Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Ordinarily these slaves belonging to the estate were sufficient; in case of need neighbours, as a matter of course, helped each other with their slaves for day's wages. Otherwise labourers from without were not usually employed, except in peculiarly unhealthy districts, where it was found advantageous to limit the amount of slaves and to employ hired persons in their room, and for the ingathering of the harvest, for which the regular supply of labour on the farm did not suffice.
At the corn and hay harvests they took in hired reapers, who often instead of wages received from the sixth to the ninth sheaf of the produce reaped, or, if they also thrashed, the fifth of the grain: Umbrian labourers, for instance, went annually in great numbers to the vale of Rieti, to help to gather in the harvest there. The grape and olive harvest was ordinarily let to a contractor, who by means of his men--hired free labourers, or slaves of his own or of others-- conducted the gleaning and pressing under the inspection of some persons appointed by the landlord for the purpose, and delivered the produce to the master;(8) very frequently the landlord sold the harvest on the tree or branch, and left the purchaser to look after the ingathering.
8. Cato does not expressly say this as to the vintage, but Varro does so (I. II. Relation of the Latins to the Umbro-Samnites), and it is implied in the nature of the case. It would have been economically an error to fix the number of the slaves on a property by the standard of the labours of harvest; and least of all, had such been the case, would the grapes have been sold on the tree, which yet was frequently done (Cato, 147).
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Reference address : https://www.ellopos.net/elpenor/rome/3-12-management-land-capital.asp?pg=9