Differences between broiler and layer chickens I

Broilers are chickens that are reared specifically for meat. They have been bred to maximise feed conversion to achieve an optimum slaughter weight within a short space of time, with meat yield to live cost ratio being a crucial factor in desirability.

Commercial breeding pyramid, ©BHWT

Resulting heavy birds are prone to hock lesions, commonly referred to as ‘hock burn’. These pressure sores may have developed as the skin on the hock has not evolved to tolerate heavy bodyweight which causes birds to be sedentary (Price, 2012). High protein diets have also been shown to increase hock lesions, whilst maintaining good quality, dry litter helps minimise the problem.

Broiler breeding

The process of breeding broilers begins with pure line breeding stock. There are only 3-4 companies across the globe who hold these pure line birds and all the chicken we eat today can be traced back to these companies.

The pure line birds are kept in high level bio secure hatcheries and their eggs produce Great Grand Parent Stock (GGPS). Birds classed as GGPS then go on to produce Grandparent Stock (GP) who then produce Parent Stock (PS).

Roughly 3% of the global population of poultry is made up from PS, GP and GGP stock, known collectively as Parent Stock. The US supplies around 25% of world GP stock. This is known as the primary breeding sector, and 417 million PS are produced every year. (Aviagen, 2011)

An individual pure line or pedigree level hen may have 25,000 breeding stock descendants that go on to produce 3 million broilers.

Pedigree stock are vigorously checked to determine their blood oxygen levels, bone and muscle make up and the industry is heavily investing in genomic research to further improve performance as limits to physiological selection do not appear to be close.

Parent stock are then used in broiler breeder farms to produce fertilized eggs; their lineage will follow the diagram above with GGP hens producing broiler breeders, which in turn produce our Sunday roast.

Males and females from parent stock are selected from different genetic lines or breeds and their chicks are hybrids. Every hen is expected to lay 150 hatching eggs over a 34–36-week period.

The gender of the chicks is governed by the female, (whereas males determine gender for mammals).

Typically, one would expect the male/female ratio to be 50/50, however, in parent stock production this is not always the case. The reason for this is that male embryos produce more heat than female embryos, and as embryos are sensitive to overheating, males are more likely to die late in the incubation process prior to hatching.

It is interesting to note that when the number of male ‘late deads’ are included within the ratio calculation, the 50/50 balance is maintained.

Conversely, layers behave in the opposite way with females producing more heat and therefore suffering more ‘late deads’ prior to hatching. (Poultry Performance, 2021)

Hybrid day old chicks are placed in rearing houses with specialist brooding equipment until 18 weeks of age. It is important to monitor the weight of the chicks during rearing as overweight birds tend to become poor layers. At this stage males and females are raised separately because males grow more rapidly, and separated feeding allows for uniformity of bodyweight across males and females.

At 18 weeks the birds are moved to broiler breeder laying houses and the two sexes are integrated and reared together. The roosters mate with the hens; typically the ratio will be 1 male to 12 females, fertilising the eggs which are collected twice daily.

As the flock ages, younger roosters will be introduced to maintain fertility and the male-to-female flock ratio will be managed to ensure every hen is mated every 2-3 days. The hen will store sperm in her oviduct but will only have maximum fertility for 3 to 4 days following mating.