Early-age cold conditioning of broilers: effects of timing and temperature

1. The aim of the present study was to determine the effects of early-age cold conditioning (CC) on performance, ascites mortality, thyroid hormones status and immune response (leucocytes count) of broiler chickens. 2. A total of 336 chicks at 2 and 3 d of age were exposed to 20 or 25°C (for 3 or 6 h) in a 2 × 2 × 2 factorial experiment, while a control group was kept under normal temperature conditions (30°C). Thereafter, both control and cold conditioned birds were reared under standard conditions until 42 d of age. 3. The results showed that performance (weight gain and feed efficiency) was improved by CC at the end of the rearing period. Carcase traits (breast, thigh and abdominal fat percentage) were not affected by different treatments. Heart weight was lower in cold conditioned birds accompanied with lower ascites mortality. Total leucocyte count was higher in CC birds. Higher concentrations of thyroxin (T(4)) were found in plasma of treated groups, while triiodothyronine (T(3)) to T(4) ratio was decreased. 4. In conclusion, it seems that early age CC improves performance and reduces ascites mortality of broilers through altered thyroid hormones metabolism and leucocyte function. According to the results, the best timing for CC of broilers was 20°C for 6 h at the age of 2 d, and no significant benefit was observed by repeated CC.

Influence of temperature stimulation during the last 4 days of incubation on secondary sex ratio and later performance in male and female broiler chicks

1. In 6 incubation trials a total of 9883 eggs (Ross 308) were incubated from d 1 to 17 under normal incubation conditions (37.2-37.4 degrees C) and then sorted into three hatch incubators (control: 37.2-37.4 degrees C; chronic warm incubation: 38.2-38.4 degrees C, 24 h daily; short-term warm stimulation: 38.2-38.4 degrees C, 2 h daily) in incubation trials 1 and 2 or two hatch incubators (control and short-term warm stimulation) in trials 3-6. 2. The one-day-old chicks were selected by sex and chick quality was analysed in random samples using the Pasgar score. A total of 120 male and 120 female one-day-old chickens from each incubator were used for a 35-d fattening period. 3. Neither chronic nor short-term increase in incubation temperature had a negative effect on hatchability and chick quality. Short-term warm stimulation improved hatchability by more than 1.5% and was associated with a significantly higher proportion of hatched male chicks. 4. In the subsequent broiler growth trial, the mean daily weight gain of the short-term warm stimulated male broiler chicks was significant higher than for the control group, which results in a body weight increase of 2.9%. 5. Feed conversion (feed:gain ratio) of the short-term warm stimulated male and female broilers was significantly lower than in the males and females of the control and chronic warm incubated groups. 6. In conclusion, an incubation temperature profile which includes short-term temperature variation can be important in improving poultry performance (European patent pending since March 2008).

Attainment of thermoregulation as affected by environmental factors

The review addresses the development of thermoregulation in poultry embryos as well as the effect of acute and chronic changes of environmental factors on this process and the incubation temperature being the foremost. In poultry, the early development of adaptive body functions, like the thermoregulatory system, is characterized by the following peculiarities. First, the development of peripheral as well as central nervous thermoregulatory mechanisms start during the prenatal ontogeny. However, their maturity is attained during early postnatal development. In the perinatal period, environmental factors have a high effect on development of temperature regulation. Second, acute changes in the environmental conditions induce as a rule first uncoordinated and immediately nonadaptive reactions. Later, the uncoordinated nonadaptive reactions change into coordinated (adaptive) reactions. Prenatal environmental influences may have a training effect on the postnatal efficiency of the thermo-regulatory system. Third, functional systems of the organism develop from an open loop system without feedback control into a closed system controlled by a feedback mechanism. During this critical period, the actual environment modulates the development of the respective physiological control systems for the entire life period, especially by changes in neuroorganization and expression of related effector genes. Knowledge on these mechanisms might be specifically used to generate long-term adaptation of the organism to the postnatal climatic conditions (perinatal epigenetic temperature adaptation). In poultry, perinatal epigenetic temperature adaptation was developed by changes in the incubation temperature. When a comparison is made in birds, which were incubated at 37.5 degrees C, a low incubation temperature induced postnatal cold adaptation, and warm incubation temperature induced postnatal heat adaptation. Perinatal epigenetic temperature adaptation exhibited changes in the neuronal thermosensitivity in the hypothalamus as well as in the peripheral thermoregulatory mechanisms. These alterations could be already found at the end of incubation. Further, temperature-experienced embryos have a lower c-fos expression than in the control after acute heat stress.

