Most antique sources we have come from the Mediterranean regions and Middle East that were dominated by wheat cultivation. Authors writing about the agricultural practices in the Roman Empire, such as Columella in his De re rustica or Varro in his work of the same title speak extensively about wheat (Triticum aestivum) and its variety called far, under which name they most likely refer to emmer (Triticum dicoccon). Some, like Pliny, mention rye but do not speak of it in length, usually limiting themselves to note its inferiority and general lack of interest in that crop. It seems however, that wheat, ubiquitous in the southern Europe, was way less common in the northern part of the continent, for the reasons described below.

The main reason for the differences in the cereals cultivated in various regions of Europe were the climatic conditions and the requirements of the particular plants. Wheat gives large yields of highly caloric grain, but it requires fertile soils, warm climate, good hydrology and substantial amount of work invested in the fieldwork. This meant that although popular in southern provinces of Rome, such as Italy (especially in the areas near volcanoes that sported very fertile volcanic soils) and the equally good alluvial plains of Egypt (according to Varro, yields in these regions could reach 350-400 kg per aroura or iuger, i.e. roughly 0.25 ha or 5/8th of an acre).

Rye, although yielding relatively lower amount of grain and not as good according to caloric value as wheat or barley has the large advantage of durability, allowing it to be cultivated in colder climates. In addition, larger root system and smaller leaves leads to better absorption and more efficient usage of water. This means that rye fares much better than wheat or barley in colder, dryer and less fertile areas that are quite typical for the northern part of Europe. In the regions characterized by poorer soils, rye was also popular due to its relatively low propensity to deplete the soils, even in the monoculture. Conversely, rye cultivation is very hard in any substantially warm regions, like a large part of the Mediterranean area, as it requires frost to germinate and thus is illy-suited for the warmer climates. There were exceptions, of course, especially in the colder climates, as e.g. upper Po valley mentioned by Pliny or in more continental, mountainous Anatolia.

Please note that basically entire half of Europe north of Austria and Romania share latitudes with Canada and southern Siberia with northern reaches of southern Sweden, Norway and Finland (e.g. cities of Bergen, Uppsala or Helsinki sharing them with southern tip of Greenland (roughly 60 degrees north). This means that the entire region lies firmly in the colder climatic range that is only mediated by the warming influence of the Atlantic and especially the Gulf Stream.

In addition to already imperfect climatic conditions, regions in question (northwestern part of what is now Germany, and the entire northern part of Central and Eastern Europe) are a transitory region between the boreal and subboreal podological zone characterized by relatively poor soils, chiefly a mixture of luvisols and cambisols (brown soil) that turns to mainly ambisols (white soils) in northern Russia and podsols (white sandy soils) in Jutland and Scandinavia. These regions are also largest producers of rye even today, with Poland, Germany and Russia in the firm lead and Belarus, Denmark or Ukraine closely behind.

For example, contemporary sources describing the agriculture in Polish-Lithuanian Commonwealth in 16th-17th century allow us to estimate that the wheat accounted for only 5-10% of all cereals cultivated there and was largely limited to the fields in warmer, southeastern Poland. Agriculture in the Commonwealth was dominated by rye, constituting almost 40-50% of all cereals (with the northern provinces showing larger affinity to this type of grain) with oats, crucial as the popular animal nutritional fodder forming another 35-40%. Remaining 10-15% was barley, with its popularity rising along the development of the breweries. This could have been likely caused by the climatic factors, as in the aforementioned period wheat shown on average lower yield (4.5 grains per 1 grain sown) than barley or oats (5 and 4.7 grains respectively) and similar to that of rye.

The fall of Roman Empire itself does not seem to have much in common with the regions mentioned in the text, what more or less extends to anything else, as almost entire northern Europe with the exception of England (Britannia) and northern France (Galia) were located outside the Roman sphere of direct influence with lands corresponding to modern Scandinavia and Central-Eastern Europe most likely having virtually no contact with Roman culture (few Roman artifacts from the Imperial period have been found near the Baltic shore, but it is unclear, whether they have been brought by the Roman traders coming for amber or they were brought through other peoples who have better contacts with Romans, such as Germanians or Thracians). But I digress.

Although the expansion of the rye cultivation in northern part of the Europe is usually associated with the early Middle Ages, this is considered more a result of the peoples migrations that gave this period one of its common names, there is evidence that in what is now northwestern Germany, rye has already been cultivated in the Roman period (although the exact chronology is hard to ascertain, given that rye co-existed with other cereals as the common weed or incidental crop since Neolithic). Also please note that the statement 'recorded history beginning with the fall of Roman Empire' (I assume author(s) mean Western Roman Empire) does not really apply to the Central and Eastern Europe as the earliest written sources from this region generally appear only after the adoption of Christianity, i.e. 9th-10th century.

Nevertheless, the first substantial archeobotanical findings strongly suggest that rye has already been cultivated in the early pre-Roman Iron Age near what is now Merseburg in central-eastern and southern part of Germany as well as in 6th-5th century BCE in the areas near what is now Poltava in Eastern Ukraine and in 4th-3rd century BCE in the Crimea region. Traces of rye cultivation in what is now western and southern Poland are younger but still correspond to the Roman period (1st century BCE - 2nd century AD). No evidence of rye cultivation in the period not earlier than early Middle Ages comes primarily from Jutland and Pannonia.

It is true that the cultivation of rye significantly increased in the early Middle Ages, especially after the 10th century, what is coincides with the formation of new centralized states and steadily rising number of settlements and population numbers in the region. In other words, the link between the fall of Roman Empire and rye cultivation is generally incidental, as the migration and subsequent colonization and development of the regions in northern Europe called for large amounts of crops with rye being the cereal best suited to the colder climate and poorer soils typical for the region.