I have to assume that somebody, somewhere has made this argument before (because it seems to obvious to me); but Google isn't helping me find priors, and I'll jot my notion down here.
In a nutshell: Beowulf is a John Barleycorn narrative; or its first third is. Beowulf fights a humanoid creature called Grendl, who has come into the hall and drunk the blood of one of his warriors; he rips his arm off, and the creature runs away. We do not see him die; but Beowulf pins his arm to the wall like a lucky branch. Then everybody gets drunk and sings songs.
This is the evidence I'd constellate to make the theory, had I time.
1. Grendl's name is the OE for 'Green man'. Really, it is.
2. The 'Beow' in Beowulf means 'barley'. ('Scholar Kathleen Herbert draws a link between Beowa, a mythical figure stemming from Anglo-Saxon paganism that appears in early Anglo-Saxon royal genealogies whose name means "barley", and the figure of John Barleycorn. Herbert says that Beowa and Barleycorn are one and the same, noting that the folksong details the suffering, death and resurrection of Barleycorn, yet also celebrates the "reviving effects of drinking his blood."')
3. As in the ancient ballad, Grendl has one of his limbs uprooted; and seems to die; but he comes back to life (in the form of his mother). The whole thing is a symbolic narrative of the seasons and fertility.