Do We Really Know What a ‘Normal’ Broiler Breeder Looks Like?

Do We Really Know What a ‘Normal’ Broiler Breeder Looks Like?
          By Drs. Rob Renema, Frank Robinson, and Martin Zuidhof

Where Did ‘Broiler Breeders’ Come From?

The modern breeder female is a genetic compromise between two very different selection criteria. This parent must have the genetics for rapid and efficient growth, and yet exhibit a high rate of egg production to supply the next generation of broiler chicks.
Decades of genetic selection for meat production traits have impaired the reproductive ability of both broiler parents, resulting in a negative relationship between growth and reproductive fitness. As the modern broiler breeder continues 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.

Studies concerning the growth rate of poultry began to appear in the 1930’s, when the economic importance of genetic purity and ornamental breeds was shifting to the development of breeds with both meat and egg production potential. By the mid-1950’s heavy selection for growth rate and efficiency was established. Until this time, meat birds consumed as broilers are today would have consisted of surplus males from flocks being housed for egg production, spent fowl and birds from backyard flocks.

Backyard flocks would generally have used ‘dual-purpose’ breeds, which had an adequate rate of lay but also had enough fleshing to make a good meal when needed. These flocks would not get larger than 200-300 birds due to problems controlling diseases like coccidiosis.

But with the introduction of high-energy feeds and linear programming for diet formulation (done by hand), the door was opened for the development of a specialized broiler industry.

Following less than 20 years later, the negative impact of selection for growth on the reproductive capabilities of the broiler parent-stock was becoming apparent. A 1972 study compared high and low juvenile body weight lines, and reported that the high weight lines produced more eggs than the low weight lines, while settable egg production was reduced. High weight line hens showed increased rates of internal ovulation and defective egg production (36 per cent vs. 2 per cent in high and low weight birds, respectively).

This relationship is typical of many full-fed broiler breeder hens, where there is difficulty adhering to the normal models of the ovulatory cycle. Studies on arrhythmic laying patterns in meat-type hens in the late 1960s and early 1970s led to the description of a condition called “erratic oviposition and defective egg syndrome” (EODES).

Hens affected by this syndrome often lay erratically throughout the entire day. A 1992 study reported that during the first few weeks of lay, hens that had been fed ad libitum  from hatch laid 40.8 per cent of their eggs outside the normal laying period – the first 10 hours after dawn signal. The laying of a defective egg can often be the result of an inappropriately timed ovulation, either bumping into or forcing out the previous egg early. A period of at least 21 hours before or after another ovulation is needed for the production of a normal egg.

Feed Intake and Body Weight Control
While laying stocks will voluntarily consume appropriate feed quantities to meet their growth, metabolic, and production requirements, broiler stocks will consume feed well beyond their nutrient requirements. This is simply a result of their divergent genetic selection programs. The increased growth rate and very high body weights of broiler stocks are directly related to increased food and water intake, and somewhat to improved feed efficiency. 

The peripheral appetite control centers have been altered in broiler strains, causing ad libitum fed birds to eat near gut capacity. These birds don’t regulate voluntary feed intake to achieve energy balance, which throws the cumulative balance out between the regulation of meal-to-meal feed intake with the long-term maintenance of fat stores.

Ad libitum fed birds spend more of their time eating, with the increased feed intake being expressed through an increased number of meals. Ad libitum broiler breeder pullets spend the majority of their time eating and resting, whereas restricted pullets spend much of their time scratching and pecking in the litter.

A recent examination of what controls feed intake in breeder stocks has indicated that eating behavior in broilers is controlled more by satiety mechanisms than by hunger mechanisms. With only an upper set point for eating behavior apparent, broilers would then eat to their maximal physical capacity. For laying hens, eating behavior has been shown to be equally influenced by both hunger and satiety mechanisms.

The development of current, high breast-yield strains demonstrates continued increases in growth potential. The routine feed restriction of broiler breeders from an early age is considered essential for the well being of the birds and for the production of eggs and chicks. The welfare dilemma of female broiler breeder production is that if a pullet is managed using the significant degree of feed restriction essential to ensure reproductive efficiency and good livability, other aspects of welfare may be affected. 

