The recognition in the late 1960’s that excessive growth rates in broiler breeders were negatively impacting rate of lay and increasing production of unsettable eggs (Jaap and Muir, 1968) has been one of the most transformative events in the history of the hatching egg industry. The resultant feed restriction programs during rearing resolved the issue at the time. But continued genetic progress resulted in feed restriction being necessary throughout growth and production. Nutrition and management issues have evolved as genetic growth potential has increased, meaning that the monitoring and management of feed restriction programs have become much more complex.
The increasing degree of feed restriction required to keep broiler breeder pullets and cockerels on the appropriate growth curves have led to sex-separate feeding systems and technological advances in feeding equipment. In the late 1980’s, numerous research projects were still comparing ad libitum fed birds to restricted fed birds as a way to characterize the benefits of restricted feeding. Then, in the 1990’s there was increased focus on issues related to the onset of egg production. This was a mix of learning about the appropriate body condition and age to commence lay and recognition that slightly aggressive feeding during this period was triggering long-term issues with maintenance of lay (Renema and Robinson, 2004; Eitan and Soller, 2009). Today there is much more research focus on possible ways to relax feed restriction to increase feed intake without hurting egg production and hatchability.
Genetic progress for growth and feed efficiency has reduced fat deposition (Renema and Robinson, 2004). With birds more predisposed to support protein deposition than lipid deposition, the fat depots the birds used to be able to utilize during inadvertent nutrient shortages early in lay are lacking, and young hens must instead rely on the quality of the flock manager and feed delivery system to meet their daily nutrient requirements. This has led to an increase in research on delivering the appropriate nutrients to support key developmental events and daily activities. Issues with overfeeding today are generally due to complications of over-fleshing than to reproductive disorders. Emerging with this is an increase in on-farm issues with energy deficiency, particularly during the transition from pullet to hen. Recent research has shown clear links between nutrient delivery in the pullet phase and long-term reproductive success, regardless of what is fed during the laying period (van Emous et al., 2015a) and that feeding decisions in the pullet phase can even impact offspring yield traits (Moraes et al., 2014; van Emous et al., 2015b). The breeding companies have worked to maintain or even increase rates of egg production and hatchability (Laughlin, 2009), but achieving these potential results at the broiler breeder farm level on a consistent basis has been challenging. While these modern flocks have the potential to produce as many chicks as in past years, they are much more likely to falter due to feeding and management errors.
II. FINE-TUNING BROILER BREEDER NUTRIENT DELIVERY
Broiler breeder management has grown more complex as broiler growth efficiency has increased. However, despite the large changes in genetic growth potential of modern broiler breeders, body weight targets have remained relatively constant (Renema et al., 2007a). From a practical standpoint, maintaining this level of feed restriction is similar to needing more and more effort to keep a spring compressed. As the growth potential of broilers continues to increase, the degree of feed restriction required to manage parent stock body weight gains has created a more competitive feeding environment. From the perspective of parent stock managers, modern broiler strains are simply too good at depositing breast muscle. With a propensity to deposit muscle rather than fat, there may not be enough energy stored in the body to mobilize in times of energetic shortage, and as a result, broiler breeder hens may have difficulty with early chick quality and long-term maintenance of lay. Carcass fat in feed restricted birds at sexual maturity averages between 12.5 and 15% of body weight (Renema et al., 2007a, Yu et al., 1992) and has been trending downwards. Apparent reductions in fat content in current stocks are likely a reflection of the increased muscling that has occurred.
While the negative consequences of ad libitum feeding on bird health and welfare are clear (Renema and Robinson, 2004), there are welfare concerns about satiety of feed restricted birds during the rearing phase in particular. This has led to numerous studies on low-density rations, diet dilution, and use of various fiber sources to help the bird feel fuller. However, typical results describe very little difference in feed clean-up time, little effect on body weight uniformity and infrequent significant differences in welfare indicators (Renema et al., 2007a). There is a need for research on feeding preferences as it related to food quantity and quality (D’Eath, 2009) as it relates to satiety.
Bowling et al. (2018) examined the effect of feed restriction on stress and growth of the broiler offspring. They reported that a higher degree of feed restriction increased plasma corticosterone, which appeared to reduce growth of male broiler offspring between 35 and 42 days of age, while a lower degree of feed restriction increased immune response indicators of female offspring. However, these breeder hens were held at specific target weights rather than under conditions similar to what would occur in commercial barns, so care must be taken in applying these results.
