I understood. I created first a custom field but it was in products teble so It didn t appear on the product listing. I then saw
Custom item fields don't appear on the front end.
When you create a custom field, don't forget to enable the "Front-end" option.
Be careful, when you create the custom field, be sure that you set the good table , which is the "item" table, not the "product" table.
For products listings, enable the default option in System->configuration->display, enable "Display custom item fields" in the Default parameters for products.
For products listings, enable the same option, in Display->content menus/modules.
and followed the instruction. But as an item custom field I can t see it. It doesn appear anywhere