute heat waves. To ensure realistic rather than overly extreme heat waves, all birds of both phenotypes were kept (after the brooding period) at a warm AT of a constant 28°C until d 43, and were kept under 30°C thereafter. The moderate heat wave was initiated on d 47, when AT was gradually (over 12 h) elevated to 35°C and maintained at this level until the evening of d 48. The acute heat wave was initiated at the beginning of d 53, when AT was more sharply (over 3 h) elevated to 36°C and maintained at this level until the evening, when the experiment was terminated (Figure 1A). Body temperatures of all birds were measured at several time points before and after the heat waves (Figure 1B).There was no intention to induce mortality, yet 2 feathered birds died during the moderate heat wave, and 16 birds (only 1 featherless) died during the acute heat wave. In total, 34.7% of the feathered birds (10 males and 7 females) and 3.8% of the featherless birds (1 female) died during the heat waves. This difference in incidence of mortality was found (by chi-squared test) to be highly significant.
Experiment 1: A) The ambient temperatures (AT, °C) from d 43 to the end of the experiment, with a moderate heat wave on d 47 to 48 and an acute heat wave on d 53; B) least squares means of body temperature (BT, °C) at the ages of 43, 47, 48, and 53 d, presented for 3 groups of birds: featherless (●, solid line), their normally feathered siblings that survived the heat waves (■, broken line), and those that died during the heat waves (□, dotted line). Means within age without s common letter (a–c) differ significantly (P < 0.05).
Experiment 1: A) The ambient temperatures (AT, °C) from d 43 to the end of the experiment, with a moderate heat wave on d 47 to 48 and an acute heat wave on d 53; B) least squares means of body temperature (BT, °C) at the ages of 43, 47, 48, and 53 d, presented for 3 groups of birds: featherless (●, solid line), their normally feathered siblings that survived the heat waves (■, broken line), and those that died during the heat waves (□, dotted line). Means within age without s common letter (a–c) differ significantly (P < 0.05).
BT.
The means (over sex) of BT were calculated for the featherless birds and for the feathered ones, separately for the survivors compared with those that died because of the heat. In each group, the mean BT increased with AT, but the magnitude of the effect was significantly larger in the feathered than in the featherless birds (Figure 1B). Mean BT of the feathered birds that survived increased from 41.3°C on d 43 at an AT of 30°C to about 42°C at a constant AT of 30°C, then to 42.8°C because of the moderate heat wave (end of d 48), and to 43.3°C by the end of the acute heat wave (Figure 1B). Compare with the feathered birds that survived, their counterparts that died during the acute heat wave already had a significantly higher mean BT before the moderate heat wave. This difference increased by the end of this heat wave and was also maintained between the heat waves. The BT during the acute heat wave of these birds could not be measured because they were dead by the end of d 53, but it was surely higher than 43°C, known to be the lethal threshold of BT in chickens (Morrow, 2008). The mean BT of the featherless broilers was already significantly lower than that of their feathered sibs at an AT of 28°C (d 43), and it remained in the normal range of 40.7 to 41.6°C even during the moderate heat wave, and then increased to only 42°C under the acute heat wave (Figure 1B).
BW.
In all 3 groups, the mean BT increased with age between the mornings of d 49 to 53 (Figure 1B), suggesting that age, in addition to AT and phenotype, affected the BT of broilers. The age effect on BT was most probably caused by the increase in BW of birds in this age range because higher BW is associated with higher feed consumption, metabolism, and internal heat production (Cahaner, 2008). Means for BW are presented in Table 1 by phenotype (feathered vs. featherless) and separately for the survivors compared with those that died because of the heat. At all ages, the mean BW of the featherless birds was just slightly (and not significantly) lower than that of their feathered counterparts. These results suggest that higher sensible heat loss (because of the lack of feathers) rather than low BW helped the featherless broilers maintain a normal BT under high AT. It appears that BW was also not associated with the mortality of about one-third of the feathered birds. The mean BW of those that died did not differ significantly from the mean BW of their counterparts that survived at any age between d 28 and 53 (Table 1).
