
Continuous genetic selection in meat-type chicken for rapid growth resulted in increased meat production and decreased the time required for achieving the market weight. According to Havenstein et al. (2003), 42-day broiler weights have improved about 1% per year, and this trend has continued through to today.
In 1957 a 42-day broiler weighed 540 g with and FCR of 2.35; in 2003 a 42 d broiler weighed 2800 g with an FCR of 1.70; and a recent study at NC State University demonstrated a 42 d broiler can weigh over 3500 g with an FCR of about 1.40. Hence, the incubation and neonatal period of modern commercial broilers can represent less than 50% of a bird’s lifespan, depending on the age it is marketed. Because the chicken embryo undergoes development enclosed in an egg, it depends on the limited egg nutrients, particularly during the late-term embryonic developmental stage. Genetic selection has not only altered feed intake and meat production efficiency after hatch but also influenced gene expression and nutrient metabolism during incubation. The differential between the growth potential of modern broilers and the target body weight of their parent stock to optimize reproductive performance has increased substantially over the past 60 years. Male breeder genetics positively affects the in ovo growth rate and nutrient demand that the female genetics and diet must supply. Therefore, hen diets with insufficient essential nutrients may adversely affect embryonic development and consequently post-hatch performance. Intensity of broiler breeder feed restriction has consequently increased as genetic potential for growth rate and meat production increased so as to maintain reproductive performance; but limiting feed intake of broiler breeders can also influence favorable and unfavorable transgenerational effects. A lesser degree of growth restriction of broiler breeders during the prepubertal and early pubertal growth phase increased male progeny growth rate. This paper discusses the challenges in feeding and managing modern broiler breeders, emphasizing the transgenerational effects that affect the production of high-quality eggs to maximize hatchability and ensure chicks can meet performance targets. Additionally, it explores key nutritional factors in broiler breeders that impact chick quality and offspring performance.
Genetic improvement affects broiler breeder nutrition
The nutritional strategy for breeder hens is crucial, as both excess protein and insufficient energy intake can have adverse effects. Excessive protein can lead to lower fat reserves and poor egg-shell quality, while inadequate energy intake affects the immune system, feathering, and overall reproductive performance. Research has traditionally focused on studying nutrient requirements for maximizing egg production and hatchability in broiler breeders. However, a slight increase in protein intake can positively impact egg size and chick weight, thus influencing broiler chicken growth at later stages. Maintaining optimal egg size, hatchability, and body weight control in breeder hens poses challenges, especially with the use of higher protein diets after 40 weeks of age. Formulating breeder diets with minimum levels of crude protein is common, but careful consideration of digestible amino acids is necessary. Additionally, implementing a two-diet approach after 35 weeks, with lower protein and balanced amino acids in the second phase, is recommended to sustain production parameters and address issues like feed reduction post-peak. Skilled management is crucial to understand the relationship between diet specification and feed allocation, ensuring overall flock health and performance.
Breeder nutrition affects chick quality and performance
The quality of chicks in poultry farming is influenced by a myriad of factors, involving complex interactions. These factors include the physiological status of hens, hen nutrition, diet formulation, farm and hatchery management, transportation, and brooding efficiency. The interplay of these elements is crucial in determining the overall health and performance of breeder flocks, chick quality, and subsequent offspring performance. Poor flock uniformity, early photo-stimulation, improper feed allocation, and various stressors in the broiler house can significantly impact the outcomes.
