The most basic kind of transport is often small-scale overland transport, either to and from the nearest city, or in small (compared to what we’ll discuss in a moment) caravans moving up and down a region […]. The Talmud, for instance, seems to suggest that much of the overland grain trade in Palestine under the Romans was performed with itinerant donkey-drivers in small caravans – and I do mean small. Egyptian tax evidence suggests that most caravans were small; Erdkamp notes that 90% of donkey caravans and 75% of camel caravans consisted of three or less animals. These sorts of small caravans don’t usually specialize in any particular good but instead function like land-based cabotage traders, buying whatever seems likely to turn a profit at each stop and stopping in each town and market along the way. Some farmers might even do this during the off season; in Spain, peasants often worked as muleteers during the slow farming season, moving rents and taxes into town or to points of export for their wealthy landlords and neighbors.

Truly long-distance bulk grain transport overland wasn’t viable for reasons we’ve actually already discussed. There is simply nothing available in the pre-modern period to carry the grain overland that doesn’t also eat it. While moving grain short distances (especially to simply fill capacity while the main profit is in other, lower-bulk, higher value goods) can be efficient enough, at long distance, all of the grain ends up eaten by the animals or people moving it.

The seaborne version of this sort of itinerant, short-distance trade is called cabotage. Now today cabotage has a particular, technical legal meaning, but when we use this word in the past, it refers not to the legal status of a ship but a style of shipping using small boats to move mixed cargo up and down the coast. In essence, cabotage works much the same as the small caravans – the merchant buys in each port whatever looks likely to turn a profit and sells whatever [is] in demand. By keeping a mixed cargo of many different sorts of things, he protects against risk – he’s always likely to be able to sell something in his boat for a profit. Such traders generally work on very short distances, often connecting smaller ports which simply cannot accommodate larger, deeper-draft long-distance traders. Such cabotage trading was the background “hum” of commerce on many pre-modern coastlines and might serve to move grain up or down the coast, although not very much of it. Remember that grain is a bulk commodity, and cabotage traders, by definition, are moving small volumes.

But when it comes to moving large volumes, the sea changes everything. The fundamental problem with transporting food on land is that the energy to transport the food must come from food, either processed into muscle power by porters or animals. But at sea, that energy can come from the wind. So while the crew of a ship eats the food, the ship can be scaled up without scaling up the food requirements of the crew or the crew itself. At the same time, sea-transit is much faster than land transit and that speed is obtained from the wind without further inputs of food. It is hard to overstate how tremendous a change in context this is. Using the figures from the Price Edict of Diocletian, we tend to estimate that river transport was five times cheaper than land transport, and sea-transport was twenty times cheaper than land transport. So while the transport of bulk goods like grain on land was limited to fairly small amounts moving over short distances – say from the farm to the nearest town or port – grain could be moved long distances en masse by sea.

Now the scale and character of long-distance transport is heavily impacted by the political realities of the local waterways. If the seas are politically divided, or full of pirates, it is going to be hard to operate big, slow vulnerable grain-freighters and still make a profit after some of them get seized, pirated or sunk. But when we have periods of political unity and relatively safe seas, we see that this sort of transport can reach quite impressive scale. For instance the port regulations of late Hellenistic and Roman Thasos – itself a decent sized, but by no means massive port – divided its harbor into two areas, one for ships carrying 80-130 tons of cargo and one for ships 130+ tons (those regulations are SEG XVII 417). A brief bit of math indicates that the distribution of free grain in the city of Rome – likely less than a third of the total grain demands of the city – required the import, by sea of some 630 tons of grain per day through the sailing season. The scale of grain shipment in the back half of the Middle Ages (post-1000 or so) was also on a vast scale, with trade-oriented Italian cities exploding in population as they imported grain (Genoa being particularly well known for this, but by no means alone in it); with that came the reemergence of truly large grain-freighters.

Bret Devereaux, “Collections: Bread, How Did They Make It? Part IV: Markets, Merchants and the Tax Man”, A Collection of Unmitigated Pedantry, 2020-08-21.