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Maize (Zea mays L.) silage is an important source of feed for ruminant animals. However, maize monocultures are exposed to both abiotic and biotic stresses that can reduce yield and contribute to environmental degradation. Over the past century, maize + soy [Glycine max (L.) Merr.] polycultures gained traction because polycultures diversify silage cropping systems and can increase the nutritional quality of silage compared with maize monocultures. However, it remains unclear under which conditions these polycultures provide biomass yields like those of maize monocultures. We compiled paired data from 30 papers (529 records) that reported maize and maize + soy yields for silage. Using random forest, we created two models to investigate the factors determining yield and the yield differential with respect to maize monocultures. We found that the most important variables in determining the polyculture yield were the maize proportion in the mixture and year, where year represents mostly technology trends. Precipitation and temperature remain pivotal, with yields stabilizing when yearly precipitation exceeds 100 cm and average temperature exceeds 15 °C. Maize + soy polycultures may produce similarly to maize monocultures if the maize density is at least 5.3 plants m−2. This signals that whereas biomass production in polycultures is maize-driven, maize stands can be supplemented with soy or with other suitable companion crops without sacrificing yield, perhaps capturing additional production and ecosystem benefits. Research on polycultures for silage production should focus on economical ways of planting mixtures and on suitable maize companions for each region.