No abstract

Recent decades have seen significant progress in the genetic selection of fast-growing broiler chickens. Whereas in many countries the average marketing age is 5 to 6 wk, the US, French, and other markets demand heavier broilers (~4 kg) that require a longer posthatching growing period. With greater age and greater BW, the ability to cope with hot weather conditions deteriorates, which can result in increased economic losses during periods of hot weather. Recent studies have demonstrated a long-lasting effect of intermittent thermal manipulation (TM) during embryogenesis, when it was applied for 12 h/d between embryonic (E) days E7 and E16, which was shown by improved thermotolerance during acute posthatching heat stress as well as improved feed conversion ratio (FCR) and breast muscle yield. The present study was designed to elucidate the effect of TM during embryogenesis on Cobb 500 broiler performance up to 70 d of age. Hatchability and male BW were not affected by TM, but TM females demonstrated a lower (P = 0.024) BW during the entire 70-d posthatching study. However, following embryonic TM, both sexes exhibited lower (P = 0.028 and P = 0.018 for males and females, respectively) feed intake and body temperature accompanied by improved FCR and greater breast muscle weight. In light of the present and previous studies, it was concluded that intermittent TM during broiler embryonic development had a long-lasting effect on energy balance that led to improved FCR and breast muscle yield.

The potential to induce improved thermotolerance in broiler chickens is of great importance. Thermal conditioning is one of the management tools used to improve thermotolerance, enabling broilers to cope with extreme environmental conditions. This study investigated the effects of exposing chicks to low ambient temperature (T(a)) on-chick body (T(b)), surface (T(s)) temperatures and total sensible heat loss (SHL) by convection and radiation from the body and from 2 main radiative organs, the face and the legs. At 3, 4, or at both 3 and 4 d of age, chicks were exposed to 5 degrees C for 1.5 h a day (to avoid mortality) or to 10 or 15 degrees C for 3 h a day. In general, in all treatments, the results during exposure to cold differed significantly from the control. A second cold exposure (on d 4 after a first exposure on d 3) clearly enhanced the chicks' ability to maintain on-chick body surface temperatures during exposure to 15 degrees C and to recover much faster from cold exposure. A dramatic decline in average surface temperature was observed during the first 15 min of chicks' exposure to the various low ambient temperatures in all ages, reaching the lowest values in the 5 degrees C treated chicks. The face responded immediately to cold exposure by significantly increasing its SHL to a level that then remained relatively steady (15 degrees C) or declined moderately with time (10 and 5 degrees C). In the legs, however, a significant and continuous decline in SHL was exhibited in all ages. The dynamics of SHL from the legs differed from that from the face, suggesting that the legs are a major organ for vasomotor responses, whereas the face is a more conservative vasoregulatory organ. It is concluded that repetitive exposure to cold may enhance thermotolerance, and that this is partially related to the vasomotor responses. This is the first report quantifying the differentiation between the legs as a responsive vasomotor organ and the face as a conservative vasomotor one.

The rapid growth of modern broilers is associated with enhanced appetite and high metabolic rate and, consequently, high O(2) demand. Ascites syndrome (AS) develops in individuals that fail to fully supply the increasing demand for O(2) in their bodies under ascites-inducing conditions (AIC) such as high altitude or low temperatures. The tendency of broilers to develop AS is heritable, but efficacious selection against AS susceptibility (without affecting the normal expression of other important traits) requires identification of indirect selection criteria. In the present study, divergent AS-susceptible (AS-S) and AS-resistant (AS-R) lines were developed to confirm the heritability of AS and to facilitate future detection of criteria for indirect selection against AS susceptibility. The base population consisted of 85 sire families with a mean of 73 progeny per sire, reared in a commercial broiler house under low-challenge AIC (cold environment and pelleted feed). Chicks dying with AS manifestations were designated AS-susceptible, whereas the surviving birds were designated AS-resistant. By the end of the trial (d 48), AS mortality had accumulated to 17.2%, but AS incidence per family (%ASF) ranged from 0 to 49%, with a high heritability (0.57). Parents of 7 families with very high %ASF produced the first generation (S(1)) of the AS-S line, and parents of 7 families with very low %ASF produced the S(1) of the AS-R line. The S(1) males and females reproduced generation S(2) of the selected lines, whereas additional S(1) males were tested under high-challenge AIC (individual cages, cool wind, and pelleted feed). Progeny testing under this high-challenge AIC, followed by sib selection, was repeated in generations S(2) and S(3), resulting in a divergence of 86.6% in the incidence of AS between the AS-S (91.3%) and AS-R (4.7%) lines. The rapid genetic divergence, and family analysis of %ASF suggested that a single or few major genes are responsible for the difference between the 2 selected lines. These lines may facilitate more sensitive and effective genomic research aimed at detecting these genes or identifying the primary physiological cause of AS.

Under hot conditions, contemporary commercial broilers do not reach their full genetic potential for growth rate, body weight (BW), or breast meat yield because dissipation of their excessively produced internal (metabolic) heat is hindered by the feathers. Therefore, it was hypothesized that heat stress can be alleviated by using the naked-neck gene (Na) or the featherless gene (sc). The 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), and commercial broilers. 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 degrees C), and the others remained under comfortable conditions (constant 25 degrees C). Individual BW was recorded from hatch to slaughter at d 45 and 52 at 25 and 35 degrees C, respectively, when breast meat, rear part, heart, and spleen weights were recorded. Body temperature was recorded weekly from d 14 to 42. Feather coverage significantly 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 degrees C were able to minimize the elevation in body temperature. Consequently, only the featherless birds exhibited similar growth and BW under the 2 temperature treatments. The naked-neck birds at 35 degrees 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, approximately 25% advantage) than that of their partly feathered and fully feathered counterparts and the commercial birds under hot conditions. The high breast meat yield (at both 25 and 35 degrees 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, further research on genetically heat-tolerant broilers should focus on the featherless phenotype.