Six players were absent from the 49ers' OTA practice on Thursday, according to the San Jose Mercury News.
Those not present were wide receiver
OTA practices are voluntary and players are not required to attend.
Boone is scheduled to make $3.4 million in 2015, but he's seeking an extension.
Reportedly, Boone doesn't plan to skip training camp, but he would welcome a trade.
Ventrone only played eight games this season for San Francisco and recorded just three tackles.
Holliday was waived in Week 9 from the Buccaneers to promote Marcus Thigpen.
Replacement Joe Looney and the rest of the offensive line played well in Boone's absence as San Fran averaged 5.1 yards per carry and allowed one sack.
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