Demonstration of effects of what the hen was fed during rearing on broiler offspring traits is a relatively new phenomenon. Van Emous et al. (2015b) reported that hens fed on a higher growth curve during rearing produced broiler offspring early in lay that were heavier at 34 days of age. The 200 g higher growth target at 20 wk of age also resulted in higher fertility and reduced embryonic mortality than hens reared on the standard body weight curve. This group also reported that feeding pullets on a low protein ration during rearing impacted body composition of birds going into production (van Emous et al., 2015a). At 22 wk of age, breast muscle of these pullets was 4% lighter and abdominal fatpad was 97% heavier than that of birds on the high protein ration. This shift in composition led to hens reared on the low protein ration having increased hatchability during the first of three phases of lay and increased egg production during the second of three phases of lay.
Moraes et al. (2014) studied birds reared on one of two levels of dietary protein and one of three levels of dietary energy, followed by rearing diets with one of two energy levels. They reported that female offspring of 29 wk old breeder hens reared on a lower protein ration were lighter between 22 and 36 days of age. Their high energy pullet ration resulted in fatter hens and fatter broiler progeny. It was noted that if the energy:protein ratio decreased between the rearing and breeding phases, broiler offspring yield was negatively affected. As an example, moving from a higher energy ration in the rearing period to a lower energy ration during the breeder period, which results in a drop in the energy to protein ratio, also hurts broiler offspring breast muscle yield and overall carcass yield by approximately 1% (19.8% vs. 20.9% breast muscle) when compared to treatments where the energy:protein ratio remained the same or increased between the rearing and breeder diets. Weekly pullet growth was more influenced by feed energy than by feed protein. Because weekly body weight gains across treatments correlated much more closely with feed energy, there was a broad range of protein intakes across treatments. This suggests that breeder pullets could be at risk for overfeeding protein if feed mill ration specs include a crude protein buffer to ensure minimum target specs are being met. A test of broiler breeder rations collected from commercial Alberta hatching egg farms demonstrated that the actual crude protein content of these 15% CP diets ranged from 15.8 to 19% (Carney and Renema, unpublished data). Feeding excess protein to birds very adept at growing muscle is a management concern to consider when assessing production problems on commercial hatching egg farms.
To get at the core issues of what causes a broiler breeder hen to allocate nutrients to support growth compared to egg production, there needs to be work with individual birds. This removes feed competition from the assessment. Flock body weight uniformity peaks near the time birds are photostimulated. But from this point, individual body weight will be impacted initially by how long the bird takes to enter lay and subsequently by rate of lay. How much weight a bird gains or loses during lay is impacted by the balance between their energetic efficiency and rate of lay. Within a hen population some hens lose weight in time – often as a result of a high rate of lay, while some gain weight due to a poor rate of lay. But in addition to this, individually-fed birds on the same feed allocation will include a portion of the population that will gain weight while maintaining a high rate of lay, and a portion that will lose weight despite having a low rate of lay (Renema and Zuidhof, unpublished data). Bird:bird weight variability can have a behavioural component, with some birds eating more aggressively than others, as well as energetic efficiency component. Small birds in particular are often found to be less energetically efficient. Less efficient hens have a higher regulatory thermogenesis, resulting in the loss of more energy as heat (Gabarrou et al., 1998). If these less efficient birds also get behind in body weight compared to their flock-mates, they will often also mature later, and with less robust ovarian development than their larger flock-mates.
Maintaining individually-caged birds on non-traditional feed allocation profiles has demonstrated that recent feed allocations can have a larger impact on ovarian morphology parameters than current body weight does (Renema et al., 2007b; Zuidhof et al., 2007). For example, small pullets fed aggressively through the sexual maturation process will lay eggs approximately 2 g heavier than those of large birds fed sparingly through this period. This difference in egg weight is maintained throughout the production period. Thus, there is potential to use modified feeding levels either in groups or in individual birds to manipulate body composition to optimize egg and chick production. The practice of sorting pullets into size groups and re-sorting at set intervals during rearing is an industry practice in some countries that is rooted in this principle at a more moderate scale that puts this into practice.
III. ADJUSTING GROWTH TRAJECTORIES
A common assumption regarding flock body weight management is that productivity will be maximized if body weight uniformity is high – with the ideal case being that all birds had the exact same body weight. To test this, Romero et al. (2009) maintained a group of broiler breeder pullets on either a common feed allocation, or on customized feed allocations for individual birds. Allocation treatments began at 16 wk of age and with birds approaching a common body weight target at 20 wk of age. Body weights of individually managed birds had a very good uniformity (CV=1.9%) between 20 and 60 wk of age compared to the group-fed birds (CV=5.4%).
