Abstract
In M33 cockerels, the rapid phase of testicular growth and the onset of sperm production were advanced either by increasing daylengths or an interrupted-night photoschedule; they were delayed by constant short days (8L). The results observed under constant long days (16L) were intermediate. In this strain, the cockerels maintained their mean testicular weight during the adult period while their daily sperm output (DSO) gradually declined. Individual variations in the DSO results were greater than individual variations in testicular weight. These variations did not appear to be influenced by the photoschedules. The differences in DSO observed between the best and the worst semen donors at the end of testicular development or thereafter remained relatively stable in ageing cockerels, except in birds submitted to increased photoperiods. This is the basis of the method described here for reliably choosing the best semen donors just before they are used as breeders.
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