Security deposit deduction for grass destruction by dogs

Hello,

I am in CA with a class A property that was delivered to tenants at time of move in with brand new sod put in. They are now moving out and their two dogs totally destroyed the grass. I asked them to replace the sod as part of our pre-move out process but they are arguing that it is not the dogs that destroyed it but the gophers. No gopher activity is visible in the back yard and in the grass area I have asked them to replace (there are gopher dirt mounts in the front yard but again nothing like that in the back).  What is your opinion with regards to charging replacement of the grass against their SD? Thank you