Being featherless (homozygous sc/sc) provides fast-growing high-yield broilers with genetic adaptation to hot conditions Avigdor Cahaner

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1 Being featherless (homozygous sc/sc) provides fast-growing high-yield broilers with genetic adaptation to hot conditions Avigdor Cahaner The Hebrew University of Jerusalem, Faculty of Agriculture, Rehovot 76100, Israel (fax) cahaner@agri.huji.ac.il avigdor.cahaner@gmail.com Abstract The genetic enhancement in growth rate (GR) of modern broilers has increased their heat sensitivity. High GR requires higher rate of feed intake and metabolism, and therefore it is also associated with increased production of internal heat. Broilers must dissipate this excess heat, but efficiency of heat dissipation is reduced by high ambient temperatures (AT). Therefore under hot conditions, modern broilers cannot dissipate all the heat they generate. Heat stress is further increased by high stocking density, due to higher litter temperature and reduced air movement. The negative effects of heat can be countered by cooling systems, but they are associated with high costs of equipment, maintenance and electricity. Without effective cooling, modern broilers acclimatize to hot conditions by reducing feed intake in order to minimize the excess production of internal heat. This, however, leads to a significant depression of growth rate and body weight, poorer feed conversion ratio, and specific reduction in breast meat yield. Moreover, heat stress also reduces the quality of breast meat and may lead to the PSE syndrome which stands for three negative meat characteristics: Pale, Soft and Exudative. Heat dissipation from the body is hindered by feathers; this is advantageous in slow-growing chickens or in broilers reared at low AT, but under high AT the feather coverage has a negative effect on thermoregulation. It was hypothesized that genetic elimination of the feathers may provide an inherent heat tolerance to fast-growing broilers. Current research is based on cross-breeding between the Scaleless mutants (homozygous sc/sc) and contemporary fast-growing broilers. Featherless broilers, their feathered sibs, and commercial broilers, were reared at stocking densities ranging from 7 to 22 birds/m 2, under AT of 30-32ºC, without forced ventilation. Birds were slaughtered at 44 days and their breast meat was dissected and weighed. As expected, the growth of feathered broilers was depressed by the heat, and this effect increased with stocking density; their mean BW at 44 days ranged from 2.4 kg (7 birds/m 2 ) to 1.8 kg (17 birds/m 2 ). In contrast, the growth of featherless broilers was hardly affected by density, with mean BW ranging from 2.4 kg (12 birds/m 2 ) to 2.2 kg (17 birds/m 2 ) and 2.1 kg (22 birds/m 2 ), with the latter resulting in total live-weight production of 46 kg/m 2. Heat and density stresses also reduced breast yield of the feathered broilers to 15-16%, whereas in the featherless broilers, breast yield averaged 19-20% in all densities. Whereas standard broilers suffer under hot conditions, as evidenced by depressed appetite, elevated body temperature, and high incidence of heat-related and disease-related mortality -- these effects are rare in fast-growing featherless broilers. Moreover, featherless broilers can contribute to sustainable broiler production in hot regions, in two independent ways: 1. No need to use costly energy (and equipment) for climate control inside the broiler houses. In addition to saving scarce global resources, it facilitates efficient broiler production also in developing countries and in regions lacking reliable supply of energy (electricity). 2. About 15% of the nutrients in the broiler feed, mainly amino acids, are used by broilers to build the feathers that later are discarded as waste. These nutrients are saved in featherless broilers, contributing to their higher meat yield. Moreover, a 10% reduction in intake of dietary nutrients has no negative effect on growth rate, FCR and meat yield in featherless broilers. Key words: featherless broilers, heat stress, scales gene, growth rate, meat yield 1

2 Introduction The tremendous genetic progress in broiler growth rate (GR) and meat yield since the 1950 s (Havenstein et al., 2003), and the consequent reduction in time needed to achieve marketing body weight (BW) under optimal conditions, have not been accompanied by similar improvements under sub-optimal conditions, especially under high ambient temperatures (AT). The broiler industry continues to expand to hot regions, but climatic control of broiler houses is lagging due to high installation and operational costs. Nevertheless, commercial breeding is still conducted in temperate climates, thus improving the genetic potential (mainly in GR and meat yield) of broiler stocks through intensive selection under optimal conditions. However, results from several studies suggest that due to this progress in genetic potential for high GR and meat yield, broilers are becoming more sensitive to elevated AT. Hence, also in developed countries, many broilers are reared under practically hot conditions. Chickens, like all homeothermic animals, maintain a constant body temperature (BT) over a wide range of AT. In birds, heat loss is limited by the feather coverage and by the lack of sweat glands. The ability of an animal to maintain its 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 BW and feed intake, and heat dissipation rate depends on environmental factors, mainly AT, and on the broilers feather coverage. When the physiological and behavioral responses to high AT are inadequate, an elevation in BT occurs, causing a decrease in appetite and in actual GR. Consequently, the time needed to reach marketing weight is increased, leading to lower efficiency and profitability of poultry meat production (Cahaner and Leenstra, 1992; Leenstra and Cahaner, 1992; Yalcin et al., 1997a; Settar et al., 1999). Hot conditions negatively affect also the yield and quality of broiler meat (Leenstra and Cahaner, 1992; Mitchell and Sandercock, 1995, 1997; Sandercock et al., 2001), and may lead to PSA (pale, soft, exudative) meat (Barbut, 1997). 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. There are several approaches to minimize heat-related reduction in productivity and mortality, including acclimation to heat (e.g., De Basilio et al., 2003), feed intake manipulation and feed deprivation (Ait Boulahsen et al., 1993) and tunnel ventilation (May et al., 2000). But as commercial breeding continues to increase the potential GR of contemporary broilers, the rate of their internal heat production increases because they consume more feed per time unit. Hence management and nutritional practices are limited as they provide only short-term or very costly solutions, and therefore genetic solutions should be preferred. Heat dissipation is controlled by the insulation of the feathers. This insulation is advantageous in slow-growing chickens or when broilers are reared at low AT. In high-gr broilers under high AT, the feather coverage has a negative effect on thermoregulation because it hinders the dissipation of excessive internal heat (Deeb and Cahaner, 1999). This negative situation can be avoided by introducing genes that reduce or eliminate feather coverage into the genetic makeup of high-gr broiler stocks. Reduced feather mass was also expected to contribute to higher meat yield, if the saved protein goes into building more muscles, as suggested by Cahaner et al. (1987). Many studies had been conducted with the naked-neck (Na) gene, which is quite common in rural chicken populations in hot regions (Merat, 1986). This gene reduces feather coverage by about 20% and 40% in heterozygous (Na/na) and homozygous (Na/Na) chickens, respectively (Cahaner et al., 1993). Compared to their normally-feathered counterparts, naked-neck broilers have a higher rate of heat dissipation (Yahav et al., 1998) and better thermoregulation in hot conditions (Deeb and Cahaner, 1999), resulting in higher actual GR and meat production at controlled high AT (Cahaner et al., 1993; Deeb and Cahaner, 2001) and hot conditions (Yalcin et al., 1997b). Despite the relative advantage of naked-neck broilers, they also suffer substantially under heat stress. It was therefore hypothesized that total feather elimination is required in order to maximize heat-tolerance of high-gr broilers in hot conditions (Cahaner et al., 2003). Featherless chickens are derived from the recessive spontaneous mutation called Scaleless (Abbott and Asmundson, 1957) found in the New Hampshire breed, characterized by low GR 2

