I don't write about the grain markets very often, but the Commitment of Traders report for corn caught my eye this weekend.

The large speculator group has moved from a net short position to the biggest net long position since December 2012. And they did it in less than two months.

On Feb. 4, the group was short less than 5,000 contracts, but as of last week, the group was net long 308,663 contracts.

Granted, the price of corn was up more than 25 percent from the second week of January to April 1. The price jumped from a low of $406.25 to a high of $512.50 during the run.

Corn is now hitting some potential resistance at its 52-week moving average, which is at $506.18. The last time corn was able to close a week above this trendline was in March of 2013.

My concern for the bullish corn investors is that the price has jumped too much in too short a time. With the sentiment shifting so drastically in such a short time, how many investors that are interested in corn are left on the sidelines?

I personally would take a cautious approach to corn right now. In the past 10 years or so the only time we have seen this type of rally in corn was associated with a rally in oil prices. But this time around an oil rally isn't one of the drivers behind the rally in corn.