Experiment 1: least squares means (males + females) for BW on the mornings of d 28, 47, 49, and 53, and for average daily BW gain between these ages, presented for all the featherless (sc/sc) birds and all normally feathered (+/sc and +/+) siblings, and presented separately for the birds that died during the heat waves compared with the ones that survived
| Item | Feathered sibs (+/+ and +/sc) | Featherless (sc/sc) | Survived feathered − featherless2 | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| All | Survived1 | Died1 | Died − survived2 | All | Survived1 | Died1 | ||||
| n | 49 | 32 | 17 (34.7%) | — | 27 | 26 | 1 (3.8%) | |||
| Age | ||||||||||
| 28 d (g) | 710.2 | 702.3 | 737.9 | 35.6 | 680.8 | 681.0 | 676.4 | 21.3 | ||
| 47 d3 (g) | 1,591.1 | 1,593.3 | 1,586.7 | −6.6 | 1,534.6 | 1,537.8 | 1,461.3 | 55.5 | ||
| 49 d4 (g) | 1,625.1 | 1,640.1 | 1,591.1 | −49.0 | 1,583.4 | 1,588.1 | 1,460.8 | 52.0 | ||
| 53 d5 (g) | 1,796.0 | 1,801.7 | 1,783.1 | −18.6 | 1,765.4 | 1,767.1 | 1,727.1 | 34.6 | ||
| 28 to 47 d6 (g/d) | 46.2 | 46.9 | 44.7 | −2.2 | 44.5 | 44.6 | 41.3 | 2.3 | ||
| 47 to 49 d7 (g/d) | 14.7 | 23.4 | −5.0 | −28.4* | 28.9 | 30.0 | 0.3 | −6.6 | ||
| 49 to 53 d8 (g/d) | 42.7 | 40.4 | 48.0 | 7.6 | 44.2 | 43.3 | 66.6 | −2.9 | ||
| Item | Feathered sibs (+/+ and +/sc) | Featherless (sc/sc) | Survived feathered − featherless2 | |||||||
|---|---|---|---|---|---|---|---|---|---|---|
| All | Survived1 | Died1 | Died − survived2 | All | Survived1 | Died1 | ||||
| n | 49 | 32 | 17 (34.7%) | — | 27 | 26 | 1 (3.8%) | |||
| Age | ||||||||||
| 28 d (g) | 710.2 | 702.3 | 737.9 | 35.6 | 680.8 | 681.0 | 676.4 | 21.3 | ||
| 47 d3 (g) | 1,591.1 | 1,593.3 | 1,586.7 | −6.6 | 1,534.6 | 1,537.8 | 1,461.3 | 55.5 | ||
| 49 d4 (g) | 1,625.1 | 1,640.1 | 1,591.1 | −49.0 | 1,583.4 | 1,588.1 | 1,460.8 | 52.0 | ||
| 53 d5 (g) | 1,796.0 | 1,801.7 | 1,783.1 | −18.6 | 1,765.4 | 1,767.1 | 1,727.1 | 34.6 | ||
| 28 to 47 d6 (g/d) | 46.2 | 46.9 | 44.7 | −2.2 | 44.5 | 44.6 | 41.3 | 2.3 | ||
| 47 to 49 d7 (g/d) | 14.7 | 23.4 | −5.0 | −28.4* | 28.9 | 30.0 | 0.3 | −6.6 | ||
| 49 to 53 d8 (g/d) | 42.7 | 40.4 | 48.0 | 7.6 | 44.2 | 43.3 | 66.6 | −2.9 | ||
1Survived both heat waves, or died during the 47- to 48-d moderate heat wave (2 feathered birds) or the 53-d acute heat wave (15 feathered and 1 featherless birds); on d 53, the ambient temperature was elevated from 30 to 36°C within 3 h and kept at this level for 12 h.
2The difference in mean BW or daily BW gain between the birds that eventually died and those that survived the heat wave, or between the surviving feathered birds and their featherless counterparts.
3Body weight at the onset of the 47- to 48-d moderate heat wave.
4Body weight after the 47- to 48-d moderate heat wave.
5BW at the onset of the 53-d acute heat wave.
6Daily BW gain from wk 4 to 7 under an ambient temperature of 28 to 30°C (before the 47- to 48-d moderate heat wave).
7Daily BW gain during the moderate heat wave, on d 47 and 48.
8Daily BW gain between the 2 heat waves (moderate on d 47 and 48, and acute on d 53).
*P < 0.05.
In conclusion, the data from experiment 1 demonstrated that the BT of featherless broilers remained at normal levels under a constant warm AT and moderate heat wave, and was only slightly elevated under an acute heat wave. In a previous study (Cahaner et al., 2008), accidental elevation of the AT from 25 to 32°C for a few hours on d 42, which elevated the BT of commercial broilers (BW of approximately 2,500 g) and feathered sibs (BW of approximately 1,500 g) by 1.7 and 0.8°C, respectively, increased the mean BT of the featherless birds (BW of approximately 1,350 g) by only 0.1°C. The heat-wave tolerance of the featherless birds, indicated by a minimal elevation in BT, must be the reason that 25 (out 26) of them survived the acute heat wave, as compared with only 65% survival among their feathered counterparts.
