Genetic approaches to reducing heat susceptibility in meat broilers

A. Cahaner

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Modem meat broilers are characterized by high growth rate (GR), high rate of feed intake and metabolism, and elevated internal heat production, all acting as internal stressors that enhance the effects of external heat stress.

Hot ambient temperature hinders dissipation of excessive internal heat, leading to elevated body temperatures, depressed appetite and growth (which results in poorer feed conversion ratio), and higher mortality. These negative effects can be alleviated by compromising sustainability: costly climate-controlled housing or lower efficiency (low stocking density, low marketing weight). The negative effects of heat can be mitigated by reducing feather coverage thus enhancing heat dissipation. During the 1990’s it was shown that the reduced feather coverage in naked neck broilers provides partial heat tolerance; hence it was hypothesized that higher heat tolerance can be achieved by complete elimination of the feather coverage. Experimental featherless broilers were developed by repeated backcrossing of the Scaleless mutants (sc/sc) to contemporary fast- growing broilers. Featherless broilers and their feathered counterparts were compared in a series of studies under warm and hot conditions ranging from 25‘C to 35‘C without cooling or forced ventilation, on diets ranging in nutrient density, and at stocking densities between 7 to 22 birds/m’. It was demonstrated that hot conditions had no negative effects on welfare and liveability of featherless broilers. The heat tolerance of the featherless broilers was reflected also in their superior performance under hot conditions: higher growth rate, better FCR, and higher yield and quality of carcass and breast meat. Moreover, featherless broilers exhibited superior performance under heat stress without reducing stocking density and on low-density diets, thus maintaining efficient production, similar to that of standard broilers under normal conditions. With their various advantages combined, featherless broilers can markedly improve the sustainability of broiler meat production under hot conditions.

Remarkable genetic progress has been achieved in broiler growth rate (GR) and meat yield since the 1950s. Greater GR of broilers is driven by greater rate of feed intake and metabolism, and consequently there is an elevation in production of internal heat. However, the continuously increasing 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.

Hot conditions decrease the difference between ambient temperature (AT) and the average temperature of the body surface, reducing the rate at which metabolic heat can be dissipated. 

The lower rate of sensible heat loss leads to an elevation in body temperature (BT), which may lead to mortality under very high temperature conditions or acute heat waves. Under less extreme or chronic hot conditions, broilers acclimate by reducing feed intake but consequently GR is reduced, resulting in lower marketing body weight (BW) and poorer breast meat yield.

Hot conditions can be avoided in modem broiler houses equipped with efficient cooling systems. However, the global broiler industry continues to expand to hot-climate developing countries where climatic control of broiler houses is limited due to high installation and operational costs and an unreliable supply of electricity. The use of cooling systems is presently increasing also in temperate-climate countries, because contemporary commercial broilers (CCB) are continuously selected for greater GR and meat yield and reared to higher BW, and consequently generate more metabolic heat. They thus need lower AT in order to maintain normal BT and to fully express their genetic potential for rapid growth. 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 economical 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, but due to the insulation of the feathers, the temperature of the feather-covered body surface is close to AT, hence this surface contributes minimally to the overall sensible heat loss. It was shown that, in high-GR broilers under hot conditions, feather coverage impedes thermoregulation because it hinders sensible heat loss. Already in the 1980’s and 1990’s, several groups have tested the hypothesis that the negative effects of high ambient temperatures can be alleviated by introducing genes that reduce or eliminate feather coverage into the genetic makeup of CCB stocks.

Genetic and breeding aspects of heat stress

Heat stress effects on the performance of high-GR broilers
Chickens, like all homoeothermic animals, maintain a constant body temperature (BT) over a wide range of AT. In birds, heat loss is limited by feathering and by the lack of sweat glands. The ability of animals to maintain BT within the normal range depends on a balance between internally-produced heat and the rate of heat dissipation. The amount of internal heat produced by broilers depends on their BW and feed intake. The heat dissipation rate depends on environmental factors, mainly AT, and on feather coverage. When the physiological and behavioural responses to high AT are inadequate, an elevation in BT occurs, causing a decrease in appetite and in GR. Consequently, the time needed to reach marketing weight is increased, leading to poorer feed conversion and overall lower efficiency of poultry meat production. Moreover, hot conditions depress the yield and quality of broiler meat, and may lead to PSE (pale, soft, exudative) meat. Therefore high AT has been the main factor hindering broiler meat production in hot climates, especially in developing countries where farmers cannot afford costly artificial control of AT in broiler houses.

