I know this is a hot topic. This research has just come out from the University of California, Davis. Hart et al 'Assisting Decision-making on age of neutering for 35 breeds of dogs' Front. Vet. Sci. 7:388 https://www.frontiersin.org/articles/10.3389/fvets.2020.00388/full 133 Shelties were included in the study, usually caveats on data apply, and very interesting findings. "There were no evident increases in cancers in neutered males or females. There was no occurrence of MC in intact or spayed females and a 14 percent occurrence of PYO in intact females. Spaying at 6-11 mo. resulted in a 6 percent occurrence of UI, but at 1 year a 33 percent occurrence. Lacking a noticeable occurrence of increased joint disorders or cancers in neutered males, those wishing to neuter should decide on the appropriate age. However, to avoid the high level of UI occurrence in females, one could consider spaying females at, or beyond, 2 years." MC= mammary cancer, PYO=pyometra, UI= urinary incontinence I found it a little confusing because the rate of UI is lower for spay at 6-11months, increasing significantly for bitches spay at 1 year. But there's no figures given for Bitches spay at 2 years. So all I can take from that is either spay before 12 months or after two years.