Remember that cabin porch scene in “My Cousin Vinny” where Marisa Tomei frantically complains about the “tick-tick-ticking” of her biological clock? I’ll never forget Joe Pesci’s exasperated response.

Speculators ramped up bullish views in Chicago corn and soybeans last week with futures notching or nearing new highs, but despite the unusually strong optimism, investors are much less invested than ...