The phrase "to produce a surplus" means to have more than what is needed or required. It refers to making or generating an extra amount of something beyond what is necessary or expected. Full definition
And many will produce surplus babies of their own because new adopters promise to spay / neuter, but don't follow through. (lenoircountyspca.org)
It teaches kids that it's OK to produce surplus animals on a self - centered whim. (okcpetsmagazine.com)
The Ferrari LaFerrari captures excess power under braking or when the engine produces a surplus of power (which you might say is «always») to a battery pack and then to two electric motors. (extremetech.com)