James Trego and Andy Caddick frustrated Nottinghamshire with a resolute 90-partnership to earn two unexpected batting points at Trent Bridge.
Having resumed alongside Alfonso Thomas, who fell lbw to Shreck in his first over of the day, Trego reached 86 before he was caught on the boundary by Mark Wagh off Graeme Swann.
Caddick’s unbeaten 35 took Somerset to 252 before Swann bowled Willoughby to end the innings.
Bilal Shafayat and Will Jefferson survived two overs before lunch, reaching 3 without loss before a four-hour rain delay.
Jefferson edged to Kieswetter as Charles Willoughby claimed two wickets in four balls to dent Nottinghamshire's fightback. Nightwatchman Andre Adams was Willoughby's second victim, caught at gulley by Wes Durston.
Notts closed 206 runs behind at 46-2.