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Pullet to Hen Transition: Why this is the most critical time in female broiler breeder management

Most critical time in female broiler breeder management


February 12, 2008
By F.E. Robinson R.A. Renema and M.J. Zuidhof

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Some people may argue that there is no one “most critical” time period in the life of a female broiler breeder.

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Optimizing Reproductive Efficiency
Managing sexual maturation and increasing feed intake slowly is essential for follicle recruitment and egg production.

Some people may argue that there is no one “most critical” time period in the life of a female broiler breeder. After all, once feed restriction is initiated, every day, the question can be asked “how much feed should my birds get today?” When pullets are young, this question can be answered with a reliable scale and the initiative to obtain accurate body weights of a representative sample of the flock. If the birds are underweight, the feed allocation is increased. If birds are on target, the planned feed increases are acted on.  If birds are overweight, feed increases are delayed until weights are on target again. However, once pullets approach maturity the answer to this question is more complicated and a more detailed understanding of the biology of reproduction comes into the answer. The effects of a mature reproductive system and the hormones produced turn a pullet into a hen. This development also varies between individuals, and this can determine how individual birds partition nutrients between reproduction and growth impacts if they will become a hen who lays well and stays lean, or a hen who lays poorly and becomes obese.  We have come to consider a hen’s “reproductive attitude” as how she lives out her adult life.   With advances in breeder genetics, it is very important that we learn what the ideal body condition is and the effects of changes in lean mass and fat mass have on reproductive performance.

During rearing, excess feed shows up as excess body weight. When feed allocations are grossly excessive, females will deposit fat in the abdominal fat pad depot. In some cases overweight birds may reach sexual maturity earlier than normal weight counterparts. This condition really only is seen when birds are photostimulated early. Once a pullet is “mature” enough to recognize that the day length has increased, life changes dramatically. This maturation really means that the hypothalamus can respond to the increased day length and produce a small hormone (GnRH) which is transported a short distance to the anterior pituitary to bring about the release of gonadotrophins which target the immature follicles in the ovary. Under the influence of gonadotrophins, the small follicles grow into egg yolks and ideally they form a hierarchy. This hierarchy should be uniform and not contain “double or twin” sets of follicles. In an ideal setting, each day a hen would have a mature follicle and would lay an egg at daily intervals and hence, exhibit long periods of consecutive lay days. This is the description of a hen who has matured in a proper way. Unfortunately, often, broiler breeder pullets can enter lay under ideal circumstances and their lifetime performance can be impaired. 

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When pullets are exposed to circulating estrogen for the first time, they are highly sensitive to it. They often appear to be super-sensitized, and can in some cases initiate the production of a vast amount of liver lipid very quickly. In such cases of ad libitum feeding for example, the liver can become very fatty and friable. Excessive follicle development is common, as the birds have less control of the follicle hierarchy. Ad libitum-fed birds can have as many as 12 to 15 large yellow follicles. A high proportion of these follicles are destined to become double-yolked eggs.

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Sometimes two ovulations occur in a single day, but both eggs have poor shell quality.  Excessive follicle development is also associated with decreased fertility and hatchability. Overall, the reproductive efficiency of hens exposed to “fast” aggressive feeding programs under initial estrogen domination, can have very significant negative implications. This phenomenon has implications for the practice of “challenge feeding.”  Challenge feeding may prevent underfeeding, but it may also result in severe reproductive dysfunction if it is carried too far. Some   may feel that the above discussion does not refer to them, as they do not ad libitum feed their birds. However, keep in mind that when feed increases are implemented at the time of photostimulation, feed clean-up times are extended and some aggressively
eating birds come closer to ad libitum feeding than you may consider.

The main effect of feed restriction programs is to limit the number of large follicles on the ovary. “Slow” feeding programs in which feed increases are made daily conservatively can result in a reduced of the number of large follicles.  The University of Alberta research group has carried out several studies to show that hens that are fed following a slow feeding program lay about 10 more eggs than programs that are centred around “feeding the birds into production.”  We have shown that the period from two to four weeks after photostimulation is the most critical time in terms of overfeeding stimulating more follicle development. During this time period a high percentage of these extra follicles are in multiple hierarchies.

