Corn futures suffered their sharpest drop in almost three weeks on Wednesday as traders moved to lock in profits following a powerful rally that had pushed speculative positioning to its highest level in years, according to a report by Bloomberg.

The most-active Chicago corn contract fell as much as 1.8%, its biggest intraday decline since 13 August. Wheat and soybeans also came under pressure as investors took some money off the table across the US grain complex.

The pullback follows a sharp August rally in agricultural commodities, driven by a combination of geopolitical tensions and adverse weather. Attacks on Black Sea ports and shipping have raised concerns over global grain supplies, while heatwaves in key growing regions have added to fears of weaker crop output.

The Bloomberg Agriculture Spot Index, which tracks 10 major agricultural products, jumped more than 13% in August — its strongest monthly advance since July 2012.

Hedge funds have been among the beneficiaries of the move. Speculators' net bullish position in corn reached a four-year high last week, according to Commodity Futures Trading Commission data, reflecting the rapid accumulation of long exposure during the rally.

Wednesday’s decline represents more than a simple reversal in sentiment. After weeks of gains, some commodity-focused hedge funds and other speculative investors appear to be using the strength in corn and other grains to realise profits.