The grains ended Monday just like they started.

March soybeans led the slide, finishing the day 33 3/4 cents lower at $11.85 1/4 per bushel, according to Barchart.com. March wheat was 2 1/2 lower at $6.44 3/4 and March corn was 10 cents lower at $6.31 3/4.

Weekend rainfall in South America yanked out the risk premium built into the grains over the last 2 weeks, with broad-based selling of soybeans, analysts say. That's on top of a stronger U.S. Dollar Index Monday on anxieties surrounding the EU debt situation.

• Read more: Traders extract weather premium
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Despite Monday's main grain market story being the rainfall in South America, the rains that did fall (and more expected later this week) may be too little too late, crop-watchers say.

• Read more: Brazil drought worse than feared

Adds Cargill senior grain merchandiser Ray Jenkins on Monday's trade: "I think the market is telling us the rains came in time to preserve most of the soybean crop, but not in time to keep from seeing losses in the Argentine corn crop."