3 and BW. In the 1970 s, an attempt was made to derive featherless broilers by crossing the scaleless mutant with broilers of that time. Their GR and carcass composition (Somes and Johnson, 1982) and cooking (Somes and Wiedenhefft, 1982) were tested. Results suggested an advantage of the featherless birds under hot conditions, but the effects were small because GR of these birds was very low: maximum GR 30g/d (vs. 100g/d in today s broilers) and average BW of 1200g at 8 weeks whereas today s broilers reach this BW at about 4 weeks. Materials and Methods, Results, Discussion The development of our line of featherless broilers was initiated in the year 2000 by crossing original scaleless mutants with contemporary high-gr broilers, followed by backcrossing to high-gr broilers and selection on body weight (Cahaner and Deeb, 2004). The birds in this line are either normally feathered (Sc+/sc, carriers of the sc allele) or featherless (sc/sc). Currently (year 2007), the genetic potential GR of this experimental line is still slightly lower than that of contemporary commercial broilers. At that stage, the featherless broilers and their feathered sibs (brothers and sisters, sharing the same genetic background) are capable of exhibiting economically-important advantages when reared under warm or hot conditions. Featherless broilers and their feathered sibs were kept in AT-controlled chamber (Yaron et al., 2004). After brooding, AT of 28-30ºC was maintained up to Day 47, when AT was gradually elevated to 35ºC for 2 days, leading to an increase in BT to 42.8ºC in the feathered birds, and only to 41.4ºC in the featherless birds. On Day 53, when BW averaged 1900g, AT increased sharply to 36ºC,. This "heat wave" led to lethal elevation of BT and death of 17 feathered birds (35%), whereas BT of the featherless birds remained 41.4ºC. These results indicate that being featherless assures the welfare and livability of broilers under hot conditions. Maintaining normal BT even under extreme AT is probably the key for the heat resistance of the featherless broilers. Elevated BT under heat stress was shown to negatively affect GR, feed consumption and feed conversion in standard broilers (Cooper and Washburn, 1998). In tropical regions 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 8 birds/m 2. Trials were conducted to quantify the effects of stocking density in hot conditions (32±2ºC) on GR and meat yield and quality of commercial high-gr broilers vs. featherless broilers (Yadgari et al., 2006). Feathered broilers were reared in densities ranging from 7 to 17/m 2 and the featherless ones were reared in densities ranging from 12 to 22/m 2. The GR of feathered broilers was depressed by increasing stocking density; BW on Day 44 was down from 2.4 kg (7/m 2 ) to 1.8 kg (17/m 2 ), whereas GR of featherless broilers was hardly affected, with mean BW of 2.4 kg (12/m 2 ), 2.2 kg (17/m 2 ), and 2.1 kg (22/m 2 ); the latter resulted in 46 kg/m 2 total live-weight. The stress of heat and high stocking density reduced breast yield of high-gr broilers to 15%, 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% drip loss. Thus, in contrast to the negative association between high GR and meat yield and quality of feathered broilers under heat (e.g., Mitchell and Sandercock, 1995), featherless broilers produced high yield of quality breast meat also in hot conditions and high stocking density. Conclusions The results suggest that the superior GR and meat yield of featherless broilers under heat is due to their capacity to dissipate all the excessive internally-produced heat and maintain normal BT, and consequently normal feed consumption and GR. The introgression of the featherless genotype into high-gr broiler stocks is expected to adapte them to high AT, thus allowing to maintain and to further improve broiler performances in warm and hot conditions. References Abbott, U.K. and Asmundson, V.S., Scaleless, an inherited ectodermal defect in domestic fowl. J. Hered. 48: Ait Boulahsen, A., J.D. Garlich, and F.W. Edens, Calcium deficiency and food deprivation improve the response of chickens to acute heat stress. Poultry Sci. 123:

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