Experiment 2
Growth rate and mean BW (at comparable ages) of the featherless birds and their feathered sibs in experiment 1 (Table 1) were only slightly higher than those of the experimental featherless and feathered birds in an earlier study (Cahaner et al., 2008) and were far lower than those of CCB. Therefore, the birds in experiment 1 were not used to study broiler performance. Instead, 2 additional cycles of backcross to CCB stocks were conducted, and the resulting featherless and feathered progeny were used in experiment 2, along with a group of CCB as an industry reference (Comm). These 3 groups of broilers were reared under the same AT treatments as in the earlier experiment with less advanced experimental birds (Cahaner et al., 2008), namely, a control (temperate) AT with a constant 25°C (after the brooding period) and a hot AT with a constant 35°C.
GR and BW.
The growth curves of the 3 genetic groups under the 2 AT treatments are presented in Figure 2. At the control AT, the broilers with feathers exhibited their genetic potential for GR, with the Comm group reaching a maximal GR of 97 g/d and a mean BW of about 2,850 g on d 46 (Table 2). The GR of the experimental feathered broilers (sibs of the featherless ones) was lower than that of the Comm broilers, but only by about 15%, with a maximal GR of 83 g/d and a final BW of 2,336 g. These GR and BW of the feathered experimental broilers were much higher than those of their counterparts in experiment 1 (about 1,600 g on d 47; Table 1) and in the study by Cahaner et al. (2008; 1,580 g on d 45). These results demonstrate the genetic improvement in GR of the sc-segregating line after 2 cycles of backcross to CCB stocks and indicate that additional backcross cycles will further enhance the genetic potential of this experimental line. The featherless broilers under the control AT had a lower GR than their feathered counterparts (Figure 2), with a maximum of 63 g/d and a mean BW of 1,923 g on d 46 (Table 2), suggesting that an AT of a constant 25°C was below the thermoneutral zone for these broilers.
Experiment 2: Growth curves showing least squares means (males + females) of BW to 46 d of age under the control ambient temperature (constant 25°C, broken lines), and to 53 d of age under the hot ambient temperature (constant 35°C, solid lines), of the fully feathered commercial broilers (Comm, ■) and of the experimental line segregating to featherless broilers (sc/sc, ○), and their normally feathered (+/sc, ●) siblings.
Experiment 2: Growth curves showing least squares means (males + females) of BW to 46 d of age under the control ambient temperature (constant 25°C, broken lines), and to 53 d of age under the hot ambient temperature (constant 35°C, solid lines), of the fully feathered commercial broilers (Comm, ■) and of the experimental line segregating to featherless broilers (sc/sc, ○), and their normally feathered (+/sc, ●) siblings.
Experiment 2: least squares means1 of average daily BW gain (g/d) by week and of BW at 46 d of age (g) for commercial broilers (Comm), featherless (sc/sc) broilers, and their normally feathered siblings (sibs; +/sc and +/+), reared under control (constant 25°C) or hot (constant 35°C) ambient temperature (AT) conditions2
| Item | Control (constant 25°C after brooding) | Hot (constant 35°C) | Heat effect [(hot − control)/control, %] | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Featherless | Feathered sibs | Comm | Featherless | Feathered sibs | Comm | Featherless | Feathered sibs | Comm | |||
| n | 62 | 76 | 46 | 62 | 76 | 46 | |||||
| Age | |||||||||||
| 0 to 7 d | 9.6b | 10.9ab | 11.8a | 9.9b | 11.0ab | 11.9a | 3.1 | 0.9 | 0.9 | ||
| 7 to 14 d | 25.1c | 27.9b | 32.2a | 24.7b | 24.6b | 29.2a | −1.6 | −12.0** | −9.3* | ||
| 14 to 21 d | 36.8c | 43.4b | 52.3a | 41.0b | 37.7c | 47.4a | 11.4 | −13.2*** | −9.5** | ||
| 21 to 28 d | 45.3c | 53.6b | 71.5a | 46.9a | 40.1b | 49.4a | 3.4 | −25.2*** | −30.9*** | ||
| 28 to 35 d | 56.0c | 71.7b | 84.5a | 58.1a | 38.5b | 35.8b | 3.8 | −46.3*** | −57.6*** | ||
| 35 to 42 d | 56.4c | 70.8b | 92.3a | 67.4a | 33.8b | 23.4b | 19.5* | −52.3*** | −74.6*** | ||
| 42 to 46 d | 63.5c | 83.0b | 97.1a | 69.1a | 31.7b | 26.3b | 8.8 | −61.8** | −68.3*** | ||
| 46 d | 1,922.9c | 2,336.8b | 2,849.5a | 2,031.6a | 1,460.2b | 1,544.3b | 5.4 | −37.5*** | −45.8*** | ||