It has long been recognized that maintaining pullets on short (eight-hour) days, or on days of decreasing length will keep birds out of egg production. This is only true up to a point in time. Eventually all birds will start laying, even on short days. There is substantial strain variation in how breeder hens respond to light stimulation. One of the main purposes of a lighting program is to hold the birds out of egg production until the day you want them to lay. This is an important fact, as it is one way that a hatching egg producer can control flock uniformity.  This happens as when birds are photostimulated properly (old enough): the birds will all start laying at the same time. When birds are photostimulated too early, only the heaviest birds will come into production, and a very staggered entry into lay will result. Some research trials at the University of Alberta have shown that there is either an advantage (or no disadvantage) to light stimulating at 22 or 23 weeks of age. This is a case when the U.S. industry can differ from the Canadian; as with some strains, overall egg production is decreased with late lighting, but the number of 52g or greater eggs is increased. The uniformity in age at first egg (sexual maturity) is reduced significantly compared to lighting at 20 weeks, for the above-mentioned reasons. Cueing sexual maturity at the proper time also can be important, as some birds may be under-fleshed, and will respond to early light stimulation, but these birds may have a poor peak production or poor persistency of lay. Birds that are photostimulated later show longer
laying sequences, suggesting that they have better carcass composition.  Flocks that enter lay more uniform in age at first egg are likely flocks that are more uniform in terms of body weight during lay.  When you have a flock that is more uniform in body weight, your changes in feed allocation will benefit a greater percentage of the flock (not overfed or underfed). Being able to vary age at photo-stimulation may be one of the most important functions of a lighting program.

It is well established that after birds have reached an age threshold to respond to light at sexual maturity, that heavier birds reach sexual maturity sooner than lower weight birds. The higher the percentage of “ideal” weight birds, the greater is the number of birds that will reach sexual maturity at a normal age.  This is the most serious implication of variability in body weight.  

Heavy birds have a higher maintenance energy requirement than do small birds. When these heavy birds start laying eggs, they also have an energy requirement for sexual development and egg production. Therefore, these birds have significantly higher requirements than do low-weight birds. This sexual maturation is occurring at the time when feed allocation is being increased. This is an advantage for the smaller birds that have small maintenance requirements and minimal sexual maturation occurring.  Taken together with the above-mentioned increases in feed allocation, these small birds can gain considerable weight at this time. It is not uncommon for the smallest birds in the flock to get much fatter than the birds which were heavy during rearing. This results in even greater variability in carcass composition and reproductive condition. 

Variability in rate of sexual maturation can be a major limitation to achieving a high rate of egg production. The reason for this relates to that fact that each hen lays a long “prime sequence” that represents her “personal best” reproductive ability. This period of high reproductive output typically is seen about three weeks after hens starting laying.  When a flock is highly uniform in body weight, they should start laying within a relatively small range of dates. This means that most of them will be experiencing their prime sequence at the same time, resulting in a high egg production peak.  When one level of management is applied to the flock, most of the hens should be optimized. In some regions of the world, very high rates of egg production and hatchability are achieved by sorting birds into “lots” based on body weight. Individual lots of birds are fed differently, to maximize their genetic potential. This practice is labour intensive, but the very high rates of reproductive output seen in
such flocks suggest this practice is effective.

The opposite situation is seen in flocks that are highly variable in body weight. Such flocks show a very staggered entry into lay. The laying of prime sequences is spread over a longer period of time.  It is likely that some late and some early maturing hens will have been overfed or underfed, as the feed was allocated on the basis of the weight of the average weight hens. This results in a lower peak rate of production.

Ideally, we would like all birds in a flock to start laying the same day and exhibit high peak production.  Since management is directed to all birds in a flock, the higher the uniformity, the more likely that we will optimally manage all of the birds. Such a flock would be easy to manage, as all birds in a flock would respond to feed or management changes in a similar manner. Knowing reproductive maturational status is helpful for deciding when to photostimulate a flock.

The most efficient means to limit the consequences of variability in the pullet house is to delay photostimulation. When a non-uniform flock is photostimu-lated early (at less than 22 weeks), birds will enter lay as they reach thresholds for body weight.  This results in poor reproductive output. By delaying photostimulation to 22 or 23 weeks, all birds are held out of production. When photostimulation takes place, the flock will respond in a similar manner, masking differences in condition. 