Conversely, a full feeding environment results in obese birds and a general loss of vigor. When this is compounded by reduced livability and increased leg problems, these birds can have great difficulty in reproducing themselves. While there is pressure from a welfare perspective to eliminate or modify feed restriction, there is poor consensus on how far to relax feed restriction.

It is no longer clear what the normal, natural state is or should be in meat-type parent stock. Modern growth curve targets are created to maximize reproductive efficiency. But continual changes in the genetic composition of stocks by the primary breeders and strain-specific management effects may limit the potential application of a defined, ideal, welfare-friendly growth curve or feed restriction method.

The ad libitum feeding of modern broiler breeder parent stocks appears to create a bird further from the wild-type form than restricted feeding does. Years of genetic selection for growth parameters has created a bird unfit for the allowance of self-regulation of feed intake due to the numerous problems linked to over-consumption. While feed restricted birds are not free from hunger, they have the welfare benefit of reduced incidences of metabolic disorders and mortality.

While broiler parent stocks are heavily feed restricted during parts of the pullet rearing phase, the serious implications of overfeeding pullets at this time on the well being of the birds make feed restriction programs the more welfare-friendly alternative.
As stress and poor management reduce production efficiency, it also makes economic sense to maintain a welfare-friendly environment. Under current poultry industry conditions, the negative relationship between excess nutrient intake and reproduction can really only be controlled through management strategies, including feeding and lighting programs.

What is a ‘Normal’ Broiler Breeder?
With the continual change we see in modern broiler breeders, do we really know what a ‘normal’ broiler breeder is?
To some, a normal broiler breeder is one that is allowed free access to feed. However, when comparing the impact of ad libitum compared with restricted feeding of broiler breeders, feed restriction programs are considered the more welfare-friendly alternative due to the many obesity-based problems that can be avoided.1

Due to the negative consequences of ad libitum feeding for health and therefore also for animal welfare, a ‘normal’ broiler breeder requires some kind of body weight control.

Much of the concern about the welfare of broiler breeders is centred around the impact of feed restriction. Based on some specific stress indicators, the degree of feed restriction is believed to be the greatest between 8 and 16 wk of age, when feed restriction begins to relax to allow for the more rapid weight gains leading up to sexual maturation. Commercially, the 7 to 15 wk of age period has been reported to be the most critical for effective feed restriction because of its association with lasting improvement in reproductive performance.

Broiler breeders are managed using a combination of nutritional and environmental factors that contribute to a much slower growth rate than we see in regular broiler stocks. Feed form, nutrient density, and rearing on 8 hour days all help to naturally reduce the growth potential of the bird – thereby decreasing the degree of feed restriction the pullets must face.

It is not clear if future limitations to the improved efficiency of broiler production will originate in the area of management, meat quality, welfare, or some combination of these factors. There are some very interesting new research findings that focus on the satiety centre of the chicken. Helping the chicken to feel full would be an excellent way to encourage the growing birds to stay close to the body weight targets needed to allow the healthy, normal egg production cycle.

But in the meantime, the best tool we have to manage growing breeders is to have a defined body weight target profile. This profile has arisen from analysis of the best results from thousands of commercial breeder cycles. The breeding company knows these birds well, and we would be smart to follow the recommended profile for the strain of birds we have placed. Companies that use a common profile across barns with birds from several genetic companies can potentially hurt all of their flocks if none fall close enough to their summary profile.

Is defining ‘normal’ as simple as producing a target body weight profile? For now this may be the most effective tool we have.
A body weight target comes with baggage, however. We assume that this target will result in the best liveability, flock uniformity, egg production, fertility, hatchability, chick quality, and welfare. For the most part this is true.

With changes in our genetic stock causing them to have the need to be managed more specifically all the time, following strain-specific management practices had better be the norm rather than the exception.