Romero et al. (2009) reported that reducing body weight variability did not impact ovary weight or follicle numbers at sexual maturity. Furthermore, decreasing body weight of heavier pullets from 16 wk to reach the target weight did not significantly affect their egg production. Care was taken to ensure no birds lost weight during the 4 wk adjustment to individual growth curves. In contrast, however, a very pronounced effect was found when underweight pullets were forced up to the common body weight target. This group produced 14 more eggs in total compared to their low-weight counterparts in the group-fed control treatment. At issue, however, was that 91% of these additional eggs were < 52 g. While likely viable, they are below the lower weight limit accepted by Canadian hatcheries. It is clear that improving the body weight profile of underweight birds has the potential to significantly improve broiler breeder productivity. However, this needs to start earlier in life than 16 wk of age. Breeder recommendations for sorting small birds into a separate pen are to do this closer to 10 wk of age. Companies practicing routine sorting of pullets may start younger than this and resort the flock at approximately 4 wk intervals. Romero et al. (2009) suggested that hens may have different optimal body weights for support of egg production and that forcing them to a common body weight target may have provided insufficient nutrients to hens with more breast muscle or higher rates and energetic demands for maintenance.
Zuidhof followed this work with experiments using an automated precision feeding system from as early as 2 wk of age. By creating conditions of high body weight uniformity as early as possible, this group has been able to assess the possibility of photostimulating birds early (18 vs. 21 wk) (van der Klein et al., 2018) The 18 wk photostimulation age resulted in birds coming into lay later than birds photostimulated at 21 wk of age (182.8 vs. 173.5 d of age). Furthermore, egg production to 55 wk of age for the standard weight birds was 93 compared to 129 for birds maintained on a 22% heavier growth curve (van der Klein et al., 2018). While the authors conclude that current breeder recommended body weight targets may be too low, the egg production profile and carcass traits of their standard control treatment are very indicative of a flock that has been underfed. There appear to be some unintended negative effects of forcing birds onto exactly the same body weight curve. Because previous work with provision of extra feed to underweight birds has yielded positive results, it is surprising that now the standard weight birds have done poorly. It may be that these researchers ‘corrected’ the growth trajectories too early and that results would have been better if the adjustment to the growth profile had happened closer to when the ovary becomes sensitive to nutrient intake at approximately 10 wk of age. It may also be that birds have a range of normal optimal body weight profiles and that forcing all birds to the same body weight causes unintended damage to their ability to support establishment of a body composition and nutrient allocation system that is conducive to the sustained support of egg production.
Zuidhof’s group performed a subsequent study with the precision feeding system where feed restriction was relaxed on one of 10 modified target curves varying by increments of 2.5% (to a maximum of 22.5% higher body weight target) to evaluate if this would impact feedseeking behavior (Zukiwsky et al., 2020). In contrast to their hypothesis, feeding up to 22.5% above the standard body weight target did not reduce feeding or feed seeking behavior, and egg production traits were not affected. An impact on egg production traits would have been unexpected considering previous work with birds fed slightly above the target body weight being very tolerant of these conditions. At issue may be the nature of how birds are fed. Rather than in a single feeding, birds are fed small amounts throughout the day. Birds fed in this way will tend to be very lean. With minimal fat stores going into production, these birds may be at a disadvantage when it comes to tolerating any kind of rapid change in energetic requirements associated with ovary development and early lay.
IV. FEED RESTRICTION PROGRAMS TO MAXIMIZE FAT DEPOSITION
How can broiler breeders be grown at an appropriate rate while ensuring carcass stores are present to support long-term egg production traits? With broilers, eggs and chicks are managed with the goal of optimizing breast muscle development and limiting fat deposition. But with broiler breeders we would like the reverse of this. In addition to this there are concerns about the welfare implications of broiler breeder feeding programs during rearing, as demonstrated by high feed seeking behaviour related to a lack of satiety (D’Eath et al., 2009). While some countries have banned non-daily feeding schedules, they are common in North America.