| Item | Control (constant 25°C after brooding) | Hot (constant 35°C) | Heat effect [(hot − control)/control, %] | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Featherless | Feathered sibs | Comm | Featherless | Feathered sibs | Comm | Featherless | Feathered sibs | Comm | |||
| n | 62 | 76 | 46 | 62 | 76 | 46 | |||||
| Age | |||||||||||
| 0 to 7 d | 9.6b | 10.9ab | 11.8a | 9.9b | 11.0ab | 11.9a | 3.1 | 0.9 | 0.9 | ||
| 7 to 14 d | 25.1c | 27.9b | 32.2a | 24.7b | 24.6b | 29.2a | −1.6 | −12.0** | −9.3* | ||
| 14 to 21 d | 36.8c | 43.4b | 52.3a | 41.0b | 37.7c | 47.4a | 11.4 | −13.2*** | −9.5** | ||
| 21 to 28 d | 45.3c | 53.6b | 71.5a | 46.9a | 40.1b | 49.4a | 3.4 | −25.2*** | −30.9*** | ||
| 28 to 35 d | 56.0c | 71.7b | 84.5a | 58.1a | 38.5b | 35.8b | 3.8 | −46.3*** | −57.6*** | ||
| 35 to 42 d | 56.4c | 70.8b | 92.3a | 67.4a | 33.8b | 23.4b | 19.5* | −52.3*** | −74.6*** | ||
| 42 to 46 d | 63.5c | 83.0b | 97.1a | 69.1a | 31.7b | 26.3b | 8.8 | −61.8** | −68.3*** | ||
| 46 d | 1,922.9c | 2,336.8b | 2,849.5a | 2,031.6a | 1,460.2b | 1,544.3b | 5.4 | −37.5*** | −45.8*** | ||
a–cMeans within age and AT treatments (25 or 35°C) without a common superscript differ significantly (P < 0.05).
1Least squares means were calculated from males and females because there were no significant interactions with sex.
2Heat effects are presented as relative (%) differences between corresponding means under hot and control AT conditions, divided by the mean under control AT conditions.
*P < 0.05, **P < 0.01, and ***P < 0.001 for means under 25 vs. 35°C, within genetic group and age.
In the hot AT treatment (a constant 35°C), the Comm broilers exhibited the same high GR as in the control AT only in the 3 brooding weeks, when the chicks needed heating. From the fourth week, the growth curve of the Comm broilers under the hot AT began to decline compared with the curve of their counterparts under the control AT. Moreover, the average Comm broilers under a constant 35°C did not gain BW from d 46 to 49, as was found in a similar earlier study (Cahaner et al., 2008). Therefore, the AT of the hot treatment was lowered on d 49 to 30°C; consequently, the Comm broilers gained some BW from d 49 to 53 (Figure 2). As under the control AT, under the hot AT, the experimental feathered broilers had GR somewhat lower than that of the Comm broilers in the first 5 wk, but they were less affected by the heat, probably because of their lower mean BW, and they ended with the same mean BW on d 53 (Figure 2 and Table 2). In sharp contrast to the GR of the 2 groups of broilers with feathers, GR of featherless broilers was not negatively affected by the hot treatment; during the sixth week, their mean BW was higher than that of their counterparts under the control AT, and they continued to gain BW in a linear manner to the end of the experiment on d 53 (Figure 2).
The substantial interaction between the 3 genetic groups and the 2 AT treatments is apparent in Figure 2 and Table 2, which present weekly means of DBWG, along with the heat effects on each group in each week, and their significance. Already in the second and third weeks, DBWG of the feathered sibs and Comm broilers was significantly depressed by the heat, by about 12% (Table 2). In the subsequent weeks, the heat effect on GR of the broilers with feathers increased considerably, as reflected in decreasing means of DBWG. From d 35 to 46, the mean DBWG of the Comm broilers under the hot AT was only about 25 g/d, approximately 70% less than the approximately 95 g/d under the control AT. After d 42, GR of the experimental feathered broilers was also depressed by the heat; their mean DBWG under the hot AT was about 32 g/d, 62% less than under the control AT. These results are in agreement with previous reports on commercial broilers reared under a controlled 35°C (e.g., Deeb and Cahaner, 2002; Cahaner et al., 2008) and under a natural hot climate (Settar et al., 1999), confirming the substantial susceptibility of CCB to hot conditions. In contrast to the depressing effects of heat on the growth of CCB with feathers, the GR of the featherless broilers in this study was not depressed at all by heat. Their mean DBWG under the hot AT increased with age, to 69 g/d in the last 4 d, and on d 46 their BW averaged 2,031 g, 39 and 32% higher than those of their feathered sibs and the Comm broilers, respectively.
BT.