Influence of baclofen on temperature sensitive neurons in chick hypothalamus

The effects of the GABA(B)-receptor agonist baclofen on neuronal tonic activity (firing rate) and temperature sensitivity (temperature coefficient, TC) of neurons in the preoptic area/anterior hypothalamus (PO/AH) have been examined in chick brain slices, using extracellular recordings. The tonic activity of both warm-sensitive and cold-sensitive neurons, as well as temperature-insensitive neurons, was significantly inhibited by superfusion with baclofen (1 microM). It is clearly evident from the present study that there was an increasing trend in warm-sensitive neurons and a significant (p<0.05) increase in TC of cold-sensitive neurons. Relatively high percentage (31.5%) of chick PO/AH neurons was determined as cold sensitive by present investigation. Particularly, GABA(B) mechanisms modify temperature sensitivity of the PO/AH neurons and thus modulation of temperature sensitivity in PO/AH is involved in the control of body temperature.

Influence of bombesin on neuronal hypothalamic thermosensitivity during the early postnatal period in the Muscovy duck (Cairina moschata)

The influence of bombesin (1 microg/0.1 ml artificial cerebrospinal fluid) on neuronal thermosensitivity of the preoptic area of the anterior hypothalamus in brain slices of 5- (n = 7 neurons) and 10-day-old (n = 36 neurons) Muscovy ducks (Cairina moschata) was investigated. Similar to adult mammals, most of the neurons investigated increased the firing rate (FR) after bombesin application. Changes in FR were not related to changes in thermal coefficient (TC). The neurons react to bombesin also under synaptic blockade. The bombesin-induced effect on TC (increase or decrease in nearly the same number of neurons, e.g. nine neurons increased and ten decreased TC in 10-day-old ducklings) in the postnatal bird neurons investigated was different from the results described in adult mammals, where the main reaction to bombesin was an increase of TC in warm-sensitive and temperature-insensitive-neurons and a transformation of temperature-insensitive-neurons into warm-sensitive ones. This may be related to the assumption that during early ontogeny, body functions react to exogenous and endogenous factors nonspecifically. It is to speculate, that later, probably at the end of embryonic development or during the early postnatal period, the reactivity of these functions changes qualitatively, so that the reaction of an individual function to different factors becomes specific (ultimately adaptive).

Biological rhythms in birds--development, insights and perspectives

The aim of this review is to show that probably the internal clock of precocial birds is imprinted in the prenatal period by exogenous factors (zeitgeber). The activity of organ functions occurs early during embryonic development, before this function is ultimately necessary to ensure the survival of the embryo. Prenatal activation of some functional systems may have a training effect on the postnatal efficiency. The development of physiological control systems is influenced by endogenous and exogenous factors during the late prenatal and early postnatal period: epigenetic adaptation processes play an important role in the development of animals; they have acquired characteristics which are innated but not genetically fixed. As a rule, the actual value during the determination period has a very strong influence on the set-point of the system. This will be explained using the example of thermoregulation. It is shown in detail that it seems to be possible to imprint the prenatal development of circadian rhythms by periodic changes of the light-dark cycle but not by rhythmic influence of acoustic signals. Altogether, there are more questions open than solved concerning the perinatal genesis of circadian rhythms in birds. Topics are given for the future research.

Temperature guardian neurons in the preoptic area of the hypothalamus

Applying the slice method extracellular recordings of 218 hypothalamic neurons in Muscovy ducks during sinusoidal temperature changes were investigated. Seven neurons reacted in a hitherto unknown manner to temperatures very near the physiological limits. Four were exclusively sensitive to temperatures around 36.1 degrees C and three to temperatures around 42.3 degrees C. We recommend to call this kind of neurons temperature guardian neurons. The presented results suggest that the current neuronal model of temperature regulation of vertebrates should be extended by aspects of the two-tier theory of Bligh [J. Bligh, The thermosensitivity of the hypothalamus and thermoregulation in mammals, Biol. Rev. 41 (1966) 317-367].

