Managing the modern broiler: Growing heavy cockerels

Finishing broilers should be relatively straightforward, if the earlier life stages have been managed well. But groing heavy cockerels can be more challenging, as Richard Jackson, a vet with St David’s Poultry Team, explains in the last of our three-part series on managing the modern broiler.

A considerable proportion of British broiler farmers are now growing sexed birds to meet consumer demand for large chickens.

And while managing the pullets is not significantly different from keeping an “as-hatched” flock, managing their male counterparts can be more of a challenge.

With pullets, the main aim is to get them to the target kill weight of about 1.8-2kg at over 30 days. This means pushing them rather hard, both nutritionally and in management terms, including feeding and lighting programs.

But cockerels are being asked to grow to over 3kg, which can take 50 days or longer, and as such their lives are a marathon compared to the pullets’ sprint.

The main challenge arises because both sexes are almost always in the same house and as such are being asked to grow under the same conditions. Unfortunately for the marathon-running cockerels, they are being asked to sprint the first stage of their lives alongside the pullets, meaning many can run into trouble later on, after the pullets are thinned.

The most common problems when growing heavy cockerels are heart attacks, ascites and lameness, many of which stem from difficulties incurred in the first few weeks of life.


As discussed in the first article (Poultry World, June, p26), getting chicks off to a good start is essential – even more so for cockerels than for as-hatched birds. Because the cockerels are living longer and grow to a heavier weight, even a small amount of bacteria present in their joints can lead to lameness later on, whereas lighter as-hatched birds might not get heavy enough to develop clinical signs.

Paying even closer attention to hygiene and management on breeder farms, at hatcheries and on the broiler farms themselves is therefore critical. As a result, it is much more likely for a vet to recommend starter medication, to prevent infection, for sexed birds than for broilers grown on an as-hatched program. Some units do have separate water systems, so may find it possible to only medicate the cockerels, but in most cases this isn’t so easy.

Encouraging the development of a healthy cardiovascular system in chicks is vital, as any damage to heart and lungs at an early stage will predispose cockerels to heart attacks and ascites in later life. High carbon dioxide levels in the hatchery or on the farm can impair the birds’ cardiovascular system, so ensuring good ventilation from egg up to the finished bird is absolutely key.

Temperature control is also highly important, as excessively high or low temperatures mean the developing embryo or chick will use energy and oxygen to compensate for these undesirable environmental parameters, rather than to grow. Furthermore, if a chick is fighting extreme temperatures through raising its metabolic rate, its heart is going to be put under strain, again impairing its development and leading to possible failure later in life.

Of course, good ventilation and temperature control is important throughout the crop, not just in the first weeks of life, as low oxygen levels mean the birds’ hearts have to work harder at any age. An airy, temperature-controlled environment will also improve bird activity, thus reducing hock marks and encouraging the development of a healthy skeleton.


A healthy skeleton can also be supported by the strategic use of vitamin D, as it helps birds absorb and correctly utilise calcium. Although vitamin D is included in the birds’ ration, the EU has limited the amount that can legally be added. This maximum level may not meet the demands of today’s rapidly growing broiler – a problem that is likely to increase as our broilers grow faster. An alternative, to ensure the birds are getting enough vitamin D, is to add it to the drinking water as well.

It has been demonstrated that if broilers’ growth is slowed down in the first three weeks of life they can compensate for this later on, thus reaching the same end weight. For as-hatched birds there is often not sufficient time to do this, and of course it is likely to impact on pullets’ end weights, but it can be beneficial for cockerels.

Slowing them down early in life allows them to develop a better skeletal and cardiovascular system before putting on weight, thus reducing the likelihood of lameness or heart attacks. Ideally, the flock should be slightly below target weight in the first three weeks of life, then allowed to grow on more in the final couple of weeks to finishing.


In order to achieve this slower growth rate, the birds may be given more dark periods than the current Red Tractor minimum of six hours. Not only does this slow growth rates, it has also been shown to improve feed conversion ratios (FCRs) and boost the birds’ immune function.

Of course, extra darkness should not be given in the first week of life, as that is a critical time for chicks to learn to eat and drink properly, and get off to the best possible start. Their stomachs are also too small to manage a long dark period without impacting on growth rates.

Ideally, cockerels should be given more than six hours of darkness in the second and third weeks of life, but this should be evenly spread, and not in a single block. Giving any more than four hours of darkness at any one time means the birds will have emptied their stomachs and are likely to scramble for feed once the lights come on, leading to scratching and possibly cellulitis.


Reducing the birds’ feed intake can also help to slow their growth rates down. This can be done by controlled feeding programs or by offering a less nutritionally dense ration. Controlled or mealtime feeding means pans should be fully eaten out a good 30 minutes before the next feed, and should be monitored and reviewed daily. It can improve the FCR, but needs to be managed very carefully to ensure the birds are not held back too much.

Feeding a less dense ration can be achieved by increasing the wheat content, although it shouldn’t rise above 18%. Giving a lower density ration will not improve FCR, but since the ration is cheapened and the overall result is less disease in the cockerels, it still leads to better profitability.

Generally speaking, broiler health is relatively easy to manage during the finishing period, as long as the birds got a good start in the first few weeks of life. The use of probiotics in early life will aid beneficial gut flora, but growers can add organic acids to the water throughout the crop, to ensure continued gut health.


Small birds should have been culled during the transition phase, to improve the evenness of the flock, so the only culls in the finishing period will be those that are sick or lame. And litter quality is unlikely to be compromised during this period, as birds have moved on to a less nutrient-dense ration and are therefore less prone to diarrhoea.

Of course, all of the usual measures covering hygiene, ventilation and general bird health throughout every stage of the bird’s life will benefit cockerels and pullets alike. However, slowing the flock’s growth rate may mean that pullets struggle to make the target kill weight at the time they are thinned.

Unfortunately, it is rarely practical to only grow cockerels, as it would be financially unviable. So, as with all things in life, there is a happy medium. It is better to have lighter pullets and healthy cockerels than to have heavy pullets and lame cockerels that have to be culled at over 3kg in weight. You can’t sell a dead cockerel, but you can sell a light pullet.

St-David Richard Jackson is a vet with St David’s Poultry Team. For more information, contact the St David’s Poultry Team on 01392 872 932 or email

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