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Cows sometimes "hide milk", while other times produce lots of it. This is what you will observe as a farmer or milkman. What you will not see is the hormonal interplay yielding these two extremes.
Two hormones that serve two opposite roles are the reason behind these two scenarios, namely oxytocin and adrenalin, commonly called milk let down and fight or flight hormones, respectively. As the names suggest you can already know which is responsible for what outcome.
Don't bother yourself on how you will measure these hormones in the body system to know which one is at play, the signs will show and you too have a role in determining which hormone takes the day.
Oxytocin is produced when the animal is relaxed, at peace and happy. It is for this reason that we advise farmers to make sure the animal is relaxed prior to milking.
Now this hormone will be chased by adrenalin. The appearance of adrenalin completely locks the production of oxytocin and therefore milk, and you can choose to have adrenalin through your acts of commission or omission.
Adrenalin is produced whenever there is a danger or a threat to the animal. Tying both hind legs of a cow, roughing it up and an unfamiliar environment are conditions that lead to production of adrenaline. When this fight or flight hormone is released, the animal fights for its life and cannot produce milk at the same time. And that is how the cow hides the milk.
But because this milk is available, it will be released later when the cow is relaxed and that is when farmers say it has hidden the milk for the calf!