Before Mr. Barleycorn

"John Barleycorn" is an eighteenth-century ballad family and no older. But it has antecedents, like so many popular poems do - and while none of them are old enough to truly reflect some sort of pre-Christian crop worship, they are believed to date back to ca. 1500. Here are the earliest versions, in the original Scots.
This first one, "Allan o' Maut," continues a theme familiar from the later "Barleycorn" cycle. Alan (the name may be a pun on "all ane," that is, single malt) is subjected to a variety of things that, if we think of him as a person, are strange or unpleasant, but are a description of the malting process. There's even an argument to be made that it's explicitly about making whiskey rather than beer, based on the mention of the spirit at the end, although like the modern versions it stops short of describing distillation.

Gude Allan o' Maut was ance ca'd Bear
And he was cadged frae wa' to wear
And draggled wi' muck, and syne wi' rain,
Till he dee'd and cam' to life again.

He first grew green, syne he grew white,
Syne a' men thocht that he was ripe;
And wi' crookit gullies and hefts o' tree

They've hewed him down right doughtily.
Syne they've set Allan up into stooks
And casten on him many pleasant looks;
They've turs'd him up syne on a sled

Till in the grain-yard they made his bed.
Then men clamb up upon a ladder,
And happit his head frae wind and weather; 

They've ta'en him neist up in their arms
And made his shake-down in the barns
The hollin souples, that were sae snell,
His back they loundert, mell for mell; 

Mell for mell, and baff for baff
Till his hide flew about his lugs like chaff.
They stowed him up intil a seck
And o'er the horse-back broke his neck;

Syne birstled they him upon the kiln
Till he was bane-dry for the mill.
They coupit him then into the hopper
And broke his banes, gnipper for gnopper

Syne put the burn until the gleed,
And leepit the een out o' his head.
Till in cam' Barmy-breeks, his brother,
Like ae gude neighbour to crack wi' anither; 

Says, "Allan o' Maut, are ye gaun to dee?
Rise up, man, first, and dance wi' me."
They danced about frae hand to hand,
Till they danced o'er the working-stand;

Syne in cam' Jenny wi' her dish,
She gae mony a rummle and rush
And Usquebaugh n'er bure the bell,
Sae bauld as Allan bure himsel'.

This, however, is not the oldest example of the character of Allan o' Maut appearing. An earlier version appears in the (undated, but late 16th century) Bannatyne Manuscript under the title "Quhy Sowld Nocht Allane Honorit Be?" Specialists estimate its date of composition as earlier than the oldest extant text, however, as the language apparently points to it being written ca. 1500. And since the notion of "Allane" as a name for personified malt is evidently meant to be familiar to the audience, we can push the history of the ballads back at least most of a generation before even that, suggesting that Alan (and the modern John Barleycorn) belongs firmly to the fifteenth century. (It should be noted that "Qu" stood in for the modern "W" in Scots of this era, so the sequence "quh" should always be read "wh" today.)

Quhen he wes yung, and cled in grene,
Haisand his air about his ene,
Baith men and wemen did him mene,
Quhen he grew on yon hillis he:
Quhy sowld nocht Allane honorit be?

His fostir faider fure of the toun
To vissy Allane he maid him boun;
He saw him lyane, allace! in swoun,
For falt of help, and lyk to de:
Quhy sowld nocht Allane honorit be?

Thay saw his heid begin to ryse,
Syne for ane nureiss thay send belyse,
Quha brocht with hir fyfty and fyve
Of men of war full prevely:
Quhy sowld nocht Allane honorit be?

Thay ruschit furth lyk hellis rukis
And every ane of thame had hukis;
Thay cawcht him schortly in thair clukis,
Syne band him in ane creddill of tre:
Quhy sowld nocht Allane honorit be?

Thay brocht him invart in the land,
Syne every freynd maid him his band,
Quhill thay micht owdir gang or stand,
Nevir ane fute fra him to fle:
Quhy sowld nocht Allane honorit be?

The grittest cowart in this land,
Fra he with Allane entir in band,
Thocht he may nowdir gang nor stand,
Yit fowrty sall nocht gar him fle:
Quhy sowld nocht Allane honorit be?

Schir Allanis hewmond is ane cop,
With ane sege feddir in his top;
Fra hand till hand so dois he hop,
Quhill sum may nowdir speik nor se:
Quhy sowld nocht Allane honorit be?

In Yule, quhen ilk man singis his carrell,
Gud Allane lyis in to ane barrell;
Quhen he is thair, he dowtis no parrell
To cum on him be land or se:
Quhy sowld nocht Allane honorit be?

Yit wes thair nevir sa gay a gallane,
Fra he meit with our maistir Schir Allane,
Bot gif he hald him by the hallane,
Bak wart on the flure fallis he:
Quhy sowld nocht Allane honorit be?

My maistir Allane grew so stark,
Quhill he maid mony cunning clerk,
Upoun thair saiss he settis his mark,
A blud reid noiss besyd thair e:
Quhy sowld nocht Allane honorit be?

My maistir Allane I may sair curss,
He levis no mony in my purss,
At his command I mon deburss
Moir nor the twa pairt of my fe:
Quhy sowld nocht Allane honorit be?

And last, of Allane to conclude,
He is bening, courtass and gude,
And servis ws of our daly fude,
And with that liberalitie:
Quhy sowld nocht Allane honorit be?

The next page of the manuscript is missing, and it may have originally contained a sort of sequel or continuation. The same manuscript also contains a brief poem about brewsters, "Quha Hes Gud Malt And Makis Ill Drynk," which hopes for women who brew poor beer from good grain to be denied proper funerals so that they might be damned to hell. It then expresses hope that the best alewives be taken into Heaven, and rewarded there with plenty of such beer as they made on Earth. Fermentation, apparently, was serious business.

And there you have it: the earliest harvest ballads in which the barley is personified, but still treated as he is when being made into beer.