Body temperature was measured in approximately 12 birds per group and AT combination (Table 3). On d 14, no differences in BT were found between the 3 groups and between the AT treatment, apparently because the actual AT of the control treatment was still higher than 25°C, and the chicks in the Comm and feathered sibs groups were too small to suffer from the heat of 35°C. Under the control AT treatment, the broilers from the 3 groups also maintained a normal BT on d 28, 35, and 42 (Table 3). Under the hot AT treatment, the mean BT of the feathered and Comm broilers increased to 42.6 and 42.9°C, respectively, and further increased to about 43.4°C on d 35, more than 1°C above that of their counterparts reared under the control AT. On d 42, the mean BT of the feathered sibs (42.2°C) was lower than that of the Comm broilers (43.0°C), possibly because of the higher BW of the latter group. By d 49, these 2 groups under the hot AT had similar mean BW (Figure 2), and that could be the reason for their similar mean BT at this age (Table 3). Similar results were obtained in an earlier study (Cahaner et al., 2008), in agreement with reports that internal heat production is greater in heavier broilers (e.g., Sandercock et al., 1995). It is suggested that the broilers with feathers could not dissipate the excess of internal heat, hence the greater elevation in their BT, which peaked on d 35. In turn, the elevated BT of these birds led to reduced feed intake and consequently lower GR (Table 2), in agreement with previous reports (Cooper and Washburn, 1998; Deeb and Cahaner, 1999). In contrast to their counterparts with feathers, the featherless broilers also maintained normal BT under hot AT at all ages (Table 3), apparently also allowing them to maintain normal GR under heat (Figure 2).
Experiment 2: least squares means1 of body temperature (BT; °C) at 14, 28, 35, and 42 d of age, of commercial broilers (Comm), featherless (sc/sc) broilers, and their normally feathered siblings (sibs; +/sc and +/+), reared under control (constant 25°C) or hot (constant 35°C) ambient temperature (AT) conditions2
| Item | Control (constant 25°C after brooding) | Hot (constant 35°C) | Heat effect (hot − control, °C) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Featherless | Feathered sibs | Comm | Featherless | Feathered sibs | Comm | Featherless | Feathered sibs | Comm | |||
| n | 14 | 13 | 8 | 13 | 14 | 9 | |||||
| Age | |||||||||||
| 14 d | 41.9 | 41.8 | 41.9 | 41.6 | 41.8 | 42.0 | −0.3 | 0.0 | 0.1 | ||
| 28 d | 41.9 | 41.8 | 41.8 | 41.8b | 42.6a | 42.9a | −0.1 | 0.8*** | 1.1*** | ||
| 35 d | 42.0 | 42.1 | 42.2 | 42.2b | 43.4a | 43.3a | 0.2 | 1.3*** | 1.1** | ||
| 42 d | 41.7 | 41.5 | 41.8 | 41.5c | 42.2b | 43.0a | −0.2 | 0.7* | 1.2*** | ||
| 49 d | 41.2b | 42.4a | 42.1a | ||||||||
| Item | Control (constant 25°C after brooding) | Hot (constant 35°C) | Heat effect (hot − control, °C) | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Featherless | Feathered sibs | Comm | Featherless | Feathered sibs | Comm | Featherless | Feathered sibs | Comm | |||
| n | 14 | 13 | 8 | 13 | 14 | 9 | |||||
| Age | |||||||||||
| 14 d | 41.9 | 41.8 | 41.9 | 41.6 | 41.8 | 42.0 | −0.3 | 0.0 | 0.1 | ||
| 28 d | 41.9 | 41.8 | 41.8 | 41.8b | 42.6a | 42.9a | −0.1 | 0.8*** | 1.1*** | ||
| 35 d | 42.0 | 42.1 | 42.2 | 42.2b | 43.4a | 43.3a | 0.2 | 1.3*** | 1.1** | ||
| 42 d | 41.7 | 41.5 | 41.8 | 41.5c | 42.2b | 43.0a | −0.2 | 0.7* | 1.2*** | ||
| 49 d | 41.2b | 42.4a | 42.1a | ||||||||
a–cMeans within age and AT treatment (25 or 35°C) without a common superscript differ significantly (P < 0.05).
1Least squares means were calculated from males and females because there were no significant interactions with sex.
2Heat effects are presented as differences between corresponding means under hot and control AT conditions.
*P < 0.05, **P < 0.01, and ***P < 0.001 for means under 25 vs. 35°C, within genetic group and age.