Selection on GR under hot conditions
Breeding for adaptation to a specific stressful environment is the strategy of choice when GxE interaction affects economically important traits. Such breeding activity may take place in a particular stressful location (localized breeding) or under artificially induced stress. We are aware of only one published report of experimental selection of broilers under controlled hot conditions. Lack of later reports by these authors, or others, may indicate that this approach was not successful. Commercial localized breeding under suboptimal hot conditions has been applied successfully in India. When compared under local hot conditions, the imported high-GR broiler stocks were inferior to the locally-bred stock, but in absolute terms the latter’s performance was much lower than the genetic potential of contemporary high-GR stocks, i.e. their performance under optimal conditions. Thus, it could be concluded that broilers cannot be bred to exhibit high GR and high BW (in absolute terms) under hot conditions. So far, the latter has not been an important limitation in most hot-climate countries where customers traditionally prefer to buy live broilers with small body size (=1.5 kg). However, broilers that are produced for mechanical slaughtering and processing must have large BW at marketing and high yield of quality meat — the traits most depressed in high-GR broilers reared under hot conditions. Therefore, with the current trend to increase production of carcass parts and deboned meat in hot-climate countries, either for export or for local consumption, it will no longer be possible to avoid the negative effects of heat by marketing small-body broilers.

The effects of the Naked Neck gene (Na) on feather coverage and heat tolerance
Many studies had been conducted with the co-dominant ‘naked-neck’ (Na) gene, which is common in rural chicken populations in hot regions. This gene reduces feather coverage by 20% and 40% in heterozygous (Na/na) and homozygous (Na/Na) chickens, respectively. Under hot conditions, naked-neck broilers exhibited greater sensible heat loss and better thermoregulation, resulting in greater actual GR and meat yield than their fully feathered counterparts. However, in these studies, the naked- neck broilers raised at 25‘C were superior to their counterparts at hot conditions, suggesting that the 20 or 40% reduction in feather coverage provides only partial heat tolerance. Hence it was hypothesized that complete feather elimination may enhance heat tolerance of genetically fast-growing broilers.

The Scaleless gene (sc)
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 much lighter and slower-growing than contemporary commercial meat-type chickens. The featherless mutants were thus not considered for practical purposes. 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, but the effects were small because the GR of the birds in this study was very low: maximum GR of 30 g/d and average BW of approximately 1200 g at 8 wk (compared with about 100 g/d in today’s broilers that reach the same BW in about 4 weeks).

The development of a new line of featherless broilers was initiated in the year 2000 by crossing original scaleless mutants with contemporary high-GR broilers, followed by a series of backcrossing and intensive selection on BW. The birds in this line are either normally feathered (Sc +/sc, carriers of the sc allele) or featherless (sc/sc). Currently, the genetic potential GR of this experimental line is only slightly lower than that of contemporary commercial broilers. When compared to their feathered sibs (brothers and sisters with the same genetic background) the featherless broilers can exhibit the net effects of being featherless on economically-important traits under the trial’s conditions.

Viability et featherless broilers vs. their feathered counterparts
At an earlier stage of the development of the experimental line of featherless broilers, 27 featherless birds and 49 of their feathered sibs were reared in an AT-controlled chamber. On Day 47 the AT was gradually elevated from 30 to 35‘C for 2 days, leading to an increase in BT to 42.8‘C in feathered birds, but only to 41.4‘C in featherless birds. On Day 53 (BW averaged 1900g) AT was elevated to 36‘C. This led to lethal elevation of BT and death of 17 feathered birds (35%), whereas BT of the featherless birds remained at 41.4‘C, and only 1 of the 27 birds died. In a recent trial, after GR of the experimental line had been enhanced by several additional cycles of backcrossing to CCB, featherless broilers and their feathered sibs, as well as feathered commercial broilers, were kept together under constant hot conditions (32±1‘C). When the birds where 41 days old (BW of about 1750 g), AT in one room increased un-intentionally to 38’C for about 5 hours. Consequently, 20 of 28 commercial broilers died (71%), 30 of 72 feathered sibs died (42%), but only 2 of 100 featherless birds died (unpublished data). This event suggests an association between potential GR and susceptibility to heat in broilers with feathers, and demonstrated the exceptional heat resistance of the featherless birds, regardless their GR.

These two studies indicate that the welfare and liveability of featherless broilers are not compromised under acute hot conditions. Maintaining normal BT even under extreme AT is apparently the key to the heat tolerance of the featherless broilers. Elevated BT under heat stress was shown to negatively affect GR, feed consumption and feed conversion in standard broilers. The results suggest that the superior GR and meat yield of featherless broilers under high temperature conditions in comparison to high-GR standard (feathered) broilers, are due to their capacity to dissipate all the excessive internally-produced heat and maintain normal BT, and consequently normal (i.e., not-depressed) feed consumption and GR.

