The continually increasing growth rate of modern broilers allows each new generation to reach market weight approximately half-a-day faster each year.1 Despite changes in the rate of growth of broiler stocks, the target growth profiles used in broiler breeder feed restriction programs have changed little in past 30 years.2 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.
Whereas the poultry breeding companies have worked to maintain or even increase rates of egg production and hatchability, achieving these potential results at the broiler breeder farm level on a consistent has been challenging.3 Production of viable chicks ultimately defines success in a broiler breeder operation. Strategic use of feed ingredients and effective feed delivery contribute heavily to this success. The hen diet can be changed in ways that increase embryo viability, support development of the immune system, and at times even influence broiler yield. As these effects can change with hen age, it is important to understand some of the more influential maternal nutritional effects on the broiler offspring. The nutrient composition of the egg is affected by maternal nutrition, body composition, age and strain. These traits, as well as incubation conditions, can affect chick well-being, growth, and immune function. This paper examines some of the key attributes of maternal nutrition and management that can affect broiler chick quality and growth.
Selecting for Growth Affects Body Composition
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 per cent of their body weight and has been trending downwards.4,5 Apparent reductions in fat content in current stocks are likely a reflection of the increased muscling that has occurred.
How do we grow the bird at an appropriate rate while ensuring the carcass stores are present to support long-term egg production without letting egg size get out of hand? The bird used to be a lot more forgiving. The use of non-traditional feed allocation profiles has shown the large impact of current feeding level on ovarian morphology parameters. Current feeding level can be more important than body weight in its influence on egg production. Thus, there is potential to use feed to manipulate body composition to optimize egg and chick production.
Managing Lifetime Nutrition
By the time sexual maturation begins, managing nutrient intake of the bird is a combination of current feeding level within the context of previous feed allocation decisions. Because current broiler breeder stocks are less able to store fat and grow more muscle when overfed, what the bird consumes today has a much greater impact on productivity than it used to. There is less of a buffering effect from fat stores, and the bird must rely more on protein stores and on dietary nutrients. If the energy needs of the birds have been met today, the right signals proceed between the gut, the brain, and the reproductive organs to maintain a high rate of productivity. When too much is fed, additional nutrients are first shunted towards growth. When not enough is fed, cuts to reproduction now tend to be first on the list.
In previous trials we have noted that at the end of lay (approximately 60 wk of age) there is less fat and ovary mass in birds carrying a higher proportion of breast muscle. However, while examining this relationship more closely in a recent study, we noted that while breast muscle weight was negatively correlated with abdominal fatpad weight (r = -0.735; P < 0.0001), neither were correlated with ovary weight (Renema, unpublished data). In this study comparing various dietary energy:protein ratios, we found that birds were able to shift the balance from skeletal muscle to egg production to some extent. While the hen can use both carcass fat and protein as energy sources, the metabolic priority is to maintain protein, and hens will catabolize their own muscle tissue only as a last resort. A bird with more carcass fat is better equipped to tolerate day-to-day changes in feed availability.
Ekmay et al. (2010) worked with isotope-labeled lysine and found that while early in lay there is a high reliance on skeletal muscle turnover for egg formation, later in lay the reliance on dietary protein increases. In contrast, fat to support yolk formation comes primarily from lipid synthesis early in lay, but shifts to a more even division between lipid synthesis, dietary lipids and tissue fat later in lay.6 Support of the ovary appeared to be more closely tied to dietary energy level during the laying phase, with both ovary and liver weights being higher when a higher energy ration was fed (Renema, unpublished data). A bird with more carcass fat could be better equipped to tolerate day-to-day changes in feed availability.
In the broiler breeder research program at the University of Alberta we have recently confirmed that feeding in the pullet phase has a more long-term effect on productivity than previously thought. Basically, feeding program, feed restriction program, and how we follow the body weight targets in the growing phase all have a greater affect on final carcass composition at the end of egg production than the diets fed during the egg production period have. This is partly because muscle deposition is ‘set’ when they are young and frame size is ‘set’ as soon as the reproductive hormones begin to increase during sexual maturation, and these both have carry-over effect into the breeder phase.