Who is a ‘Normal’ Broiler Breeder?
Now that we know what a normal broiler breeder is – can we identify one if we saw it? Since the definition is based on a target body weight, then logically an average body weight bird would be the one we can call normal. Or is it?

To understand who the average birds are in a population, we have to remember the general balance between growth and reproduction. We normally think of birds gaining weight relative to the rest of the population if they are not laying well. It gets a bit more complicated, however, because we also have to consider the overall efficiency of the bird.

We recently began to characterize the ‘reproductive attitudes’ of individual hens. This is a measure of the individual balance between the support of egg production or growth and how efficiently the bird is while it makes this decision.

We were able to trace their growth, calculate their muscle and fat deposition, metabolic needs, and allocation of nutrients to their egg production. From this we calculated a ‘residual feed intake’. We totalled the true energetic cost of the bird’s growth, metabolism and egg production and compared that to what it was actually eating.

Birds that consumed less feed than we calculated as their requirement had a negative residual feed intake. These are great birds because they are more efficient than we calculate they should be. Birds that consume more feed than we calculated as required for their activities end up with a positive residual feed intake. While some of these birds may have underlying absorption issues or other inefficiencies, the bottom line is that they are costing us more feed than we calculate they should.

The three graphs on page 26 show the summary of the growth profiles of all of these birds. They show the weights of birds relative to the target body weight (100 per cent = target). The coloured lines show the percentage the group is far above or below the body weight target. The green lines are the best layers of each efficiency class. The yellow lines are the average birds and the red lines are the poorest layers. Notice the way that a high rate of lay means that the hens will tend to give up some growth while the low rates of lay are closely linked to weight gain. With less egg production to support, the poor producing birds allocate the nutrients they consume to their own growth.

In each of the efficiency classes (Figures 1 to 3), we see an interaction between rate of lay and body weight. Birds in each efficiency class that lay well will always lose some weight relative to that of the other birds in their group. Poor layers will conversely gain weight relative to that of the others in their group. When nutrients are not being diverted into egg production, they are available for diversion into body mass. The simple way to interpret the graphs is to realize that the bottom line refers to those laying the best, and the top line to those laying the worst.

A hen with poor overall efficiency starts off lighter already from an early age (Figure 1). The inefficiency of these birds has caused them to be some of the smaller than average already in the rearing period. We know that small pullets also tend to lay fewer eggs and begin laying eggs later than the larger birds. Often they also enter lay with fewer large yellow follicles on the ovary, which can limit potential egg production.

Some of them can lay just fine, however, but end up being the smallest hens in the flock once you get to 40 or 50 wk of age. Are these the birds producing the smallest eggs in the flock as well? A high-producing hen will tend to have smaller eggs regardless of the overall nutrient utilization efficiency of the bird. The larger eggs come from both larger hens and hens not laying that well. They generally have more yolk material available to divide among a smaller number of growing follicles on the ovary, which leads to larger yolk and egg sizes.

The highly efficient hens were the largest pullets at the time of housing (Figure 3). An efficient hen can express this trait even when you change its body weight target. There is a trial underway at the University of Alberta in which individual birds have been shifted onto an identical target body weight profile. Even though they all have exactly the same body weight now, the birds that were the most efficient at the start of the study continue to require less feed than the others to maintain the growth targets.

Based on these graphs, we can see that it’s not just average birds that hover over the 100 per cent, target body weight line. Also in this group we have the worst layers of the most inefficient hens. These initially small birds managed to gain enough weight to hit the flock body weight average.

The most interesting group that sits at the flock body weight average are best producing hens of the most efficient hen class. These hens sacrificed some growth to maintain a high rate of lay and may even have lost some weight in the process.

We assume that birds at the flock body weight average are pretty much all normal birds. However, this analysis shows that at the flock body weight target we actually have the best of the most efficient birds, the most average of the average (ones we had hoped to find here) and the worst producing of the inefficient hens.

1. RENEMA, R.A. and ROBINSON, F.E. (2004) Defining normal: Comparison of feed restriction and full feeding of female broiler breeders. World’s Poultry Science Journal 60: 511-525.