Nutrient digestibility, nitrogen balance, and energy partitioning in laying hens subjected to different molt...

Induced molting is a widely used strategy to extend the productive lifespan of laying hens; however, limited information is available regarding the effects of different molting programs on nutrient utilization and energy metabolism. The objective of this study was to compare nutrient digestibility, nitrogen balance, and energy partitioning among nutritionally distinct molt induction programs (MIP), which differed in ingredient composition and metabolizable energy availability, during molt induction and early post-molt recovery. A total of 75 Rhode Island Red laying hens were assigned to 1 of 5 MIP: fasting, restricted commercial feed (CF), wheat bran (WB), alfalfa, and sorghum. MIP significantly affected body weight loss, nutrient digestibility, and energy utilization (P < 0.05). Metabolic body weight loss (BW^0.75) at the target molt endpoint ranged from 15.2 to 20.7%, being lowest in CF and highest in alfalfa (P < 0.05). During the molting phase, alfalfa showed the lowest dry matter digestibility (68.1%) and among the lowest energy digestibility values (78.7%) compared with CF (77.9 and 84.1%, respectively). Likewise, energy utilization, expressed as the ratio of apparent metabolizable energy to gross energy intake (AME/GE), was lower in alfalfa (78.6%) than in CF (84.1%). Total nitrogen retention decreased from approximately 1.3-1.4 g/hen/day during the pre-molt phase to 0.34 g/hen/day in alfalfa and negative values in fasting hens (-0.18 g/hen/day) during molt induction. Similarly, total retained energy became negative across all molt induction programs during molting, ranging from -67.2 to -112.8 kcal/kg BW^0.75/day. During post-molt recovery, nutrient digestibility, nitrogen retention, and energy balance progressively improved. In conclusion, induced molting promoted a metabolic transition from an anabolic to a catabolic state, which was necessary to support reproductive regression. Among the evaluated strategies, the alfalfa-based MIP produced one of the greatest reductions in metabolizable energy availability while maintaining voluntary feed intake, demonstrating that severe functional energy restriction can be achieved without complete feed withdrawal. However, the present results indicate that this response was achieved through a physiological mechanism distinct from fasting, rather than representing an equivalent metabolic state.