Useful medieval data are pretty much limited to cereals, from the 13th century for a sample of English estates and only later for most countries, and are often in the form of returns to seed (eg 4 or so bushels harvested per bushel of seed sown), with seed use itself often unknown or subject to wide variation. The estimates of BMS Campbell, EA Wrigley and Robert Allen suggest average English yields c.1300 of about 750-800 kg per hectare gross or perhaps 550-600 kg net of seed, with similar returns in other parts of northwestern Europe: for most of the Continent, though, yields are likely to have been considerably lower (see BH Slicher van Bath, The agrarian history of western Europe AD 500-1850), suggesting an overall average nearer 700 kg gross or 500-550 kg net.

How does that compare to today? The FAO reports an average gross yield of 4,300 kg for the whole of Europe including Russia or 5,200 kg/ha without it, rising to 7,000 kg in western Europe. Seed use meanwhile has changed little, averaging 170 kg or so in recent decades, so that while gross yields have risen around sevenfold net yields have risen eight- or ninefold. Most of this increase has occurred in the last century, with yields roughly quadrupling since the eve of the First World War.