Or HaChaim on Exodus 23:16/אור החיים על שמות כ״ג:ט״ז

1 בכורי מעשיך, "the first fruits of your labours." This excludes crops raised by a Gentile. His work is not considered a Jew's accomplishment. We are taught in Rosh Hashanah 13 that if the crop has grown to one third of its final size while still under the care or ownership of a Gentile, it does not qualify as something from which this gift of בכורים can be presented to the Priest. This ruling is based on Leviticus 23,10: "the first of your harvests."