Tryptophan as a modulator of behavior and productivity in sows: a systematic review

Dietary supplementation with tryptophan in sows is a promising strategy for optimizing both performance and animal welfare. In addition to its role in protein synthesis, this essential amino acid is a precursor of serotonin and melatonin, which are bioactive compounds that influence appetite, social behavior, and stress regulation. However, the reported effects on reproductive outcomes and maternal metabolism remain inconsistent. This systematic review evaluated the evidence of tryptophan supplementation as a modulator of behavior and productivity in sows. Searches were conducted in Scopus and Web of Science databases. Experimental peer-reviewed studies that assessed tryptophan supplementation across different physiological phases (gestation and lactation), dietary levels, and environmental contexts were included. This review followed the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) 2020 guidelines, and the protocol was preregistered. Screening and data extraction were performed by two independent reviewers, and the risk of bias was assessed using the SYRCLE's (Systematic Review Center for Laboratory Animal Experimentation) risk of bias tool. Eleven studies met the eligibility criteria of this review. Tryptophan supplementation consistently reduced aggressiveness in group-housing systems and supported piglet survival and growth. However, the effects on reproductive performance and maternal metabolism vary depending on the dose, parity, and environmental conditions. The heterogeneity of the study designs and the absence of a meta-analysis due to data variability were identified as the main limitations. Overall, tryptophan supplementation demonstrated potential benefits for sow welfare and productivity, but the results remained context dependent. Long-term and standardized studies are required to clarify its impact, particularly on reproductive efficiency and maternal physiology.