Comparing slow-growing featherless broilers vs. naked neck and feathered counterparts
A unique study consisted of 4 experimental genetic groups (fully feathered, heterozygous naked neck, homozygous naked neck, featherless), progeny of the same double-heterozygous parents (Na/na +/sc), as well as commercial broilers as industry reference. Birds from all 5 groups were brooded together until d 21 when one-half of the birds from each group were moved to hot conditions (constant 35‘C), whereas the others remained under comfortable conditions (constant 25‘C). Individual BW was recorded from hatch to slaughter at d 45 and 52 at 25 and 35‘C, respectively, when breast meat, rear part, and heart weights were recorded. Body temperature was recorded weekly from d 14 to 42.

Feather coverage substantially affected the thermoregulatory capacity of the broilers under hot conditions. With reduced feather coverage (naked-neck), and more so without any feathers (featherless), the birds at 35‘C were able to minimize the elevation in body temperature. Consequently, only the featherless birds exhibited similar growth and BW under the two temperature treatments. The naked-neck birds at 35‘C showed only a marginal advantage over their fully feathered counterparts, indicating that 20 to 40% reduction in feather coverage provided only limited tolerance to the heat stress imposed by hot conditions. Breast meat yield of the featherless birds was much greater (3.5% of BW, an 25% advantage) than that of them partly feathered and fully feathered counterparts and the commercial birds under hot conditions. The high breast meat yield (at both 25 and 35‘C) of the featherless broilers suggests that the saved feather-building nutrients and greater oxygen-carrying capacity contribute to their greater breast meat yield. Because of these results, it was concluded that further research on genetically heat-tolerant broilers should focus on the featherless phenotype rather than naked neck ones.

Comparing fast-growing featherless broilers vs. feathered counterparts
The experimental birds in the previous study (Cahaner et al., 2008) reached a mean BW of only about 1200 g at 6 weeks of age with maximal GR of about 55 g/d. This GR was higher than that of the birds studied by Somes and Johnson (1982), yet substantially lower than that of contemporary commercial broilers (CCB). Hence the practical relevance of the conclusions regarding the advantages of featherless broilers remained questionable. Therefore the genetic potential of the featherless experimental line was enhanced by additional cycles of backcross to CCB stocks. The backcross progeny were used in several studies, with the objective to compare actual GR and performance-related traits of featherless broilers vs. their normally feathered siblings (sibs) and also a group of CCB as industry reference. The AT treatments, after the brooding period, were either constant 35‘C (Hot-AT) or constant 25‘C (Control-AT).

The broilers from all groups were reared intermingled to 46 or 53 days at Control- and Hot-AT, respectively, and the measured traits included body temperature (BT), growth, and weight of whole-body and carcass parts: breast meat, legs, wings, and skin. At Hot-AT, only the featherless broilers maintained normal BT; their mean day-46 body weight (203l g) was significantly higher than at Control-AT, and it increased to 2400g on day-53, much higher than the corresponding means of all feathered broilers (=1700g only). The featherless broilers had significantly higher breast meat yield (=20% in both ATs), lower skin weight and supposedly better wing quality (Azoulay et al., 2011). These results confirmed that being featherless improves the performance of fast-growing broilers at hot conditions and suggest that introduction of the featherless phenotype into commercial broiler stocks facilitates highly-efficient yet low-cost production of broiler meat at hot conditions.

The effects of being featherless on meat yield and quality in normal and hot condition
In optimal conditions, the cardiovascular system in contemporary feathered broilers develops simultaneously with the muscles growth allowing adequate levels of oxygen and nutrients supply, and clearing metabolism waste products. Hot conditions reduce the feed intake as well as cardiovascular capacity in these broilers, and consequently reduction in breast meat yield and its quality are commonly observed. The latter may lead to Pale, Soft and Exudative (PSE) meat, possibly due to insufficient capillary support. Accordingly, featherless broilers should have higher meat yield and quality under hot conditions. This hypothesis has been tested in 2 trials with 4 groups of broilers: featherless, their feathered siblings, contemporary commercial (Comm), and experimental line representing commercial broilers of the 1980’s. Half of the chicks from each group were reared under moderate ambient temperature (constant 26‘C). The remaining birds were reared under hot condition (constant 32‘C) to 45 d and 48 d of age in trials 1 and 2 respectably.

At trial’s end (45d and 48d in Trials 1 and 2, respectively) about 100 birds per room, equally representing all genetic groups, were randomly selected for carcass measurements.

