Udder

An udder is a body part made up of two or four mammary glands found on female dairy animals and ruminants like cattle, goats, and sheep. It serves the same function as breasts do in primates and other mammals, appearing as a single mass that hangs beneath the animal, with pairs of glands and protruding teats. Cattle, camels, and deer typically have two pairs of teats, while sheep and goats have one pair; some animals have many pairs. In species with udders, the mammary glands develop along the milk line near the groin, whereas glands on the chest—like those in primates and elephants—are usually called breasts.

For cows, keeping the udder clean and well-cared-for is crucial during milking. This helps ensure steady, uncontaminated milk production and prevents mastitis. Special products exist to soothe chapped udder skin, which reduces the risk of bacterial infection and lessens irritation from milking cups, making the cow less likely to kick them off. Adding nutritional supplements like vitamin E to the diet has also been shown to improve udder health and lower infection rates.

The word "udder" appears in Middle English as udder or uddyr (also uther, udre), and in Old English as ūder. It comes from the Proto-Germanic root eudrą or ūdrą, which traces back to the Proto-Indo-European h₁ówHdʰr̥ (“udder”). Cognates include Saterland Frisian uure, Dutch uier, German Euter, Swedish juver, Icelandic júgur, Vedic Sanskrit ū́dhar, Ancient Greek oûthar, and Latin ūber.

Historically, the udder of a slaughtered cow—called "elder" in Ireland, Scotland, and northern England—was prepared and eaten. Today, it is still consumed in countries like Italy, parts of Pakistan, Kenya, and some South American nations, appearing in dishes such as traditional ubres and ubres asada.

The microbial community on a cow's udder can act as a natural defense against infection and influences milk production and quality. Staphylococcus bacteria—specifically S. aureus, S. uberis, and S. agalactiae—on the teat tip can cause intramammary infections (IMI) if they move into the teat canal and mammary gland. These infections trigger an immune response and can reduce milk output. The four main bacterial groups found on a healthy teat tip are Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria. Some non-aureus staphylococci (NAS) release bacteriocins that stop S. aureus growth and prevent infection, while the microbiome of cows with IMI shows more Staphylococcus and less diversity. IMIs also affect milk protein quality and composition, and the resulting loss of production and wasted milk make them a major cause of economic loss in the dairy industry.

Environmental factors shape the diversity and makeup of these microbial communities. The udder is fully exposed to both indoor and outdoor surroundings, coming into direct contact with bedding, milking equipment, and feces. These bacteria can then contaminate raw milk with pathogens that affect safety and spoilage organisms that affect quality. For example, S. aureus naturally lives on the outer teat skin, Listeria monocytogenes is common in the environment and linked to poor cleaning, and Pseudomonas species—known spoilage organisms in milk—are often found in soil. Pasteurization can kill or reduce these bacteria, but high initial loads may allow some to survive. Thorough cleaning of the cow's housing and udder surface helps prevent raw milk contamination. Antimicrobials can also be used on the udder surface, but their use must be carefully managed to avoid antimicrobial resistance.