Feed Conversion Ratio

Feed conversion ratio is feed in divided by weight out, and it is the cleanest single measure of how well a growing animal is doing its job. Broilers convert at about 1.6 kg of feed per kilogram of gain, pigs at 2.5–3, feedlot cattle at 6–7, and the spread across species tells most of the story of why chicken is cheap. Lower is better, which is the opposite of most efficiency measures and a frequent source of confused conversation.

The ratio is only comparable if the two figures behind it are on the same basis, and this is where most published comparisons fall apart. Feed can be quoted as fed or as dry matter, and the two differ by up to a factor of three on wet rations. Gain can be liveweight or carcass weight, and the two differ by 40% or more. A feedlot quoting 6:1 on dry matter against liveweight and a competitor quoting 6:1 on as-fed against carcass are not describing the same performance at all. Always state the basis, and be suspicious of any figure that does not.

The ratio worsens as an animal grows, and that is biology rather than management slipping. A young animal lays down mostly protein and water; a finished one lays down fat, which takes far more energy per kilogram. Maintenance also takes a fixed bite out of the feed before any gain happens, and that bite grows with body weight. So conversion for a single animal deteriorates steadily from weaning to slaughter, and the economically right moment to sell arrives when the marginal cost of the next kilogram exceeds what the market pays for it — well before the animal stops growing.

A ratio below 1 is worth investigating rather than dismissing. It is genuine where unmeasured feed is being consumed, which is the normal state of affairs on pasture or with a calf still on milk. It is an error where an as-fed gain has been set against a dry matter intake, or where the weighing periods for feed and animals do not line up. Gut fill alone can move a single weighing by 4% of body weight, which is why serious trials shrink animals before weighing or use long enough periods that the noise averages out.