Birds fed every day get just enough nutrients to support target growth. With modern broiler breeders prioritizing protein deposition over fat storage, birds on this treatment would be expected to have the largest breast muscle weight and lowest abdominal fatpad (a key indicator of carcass fat stores). Zuidhof et al. (2015) reported the highest abdominal fatpad weight and lowest breast muscle weight in birds fed on a skip-a-day program. However, these birds also had the lowest ovary weight, large yellow follicle number and weight. Growth hormone will dampen the stimulatory effect of estrogen on the yolk lipid biosynthesis pathway (Walzem, 1996), so this may simply be the result of a redirection of available lipids into storage.
It is suggested that birds on a skip-a-day feeding regimen are metabolically less efficient because of the energetic cost of cycling between nutrient storage and mobilization (de Beer and Coon, 2007). As a result, skip-a-day fed birds will compromise breast muscle growth and divert more energy to storage in the abdominal fat pad. de Beer et al. (2007) reported a 5-fold increase in glycogen and total lipid levels in skip-a-day birds 24 h after refeeding. During fasting, fatty acids are released from adipose tissue, glucose is released from liver glycogen stores, and further energy may be provided through catabolism of muscle protein through gluconeogenesis (de Beer and Coon, 2007; de Beer et al., 2007).
Arrazola et al. (2019) studied the welfare and growth of broiler breeder pullets reared using every day feeding, a diet diluted with oat hulls and calcium propionate, a 4:3 system (4 days on, 3 days off), or a graduated diet where feeding frequency varied with age. They reported that birds in all of the alternatives to every day feeding demonstrated decreased feeding motivation and lower stress. Birds in the every day feeding group also had more feather fault bars (indicator of stress response) and worse feather coverage. Birds on the 4:3 schedule received the highest daily feed allocations which possibly allowed increased lipid storage, thereby allowing them to habituate for off feed days better than the birds on the graduated diet.
V. MANAGING MALE FEEDING
Broiler breeder males have a low nutrient requirement to meet their growth targets because they are so optimized for growth. This continuous increase in broiler breeder growth potential has elevated the importance of sex-separate feeding as a way to limit male protein intake with a low-protein ration to limit breast muscle deposition and support male fertility. Unless a producer has a reliable supply of spiking males to replace poor-performing males with, extending the duration of the period males are contributing to flock fertility is essential.
Ultimately, male reproductive success results from a mixture of social status and semen production. The job of the manager is to maintain an environment that supports slow, steady growth to maintain testicular condition. This is exceptionally difficult as birds enter the breeder house. Rising testosterone has made them much more competitive, they may be stealing feed from hen feeders, they have a big spike in energy requirements as they learn effective mating behaviour, and they are put into a new environment where there are a limited number of feeders available to them.
Feeder space per bird is a simple and effective management tool available to maximize uniformity of feed consumption by the males in rearing. The goal is to provide enough space that males are not being excluded, while at the same time not having surplus space so the larger and/or more aggressive males have the option of moving to multiple spots to eat before the feed is gone. The heavier males are also the birds with the largest testes and strongest sexual drive. But after peak, male size can start to become an issue due to over-fleshing and development of dexterity issues (Renema and Robinson, 2004). This means that the males that are most valuable to us in a younger breeder flock may end up being the most problematic later in life, when their excessive fleshing is interfering with successful cloacal contact.
While excess weight in male broiler breeders may impact flock fertility for mechanical and behavioral reasons, insufficient weight or nutrient can impact fertility for physiological and behavioral reasons. Cerolini et al. (1995) found evidence that fertility can also be decreased in males due to ME deficiency and that problems would happen more often at the end of the production period. Under conditions of artificial insemination to remove mating behavior effects, they fed Ross broiler breeder males with a standard diet (12% CP and approx. 2,746 kcal ME/kg) from 23 to 54 wk of age with 110, 120, 130 g/bird/d or ad libitum. Body weight ranged by 200 grams among treatments and fertility was increased for males fed increasing amounts of feed (59%, 72%, 79.2% and 79.2% for males fed 110, 120, 130 g/bird/day or ad libitum, respectively. Buckner et al. (1986) found that limiting feed intake decreased body weight, semen volume, number of spermatozoa in the ejaculate, testicle weight, hematocrit and the percentage of males producing semen. They concluded that 113 g/bird/d of their 13.1% CP and 3,167 kcal ME/kg ration was the minimum feed allocation that did not negatively affect the reproductive traits. While recommended composition of modern male diets has changed to include less protein than this, the need to monitor our flocks for overfeeding or underfeeding is still very relevant today. This is all much easier to do when flock body weights are uniform.