Chick embryos rely on nutrients stored in the egg for normal growth and development, but the true nutritional requirements of the embryo remain largely unknown. Egg composition, influenced by factors like nutrient allowances, diet fatty acid profile, hen age, health status, storage conditions, vitamin D metabolism, calcium allowances and sources, as well as mineral and vitamin supplementation, plays a crucial role. Flocks initiating egg production without meeting minimal development conditions may produce eggs of lower quality, characterized by lower weight, smaller yolk to albumen ratio, smaller fat content, and thicker shells. Although there are reviews on breeder nutritional factors of affecting chick quality and offspring performance, the applicability of these studies may be considered less relevant with the introduction of modern genotypes and commercial conditions. Overall, a holistic approach considering various incubation conditions is essential in optimizing chick quality and ensuring successful poultry production and meat quality.
a) Dietary protein and energy intake of broiler breeders affect progeny performance
Aitken et al. (1969) first demonstrated the effects of protein and energy concentrations of broiler breeder hens on egg size and offspring performance. It was observed that offspring from parents fed a high-density diet had heavier hatching eggs, and their bodyweights were significantly greater at 42 and 63 days of age. The protein to energy ratio of the breeder diet was found to influence chick weight, with reduced chick size when the energy to protein ratio was low. Spratt and Leeson (1987) further investigated the impact of different concentrations of crude protein and energy on broiler breeders. Male chicks from hens fed high energy had improved early growth, while female chicks did not show the same result. Maternal diet effects on progeny were found to depend on the sex of the offspring, with high energy increasing male offspring carcass protein and decreasing carcass fat. Low-density diets can improve offspring growth and mortality, especially in older breeders. Enting et al. (2007) determined that low- density diets of broiler breeders can improve offspring growth rates, reduce mortality, and affect immune responses depending on breeder age and egg weight. Similarly, Hocking (2006) reported progeny of parents fed low-density diets diluted with oat hulls showed lower drinking behavior, improved litter quality, and differences in egg and chick size. In contrast, Moraes et al. (2014) found that when energy to protein ratio was increased to 18.25 kcal ME/g protein in the diets of young breeder hens, growth and breast-meat yield of progeny females increased. Similarly, van Emous et al. (2015) investigated the influence of dietary protein concentrations during rearing on embryonic and offspring performance. The study supported earlier findings that maternal diet effects on progeny performance depend on the sex of the offspring, and higher growth patterns during the rearing period had positive effects on fertility and offspring performance.
Energy and protein intake of male broiler breeders may also have transgenerational effects on offspring performance. Attia et al. (1993, 1995) observed broiler breeder males with varying daily energy intake levels demonstrated a significant increase in the 6-week bodyweight of their offspring when provided with high-energy diets. The authors proposed this outcome may be linked to the presence of supernumerary sperm in eggs laid by hens inseminated with sperm from males on high-energy diets. Another study reported a reduction in broiler male fertility due to inadequate feed allocation, resulting in a loss of mating activity in males and a subsequent 100 g reduction in the bodyweight of the progeny. As a recommendation, it is advised to ensure breeder males receive sufficient cumulative energy (minimum 29,600 kcal metabolizable energy) and crude protein (minimum 1470 g) until photostimulation for optimal offspring growth.
b) Amino acid nutrition of broiler breeders affects the performance of their progeny
Adequate levels and proper balance of amino acid in the diet of broiler breeders is crucial for optimal egg production, fertility, hatchability, and the health of the offspring. Dietary lysine of breeder-hen can affect progeny outcomes. Mejia et al. (2013) utilized corn-based distiller grains to decrease breeder-hen dietary lysine and found the progeny from young breeders exhibited low bodyweight and breast yield but higher dark-meat yields under the lowest lysine condition (600 mg lysine/bird.day). Ciacciariello & Tyler (2013) also observed a significant correlation between hen digestible lysine and offspring live performance on Day 21 and concluded that changes in hen feed allocation to boost egg production over time could negatively impact offspring live performance. Additionally, another study by Kidd et al. (2005) suggested that breeder-hen dietary L-carnitine influences progeny carcass traits, with hens fed 25 mg L-carnitine/kg from 21 weeks of age showing decreased abdominal fat and increased breast meat in the progeny.
