Nelson Peltz's Trian Fund Management began a proxy fight to win a board seat at Procter & Gamble Co., characterizing the maker of detergents and diapers as a lumbering giant whose stock has underperformed its peers.

Trian will seek a seat for Peltz at P&G's annual shareholder meeting, according to a proxy filing Monday with the U.S. Securities and Exchange Commission. The firm, which initially revealed its position in February, now holds 37.6 million P&G shares, or about 1.5%. It's not seeking a breakup of the company or a new chief executive officer, but rather to shake up its “slow-moving and insular” culture, according to the filing.

P&G CEO David Taylor is struggling to reignite sales growth at the maker of Tide laundry detergent and Pampers diapers as it faces assaults from cheaper rival brands and retailers that are keeping a tighter rein on inventory to defend themselves from online competition. P&G shares are showing a 5.1% total return for shareholders since the start of the year, trailing gains of 29% for Unilever in local currency terms, and 12% for Colgate-Palmolive Co., according to data compiled by Bloomberg.