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
Even in the more civilized south it was reckoned not
becoming for the free Celts to handle the plough. In far higher
estimation among the Celts stood pastoral husbandry, for which
the Roman landholders of this epoch very gladly availed themselves
both of the Celtic breed of cattle, and of the brave Celtic slaves
skilled in riding and familiar with the rearing of animals.(13)
Particularly in the northern Celtic districts pastoral husbandry
was thoroughly predominant.
13. "The Gallic oxen especially are of good repute in Italy, for
field labour forsooth; whereas the Ligurian are good for nothing."
(Varro, De R. R. ii. 5, 9). Here, no doubt, Cisalpine Gaul is
referred to, but the cattle-husbandry there doubtless goes back to
the Celtic epoch. Plautus already mentions the "Gallic ponies"
(-Gallici canterii-, Aul. iii. 5. 21). "It is not every race that
is suited for the business of herdsmen; neither the Bastulians nor
the Turdulians" (both in Andalusia) "are fit for it; the Celts are
the best, especially as respects beasts for riding and burden
(-iumenta-)" (Varro, De R. R. ii. 10, 4).