c) Transgenerational effects of dietary vitamins have been observed
The impact of dietary vitamins of breeding poultry and effects on hatchability and progeny health has been extensively discussed in comprehensive reviews by Calini and Sirri (2007) and Oviedo-Rondón et al. (2023). Deficiencies of vitamins in breeder diets have shown to have substantial consequences. Vitamin A deficiency compromises the develop the normal blood system and causes embryonic malposition. Vitamin D3 deficiency causes improper calcification of eggshells, possible symptoms of calcium tetany in young breeders, rickets, stunted chicks with soft bones. Vitamin E deficiency reduces fertility, causes inadequate embryonic vascularization, early embryonic mortality, and exudative diathesis in chicks. Vitamin K deficiency prolongs embryonic blood-clotting time, embryonic hemorrhages, and extra embryonic blood vessels. Riboflavin deficiency increases embryonic mortality rates from 9 to 14 days of incubation, atrophied leg muscles, clubbed down, and curled toes. Vitamin B12 deficiency increases embryonic malposition with head between the legs, short beaks, poor muscle development, and high embryonic mortality rates from 8 to 14 days of incubation. Pyridoxine deficiency reduces hatchability. Biotin deficiency causes perosis, shortened or twisted bones, and excessive early embryonic mortality. Folic acid deficiency causes perosis and twisted toes and high mortality rate during pipping. Pantothenic acid deficiency causes abnormal feathering of chicks, subcutaneous hemorrhages of the embryo, weak hatchlings. Although dietary vitamin deficiencies rarely occur in commercial practice, premix supplementation errors do occasionally occur that result in marginal deficiencies because of low supplementation, imbalances, and excesses, poor quality of the vitamin source, and poor storage and feed manufacturing conditions. Sometimes the less dominant breeders consume less than their estimated feed allotment, and therefore may result in marginal vitamin deficiencies, particularly during peak of lay. Most likely, the progeny will not exhibit classical vitamin deficiency symptoms, but they will not perform at their genetic potential, especially during the challenges of the first 10 days post-hatch.
Among all the vitamins, Vitamin D and E nutrition of broiler breeders have the most significant transgenerational effect. Vitamin D3 status in hens is particularly important for optimal progeny development. Higher maternal dietary concentrations of 2000 to 4000 IU vitamin D3/kg result in improved progeny weight gain and reduced incidence of tibial dyschondroplasia from hens during peak lay, but not after 45 weeks of age. The more bioavailable form, 25-OH-D3, has gained popularity for its positive effects on reducing embryo mortality and enhancing bone ash in progeny. The antioxidant status of broiler breeders and consequential effect on disease prevention in offspring is of increased commercial interest. Vitamin E has been associated with improved adaptive antibody transfer from parent to offspring.
d) Dietary trace minerals of breeders have significant transgenerational effects
Although mineral requirements are well established for egg production of poultry, the trace minerals that have the greatest effect on progeny are selenium (Se), zinc (Zn), manganese (Mn), and perhaps iodine (I). The significance of Se, particularly in its organic form, as an antioxidant co-factor has been extensively studied. Jlali et al. (2013) demonstrated that dietary organic Se improves concentrations in eggs and enhanced the levels in the tissue of progeny. Chicks from hens fed 0.5 mg/kg organic Se exhibited higher tissue concentrations than those from hens fed lower amounts. Moreover, progeny from parents fed seleno-hydroxy-methionine showed a 1.25% improvement in feed conversion ratio (FCR) as compared to offspring from other Se sources. The higher muscle Se content at hatch suggested improved Se reserves, influencing the transition of the antioxidant system during the early days of chicks’ lives.
The role of Zn in chick quality, feathering, progeny growth, and viability has been explored in studies by Turk et al. (1959), Edwards et al. (1959), and Kidd (2003). Higher concentrations of Zn supplements were found to enhance cellular immune function and early survival. When combined with organic Mn, maternal diets with these trace minerals improved progeny livability, immune parameters, and cardiac function. Progeny from hens fed organic Mn and Zn also tended to have improved breast-meat yield compared to those fed inorganic forms. Hocking (2007) suggests that significantly higher maternal dietary concentrations of Se, Zn, and Mn than typically recommended may positively impact immune function and livability when provided in combination. These findings highlight the importance of Se and Zn, especially in organic forms, in influencing the health and development of poultry progeny.