Ontogeny of thermoregulation during the prenatal period in birds

In contrast to the well-known thermoregulatory processes in the postnatal period the ontogeny of the thermoregulatory effector system as well as the central nervous control elements in the prenatal period is underinvestigated. Because of this, the aim of this paper is to characterize the development of thermoregulatory mechanisms in the embryonic period of one avian species, the Muscovy duck, Cairina moschata. The experiments were carried out in three series. In the first the influence of low ambient temperatures on call producing activity was investigated, in the second the oxygen consumption of single or pooled embryos was measured in the temperature range between 34.0 and 40.5 degrees C and in the last oxygen consumption, colonic temperature and preferred temperature were examined in birds incubated in reduced temperatures (34.5 degrees C) during the last 7 days before hatching. The results of experiments led to the conclusion, that (1) some days before hatching temperature perception is very well developed, (2) endothermic reactions occur in the last quarter of embryonic development, (3) the thermoregulatory setpoint is adjustable during the perinatal period. Altogether, embryos of the Muscovy duck show in the last days of incubation typical endothermic reactions. They activate the heat production as well as the heat loss mechanisms to control the body temperature. Besides this, when vocalization is used, behavioral thermoregulatory mechanisms support the autonomic reactions.

Influence of prenatal and postnatal acclimation on nervous and peripheral thermoregulation

The aim of the present study was to investigate whether prenatal and postnatal adaptation to different ambient temperatures affects the autonomic (heat production, heat loss, rectal or colonic temperature), behavioral (preferred ambient temperature) and nervous mechanisms (neuronal thermosensitivity of the preoptical area of the anterior hypothalamus) of thermoregulation. The experiments were carried out in postnatal, differently acclimated adult rabbits (60 days at 6-7, 20 and 30 degrees C) and adult rats (3 to 6 weeks at 5 and 21 degrees C) and in differently incubated 1- to 10-day-old Muscovy ducklings and turkeys (last week of incubation at 34.5, 37.5 and 38.5 degrees C). The results of the experiments are summarized as follows: (1) Postnatal acclimation changes the threshold ambient temperature of heat loss and heat production. For example, cold-acclimated rabbits have a lower threshold temperature for evaporative heat loss and thermoregulatory heat production than heat-acclimated ones. (2) Prenatal acclimation changes postnatal thermoregulatory behavior as well as autonomic thermoregulatory mechanisms. Birds incubated at higher (38.5 degrees C) or lower (34.5 degrees C) temperatures than the usual 37.5 degrees C for the last week of embryonic development have higher or lower preferred ambient temperatures during the first 10 days post hatching. Besides this, cold-incubated birds have a higher heat/ production and clonic temperature in the first days post hatching than normally/ incubated or heat-incubated ones. (3) Extracellular recordings from hypothalamic neurons in brain slices from differently acclimated rats have shown that adaptation to different ambient temperatures changes firstly the temperature sensitivity of the hypothalamic neurons and secondly the modulatory action of the neuropeptides bombesin and thyrotropin releasing hormone.

Effects of ambient temperature, age and wind speed on the thermal balance of layer-strain fowls

1. In White Leghorn laying hybrids aged 14 to 399 d the effects were measured of ambient temperature (Ta) and wind speed (WS) on heat production (HP), evaporative heat loss (EHL), nonevaporative heat loss with or without considering heat storage (NEHL1 or NEHL2), conductance (c), total insulation (I) and colonic temperature (Tc) as well as on biological optimum temperature (BOT), thermoneutral temperature (TNT) and threshold temperature for evaporative heat loss (Te). 2. All thermal balance variables were affected by age, Ta and WS. 3. The relationships between Ta and HP or NEHL2 were best expressed in terms of a polynomial function at low WS, and in terms of a linear function at medium and high WS; the relationship between Ta and NEHL1 at all WS studied was best described in terms of linear functions, the slope of which increased with decreasing Ta and rising WS; those between Ta, EHL and c were best described in terms of exponential functions and between Ta and I in terms of quadratic functions, the rate of increase of which was higher at low Ta and medium and high WS. 4. The description of relationships between the age of birds and their HP, EHL, NEHL1 and NEHL2 was performed after logarithmic transformation and yielded a linearly decreasing function with advancing age. 5. Threshold temperatures increased as WS increased from 0.2 to 1.2 m/s; for EHL (Te) by up to 7.5 K, for Tc (BOT) and for HP (TNT) by 10 K. 6. The effects of age, Ta and WS on HP and both forms of NEHL were summarised by multiple regression equations.