Most research trials dealing with reproductive efficiency of broiler breeders have focused on mean population performance, rather than evaluating individuals through specific quantitative indicators. In some reports, interrelationships between parameters have been defined through correlation analyses. Because of differences in presumably genetically programmed nutrient partitioning priorities, between-bird variability in both reproductive output and in body weight (BW) gain is an inherent problem in breeder hen management. This variation is often masked when management decisions are based on data from population means. If detailed reproduction and body weight data were collected from individual hens, these data could be summarized and used as selection criteria by the primary breeders. For example, classification of individual hens based on egg production and biological efficiency would link desirable broiler efficiency traits with total egg output. Hens that lay more eggs and gain BW on the same amount of feed would arguably have superior breeding value than hens that lay the same number of eggs, but do not gain weight. In this manner, breeder hens could be selected for efficiency indicators, with benefits that would accrue along the whole broiler supply chain.

To look at how individual birds handled the feed and photostimulation treatments, an individual snapshot of production was compiled for each hen in the study. This was a composite graphical view of feed intake, target and actual BW, BW gains and losses, and daily egg production per bird. Each hen appears to have had a unique balance between the various potential “energy sinks” or terminal uses for energy in their normal activity. While some hens do not maintain a steady state, most find a fairly stable equilibrium between energy supply and the pulls to growth and reproduction. Some hens lay well at the expense of growth – causing them to progressively slip below the average BW of the flock, even losing BW to produce eggs. These “martyr” hens give of themselves to support their progeny. For some, this does not end well. At some point, their bodies can no longer preferentially divert nutrients towards egg production, and they cease laying altogether (“burn-out” hens). Other hens are more conservative with nutrients, and increase in BW relative to the rest of the flock, but do so at the expense of egg production (“tight-fisted” hens). Scattered throughout this group are the “yo-yo” hens. These hens may have repeated stoppages in egg production when growth is interfered with. At this point, they partition nutrients back towards growth. The reproductive control centre of these birds soon gets the message that the nutrient status will allow support of reproduction, and egg production restarts. Though some basic rules apply to nutrient partitioning there are clear differences between individual hens. In every population, sub-groups that did not fit the model were present. One group were “bad” hens that did not lay well and did not grow well. The most valuable hens may be the highly efficient “super” hens that lay eggs prolifically while continuing to grow on the same nutrient intake as not-so-productive neighbours. They don’t burn out despite high rates of egg production and can maintain or even beat the average flock growth rate.

These same reproductive attitudes also exist in commercial flocks. By thinking about what is driving the reproductive effectiveness of different groups of hens within the flock, hopefully we can work towards making better flock management decisions. Breeding companies have put a lot of emphasis on achieving early body weight targets in pullets because of the long-term implications for frame size. While genetic selection programs tend to be similar, it is important to know if there is a wide range of responses to specific management methods, or if our new breeder varieties act similarly. Sensitivity to overfeeding, altered growth profiles, or photostimulation ages are key components of this type of analysis. Identification of differences would underscore the need to move towards strain-specific management.

SUMMARY
Optimizing reproductive efficiency in modern high-yielding females requires attention to detail and skill in managing the metabolism of the bird. It is essential that the particular strain (genotype) is known well, as well as the particular flock is known well. A key point in knowing a flock well is to be familiar with not only body weight, but also the weekly or daily change in body weight (gain). It is established that providing small daily increases in feed allocation (or avoiding sudden large increases in feed intake) can result in gains in body weight following the target recommendations while not eliciting a metabolic response of “storing” excess nutrients in the liver, which ultimately will result in increases follicle recruitment. By making the feed increases as small and frequent as possible, it is more likely to trick the bird’s metabolism and as a result not negatively influence follicle recruitment and hence egg production.  Another key point is managing sexual maturation.  Delaying light stimulation until such time as most of the flock is mature enough to respond to it is an important factor in the uniformity in sexual maturation, and hence peak egg production.  Management of sexual maturation implies that puberty is stimulated by changing the day length (not changing the feed).  Supporting the necessary body weight gains with feed is logical point, while trying to encourage early sexual maturation by adding more feed, is not always reliable, and can often lead to overfeeding and loss of the ovary control processes. Finally, don’t forget that if you consider breeder management to be ovary management, you are managing something that you cannot see. Know the signs to look for and learn how to monitor breeder hen metabolism.

This article was presented at the Poultry Industry Health Conference