While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Monsanto Company (NYSE:MON) slipped 2% this morning after Topeka Capital downgraded the seed giant from buy to hold.

So what: Along with the downgrade, analyst Chris Kapsch planted a price target of $127 on the stock, representing about 10% worth of upside to yesterday's close. Although Kapsch remains confident in Monsanto's ability to deliver solid growth in 2014, he believes that the current valuation doesn't discount enough of the potential challenges ahead.

Now what: Topeka maintained its 2014 EPS forecast of $5.15 for Monsanto, but thinks that anything higher is unlikely. "[A]chieving EPS beyond the high-end of MON's current guidance, which is expected by the Street's consensus, may increasingly become a challenge, we believe, given the potentially meaningful shift this year to domestic soy from corn acreage," noted Topeka. "Given an incrementally higher fundamental risk profile, and considering the likelihood that Ag-related sentiment remains more circumspect, we are downgrading from Buy to Hold." When you couple that uncertainty with Monsanto's forward P/E of 20, it's tough to disagree with Topeka's cautious stance.