Warren Buffett's Berkshire Hathaway was dramatically increasing its holdings of Deere & Co. during the second half of last year, but the buying was kept secret until now.

Berkshire is required to reveal a snapshot of its U.S. publicly traded stock holdings once a quarter.

Tonight's filing with the SEC shows what Berkshire owned on Dec. 31, the end of last year's fourth quarter.

It lists a Deere stake of 17,096,886 shares, currently worth just over $1.5 billion.

That appeared to be a new position, since there was no mention of Deere shares in the company's Q3 filing.

When that filing was made in mid-November, it was assumed Berkshire had sold the almost 4 million shares included in its Q2 filing and in several quarterly filings before that.

That assumption turned out to be false. Another filing tonight reveals Berkshire held almost 7.6 million Deere shares when the third quarter ended on Sept. 30, a 90 percent increase from the end of the second quarter.

The stake grew by 126 percent in the fourth quarter.

Why didn't the third-quarter filing include any Deere stock? At the time, Berkshire noted that some unspecified "confidential information" had been omitted from the public document and filed separately with the SEC.

The SEC sometimes allows this to happen so Buffett can build a stake over time without copycat buyers driving up the price.

Today, the SEC's "confidential treatment" expired and the secret was finally revealed.