In addition, we have found that the change in energy:protein ratio during the transition between rearing and breeding phase can also affect long-term breeding success. It is possible to hurt long-term egg production and even broiler offspring yield based on choice of pullet and layer diets. Moraes et al. (University of Alberta, unpublished data), reported 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 (Moraes, unpublished data). The bottom line recommendation is not to overfeed protein when transitioning from rearing to lay.
Low protein in the layer ration may affect gene expression related to breast muscle development in the offspring. This is known as an epigenetic effect. Rao et al. (2009) reported that offspring of Langshan breeders fed 10% vs. 15% CP diets had heavier breast muscle by 4 wk of age. Offspring of the 10% CP hens had an up-regulated expression of insulin-like growth factor 1 (IGF-I) and type 1 insulin-like growth factor receptor (IGF-IR) mRNA in the breast muscle. IGF-I is a regulator of bird metabolism and muscle development and increased expression of IGF-I will result in increased breast muscle.8 Our observation that pullet phase nutrition had more influence on broiler offspring than the nutrition during the laying phase (Moraes, unpublished) supports the idea that there may be an
Who Benefits from High Flock Uniformity?
Good body weight uniformity in the pullet flock is one of the ways we can increase the predictability of the response of the pullet flock to both photostimulation and the slightly more aggressive feed changes associated with the sexual maturation period. While not perfect due to the existence of plenty of bird:bird variability in feed intake and growth patterns, uniformity can help to ensure we are over- or under-feeding as few birds as possible as egg production starts and subsequently when post-peak feed reductions are imposed. The bird:bird weight variability can have a behavioural component, with some birds eating more aggressively than others, and an 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.9 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.
What happens to the ovary development and egg production traits of the outlier pullets if their growth profile is allowed to continue in parallel to the target flock body weight curve? To test this we randomly divided pullets from all over the flock body weight distribution onto BW target profiles either at target or 150 g above or below target. For the offspring, the biggest impact of modifying BW targets was with egg size and subsequent chick size. No egg production traits were affected and all broiler trait differences could be explained by the treatment affects on egg size (Renema, unpublished data).
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, we maintained a group of broiler breeder pullets on a common feed allocation, or individually managed birds from 16 wk of age to all be at the target body weight. Body weights of individually managed birds had a very good uniformity (CV=1.9%) from 20 to 60 wk of age compared to the group-fed birds (CV=5.4%). With the larger birds, egg size will be an issue.
Decreasing body weight of heavier pullets from 16 wk to reach the target weight did not significantly affect their egg production. However, a very pronounced effect was found when underweight pullets were forced to the target. These birds produced as much 15 total eggs more than control underweight hens (Figure 1). The problem, for Canadians at least, was that 11 of these 15 eggs were lighter than 52 g – the threshold for incubation. It is clear that improving the body weight profile of underweight birds have the potential to significantly improve broiler breeder productivity.
The increased egg production results for the low efficiency birds fits with hormone profile work of underweight pullets during sexual maturation. In this work, pullets beginning 20% lighter than the flock mean will mature more slowly than standard pullets or 20% heavy pullets unless they are given a 20% boost in their feed allocation. Plasma estradiol-17b concentrations demonstrated that ovary development in the overfed small pullets was proceeding like that of their standard and high weight counterparts.
Feeding the entire flock at a higher level would result in overfeeding in the Standard and High weight birds.10 At some point the practice of sorting small birds into a separate area and feeding them either without competition from larger birds or possibly at a higher level may become cost-effective to consider. From a management perspective, correcting the body weight profile of higher weight birds has no impact on flock productivity while correcting the weight of the underweight pullets did have a positive impact on overall productivity -- provided the mean body weight of the population is under control, i.e. close to the body weight target.