Egg nutrient supplementation by in ovo feeding (IOF)
As discussed above, broiler breeder hen diet and nutritional status can have a significant effect on nutrient deposition in their eggs, especially during peak egg production of nutrient mobilization from the feed-restricted diet and body reserves are constrained. Alleviating these nutrient constraints is possible by IOF as defined by Uni and Ferket (2003). IOF technology supplements into the amnion of oviparous embryos soluble nutrients that play a crucial role in improving various aspects of perinatal metabolism and development. The glycogen stores, utilized as the primary energy source by the embryo, tend to be depleted by the end of the hatching process. IOF addresses this by enhancing glycogen reserves in the liver and muscles, serving as a vital energy source for the hatching process.
Studies conducted over approximately 20 years have delved into the efficacy of IOF with diverse nutrient supplements, including NaCl, sucrose, maltose, dextrin, and disaccharide, β-hydroxy-β-methyl-butyrate, eggwhite protein, and carbohydrate, glycerol and insulin-like growth factor, creatine monohydrate, linoleic acid, γ-aminobutyric acid, threonine, cysteine, arginine, methionine, L-leucine, vitamin E, vitamin B12, folic acid, Bacillus subtilis or raffinose, zinc and copper, manganese, and zinc-methionine, and IOF has become an excellent method of evaluating in ovo nutrition and epigenetic effects. The positive effects extend across a spectrum of factors influencing early growth and development in poultry. Notable improvements include increased body weight at hatch, advanced morphometric development of the intestinal tract, enhanced expression of brush border digestive enzymes (such as sucrase-isomaltase and leucine aminopeptidase) and increased biological activity of these enzymes. Furthermore, there’s enhanced expression of nutrient transporters, SGLT-1, PEPT-1, and NaK ATPase, contributing to improved nutrient absorption. The positive outcomes of IOF extend to various aspects, including increased breast muscle size at hatch, improved bone development, and enhanced immune response. Additionally, the technique has been associated with decreased cellular stress, improved oxidative status, and increased liver glycogen status. This multifaceted approach to supplementation not only influences the immediate post-hatch performance but also affects the development of critical tissues and bone of the neonate by approximately 2 days at the time of hatch. In summary, IOF emerges as a comprehensive strategy with far-reaching benefits for poultry production, encompassing aspects of growth, development, immune response, and overall physiological well-being.
Nutritional affects transgenerational epigenetic responses
The most recent research related to the transgenerational impact of nutrition in poultry focuses on epigenetic mechanisms, which are genomic and metabolomic adaptations to maternal nutrient status and environmental stressors. Dunislawska et al. (2022) presents an excellent review of pre-hatching and post-hatching environmental factors related to epigenetic mechanisms in poultry. These authors present evidence that maternal nutrition and environmental factors may have transgenerational epigenetic effects. Using the quail as a model, Phillips (2020) demonstrated that maternal diets containing increased levels of methyl catalysts (choline, betaine, vitamin B12, folic acid, pyridoxine, and zinc) significantly modified specific DNA methylations at the cytosine residues of cytosine-phosphate-guanine dinucleotides (CpG) under the action of DNA methyltransferases. Other maternal dietary nutrients that can be transferred to the egg that affect methyl metabolism and gene expression include selenium, vitamin D, and vitamin A. Indeed, epigenetic programming is a new avenue of research.
