Thus, in the same flock, there are chicks with a biological age of two days, and consequently a longer fasting period than those hatched in the last hours of the hatching window, whose biological age is 0 days. The difference becomes evident in practice, especially if the length of the hatching window decreases or increases. Different conditions can affect the length of the hatch window, including incubation temperature, egg weight, storage conditions, gaseous environment, as well as the age of the breeders.
The yolk, rich in lipids, is the main source of energy during the development of the embryo, while the albumen is the main source of protein. Given the low level of carbohydrate in the egg, it is very likely that part of the yolk sac albumen provides gluconeogenesis after absorption from the yolk sac membrane. Therefore, protein metabolism via gluconeogenesis appears to be the source of glucose for glycogen accumulation that subsequently fuels egg hatch. However, after hatching, if the chick does not have access to glucose sources for energy generation, reliance on gluconeogenesis involves the consumption of protein resources that would otherwise be used for development and growth. After hatching, the predominant energy source for a chick shifts from yolk lipids to exogenous carbohydrates introduced through the diet, in order to increase glucose levels in blood and consequently gluconeogenesis activity. However, under fasting conditions, when access to external carbohydrate sources is delayed, the increased reliance on fat for energy increases the likelihood of ketosis and dehydration. Therefore, it is necessary to ensure that the chick has access as soon as possible after hatching to the main source of energy, starchy carbohydrates.
Chicks fasted for prolonged periods show an increase in hematocrit level of up to 24%. It is also observed that hemoglobin is higher with decreasing body weight. One possible explanation for these elevated levels is the use of body fluids to cope with dehydration caused by delayed access to food and water. Furthermore, the high plasma corticosterone levels confirm that delayed feed access is, along with many other factors, a significant stressor for the chick. These stress factors will affect the final performance of the broiler, leading to decreased final body weight, higher feed conversion and, in some cases, higher mortality.
Hormonally, chicks that undergo long hours of fasting have lower than normal plasma T3 levels, indicating a lower overall metabolic rate. Thyroid hormones have also been shown to play a crucial role in thermoregulation. Chicks fasted for more than 16 hours and exposed to cold for a short time have a lower body temperature than chicks fed immediately after hatching, and this temperature seems to be maintained during the first week of life. At the same time, fasting and lower body temperature appear to correspond with lower serum T3 levels and higher serum T4 levels; opposite data were observed in chicks fed before cold exposure. These results suggest that the delay in access to feed and water after hatching impairs the thermoregulatory capacities of chicks, which implies the need to regulate the climatic conditions of the environment to preserve the quality of fasting chicks when they reach the farm.
Growth and development
Delayed access to feed and water after hatching causes significant weight loss during the incubation period, mainly due to dehydration and yolk use, but as soon as the chicks begin to feed, weight body begins to increase. Early access to feed improves growth performance, body weight gain, pectoral muscle size and promotes intestinal development.
A meta-analysis study verified the effects of fasting on the productive performance of broilers. Although the chicks were able to compensate for the growth retardation caused by starvation, this compensation is not sufficient to match the development of chicks fed immediately after hatching. In fact, statistically significant differences in the live weight of the chickens at 42 days of age were highlighted between those fed immediately and those that were late in accessing the feed.
On the other hand, in addition to the need to provide feed and water as soon as possible, it is important that the macronutrients in the diet are perfectly balanced to promote efficient growth. The role of proteins in muscle development is fundamental, for this reason the importance of giving chicks a diet containing highly digestible proteins to promote growth from the first hours of life is emphasized. Feeding chicks immediately after hatching is essential for muscle development as glycogen stores are used immediately after hatching, leaving the chick in a state of energy deficit.
The researchers speculate that prolonged fasting may result in less muscle development due to lower satellite cell activity. Poultry show less mitotic activity of satellite cells when they are deprived of food. Although mitotic activity resumes once feed is given, the response is not sufficient to compensate for that of fed chickens, neither with respect to satellite cell activity nor with respect to muscle growth. This is further confirmed by the increase in Pax7 protein expression in fasted chickens which, after a period of stress, appears to be related to low levels of satellite cell mitotic activity.
The physical, morphological and physiological development of the gastrointestinal tract of birds occurs during the first weeks of life. It has been observed that the yolk favors the initial development of the small intestine during the first 48 hours, but the introduction of an exogenous diet just after hatching favors a more rapid development of the gastrointestinal tract. Therefore, the introduction of feed is essential for intestinal development and can affect the ability to digest and absorb nutrients throughout the life of the chicken. Numerous studies record the negative effects that fasting has on the length of the duodenum, jejunum, and ileum. Furthermore, during the first two weeks of life, intestinal villus height appears to be significantly lower in fasted than in fed chicks, while less crypt depth has been found in the jejunum from the first week of life. Although the effects of prolonged fasting on intestinal morphology do not appear to be long term, the incidence and severity of side effects cannot be excluded.