Towards a more reliable comparison of the FCR

Luc Maertens
Luc Maertens

Luc Maertens, the newest member of World Poultry’s panel of experts, is a senior poultry researcher at ILVO, Belgium. In this column Luc looks at the feed conversion ratio (FCR) calculation for broilers and the need for change.


The feed conversion ratio (FCR) in broilers is largely dependent on their weight. To be able to compare different flocks, houses or diets, the FCR is corrected for weight differences. It is still common to recalculate the FCR towards a weight of 1500 grammes and then using a correction of 4 points for each 100 g deviation from this weight.However, slaughter weight and carcass composition of the modern broiler has changed dramatically over the last decades and a recalculation to a FCR 1500 makes no sense anymore.

A correction with 2 points per 100 grammes deviation from a reference weight of 2500 grammes is proposed.

The fast growing broiler of today is slaughtered at a weight of around 2.5 kg. For this fact alone a comparison and correction of the FCR in the range of the average slaughter weight is desirable. Moreover, a recalculated FCR 1500 favours highly the heavy flocks. An unrealistic FCR 1500 close to 1.0 can be obtained for a flock with a weight of 3.0 kg. This can – to some extent – be explained because the relationship between weight and FCR is not linear. By consequence; how larger the difference with the reference weight of 1500 grammes how bigger the error.

Carcass composition of modern broilers

Furthermore, the improvement of performance has led to a changed carcass composition of broilers. Modern broilers have much less fat and more muscles deposits at heavier weights than in the past. By consequence the effect on FCR is strongly overestimated when a correction is applied of 4 points (0.04) for each 100 grammes above or below the weight difference. By using the current FCR mentioned in the manuals of Cobb 500 and Ross 308, we calculated an effect on the FCR of about 2 points for each 100 g difference in the weight range of 2.3 – 2.7 kg. A comparison at a standard reference weight of 2.5 kg with this correction factor leads to a proper FCR 2500.

Therefore, we propose to no longer use the FCR 1500 but the FCR 2500 with the following formula:

FCR 2500 = FCR – Y with Y = (Average slaughter weight – 2500)/50/100.

Depending on the actual local slaughter weight a reference weight somewhat below or above 2500 grammes may also be used with our proposed correction factor of 2 points/100 grammes deviating weight.