Activist hedge funds are launching more campaigns against larger companies this year, but settlements dropped 53 percent in the first half of this year, according to ActivistMonitor, a division of research firm Acuris. In recent high-profile case, P&G said in October that its shareholders rejected a bid from Trian Partners' Nelson Peltz for a seat on the company's board. Peltz has not conceded the vote. P&G is the largest company in history to face a proxy fight.
Rodriguez and Ackman have been engaged in a heated war of words over the last few months since the latter revealed his stake in August. Ackman has said ADP is "very inefficient" and suggested Rodriguez is not the right person to lead the company. Rodriguez has compared the activist investor's efforts to that of a "spoiled brat" who "doesn't know what he's talking about."
"We didn't learn a lot of new things as a result of Bill's presentation and also his ongoing comments, but there's really no dispute about the company's path and what it needs to do," Rodriguez said. In response to a question, he added that he didn't think Ackman's sometimes fierce approach made a difference.
Omega Advisors Chairman and CEO Leon Cooperman, who served on ADP's board for 20 years through 2012, told Ackman his activist campaign was not "intelligent." ADP has "quality management that has done a great job over many years for the shareholders," Cooperman said in August.
Ackman's Pershing Square Capital Management has a nearly 2 percent stake in ADP common shares, according to FactSet. Including derivatives, Ackman has an 8.3 percent stake. The hedge fund manager told CNBC on Monday that he would still keep a large position in the stock even if he lost the vote.
Pershing Square was down 3.3 percent for the year as of Oct. 31, according to its website. The S&P 500 has risen about 15 percent this year.
— With reporting by CNBC's Leslie Picker.