Weight of Slaughtered Birds
At the end of the experiment (d 46 for the control AT groups and d 53 for the hot AT groups), yields of breast meat, legs, wings, and skin were determined for samples of slaughtered birds from all groups. Table 4 presents the exact number of birds per group and their mean weight upon slaughter after 10 h of feed withdrawal. These means were similar to the corresponding means of the entire groups at these ages (Figure 2), indicating that meat yield was measured on representative birds from each genetic group and AT treatment. Aiming to separate the net effect of heat on meat yield from its effect on BW, the hot AT groups were reared for 1 more week (to d 53) because mean BW on d 46 of the Comm and feathered sibs groups were far lower than those of their counterparts at the control AT (Table 2). To further enhance the growth of the feathered broilers before slaughter, AT from d 49 to 53 was lowered from 35 to 30°C. Nevertheless, the mean slaughter weights on d 53 of the birds in the feathered sibs and Comm groups were only 1,754 and 1,704 g, 24 and 36% lower than the mean slaughter weight of their control AT counterparts on d 46. The difference in mean BW between the AT treatments was reversed in the featherless groups of slaughtered birds; it was 1,948 g on d 46 at the control AT and 2,338 g (greater by 20%) on d 53 at the hot AT. Because the broilers from the 2 AT treatments were slaughtered at different ages and mean BW, the AT effects were tested only on yields (i.e., the weight of a given part or organ as a percentage of BW), whereas means of absolute as well as relative (%) weights were compared between genetic groups within each AT treatment.
Experiment 2: least squares means1 of BW at slaughter,2 of weight and percentage (of BW) of breast meat,3 legs,4 breast + legs, wings, and skin, of commercial broilers (Comm), featherless (sc/sc) broilers, and their normally feathered siblings (sibs; +/sc and +/+), reared under control (constant 25°C) or hot (constant 35°C) ambient temperature (AT) conditions5
| Item | Control (slaughter at 46 d of age) | Hot (slaughter at 53 d of age) | Heat effect6 [(hot − control)/control, %] | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Featherless | Feathered sibs | Comm | Featherless | Feathered sibs | Comm | Featherless | Feathered sibs | Comm | |||
| n | 35 | 33 | 31 | 48 | 31 | 8 | |||||
| Variable | |||||||||||
| Slaughter BW (g) | 1,948.1c | 2,302.7b | 2,663.8a | 2,338.1a | 1,754.4b | 1,704.2b | |||||
| Breast meat (g) | 388.4b | 392.9b | 503.3a | 456.5a | 239.7b | 255.5b | |||||
| Breast meat (%) | 19.9a | 17.1c | 18.9b | 19.6a | 13.6b | 14.9b | −1.5 | −20.5*** | −21.2*** | ||
| Legs (g) | 392.4c | 489.9b | 546.3a | 525.6a | 413.0b | 428.0b | |||||
| Legs (%) | 20.0b | 21.2a | 20.5b | 22.4c | 23.5b | 24.8a | 12.0*** | 10.8*** | 21.0*** | ||
| Breast + legs (g) | 780.9c | 882.8b | 1,049.5a | 984.5a | 653.0b | 684.3b | |||||
| Breast + legs (%) | 40.0a | 38.3b | 39.3ab | 42.0a | 37.1c | 39.8b | 5.0*** | −3.1* | 1.3 | ||
| Wings (g) | 148.2c | 195.0b | 216.3a | 179.1a | 164.2b | 169.5ab | |||||
| Wings (%) | 7.6c | 8.5a | 8.1b | 7.7c | 9.4b | 9.9a | 1.3 | 10.6*** | 22.2*** | ||
| Skin (g) | 88.0b | 164.3a | 173.2a | 129.1a | 118.9a | 102.9a | |||||
| Skin (%) | 4.5c | 7.2a | 6.5b | 5.4c | 6.8a | 6.0b | 20.0*** | −5.6 | −7.7 | ||
| Item | Control (slaughter at 46 d of age) | Hot (slaughter at 53 d of age) | Heat effect6 [(hot − control)/control, %] | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Featherless | Feathered sibs | Comm | Featherless | Feathered sibs | Comm | Featherless | Feathered sibs | Comm | |||
| n | 35 | 33 | 31 | 48 | 31 | 8 | |||||
| Variable | |||||||||||
| Slaughter BW (g) | 1,948.1c | 2,302.7b | 2,663.8a | 2,338.1a | 1,754.4b | 1,704.2b | |||||
| Breast meat (g) | 388.4b | 392.9b | 503.3a | 456.5a | 239.7b | 255.5b | |||||
| Breast meat (%) | 19.9a | 17.1c | 18.9b | 19.6a | 13.6b | 14.9b | −1.5 | −20.5*** | −21.2*** | ||
| Legs (g) | 392.4c | 489.9b | 546.3a | 525.6a | 413.0b | 428.0b | |||||
| Legs (%) | 20.0b | 21.2a | 20.5b | 22.4c | 23.5b | 24.8a | 12.0*** | 10.8*** | 21.0*** | ||
| Breast + legs (g) | 780.9c | 882.8b | 1,049.5a | 984.5a | 653.0b | 684.3b | |||||
| Breast + legs (%) | 40.0a | 38.3b | 39.3ab | 42.0a | 37.1c | 39.8b | 5.0*** | −3.1* | 1.3 | ||
| Wings (g) | 148.2c | 195.0b | 216.3a | 179.1a | 164.2b | 169.5ab | |||||
| Wings (%) | 7.6c | 8.5a | 8.1b | 7.7c | 9.4b | 9.9a | 1.3 | 10.6*** | 22.2*** | ||
| Skin (g) | 88.0b | 164.3a | 173.2a | 129.1a | 118.9a | 102.9a | |||||
| Skin (%) | 4.5c | 7.2a | 6.5b | 5.4c | 6.8a | 6.0b | 20.0*** | −5.6 | −7.7 | ||
a–cMeans within variables and AT treatments (25 or 35°C) without a common superscript differ significantly (P < 0.05).
