It's a bit like an aeroplane with a hole in it's wing the engines have to work harder to maintain height and speed. The benefits from these bodybuilding exercises are reaped when the peak of the moult is over, or when regrowing feathers are up to, or over, three-quarters grown, just before the next flight is dropped. For suddenly you have a pigeon with muscles and heart built to cope with inadequately feathered wings, now working with an almost full wing. Race results can improve dramatically when this condition occurs, and can explain why youngsters can suddenly pull a spectacular race result out of the bag. Another disadvantage with the darkness system is that it denies the pigeons access to a full days exposure to the sun's rays.
Sunlight is a great sexual stimulator.
So birds that are denied the full days exposure to the sun, can have a reduced sexual drive. This will have a detrimental effect on the racing performance of some of the older youngbirds, in particular those youngbird cocks that are raced on a semi widowhood system, in which they are allowed to mix with the hens prior to sending. In addition to all this, when fliers change to the darkness system they frequently just darken their existing loft, with little or no modifications, except for a curtain or shutter. Little regard is paid to the sudden change in airflow, in and out of the loft. Lofts become stuffy, the oxygen levels inside can drop and exhaled carbon dioxide will increase.