• by Steve McClellan  @mp_mcclellan, October 14, 2011

Add Nomura Securities to the list of Wall Street firms lowering their organic growth estimates for Interpublic Group and Omnicom Group in the fourth quarter as well as next year.

The good news: Nomura expects both companies to report solid returns for the third quarter, ended Sept. 30. IPG and Omnicom will report those results later this month.

The bet is the economy will continue to be uncertain, prompting caution and perhaps less aggressive advertising and marketing spending on the part of clients for the holding companies in the foreseeable future, Nomura said.

"We do not think the agencies will show a slowdown in revenue," in the third quarter, Nomura said in a report. "Yet if the past is any indicator of the future, we would expect a deceleration in agency [revenue growth] to appear in the coming quarters."

Nomura is knocking roughly a percentage point off its previous organic growth estimates for both holding companies in the fourth quarter and another roughly three percentage points off previous 2012 estimates for both firms.

The securities firm is now forecasting 5.5% organic growth for Omnicom in Q4, followed by 3% growth next year. For IPG, the comparable figures are 3% and 1.7%.

Nomura's recalculation follows a similar move last month by UBS, citing "global macro concerns." UBS also trimmed its U.S. ad spending growth estimate next to 3.6% from 4%.

A number of other firms have also downgraded ad spending estimates. Just last week, ZenithOptimedia, the Publicis-owned media agency network, cut its 2011 global advertising expenditure forecast by half a percentage point to 3.6%, resulting in total expenditures of $466 billion.