Calves headbutt, or “bunt,” the udder to trigger their mother’s milk release. This forceful nudging isn’t random or playful. It activates pressure-sensitive receptors in the udder that send a signal to the cow’s brain, ultimately causing milk to flow. Without this physical stimulation, calves would get far less milk from each nursing session.
How Bunting Triggers Milk Flow
The udder contains pressure-sensitive receptors, particularly around the teats and the tissue connecting to the body. When a calf pushes its head into the udder, these receptors fire and send signals up through the spinal cord to a region of the brain called the hypothalamus. The hypothalamus then signals the pituitary gland to release oxytocin into the bloodstream.
Oxytocin is the key player here. Once it reaches the mammary gland, it causes tiny muscle-like cells surrounding the milk-producing sacs (called alveoli) to contract. This squeezes stored milk down into the udder cistern and out through the teats. The whole chain, from headbutt to milk flow, takes only about a minute. Without that initial mechanical stimulation, much of the milk stays locked in the upper portions of the udder where the calf can’t access it through suckling alone.
This is a positive feedback loop. The more the calf bunts and suckles, the more oxytocin the mother releases, and the more milk flows. It’s why calves tend to bunt repeatedly during a single nursing session rather than just once at the beginning.
Why the Milk Is Worth the Effort
Bunting doesn’t just increase the volume of milk a calf gets. Research on udder stimulation in cows shows that physical manipulation before milk release significantly increases the fat and solids content of the milk. In studies measuring milk quality after different durations of pre-milking stimulation, fat percentage and solids-not-fat were both measurably higher in stimulated groups compared to unstimulated controls. For a growing calf that depends entirely on milk for its first weeks of life, that extra fat means more calories and more energy per feeding.
This makes sense biologically. The fattiest milk tends to be stored deeper in the alveoli, and oxytocin-driven contraction is what pushes that richer milk down. A calf that bunts vigorously gets access to this higher-calorie “hindmilk,” not just the thinner milk already sitting in the cistern.
An Instinct, Not a Learned Behavior
Calves begin bunting within minutes of birth, before they’ve had any opportunity to learn. It’s a hardwired behavior, part of the same suite of neonatal reflexes that includes standing, rooting toward the udder, and suckling. Newborn calves will headbutt anything in the general vicinity of where an udder should be as they search for their first meal, and they refine their aim quickly.
The behavior is strongest in very young calves and tends to decrease in intensity as they grow older and begin eating solid food. But even older calves that still nurse will bunt before latching on. The force can be surprisingly strong, and cows occasionally kick or step away from particularly aggressive bunters, which is one reason dairy farmers monitor nursing interactions.
What Happens When Calves Can’t Bunt an Udder
In modern dairy operations, calves are often separated from their mothers and fed by bottle or bucket. The bunting instinct doesn’t disappear just because there’s no udder available. Instead, it redirects. Calves raised in groups commonly “cross-suck,” meaning they bunt and suckle on the ears, navels, or prepuces of other calves. This misdirected behavior can cause skin irritation and health problems in the calves being suckled.
Research from the Journal of Dairy Science found that feeding method makes a significant difference. Calves fed from open buckets spent roughly three times as long cross-sucking compared to calves fed from slow-flow teat buckets. The teat buckets gave calves an outlet for both their suckling and bunting drives, reducing the urge to redirect those behaviors toward penmates. During the weaning period, cross-sucking increased across all groups, but calves with teat buckets still showed substantially less of it.
This tells us something important about the nature of the behavior. Bunting isn’t just about getting milk. It’s a deeply embedded motor pattern that calves are driven to perform. When the natural target isn’t available, they’ll find a substitute. Providing a rubber teat to push against satisfies much of that drive, even though there’s no udder behind it.
The Oxytocin Connection Goes Both Ways
Oxytocin isn’t just a milk-release hormone. It also reinforces the bond between cow and calf. When a calf bunts and suckles, the oxytocin released in the mother’s brain promotes nurturing behavior, encouraging her to stand still and allow nursing rather than walking away. In sows, a similar mechanism has been documented: piglets stimulating teat receptors triggers oxytocin release that directly promotes maternal behavior and willingness to nurse.
For the calf, this creates a reliable system. Bunt the udder, get milk, and simultaneously make your mother more tolerant of your presence. It’s a neat evolutionary solution to the problem of convincing a 1,200-pound animal to stand still while a wobbly newborn figures out how to eat.