TV just isn’t what it used to be.

That’s what marketers are saying, with a new study showing a majority of them feel TV advertising has become less effective in the past two years. That shouldn’t come as a surprise given that nearly 25 percent of U.S. households are now equipped with DVRs that let viewers zip through those pricey commercials.

For the TV networks — who sell more than $9 billion in prime-time commercial spots each year — the really bad news is that when DVR penetration reaches 50 percent marketers are going to start cutting their spending. In fact, once DVR penetration hits that level, the study found, that more than 50 percent of marketers surveyed said they would slice spending on TV advertising by 12 percent.

Here’s some other major findings from National Association of Advertisers and Forrester Research, which surveyed 78 leading advertisers in January (which, of course, was right smack in the middle of the screenwriters’ strike, when TV had all the bad press it could handle).

• Close to half of the advertisers surveyed are experimenting with new ad formats for  DVRs and VOD. Eighty-seven percent of advertisers say branded entertainment will play a stronger role in TV advertising in the coming year.
•  Advertisers are eager to try new ad formats, including ads in online TV shows (65 percent), ads embedded in VOD (55 percent), interactive television ads (43 percent), and ads within the set top box menu (32 percent).
• Eighty-seven percent of respondents said they intend to spend more on Web advertising this year.
• Seventy-two percent of marketers are very interested in having individual commercial ratings rather than average commercial ratings.