It was the slight smile as she was feeding it that convinced Hachiken that Tamako loves her animals too, just in a different way. Sure, it's a ruthless kind of love, because Tamako is a ruthless person.
The other way you could view it is that both Mikage and Komaba are almost on the verge of losing their farms, particularly the latter. Mikage doesn't have the heart to carry on the dairy side of their farm, so if she does follow her dream, the family farm will get sold to someone else (unless she marries Hachiken, like you said). Komaba knows that the only way to save his farm is to follow his dream for the cash infusions it will bring.
So, you could say that Tamako is ruthless because she loves her farm. The business isn't an easy one, she calls it the Warring States Era of Agriculture, and she thinks her parents can't keep it running at peak efficiency with a good profit margin. That size allows them to rotate the workers and even give them breaks (leaving them with excellent quality of life) and have a stable revenue stream. She also mentioned that if the cows get diseased, Hachiken would be responsible for making all of the workers lose their jobs. Tamako is actually a very caring person, she's just incredibly ruthless about it.