1Least squares means were calculated from males and females because there were no significant interactions with sex.
2Body weight on the day of slaughter (46 d for the control room and 53 d for the hot room) after 10 h of feed withdrawal.
3Pectoralis major and pectoralis minor.
4Drumsticks with bone and thighs without bone, both without skin.
5Heat effects are presented as the relative (%) differences between corresponding means under hot and control conditions, divided by the mean under control AT.
6Heat effect was not calculated for absolute weights because the birds from the 2 temperature treatments were slaughtered at different ages.
*P < 0.05 and ***P < 0.001 for means under 25 vs. 35°C, within genetic group.
Breast Meat Under the Control AT
Under the control AT, the high breast meat yield (BR%, % of BW) of Comm broilers reared under the control AT (18.9% of BW; Table 4) agrees with recent studies with CCB (e.g., Havenstein et al., 2003b) and reflects the intensive commercial breeding for this trait since the early 1990s. The mean BR% of the feathered sibs was 17.1%, indicating the elevation in genetic potential (after 2 backcross cycles) for this trait in the sc-segregating experimental line, compared with 14.6% of the feathered sibs in the earlier study (Cahaner et al., 2008). The featherless broilers had significantly higher BR% than their feathered sibs (19.9 vs. 17.1%). The featherless broilers and their feathered sibs shared the same genetic background; therefore, the difference between 19.9 and 17.1% can be considered the net effect on BR% of being featherless, possibly because the nutrients that otherwise would have been used to build feathers were deposited in the breast muscle. A possible trade-off between lower feather mass and greater BR% was suggested by Cahaner et al. (1987) and Ajang et al. (1993). The mean BR% of the featherless broilers was higher than that of the Comm broilers, suggesting that the spared feather-building nutrients more than fully compensated for the lower genetic potential of the former, as indicated by the mean BR% of the feathered sibs.
Breast Meat Under the Hot AT
The favorable effect on BR% of being featherless was accentuated under the hot AT, in which the mean BR% of the featherless broilers was 19.6%, similar to that under the control AT. In contrast, and as was shown in previous studies (Leenstra and Cahaner, 1992; Cahaner et al., 1993, 2008; Deeb and Cahaner, 2001, 2002), the negative effect of hot conditions on BR% of broilers with feathers was larger than the effect on BW. Consequently, the mean BR% under the hot AT of the feathered sibs and the Comm broilers was only 13.6 and 14.9%, respectively, reflecting a heat effect of about −21% compared with the corresponding BR% means at the control AT (Table 4). In absolute terms, the mean breast meat weight of Comm broilers was 503 g on d 46 at the control AT and was only 255 g on d 53 at the hot AT, a reduction of about 50%. In contrast, the mean breast meat weight of the featherless birds increased from 388 g at the control AT to 456 g at the hot AT, in which being featherless increased breast meat production per bird by 220 and 200 g over the breast meat weight of their feathered sibs and the Comm broilers, respectively, an advantage of about 85%.
It should be noted that in a previous study (Cahaner et al., 2008), the mean BR% of featherless birds was already found to be substantially higher (by 4% of BW) than that of their feathered sibs under both normal and hot AT. In that study, however, the mean slaughter BW of the featherless broilers (<1,400 and <1,700 g at the normal and hot AT, respectively) was lower than the typical industrial slaughter weight; hence, the practical relevance of that finding could be questioned. In the present study, the slaughter weight of the featherless broilers averaged almost 2,000 g at the control AT and more than 2,300 g at the hot AT, thus confirming the reliability and proving the practical relevance of the dramatic effect of being featherless on BR%. Moreover, the elevation in BR% caused by being featherless, a phenotype determined by the single gene sc (scaleless), was at least 2-fold larger than the higher BR% reported in the commercial high-yield broiler lines, which has been gradually achieved by generations of costly intensive selection.
