PROBLEM: Determination of lactation stage‐dependent changes in levels of tumor neurosis factor (TNF) in milk.

METHOD: Bioassay and immunoblocking assay were used to identify and assay tumor necrosis factor (TNF; mostly TNFα) in bovine milk at different stages of lactation.

RESULTS: TNFα levels in milk started to increase steadily after the onset of drying‐off (weaning/involution), peaked at 4 to 6 wk prior to parturition and precipitously decreased to undetectable levels at parturition (colostrum). Thereafter, TNFα reappeared and maintained midlevel concentration in the mature (normal) milk throughout the rest of the lactation cycle. Analysis of cells in mammary secretions by flow cytometry revealed that elevated TNFα levels coincided with an increase in macrophages in the secretion from the dry period.

CONCLUSIONS: These lactation stage‐dependent changes in TNFα levels reflect differential effects that TNFα have on involution and prepartum remodeling of the mammary gland of the dam and on gastrointestinal development and immunoregulatory function of the suckling.