I feel summoned.

The first thing to say is that, in the Middle Ages as a whole, it's unfair to say that Ukraine was not given credit for its agriculture. During the Black Sea Boom (roughly mid 13th-15th centuries), when Venetians and Genoese merchants are making a killing in the region, grain comprises a significant commodity which they export from the northern coastline. Indeed, there's so much of it available that merchants are able to refer to handbooks (such as the famous one by Pegolotti) to tell them which specific regions provide the best grain. Venetian and Genoese control of this grain trade allowed them to blackmail the ever-weakening Byzantine Empire with threats of starvation - because the grain supply of the modern Ukraine was essential for them. I think it is fair to say in this period that what is now Ukraine was in fact acknowledged as an excellent grain resource, although its fame in this regard was overshadowed by the exciting Silk Road commodities available in the region, particularly at the mouth of the Don.

However, for the earlier period - you are right. Ukraine was surprisingly underappreciated during the pre-boom years, when it was ruled by the Kyivan Rus. That's such an interesting question that someone ought to have done a PhD thesis on that very topic.

It's first necessary to prove that there was grain there. The Kyivan Rus territory, whilst not identical with modern Ukraine, covered a large portion of the relevant area - the 'Chernozem Belt'. Chernozem, literally meaning 'black earth', is a particularly fertile soil; it's found in only two places - the Pontic-Caspian steppe, and the North American plains. To give you an idea of how fertile the soil is, it's worth noting that before the first Russian invasion in 2014, Ukraine had been obliged to put a law forbidding the export of Chernozem, but this only led to the establishment of a black market worth approximately one billion dollars.

Before the Kyivan Rus, the area was inhabited largely by nomads, who were noted for being non-agriculturalists; as a result, the Chernozem was not exploited. However, when the Kyivan Rus arrived, this circumstance changed. The Rus were not oblivious to this soil. We know that they engaged in significant agriculture. They had a form of field-rotation called perelog, they had punishments for ploughing beyond the limits of ones own estate, and in their markets you could find all kinds of cereal grains for sale, indicating that this was not merely subsistance farming. The sources are a bit quieter on agriculture than other topics (i.e. church and war), but there is no doubt that agriculture comprised a large part of the Rus economy.

Yet, as you noted, and as other historians have noted before you (the first of which was probably Teall), we don't have any record of Rus grain exports. There's a whole list of people (mostly arabic sources) who discuss Rus merchants, and the commodities they export - amber, fur, slaves, even swords which can bend in two - but there is no source which mentions Rus merchants selling grain.

We can, in part, attribute this to the nature of historical sources - bendy swords are interesting, but grain is boring, and so no-one comments on grain at all, unless they're starving. So most mention of grain in medieval sources at this time is either completely generic, giving no indication of the grain's provenance; or refers to grain drawn on in emergency and unusual circumstances, meaning that we can't easily draw on such mentions to deduce the day-to-day grain trade. The absence of grain from most sources can be seen by the fact that there is no Byzantine source from this period describing the Rus lands as fertile or well-farmed or anything similar. This is despite the fact that there were significant Byzantine-Rus connections, with bishops, diplomats and artisans flying to and fro. It was just that grain and agriculture was often such a banality that people (at least, rich, literate people) very seldom commented on it.

However, I postulate that there's another reason we don't hear much about Rus grain. If we compare Kyiv to other potential 'breadbasket' regions, we'll notice something unique. Sicily and Egypt are often praised for their agricultural produce - they send ships out to Rome and Constantinople and Genoa and Venice and anywhere people wanted toast. That was the key to their success - their maritime connections. By contrast, Kyiv is landlocked. If the Rus wished to export large quantites of grain, they'd be forced to rely on cart exports, over long distances, through often hostile regions. Frankly? Not worth the effort - not when you can make a bigger profit with your bendy swords and walrus-tusk boxes, which the Byzantines down south love dearly.

"But hang on, echoswolf - what about 'the route from the Varangians to the Greeks'? The most famous thing about the Rus? Their ship-borne trade route down south? Surely that can be an export route?" Not so, my friend. The Rus travel this route in special ships called 'monoxyla' - very small ships, which are not built but carved from tree-trunks. Their draught is shallow enough that you can walk alongside them in the river; their weight is such that they can be easily carried. These are not quite dugout canoes - but they cannot have been far off. These small ships braved dangerous rapids, then travelled open seas along the Black Sea coast. For most of the way, their crew would have been in danger of attack from nomads - either Pechenegs or Kipchaks. Furthermore, these monoxyla didn't have enclosed holds. In such ships, transporting grain - especially in sufficient quantities to make good profit - is very difficult. It's bulky, it gets in the way when you're fighting, and if it gets wet it rots. Again - goods like amber and slaves are easier, more profitable, and require less effort than grain. So why bother?

So the reason we don't hear about Rus exporting grain, is because they didn't. They had more fun, saga-rific exploits to engage in, and didn't export large quantities of grain to foreign nations.

But this begs the question - where did the excess Rus grain go? We know there was some internal trade, with grain heading north towards Novgorod, but that's not sufficient to account for the presumable surplus the chernozem-fields supplied. It's possible that there was an alternative, overland route between the Rus and the Crimea. We do know that, on occasion, there are 'Rus carts' in Crimean cities in the mid-13th century; we also know there's a ford where Byzantines 'cross over' from the lands of the Rus. There's some sigillographic evidence, of a Byzantine grain official, that potentially indicates grain exports from Crimea to Constantinople. It's not conclusive - but it does fit the data.

If there was this second route, it would solve the logistical problems the Rus faced. Grain could be exported a short distance overland to Byzantine Cherson, in the Crimea; from there it would travel in larger, enclosed ships to the various cities of the Black Sea. When grain arrived in Constantinople, it would effectively be anonymous - it had been transported by Greek sailors in Greek ships from a Greek port, with no real evidence to its eventual eaters that it had ever been Rus. And given how rarely the written sources comment on the provenance of their grain, we'd be lucky to find any evidence of it in the literature.

So, to summarize - Why don't we hear about Ukraine as a fertile region in the middle ages? Because at first nomads didn't farm it; then, when the agricultural Rus take over, they do farm, but apparently don't export. Any exports that did take place, wouldn't have been associated with the Rus and Ukraine. Once a maritime and economic revolution takes place, and western Europe has access to Ukraine, then people start to realize how fertile the region is, and it develops its modern reputation.