Commentary for Avodah Zarah 48:10
ר' יוחנן אמר גבול יש לה פחותה מבת ג' שנים נעקרת בת ג' שנים אינה נעקרת
R. Yohanan said: There are limits: under the age of three years [an animal] becomes infertile, but from the age of three years it does not become infertile.
Daf Shevui to Avodah Zarah
According to R. Yohanan whether or not the animal is made infertile by having sex with a human depends on the age of the animal. If the animal is over three, it will not be made infertile, and therefore non-Jews will copulate with it. But if the animal is under three, they will refrain from copulating with it.
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