The critical epigenetic reprogramming events occur during germ cell development in adolescent breeding stock, and chromatin remodeling due to events such as demethylation and remethylation of the embryonic genome during early embryogenesis. Increased methylation of CpG and histone acetylation can also occur during the early post- hatch period. Key epigenetic mechanisms include microRNA (miRNA) activity, DNA methylation, and histone modification. Small RNA molecules encoded in the genome, miRNAs, play a crucial role in gene expression and epigenetic response. They bind to the 3’-UTRs end of target gene mRNA, destabilizing it and preventing translation, thereby silencing target genes. DNA methylation involves adding methyl residues to cytosines within CpG islands, inhibiting the transcription of genes from DNA into mRNA. The methylation process is influenced by nutritional components and supplementation, as DNA requires methyl donors and cofactors from the external environment. Histone modification, regulated by enzymes sensitive to endogenous small molecule metabolites, affects transcription and responds to environmental changes. For instance, changes in intestinal microbiota regulate histone methylation and acetylation in host tissues in a diet-dependent manner.
In ovo feeding provides a valuable approach for early embryo support and allows for the assessment of nutrient effects on epigenetic changes in adult birds. A study administering folic acid to the yolk sac of broiler chicken embryos on the 11th day of incubation revealed induced methylation of histones in IL2 and IL4 promoters, with post-hatch effects on histone H3 lysine 4 (H3K4me2) enrichment and loss of histone H3 lysine 9 (H3K9me2) in growing chickens (Li et al., 2016). Conversely, the IL6 promoter showed decreased H3K4me2 and increased H3K9me2. H3K4me2 participates in euchromatin formation and ongoing gene expression; whereas H3K9me2 is a repressive histone mark that negatively regulates transcription by promoting a compact chromatin structure. Thus, folic acid administered with IOF impacts immune functions through epigenetic regulation of immune genes. Another investigation found that in ovo administration of Zn to Zn-deficient chicken eggs reduced embryo mortality and increased hatchability, with organic Zn showing higher efficiency in enhancing methylation and acetylation compared to inorganic Zn. Additionally, in ovo injection of betaine was shown to regulate cholesterol metabolism in chicken livers through epigenetic mechanisms, alleviating effects related to diet and corticosterone exposure, and influencing gene expression and methylation modifications associated with CpG methylation in key genes.
The perinatal period is vital for programming the microbiota to facilitate the colonization of the embryo’s intestines with beneficial bacteria before hatching. Notably, the administration of a single dose of prebiotic or synbiotic suspension to the egg’s air chamber on the 12th day of incubation has enduring effects on the chicken’s lifespan, with significant molecular changes observed in the liver and spleen. In a study by Dunislawska et al. (2020), synbiotics based on Lactobacillus strains were administered on the 12th day of egg incubation, resulting in hypermethylation of the ANGPTL4 gene in the liver. This hypermethylation was associated with a substantial decrease in gene expression, emphasizing the gene’s role in lipid metabolism, insulin sensitivity, and glucose homeostasis. The epigenetic regulation of gene expression through early microbiota stimulation is also dependent on liver miRNA activity, suggesting miRNA is a crucial element in the molecular mechanism of host-microbiota interaction, particularly in the context of gene expression silencing. Maternal nutrition plays a crucial role in shaping the epigenome of future offspring through a process known as the maternal effect, involving non-genetic interference by the mother on the offspring’s phenotype. In poultry production, maternal substances like antibodies, hormones, and antioxidants transferred through the yolk sac impact the immune response and microbiome in young birds.
Conclusion
The continuous genetic selection in meat-type chickens for rapid growth has significantly transformed the poultry industry over the past decades. The evolution in broiler weights and feed efficiency reflects the success of these breeding programs. However, this progress has introduced challenges in feeding and managing modern broiler breeders. The intricate interplay between genetics, nutrition, and environmental factors shapes the quality and performance of chicks. The essay delves into the nuanced realm of broiler breeder nutrition, emphasizing the transgenerational effects that influence egg quality, hatchability, and offspring performance. Furthermore, the exploration of IOF and its impact on early development, along with the emerging field of epigenetics, underscores the complexity of optimizing poultry production for both current and future generations.