Romans carrying 17 days rations

[Nathan Ross]

17 days food and fodder is an enormous weight.  How could a soldier carry it - when he's always got some 60lbs of other things...?

Vegetius (1.19) has Roman troops trained to carry 60 pounds (modern 43lbs), in addition to their arms. What this load would have comprised is up for debate. One day's ration would be about 1-1.3kg (2.5-2.8lbs) per man (as per Roth, Logistics of the Roman Army at War).

Here are the quotes in question, for discussion purposes:

Ammianus Marcellinus 17.8.2: “[Julian] had the grain allowance for twenty days taken from what was to be consumed in the winter quarters, and baked up to serve for some time; he put this hard-tack (as they commonly call it) on the backs of his willing soldiers…"

(Then some time later):
 
Ammianus Marcellinus 17.9.1: "he planned to repair (as time would permit) three forts situated in a straight line along the banks overhanging the river Meuse [and so] he took a part of the seventeen days' provisions, which the soldiers, when they marched forward on their expedition carried about their necks, and stored it in those same forts, hoping that what had been deducted might be replaced from the harvests of the Chamavi."

Historia Augusta, Alexander Severus, 2.47.1: "During his campaigns he [Alexander Sev] made such careful provision for the soldiers that they were furnished with supplies at each halting-place and were never compelled to carry food for the usual period of seventeen days, except in the enemy's country. And even then he lightened their burdens by using mules and camels..."

Cicero, Tusculan Disputations, II.17:"how great the labor is of an army on its march... consider that they carry more than a fortnight’s provision, and whatever else they may want; that they carry the burden of the stakes, for as to shield, sword, or helmet, they look on them as no more encumbrance than their own limbs."

Caesar, Bellum Civ 1.78: [Caesar's legionaries] "had been ordered to bring a twenty-two days' supply [of grain] from Ilerda; the light-armed and auxiliaries had none, since... their bodies were not trained to carry burdens."