Regulation of colostrum formation in beef and dairy cows.

Fifteen beef cows and 13 dairy cows were used to investigate the physiological basis of breed differences in IgG1 concentration in colostrum. Concentrations of progesterone and alpha-LA in sera and IgG1 in sera and secretion were quantified in samples collected prepartum. Serum progesterone concentration decreased significantly between 30 and 10 d prepartum, but breeds did not differ. Serum alpha-LA concentration was initially low but increased to 150 ng/ml in beef cows and 720 ng/ml in dairy cows at parturition. Serum IgG1 concentrations averaged 8.4 in beef cows and 5.7 mg/ml in dairy cows between 28 d and 24 d prepartum and subsequently declined to 6.7 in beef and 1.4 mg/ml in dairy cows between 4 d prepartum and calving. Overall, IgG1 concentration was greater in secretion from beef cows (113.4 mg/ml) than from dairy cows (42.7 mg/ml). The greater disappearance of IgG1 from serum suggested that more IgG1 was transferred into secretion of dairy cows, but concentration in secretion was lower. The reduced IgG1 concentration in secretion was associated with greater lactogenic activity in dairy cows, evidenced by fivefold higher alpha-LA concentration in sera. Dilution of IgG1 in secretion may be responsible for breed differences in colostral IgG1 concentration.