OMAHA, Neb. » Warren Buffett’s company has partially rebuilt the stake in Johnson & Johnson he reduced over the past two years to raise cash for other investments, and Berkshire Hathaway Inc. boosted its investment in Walmart Stores Inc. during the second quarter.

Berkshire detailed its $46.4 billion U.S. stock holdings yesterday in a filing with the Securities and Exchange Commission.

The document revealed several changes in the Omaha-based company’s portfolio between March 31 and the end of June, including decreases in Berkshire’s holdings in Kraft Foods, ConocoPhillips, Procter & Gamble and M&T Bank. Berkshire also increased its stakes in Becton Dickinson & Co., Nalco Holding Co. and Sanofi Aventis.

Berkshire also revealed a 4.4 million-share stake in Fiserv Inc. That new investment in the Brookfield, Wis.-based information management and payment processing company was worth roughly $200 million at the end of June.

But the biggest change came in Berkshire’s J&J stake, which grew to 41.3 million shares at the end of June. That’s up from 23.9 million shares in March. In 2008 and 2009, Buffett sold off some of Berkshire’s Johnson & Johnson stock to help pay for more attractive investments.

Berkshire held 64.3 million shares of New Brunswick, N.J.-based Johnson & Johnson at the end of 2007. Buffett, who is Berkshire’s CEO and chairman, has said he sold some stock in the drug and medical products maker in 2008 to free up capital for Buffett’s $14.5 billion fixed income investments in Goldman Sachs Group Inc. and General Electric Co.

Later in 2009, Buffett again sold some J&J stock to help raise cash for Berkshire’s $26.7 billion acquisition of the Burlington Northern Santa Fe railroad.

Berkshire officials did not immediately respond to a request for comment yesterday, but they don’t typically comment on the company’s stock holdings beyond what it is legally required to disclose.

Berkshire added 4.2 million Walmart shares during the quarter to give it 38.1 million shares of the discount retailer.