To truly see the impact of a tight uniformity, a treatment like this should be started at a much younger age to eliminate biases that might be introduced by early growth profile. Careful attention to feeder space and even initiating a sorting program during the pullet phase can help generate a group of birds with uniform BW going into the breeder house. With females maturing within a shorter age range today, there may be fewer issues with male intimidation of females that are not yet receptive to mating. This can contribute to a more stable, long-term sexual behavior in the flock.
A flock that has high body weight uniformity values coming into lay may not continue this way. 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. However, other groups exist within the population that can both gain weight and produce large numbers of eggs, or do the opposite (Renema and Zuidhof, unpublished data). As a result, the average weight birds at the end of lay include the best layers of the most energetically efficient birds (lost weight), the worst layers of the least energetically efficient birds (gained weight), and the average layers of the average efficiency birds (remained average weight throughout). As a result of this variability, later in the egg production period it is much easier to interpret the relationship between male size, appearance and reproductive effectiveness than it is for the females.
How has Genetic Change Impacted Flock Management?
Egg Size: Genetic selection programs in table egg stocks compared to broiler stocks have affected reproductive traits differently. In laying hens, earlier maturation and higher rates of lay have led to potential skeletal issues due to the challenge of maintaining support for shell formation. While increasing egg size with age is an issue in both laying and broiler breeder stocks, in table egg production this is much easier to manage using nutritional tools. Unfortunately in broiler breeders, once you move beyond methionine and start reducing various combinations of choline, folic acid, and vitamin B12 that can work well in laying hens), you are reducing micro-ingredients essential for broiler hatchability.11
A general uneasiness to commit to a defined post-peak feed withdrawal program in broiler breeder flocks could be largely responsible for current issues with large egg size in older broiler breeder flocks. Issues with late egg weight within the breeding companies may not be the same as what is faced on commercial farms. Under conditions of overfeeding, egg weight was much more responsive in commercial strain crosses than in pure lines (Figure 2).
Figure 2. Egg weight of pure lines (1 to 4) or of commercial and experimental strain crosses (5 to 8) fed a standard ration (R) or overfed 20% from placement in the layer barn (OF)
The egg can be affected very quickly by fluctuations in feed intake. There is a short term effect of changes to feeding level on egg size, for example. The albumen content reacts to changes in energy intake immediately, while yolk size is slower to respond. Unfortunately, the yolk tends to only trend upwards in size. A reduction in rate of lay means the hen has more yolk material available to spread across fewer yolks, thereby increasing egg size. As a result, the most effective approach to controlling egg size is still to maintain as high as possible a rate of lay later in production.
In contrast to table egg laying hens, broiler breeder hens lay at a lower rate and have a higher body mass – both of which contribute to less stress on calcium supplied by the diet or skeleton. The shell quality issues that have appeared in some flocks after 40 to 45 wk of age can typically be easily remedied by the supply of some large particle calcium. There may be a feed formulation or diet density trigger in flocks where shell issues appear. We have recently begun to see examples of shell quality issues confined to specific feeding treatments with no obvious reason for the shell quality differences among groups.
Can feed restriction be relaxed and birds allowed a less restrictive growth profile? In a comparison of a range of both pure lines and commercial lines, providing 20% extra feed reduced productivity and shell quality (Table 1). On average, egg production was reduced by 12.5 eggs (8.3%) under these conditions. This is in contrast to underfed birds, which we have shown will cease egg production all together with just a 9% drop in feed allocation (86% vs. 63% of birds still in production at 65 wk in Control and -9% groups) (Renema, unpublished). In time of energetic stress, reproduction is one of the first things the bird will sacrifice – instead diverting nutrients to maintenance and survival.