In 1987, Wilson et al. demonstrated that feeding caged breeder males 12, 14, 16, or 18% protein isocaloric diets did not affect bird weight, testes weight, or semen quality. However, more males on the 12 or 14% protein diets were able to produce more semen. More recently, Romero-Sanchez et al. (2007) compared 12% and 17% CP male rearing diets fed in a concave or sigmoid pattern to 26 wk of age. The 17 % CP diet increased body weight, but this was limited to the 8 to 32 wk of age period. More importantly, the 12% CP rearing diet improved both weekly and cumulative fertility. The concave feeding program had larger feed increases toward the end of the rearing period. Like the 17% CP diet, this excess nutrition as birds were coming into production led to males that were unable to sustain fertility after 40 wk of age without additional increases in feed allocation. This is a similar effect to what used to happen with overfed hens. The flock would come into lay strongly, there would often be a high peak rate of egg production, and then after 40 to 45 wk of age production would tail off as hens were unable to sustain egg production. Current male stocks appear to still be very responsive to excess nutrition. With their propensity to lay down muscle, this can quickly become an issue as it starts to interfere with fertility due to incomplete matings.
VI. UNDERSTANDING FLOCK MATING DYNAMICS
There has been very little work done in broiler breeders on mating behaviour and how it can impact flock fertility. Modern broiler stocks have been selected for a shorter, wider-legged stance to support rapid broiler growth. In the breeder, shifts in body conformation have the potential to affect how well the male and female are able to make sexual contact during the act of mating in heavy flocks (McGary et al., 2003). The behaviour of these birds suggests they think a complete mating has happened, when no semen transfer occurred. As this likely affects mostly older, heavily muscled males, this could become a criterion for male culling. Unlike underweight males who may express less sexual behavior due to decreased testicular mass and testosterone production, these large males are often still perfectly functional, and only serve to disrupt mating activity of subordinate males. Managing flock fertility requires spending time observing flock mating activity and assessing all males for potential culling. The best males in the younger flock could be the ones causing the most trouble in the older flock if they are not able to complete matings.
Specific recommendations on aligning male and female sexual maturation, modern flock sex-ratios that optimize fertility while reducing issues with aggressive males, use of a male hospital or holding pen in the breeder house, and implementation of a fully separated male feeding area are all examples of current recommendations have been derived from practical field research. Shifts in practice can come quickly due to regulatory changes or from sharing of innovative ideas. For example, biosecurity concerns have limited access to spiking males in some countries. Development of male holding/rest pens at the end of the barn and intra-spiking programs among barns are two techniques that have become much more common. Mphepya et al. (2019) tested exchange of 25, 35, or 45% of males at both 40 and 48 wk of age in a double intra-spiking program. Male competition and flock fertility were increased the most with the 45% exchange of males, with this being the only treatment to return the flock to the breeding company standard. This is a good example of establishment of field practices that are later corroborated in a formal research study.
Understanding the dynamics of rooster and hen sexual behaviour has been enhanced by previous work performed with laying hen stocks on choice testing and male courtship behaviour and, more recently, with in-depth research in wild populations of Red Jungle Fowl. Pizzari et al. (2002) provides an early summary of this ongoing research program. Their work on male:female behavioural interactions during mating has helped increase the understanding of the role of the hen in the acceptance of males and in what attracts roosters to hens. This adds depth to the understanding of normal breeding behaviour. The elements of attraction and rejection among breeding chickens adds a fascinating layer of complexity to flock management. They describe the impact of social status and on male strategies to inseminate sperm of higher or lower quality, as well as female strategies to eject semen or to attract interference to a mating from a male perceived as less desirable. While only part of this may translate to a commercial broiler breeder barn setting, it clearly demonstrates that controlling weight is just one piece of the intricate puzzle leading to the successful production of viable chicks.
In conclusion, as broiler breeders continue to change due to the impact of genetic selection for improved growth efficiency and meat yield, there is value in understanding how our management priorities have changed along with the bird. Both male and female broiler breeders need a positive growth profile in order to maintain reproductive effectiveness, and are now specialized to the point that they are less able to tolerate nutrient deficiencies when demands for growth and reproduction are high. Success in a broiler breeder operation is measured by the production of viable chicks. After providing the best possible conditions, the managers must leave it up to the bird:bird interactions to sort out the intricacies of dominance and mating behaviours.
Presented at the 32th Annual Australian Poultry Science Symposium 2021. For information on the next edition, click here.