Abstract
Background:
Kedu chicken is a local variety found in Indonesia. Its development is mostly limited to its natural habitat in Temanggung.

Aim:
This study aimed to investigate the effects of rearing altitude and dietary betaine supplementation on the growth performance, nutrient digestibility, and intestinal morphometrics of Kedu chickens.

Methods:
A total of 240 male Kedu chickens were allocated to a 2 × 2 factorial design with 6 replicates, each consisting of 10 birds. The first factor was rearing altitude (HA vs LA in situ vs LA in situ), and the second factor was betaine supplementation (0 vs. 0.15%). Growth performances, intestinal morphometric data, and nutrient digestibility data were analyzed using two-way analysis of variance (ANOVA).

Results:
There was no significant interaction between altitude and betaine supplementation (p > 0.05). Chickens reared in HA exhibited lower feed intake and feed conversion than those reared in LA (p < 0.05). They also showed longer ileum and greater intestinal weights, as well as higher dry matter, organic matter (OM), and crude protein (CP) digestibility (p < 0.05). Dietary betaine increased the average and final body weights and reduced feed conversion compared with LA (p < 0.05). These improvements are associated with enhanced OM and CP digestibility and increased jejunal weight and villus height (p < 0.05).

Conclusion:
Both rearing altitude and dietary betaine supplementation independently affected the performance and digestive physiology of Kedu chickens, with no interactive effects observed. Betaine can be considered an effective nutritional strategy for enhancing growth and improving digestive efficiency in Kedu chickens reared at various altitudes.

Key words: Altitudes; Betaine; Intestinal morphometry; Kedu chicken; Nutrient digestibility.