Ask a herder to describe their job, and they'll send you a poem. If you could follow one, day in and day out, you'd realize just how much they need to know.
Herders have to know about genetics: the strengths and weaknesses of their breed, possible crosses with other breeds (necessary if numbers are low), which animals are best suited to reproduction. Rove goat herders, for example, know that their goats are more maternal and can easily adopt kids from other mothers, and that their digestive system is more resistant than that of other goats.
A herder also has to be a bit of a veterinarian, able to recognize a healthy animal and often to cure sick ones. Visiting a real veterinarian for wounds or minor ailments would cost too much, and so herders must learn to do what they can themselves. They know how to help their animals give birth, and when to shelter them from the cold. They understand from the plants the animals choose to eat what they need. If the animals start eating the bark from certain trees, it's time to give them blocks of mineral salts, which they can lick if needed.
They can milk their animals and recognize a case of mastitis from the animal's appearance and behavior and from tasting the milk if necessary.
They know if an animal has woken up in an aggressive mood, and so they keep an eye on it, separating it from the others if necessary. Similarly they will notice if one animal is being harassed by the others from one day to the next.
The herder knows how to predict and interpret the weather, and adapt accordingly, and how much food to set aside for the winter in colder countries.
Herders are also experts in botany, knowing how recognize foreign plants, which perhaps came from seeds carried in the wool of a New Zealand sheep, as well as harmful plants that can kill. They can name all the plants their sheep, cows or goats feed on, or the different mosses found under the snow in the case of elks. They also know which wild plants are good for humans to eat.
They know how to train dogs to help them direct and group their flock or herd.
Herders know exactly how to use and manage the environment. They will rarely bring their animals to graze twice in a row in the same place, and they need to know the best places to bring them depending on the rain, the sunlight and the animals' needs. They keep mountain meadows in good condition, encouraging the growth of a wide variety of plants the following year and limiting possible damage from forest fires.
Herders must also be able to live on their own for long periods.
This is just a small part of what they know. Herders are walking encyclopedias of nature.