Monsanto Says Justice Dept Closes Inquiry  NEW YORK (Reuters) - Monsanto Co. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=MON.N target=/stocks/quickinfo/fullquote"&gt;MON.N&lt;/A&gt; on Wednesday said  the U.S. Justice Department has closed an inquiry into  potential antitrust issues regarding a key ingredient used in  its Roundup herbicide.