「Pricks」with「horizontal head whacked」describes the process today known as threshing, and the results achieved after removing the awns (grass spikelets) and rachis from the tips of grain stalks.「Cereal」simply means EDIBLE SEEDS, i.e., seeds that are to be eaten. Throughout Chinese glyphs, cereal grain (禾 seeds of some sort) is used as a euphemism for ejaculated semen, and, as symptoms of gonorrhea, both「descending slowly between feet below prick」and「crotch head prick.」Nonetheless, ingesting gonorrhea bacteria is not recommended generally for most folks to try at home, however dynastic Chinese service providers sometimes had little, if any, choice in that matter.
「Pricks」with「lying prone and spread feet, head in crotch」with the ends of the legs (that is the feet, or, what remains of them) positioned above or over the head beside the ears…consider that positioning and what might possibly drip down into one's mouth after researching some samples of erotic Chinese art. Interestingly, in English also, reference is made to 'heads' of grain, 'ears' of corn. Refers to the tops of the plant being very heavy and hanging down 'with the head inverted' (and what head hangs inverted most of the time?) with spiked bristles and ripened grain, meaning BARLEY. Heads of barley, rye, and wheat appear quite bristly, with long-awned, bristly spikes (rachis, the「pricks」) surrounding the seed grains, compared to heads of rice grain with much shorter rachis. Top element of「pricks, thorns」and the lower element is inverted feet, meaning「slowly advancing, very obvious, hanging upside down」as a reference to the condition of heavy heads of grain. Original glyph used the old form of「coming」as the top element (麥), perhaps since it resembles the spiked rachis of barley and rye,…or because seeds are「coming, on the way」as a comparison of the similarities shared by both cooked barley grains and droplets other than urine from a prick.

External source definitions …excluding politically incorrect concepts and other meanings deemed offensive today; may include extended definitions, euphemisms, and mnemonics)
Chinese: wheat; barley; oats
Japanese EDRDG: barley, wheat
Unihan extended: wheat, barley, oats