the reason he wouldn't have had a dog with him would be because the cow would've instantly gone nuts on first sight of the dog, the cow and calf were most likely in a shed or separated field so no real need for a dog.
what a lot of folks don't realise is "suckler cows" a e left to there own devices for the most part and only interacted with when dosing/drenching or treatment is needed, whereas milking cows are in contact with the farmer twice a day, but also a main factor is the breed, every cow gets protective of there new borns regardless of breed but the the "suckler cow" is in a different