Gains in wheat ignited the rally as hopes for a pickup in US wheat export demand pushed prices through key technical levels. Corn and soybeans promptly followed and also broke through chart resistance points.
"We're still seeing some short covering after that large sell-off on Tuesday," said Tim Marsh, analyst with Summit Commodities.
Grain futures fell to multi-week lows earlier in the week on concerns about slow progress in talks to resolve the US-China trade war that has slashed Chinese demand for US soybeans and grains.
Markets have since been clawing back from those lows, awaiting any news of grain purchases promised by Beijing or signs that a deal to end the bitter trade fight could be reached by a March 2 deadline, when US tariffs on Chinese products are due to jump.
Adverse weather in South America continued to underpin corn and soy markets. Numerous crop watchers have scaled back their harvest forecasts for Argentina and Brazil due to stressful weather.
The Buenos Aires Grains exchange cut its forecast for Argentine soybean plantings to 17.7 million hectares, from 17.9 million hectares previously, due to excessive rains.
Chicago Board of Trade March soybeans jumped 15-1/2 cents to $9.10 a bushel by 12:36 p.m. CST (1836 GMT), breaking through chart resistance at its 20- and 50-day moving averages.
March corn rose 6-1/2 cents to $3.80-1/2 a bushel after breaching resistance at its 20-, 50- and 100-day averages.
CBOT March wheat was up 9 cents at $5.21-1/2 a bushel, rising above its 50-day moving average but holding just below its 100-day.
Concerns that harsh cold could threaten vulnerable wheat crops in the US Plains that do not have a protective blanket of snow supported the wheat market, traders said.
Continued speculation that top wheat exporter Russia may move to curb exports also supported futures, although Moscow has not officially changed its export outlook.