(Reuters) - Billionaire investor William Ackman on Friday said his hedge fund had taken an 8 percent stake in Automatic Data Processing Inc ADP.O and plans to ask for board seats so he can push the U.S. human resources outsourcing company to cut costs and improve its returns.

FILE PHOTO: Bill Ackman, chief executive officer and portfolio manager at Pershing Square Capital Management, speaks during the SALT conference in Las Vegas, Nevada, U.S. May 18, 2017. REUTERS/Richard Brian/File Photo

Ackman's move on ADP, which has a market value of about $50 billion, comes as the leader of Pershing Square Capital Management looks for a big win following money-losing investments in debt-laden drugmaker Valeant Pharmaceuticals International Inc VRX.TO and norovirus-stricken restaurant chain Chipotle Mexican Grill Inc CMG.N.

The 51-year-old activist investor said he would seek minority representation on ADP’s 10-member board. ADP said earlier on Friday that Ackman wanted five board directors, including a seat for himself.

ADP also said Ackman wanted to oust Carlos Rodriguez, ADP’s chief executive since 2011. Pershing Square said in a statement later in the day that it was willing to work with existing management at ADP or a new external CEO.

Ackman said he had asked ADP to extend its Aug. 10 deadline for board nominations to facilitate negotiations with the company. ADP said it declined this request, which would have pushed back the nomination deadline by as much as 45 days.

Ackman did not disclose his board nominees in his conversations with ADP, people familiar with the discussions told Reuters on condition of anonymity.

He could disclose them as early as next week, in a filing with the U.S. Securities and Exchange Commission, which will have details on his position in ADP, one of the people added.

Ackman believes ADP can improve its operating performance by accelerating growth, improving the quality of its software and service offerings, slashing operating costs and increasing efficiency, Pershing Square said in its statement.

The stakes are high for Ackman. His fund lost 10.2 percent last year after losing 16.6 percent in 2015. It had posted a 37.2 percent gain in 2014.

If Ackman misses the nomination deadline, ADP’s bylaws dictate that he would have to muster the support of 33 percent of the company’s shareholders to call for a special shareholder meeting, if he does not want to wait for next year’s annual meeting.

SHORTED STOCK

While Ackman is betting ADP’s stock, which he previously owned between 2009 and 2011, has room to rise, others are betting it will fall.

ADP is the third-most shorted company in the data processing and outsourced services sector behind Visa Inc V.N and PayPal Holdings Inc PYPL.O, according to data analytics firm S3 Partners, amid concerns that new business bookings are slowing down.

ADP said that during Rodriguez’ six-year tenure, ADP has yielded a total shareholder return, which includes dividends, of 202 percent. That is well ahead of the Standard & Poor’s 500 Index, which has returned 128 percent on that basis over the same period, and Pershing Square itself, which has achieved only 29 percent.

Ackman may find it hard to challenge Rodriguez, given his track record as CEO, some analysts said.

“Rodriguez has very broad-based investor support and has factually delivered very compelling returns for shareholders so Pershing Square faces an uphill battle in this case,” said Bernstein Research analyst Lisa Ellis.

Ackman built his 8 percent stake in the company largely through derivatives, rather than common stock, according to ADP.

Accumulating a position in such a way allows activist investors to build large stakes more cheaply, because the stock does not run up once other company shareholders find out about their bet.

However, reports of Ackman accumulating a stake in the company surfaced last week, and ADP’s stock has risen 8 percent since then.

Pushing for new CEO has been a hallmark among activists recently. This week, hedge fund Barington Capital notched a victory as Avon Products Inc AVP.N CEO Sheri McCoy announced she would step down. Carl Icahn won an even bigger prize by calling for the ouster of American International Group Inc AIG.N CEO Peter Hancock, who left the company earlier this year.

Ackman himself has forced out CEOs at companies including Air Products & Chemicals Inc and Canadian Pacific Railway, where he earned large gains on his investments.