The breast meat (pectoralis major and pectoralis minor) was removed from each carcass (by a single operator) and weighed. The rear part of each carcass, consisting of pelvic, thighs and drumsticks (bones included) was also weighed. The heart of each bird was also removed and weighed. Colour of breast meat was measured by Minolta spectro-colorimeter with the CIELAB (L*, a*, b*) system. The L* (lightness) describes the relationship between light reflection and absorption, which relates to the liquids exudates from the meat. The a* (redness) indicates redness when positive and greenness when negative. The b* indicates yellowness when positive or blueness when negative. These measures are commonly used to evaluate variations in meat quality. Colour was measured 24 h post-mortem. Drip loss was determined from the reduction in the weight of pectoralis major during 48 hours (from 24 to 72 hours post-mortem) of storage in plastic bags at 4‘c.

Average BW of the Comm broilers reared under heat (32‘C) was 2050g at 47d, 650g lower than their counterparts under moderate temperature (25‘C) (Fig. 1). The 1980’s broilers had a lower growth rate, and the heat reduced their mean BW by only 380g (1420 vs. 1800 g). Heat did not affect significantly the growth rate of the featherless broilers; their BW averaged 2000g and 2150g in under hot and moderate conditions, respectively. In moderate conditions, breast meat yield averaged 23’7e, 22%, 18% and 13’7e for the Featherless, Comm, feathered and 1980’s broilers, respectively. The hot conditions significantly reduced breast meat yield of Comm broilers, from 22% to 19%, and also negatively affect the quality of their meat, e.g. drip loss was 2-fold higher than under moderate temperature. The featherless broilers had the highest breast meat yield (23 and 22%) and also highest quality, reflected by lower drip loss and lightness (L*): 2.6% and 54 vs. over 4.4% and 58 in the feathered broilers. Significantly larger hearts (%BW) and higher breast meat redness (a*) in the featherless broilers (0.6% and 4.03 vs. about 0.4% and 3 in the other groups) suggest that the superior breast meat yield and quality in the featherless broilers is associated with better capillary support.

Nutrition of featherless vs. feathered broilers under normal and hot condition
Trials on the effects of dietary protein and energy content on performance of feathered vs featherless broilers were conducted under temperate (26‘C) and hot (32‘C) conditions and varying stocking densities. Commercial 3rd (days 17-31) and 4th (days 3 1-46) diets were used as Control. Experimental diets had lower contents (down to 80% of control diets) of protein or energy or both. Body weight (BW) gain and feed consumption were recorded from day 17 to end of trial, when breast meat yield was determined.

Under temperate conditions, lower protein and energy contents in the diluted diets reduced body weight and breast meat yield in the feathered broilers, but not in the featherless broilers. It appears that the featherless broilers have lower protein requirement, as could be expected because they do not need the amino acids used to build the feathers in standard broilers. With the lower requirement for protein and energy, being featherless improves also the economic FCR (lower feed costs), and also reduces the environmental impacts of processing, by avoiding the plucking and dumping of the feathers. The hot conditions reduced the performance of standard broilers to a similar extent in all diets, due to lower feed intake of all diets. The heat did not depress feed intake and performance of the featherless broilers.

j) The effects of stocking density on feathered vs. featherless broilers under hot condition
In tropical developing countries (e.g. Indonesia) where broiler producers cannot afford costly cooling and ventilation, production of relatively large and meaty broilers is based on high-GR stocks reared at low stocking density of about 7 to 8 birds/m’. Trials were conducted to quantify the effects of stocking density under hot conditions (constant 32±1‘C) on GR and meat yield and quality of commercial high-GR broilers vs. featherless broilers. Feathered broilers were reared at densities ranging from 7 to 17/m’ and the featherless birds were reared at densities ranging from 12 to 22/m’. GR of feathered broilers was depressed by increasing stocking density; BW on Day 44 decreased from 2.4 kg (7/m’) to 1.8 kg (17/m’). GR of featherless broilers was only marginally affected by stocking density, with mean BW of 2.4 kg (12/m’), 2.2 kg (17/m’), and 2.1 kg (22/m’); the latter resulting in live- weight production of 46 kg/m’. The stress of heat and high stocking density reduced breast yield of high-GR broilers to 15′>e, with pale meat (L*=50, a*=4) and 4% drip loss. In the featherless broilers, breast yield was 19% in all stocking densities, with darker meat (L*=44 and a*=5) and less than 2’Ze drip loss. Thus, in contrast to the negative association between high GR and meat yield and quality of feathered broilers under heat featherless broilers produced high yield of quality breast meat also in hot conditions and at high stocking density.

From the Australian Poultry Science Symposium