The phrase "small herd" is an idiomatic expression that refers to a group or collection of something, in this case, cows. The term can also be used metaphorically to describe any small number of people or things gathered together.
Sentences with «small herd»
Zimbabwe, she notes, had some 82,000 elephants according to the recent census, but many of the animals «live in small herds of only 5000 to 7000» animals each, closer to the study scenario. (sciencemag.org)
Welsh Corgis were bred as small herding dogs for cattle, which could nip at the animals» ankles to get them moving. (woofwhiskers.com)
Authors tend to be solitary critters, although they do gather in small herds (er, writing groups) sometimes. (davidkeener.org)