Legs
The mean weight of legs at the control AT reflected BW means, with the feathered sibs having a slight advantage and with the mean leg weight relative to BW being higher than those of the featherless and Comm groups (21.2% vs. 20.0 and 20.5%; Table 4). In the birds reared under the hot AT, the featherless broilers had the highest mean leg weight but the lowest mean leg weight relative to BW because of their highest BR%. Therefore, the weights of the 2 main edible parts, breast meat and legs, were combined into one item (Table 4). Under the control AT, the combined weight of these parts averaged between 38.3% (feathered sibs) and 40% (featherless) of BW. Under the hot AT, the relative combined weight of these 2 parts averaged 42% in the featherless broilers, significantly higher than those of their feathered sibs and the Comm broilers (37.1 and 39.8%, respectively).
Wings
The mean relative weight (% of BW) of wings in the featherless broilers was almost the same (7.6 and 7.7%) in the control and hot AT, and was significantly lower than the corresponding means of their feathered sibs and the Comm birds at the control AT (8.5 and 8.1%) and hot AT (9.4 and 9.9%; Table 4). In the feathered groups (sibs and Comm), the relative weight of wings was higher at the hot AT compared with the control AT because of the greater heat-induced reduction in BR% in these 2 groups. However, the absolute weight of the wings was much lower at the hot AT (despite the 1-wk longer rearing period) than at the control AT in the 2 groups of broilers with feathers, and was also lower than the mean wing weight of their featherless counterparts.
Skin
The skin relative weight (% of BW) in the featherless broilers was significantly lower than the corresponding means of the broilers with feathers in both AT treatments (Table 4). The skin of the former was indeed almost transparent, and its lower relative weight suggests that this skin was thinner than the skin of broilers with feathers. Skin weight and composition were evaluated in broilers segregating for the 3 genotypes of the codominant naked-neck gene (Na), which reduces the number of feathers by 40% in the homozygous naked neck (Na/Na) and 20% in the heterozygous naked neck (Na/na), compared with normally feathered homozygous (na/na) birds (Crawford, 1976). The means for relative skin weight (% of BW) of these 3 genotypes, respectively, were 8.5, 7.6, and 7.1%, reflecting the differences in feathering between the 3 groups, and further analyses indicated that skin fat content also decreased along with the reduced feathering (Cahaner et al., 1993). Although skin composition was not determined in the present study, it is suggested that the lower skin weight in featherless broilers may be attributed to the lack of feather follicles and the reduced amount of cutaneous fat that accumulates around these follicles. With the skin of standard broilers being considered either as having less nutritive value because of its high fat content or as a low-priced by-product in deboned broilers, the lower skin weight in featherless broilers, and its assumed lower fat content, could be translated into nutritional and economic advantages. The lower relative weight of the wings in the featherless broilers can be explained by less cutaneous fat and the lack of feather follicles. Because of the large wing feathers, their follicles contribute considerably to wing weight in standard broilers, whereas this weight is missing in the wings of featherless broilers. Indeed, preliminary analyses of wing composition (A. Cahaner, unpublished data) indicated that the amount of meat in the wings of featherless broilers is similar to that in the wings of feathered broilers of similar BW, and the higher wing weight in the latter is due only to the extra weight of skin and cutaneous fat.
Summary
The feather coverage clearly affected the thermoregulatory capacity of broilers. In experiment 1, the featherless birds maintained normal BT and livability under heat waves, in contrast to the marked elevation in mean BT of their feathered sibs, and this also led to 34% mortality under the acute heat wave. In experiment 2, the genetic potential for the performance traits of the featherless broilers and their feathered sibs was only about 15% lower than that of CCB, represented by the Comm group. In this experiment, only the featherless broilers were able to maintain normal levels of BT at the hot AT as well as the control (standard) AT, and consequently were able to reach similar BW in both AT treatments. Compared with the 2 groups with feathers, the featherless broilers produced substantially higher yields of breast meat, legs, and wings under severely hot conditions of a constant 35°C and no mechanical ventilation. Being featherless was also found to lead to lower skin weight, which should further increase the yield of deboned meat and improve the quality of wings for human consumption.
Given the superior livability and performance of the featherless birds, the results of the present study further support the suggestion that introducing the sc gene and the featherless phenotype into CCB stocks is a promising approach to facilitate highly efficient yet low-cost production of broiler meat under hot conditions. More research should be conducted to determine the effects of birds being featherless on their nutritional requirements, responses to stocking density, welfare, and meat quality.