A flock can transition from being on the target body weight profile to overweight over just a few weeks time – often as the birds reach peak production and ‘overshoot’ the weight targets. As the birds are transitioned from feed increases during sexual maturation to post-peak feed decreases, they grow more energetically efficient. This same phenomenon occurs during the transition onto feed restriction from full feeding in the first few weeks after breeder chick placement. As these hens are able to utilize the feed more efficiently in the short term, the initial feed withdrawals may not be as effective as hoped, leading to the hens getting too heavy.
In warm environments, overweight birds can be the result of not compensating for the higher barn temperature with a lower feed allocation. As long as the feed is formulated to ensure adequate supply of the micro-ingredients on a daily basis, focusing on a body weight target rather than a feeding program can help ensure body weight does not become excessive.
Lighting: The majority of research on daylength and light intensity has occurred in laying hens. At current commercial light intensity levels, we have not been able to demonstrate any significant effects on reproductive traits. Concerns with high light intensity in broiler breeder barns has so far proven to be of little consequence. However, the results we have seen demonstrate that ovary development is affected in extreme cases (particularly low light intensity), demonstrating that these effects should continue to be monitored.
New LED lighting systems have the potential to be produced with very specific blends of light wavelengths. New lights are being produced that have claims of encouraging more efficient growth, for example. This is presumably achieved in part through behavioral modification, as evidenced by anecdotal reports of ‘calmer flocks’. Some red light will always be necessary to support reproduction since these wavelength have the greatest ability to penetrate through the feathers and skull to the light-sensitive neurons associated with gonadotrophin producing neurons. Too much red light has anecdotally been shown to cause undesirable behaviour aviary-housed laying hens, demonstrating it is important to work with companies familiar with how their products have been tested in agricultural environments.
Fertility: Assessing flock fertility comes down to one main theme – if you don’t have mating, you won’t get fertile eggs. A good female flock can come out just average for chick production if the males have been ineffectively managed. While there are some nutritional components to male fertility (antioxidants and minerals like Zinc, Choline and Selenium that contribute to both sperm production and sperm survival in the female reproductive tract), reproductive behavior of the flock must be managed appropriately to maintain long-term flock fertility.
Heavy birds are an issue, as it can impact physical traits such as footpad condition and cause pain. If the male is sore, the last thing it wants to do is mate, and if it is mating it will be much less successful at it. Rapid declines in flock fertility are often due to insufficient bodyweight control. Hocking et al. (2002) reported that feed restricted and overfed hens have similar fertility when provided a similar semen source, but overfed hens have a reduced hatchability due to an increase in late embryonic death. Duration of fertility (measured by monitoring fertility in consecutive eggs) is also reduced under conditions of overfeeding.13 Nutritionally, too much protein is bad for yolk membrane strength and embryo survival. Underfeeding hens, while being potentially detrimental to rate of lay, does not appear to hurt fertility or hatchability.
Many aspects of mating and dominance behavior cross the boundaries of breed. We can learn a lot from table egg laying hens reproduction and even from wild poultry species. Female preferences for dominant males can be problematic in flocks with heavy males. 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. The behaviour of these birds suggests they think it was a completed mating 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. Flock fertility results don’t show which males are working and which ones are lame, too big, or just sore enough in the feet and leg joints to not want to bother to mate. 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.
The broiler breeder of tomorrow will require a higher degree of precision in its feeding. Increasing vigilance is needed in the areas of feed composition and maintaining consistent body weight gains through careful decisions about how much and how often to change feed allocations. Extra attention to detail can make it possible to change body weight targets, but make sure the intended consequences actually do occur rather than negative unintended consequences. Effective management of these flocks needs to ensure managers are able to deliver the right nutrition to the bird WHEN they need it. Using this approach can enhance late egg production, control egg size and contribute to improved embryo survival and even broiler yield traits. The ability to think of daily nutritional decisions in a broiler breeder operation within the context of the entire life history of the flock will become a more important aspect of broiler breeder management and feeding.
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