Broilers 10

Summary

Intense selection for meat production and efficiency in modern broilers has impaired the reproduction performance of their parent stock. In breeder hens and roosters, fast growth rate and heavy body weight result in low laying rate and poor hatchability under ad libitum feeding. Recent research has highlighted that excessive weight and poor body condition before photostimulation are determinant factors of asynchronous onset of lay and poor laying persistency. Therefore, quantitative feed restriction throughout rearing and lay is necessary to slow down growth rate and achieve optimal reproductive performance in broiler breeders.

Feed restriction during rearing aims to provide enough nutrients for skeletal development and sexual maturation while maintaining healthy body composition (having enough protein content at low-fat accumulation). Optimal body composition at the end of rearing is essential for proper ovary functioning and regulation during lay and to avoid problems associated with excessive fat deposition. However, feed restriction also leads to poor body weight uniformity and chronic stress due to feeding competition and frustrated feeding, respectively. Poor body weight uniformity and chronic stress are linked with reduced reproduction performance in broiler breeders, and finding solutions to these problems can particularly maximize laying rate and persistency in broiler breeder hens. This chapter covers into more detail the need for feed restriction and the rationale behind commercial feed restriction programs for broiler breeders. Then, practical solutions to prevent poor body weight and alleviate chronic feed restriction are discussed to, ultimately, improve the well-being and reproductive performance of broiler breeder under commercial conditions. Concluding remarks and future trends about feed restriction and the reproductive performance of broiler breeders are summarized at the end, including public awareness and welfare concerns about broiler breeder management.

Introduction

The genetic selection of fast-growing broiler chickens is linked with limited reproductive performance and breeding problems when broiler breeders are fed to satiety (Mench, 2002). Broiler breeders share the same genetic potential for a fast growth rate with their progeny (Havenstein et al., 2003), but selection traits that optimize broiler performance have negative consequences on the reproductive performance of modern broiler breeders fed ad libitum (Mench, 2002). Consequently, feed restriction in

conventional broiler breeders is a common on-farm practice used to avoid high mortality, health problems, and poor reproductive performance during lay (D’Eath et al., 2009).

The growth rate of broiler breeder hens and roosters is controlled using feed restriction throughout the entire life cycle. Under commercial practice, feed restriction provides enough nutrients to meet metabolic demands and support skeletal development at a slow growth rate without the negative consequences of overfeeding. Controlled growth rate prevents health and reproductive problems related to high body weight and excessive fat content in conventional broiler breeders fed ad libitum (Mench, 2002). However, chronic feed restriction also raises welfare concerns and production challenges (D’Eath et al., 2009; de Jong and Guémené, 2011).

Need for feed restriction

Intensive genetic selection in pedigree lines and grandparent stock has resulted in broilers with a fast growth rate, capable of achieving their target body weight in less than five weeks. This remarkable growth rate in broilers is attributed to high feed intake and improved feed efficiency (Havenstein et al., 2003). Over time, broilers are becoming more feed efficient, grow faster per unit of time, and achieve a market body weight at a younger age (Tesseraud et al., 2000). This selection for fast growth has dramatically changed the metabolism and behaviour of modern broilers and, therefore, their parent stock (Tesseraud et al., 2000; Bokkers and Koene, 2003; Havenstein et al., 2003).

Broiler breeders are the parent stock of broilers and consequently share same strong predisposition for feeding motivation and feed efficiency as broilers. Excessive feed intake (overfeeding) and the fast growth rate in broiler breeders ultimately lead to metabolic disorders and pathologies related to overweight and obesity. Indeed, excessive body weight is associated with cardiovascular problems, skeletal and metabolic disorders, leg weakness, pododermatitis and hock burn, low immunocompetence, and sudden death syndrome (see review by Julian, 2005). As broiler breeders stay in production for over a year, excessive body weight gain and fat accumulation result in elevated mortality, the aforementioned pathologies, and poor reproductive performance when hens and roosters are fed to satiety (Walzem and Chen, 2014).

Factors associated with poor reproduction performance

Research from the early 2000s concluded that conventional broiler breeders (hens and roosters) need feed restriction to achieve optimal performance during lay due to their propensity for a fast growth rate and to be overweight (Bruggeman et al., 1999; Heck et al., 2004). During rearing, fast growth rate and body composition before the onset of lay are determining factors of subsequent laying rate (onset, peak, and persistency) during rearing. During lay, excessive body weight and elevated fat accumulation are risk factors for poor reproductive performance (Ramachandran, 2014; Walzem and Chen, 2014).

Excessive body weight is ultimately responsible for a low laying rate of hens and

a low fertility rate in roosters (Chen et al., 2006). In hens, being overweight is linked with dysfunctional ovaries that, in the end, result in a poor laying rate. Excessive body fat contributes to a high number of mature yellow follicles, heavier residual ovary, and multiple follicle hierarchies that interfere with optimal follicle regulation. Problems establishing follicle hierarchy can result in ovulation abnormalities, such as higher incidence of double-yolks eggs, follicle dysregulation, poor follicle recruitment, and a low laying rate (Renema et al., 2001a,b). In roosters, high body weight is associated with low fertility and hatchability related to low breeding activity and poor sperm quality (Hocking and Bernard, 2000). Poor sperm quality directly compromises fertility, and low breeding activity in heavier roosters can be explained by the high prevalence and severity of foot problems and lameness. However, the link between excessive body weight in roosters and a low percentage of hatched or fertile eggs is unclear.

Problems related to poor reproductive performance in overweight hens often refers to a low laying rate, but heavy body weight in hens is also a risk factor for low fertility, embryo mortality, and reduced percentage of hatched or fertile eggs (Walzem and Chen, 2014). The causal factors behind this relationship are not well-understood. The negative effect of excess body weight on embryo mortality is hypothesized to be mediated by an indirect effect of hens’ body condition and age on egg composition (van Emous et al., 2015). Hatching eggs with relatively heavier yolk laid by older hens can be at risk for embryo mortality, particularly in male embryos, due to excessive endogenous heat production from lipid oxidation (Arrazola et al., 2019). However, the negative consequences of excessive fat content in breeder hens on embryo development are yet to be determined.

Quantitative feed restriction in broiler breeders

Feed restriction during rearing and lay mitigates the severity and incidence of disorders associated with poor liability, health problems, and impaired reproductive performance. To avoid these problems, a controlled growth rate in broiler breeder pullets and cockerels is designed to achieve the target body weight, optimal body condition, and proper frame size before the development of the reproductive organs (Vignale et al., 2016). Therefore, conventional broiler breeders are quantitatively feed restricted throughout the entire production following slow growth curves.

Quantitative feed restriction refers to limited weekly feed allotment to slow body weight gain and reach age-specific target body weight using standard broiler breeder diets (D’Eath et al., 2009; Zuidhof et al., 2015). Weekly feed allotment for broiler breeders slightly increases with age to support steady body weight gain as feed restriction is not intended to result in body weight loss or growth checks. For this reason, limited calorie intake aims to satisfy the nutritional requirements needed for a healthy growth rate while avoiding the negative consequences of overfeeding and being overweight. In other words, quantitative feed restriction provides enough nutrients to promote skeletal