Pet food ingredients are listed in descending order of predominance by weight. This means the first ingredient is the heaviest (by weight) before processing, and each subsequent ingredient contributes less to the total weight of the product. This system is standard for labeling in many countries, including under AAFCO (Association of American Feed Control Officials) regulations.
Example: If chicken is listed first, it means that chicken, in its raw form (which contains water weight), is the most abundant ingredient in the food before cooking. However, after cooking, the moisture is reduced, and its prominence in the final product may decrease.