In Vivo Responses of Ruminants to Dietary Tomato Pomace: A Systematic Review and Multilevel Meta-Analysis

Abstract

Tomato pomace is a fiber‐, lipid‐, and bioactive‐rich agro‐industrial byproduct with potential application in ruminant diets, but animal responses vary across species, processing methods, inclusion levels, and feeding systems. This systematic review and multilevel meta‐analysis quantified in vivo responses while accounting for dependent effect sizes, shared controls, and crossover designs. The literature search was conducted across Scopus, PubMed, AGRIS, and Google Scholar. The systematic‐review evidence base comprised 32 reports representing 33 independent in vivo experiments; the primary quantitative dataset contained 456 effect‐size observations from 27 independent biological cohorts. The primary effect size was the natural logarithm of the response ratio (lnRR), expressed as percentage change, except for rumen pH, which was analyzed as a raw mean difference. Multilevel REML models used full sampling variance‐covariance matrices and CR2 cluster‐robust inference. Among the 26 experiments with complete domain‐level risk‐of‐bias appraisal, 19 (73.1%) were classified as having some concerns and seven (26.9%) as high risk; none was classified as overall low risk. Across conventional performance outcomes, tomato pomace was associated with a small average decrease in dry matter (DM) intake and average daily gain, a slight decrease in feed conversion ratio, and a small increase in milk yield and milk fat, although these responses were not statistically consistent. DM digestibility tended to increase slightly, whereas organic matter and crude protein digestibility showed small negative average changes. In contrast, NDF digestibility showed a possible small increase (+2.6%), suggesting a potentially favorable fiber‐utilization response, although its prediction interval crossed the null and statistical support was sensitive to variance‐quality assumptions. Ruminal fermentation responses were modest and heterogeneous: rumen pH tended to decrease slightly, whereas ammonia–N and total VFA tended to increase. Milk fatty‐acid composition showed the clearest directional changes, with C16:0 tending to decrease, while C18:0, C18:1 cis‐9, and total MUFA increased. Exploratory moderators did not identify consistent effects of species, physiological stage, or replacement category, and no linear dose–response relationship was detected for DM intake. Overall, tomato pomace appears compatible with maintenance of conventional productive responses, while effects on fiber digestibility and milk fatty‐acid composition remain context‐dependent.

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