This study aims to determine the effect of using brown seaweed extract (Sargassum crassifolium) in drinking water on the accessory organs of finisher phase broiler chickens. The research used a Completely Randomized Design (CRD) with 4 treatments and 5 replications. Treatment consisted of levels of brown seaweed extract, namely 0% (0.00gr in 3L water), 5% (0.18gr in 3L water), 10% (0.37gr in 3L water), 15% (0.56gr in 3L water). The parameters measured were the weight of accessory organs consisting of the crop, heart, liver, pancreas, gizzard and spleen. The results of analysis of variance showed that the treatment of giving brown seaweed extract in drinking water had a very significantly different effect (P<0.01) on gizzard weight, pancreas weight, heart weight and crop weight, and had a significantly different effect (P<0.01). 05) against heart weight and spleen weight. The results of the Honestly Significant Difference (HSD) test showed that administration of 15% seaweed extract resulted in the weight of the pancreas and spleen being very significantly (P<0.01) higher than without administration of seaweed extract; Without giving seaweed extract, the heart weight and crop weight were very significantly (P<0.01) higher than when giving 15% seaweed extract; Giving 5% seaweed extract resulted in a liver weight that was very significantly (P<0.01) higher than giving 15% seaweed extract. It can be concluded that the use of brown seaweed extract in finishing phase broiler chicken drinking water can be given up to 10% because it can maintain the weight of finisher phase boiler chicken accessories organs.

Key words: Broiler chickens, Sargassum crassifolium, accessory organs