To compare Feed Conversion Ratio’s (FCR, kg of feed divided by kg of life weight) between flocks of broilers of different body weight, we have to correct the FCR to the same body weight. This is because there is an influence of growth on FCR. A bird uses feed for maintenance and growth. The faster a bird grows, the efficienter it will be as relatively less feed is used for maintenance to get to the same body weight. But as the bird produces more meat (higher body weight) it uses in total more feed, as it uses more feed to be converted in more meat. That means that at a higher body weight it looks less efficient (higher feed consumption).

Traditionally we often correct FCR back to an equal body weight of 1400 or 1500 g, with a correction factor of 4 points of FCR per 100 gram of body weight. This would mean that if one flock has a body weight of 1500 gram with an FCR of 1.40 and another flock has a body weight of 1700 gram with a FCR of 1.46, we distract 2 x 0.04 from the FCR of the heaviest flock, which gives a corrected FCR for the heaviest flock of 1.46 – 0.08 = 1.38.

Nowadays we see that normal slaughter weights are much more in the range of 2.2 to 2.4 kg or even higher, so its questionable if correcting FCR to a level of 1500 gram life weight makes a lot of sense. It helps to be able to have one standard that makes FCR between for instance companies in a region comparable, but if we are too far off from the real body weight the change that the adjustment is not totally correct anymore is increasing.

We see that especially with higher body weights. Comparing FCR from flocks of different bodyweight assumes that a bird is growing in a sort of “balanced” way, that the composition of a bird of 1500 gram is equal to a bird of 2300 gram. However, we see that birds of higher body weight have relatively more meat and especially more breast meat, and the percentage of bone, fat, intestines and so on is going down. As putting on a gram of breast meat is a much more efficient program than putting on a gram of bone or fat, it is questionable if a FCR correction of 0.04 is correct at all bodyweights. At higher body weights, 2.2 kg and higher, a correction factor of 0.03 per 100 gram is probably more realistic, and when the birds are significantly heavier than that it might even be that the correction factor should be closer to 0.02 per 100 gram of extra body weight.