The Auckland-born centre - who has made nine appearances for his country - has been an almost ever present in the starting line-up over the last eight seasons. The ultra-consistent back is also a leader away from the action as a superb role model for younger players who epitomises club values with his attitude and work ethic.
Rock solid back row performer Horstmann, 30, has made 171 appearances since joining the Warriors from Harlequins in 2005 and has been an integral part of the side at Sixways.
The forward has captained the side on a number of occasions and was awarded the Player's Player-of-the-Year at the club's annual awards at the end of the 2005/06 season. Horstmann repeated that feat at the end of the 2006/07 season when he was named Player of the Year and secured a dream double with both the Player of the Year and Player's Player of the Year award at the 2008/09 awards dinner.
Head Coach Richard Hill paid tribute to both players after it was confirmed they would leave the club.
"It has been two tough decisions," he said. "I have got to look to change the squad and bring my own players in, it takes three years before you can develop the squad you want.
"With Kai I have got to consider we have a number of young players like Jake Abbott, Sam Betty and Matt Kvesic and the likes of Blair Cowan and Semisi Taulava coming in.
"Kai has been a superb player for us, a superb leader. But we have got to look to the future of Worcester Warriors and so we have to wish him all the very best for the future.
"Dale is also a hard call," he added. "He has been outstanding for us and he always gives 100 percent and frightens other attacking players when he is running at them.
"I couldn't fault him at all, he is a great character who has captained the side when we have needed him.
"But I have got to look at Ravai Fatiaki, who we will give opportunities to, and it gives me an opportunity to bring in another centre who will be here for the next five to six years as we look to the future and who we can mould into how we want them to play.
"It is a tough decision as a coach to release two very good players. But I have got to be hard and look to the future of Worcester Warriors and the long-term product we put on the field."