Bovine IgG(1) is thought to be specifically transported by a process of transcytosis across the mammary epithelial cells during colostrogenesis. Mammary IgG(1) appearance in cow colostrum has typically been reported as a concentration and shows IgG(1) concentration to be extremely variable because of animal variation, colostrum milking time, and water dilution effects. To identify animal IgG(1) transfer capacity and separate it from the other effects, our objective was to determine first colostrum IgG(1) total mass. We collected 214 samples of totally milked first colostrum with recorded colostrum weights from 11 Pennsylvania dairy farms that participated in Pennsylvania Dairy Herd Improvement Association, analyzed colostrum for IgG(1) by ELISA, and calculated total IgG(1) mass. Median and mean concentrations of IgG(1) were 29.4 mg/mL and 37.5+/-30.2 mg/mL, respectively, with a range of 9 to 166 mg/mL. However, total mass of IgG(1) had a median of 209.1g, mean of 291.6+/-315.8 g, and a range of 14 to 2,223 g. Colostrum IgG(1) concentration showed no relationship with colostrum volume, but IgG(1) mass had a positive relationship with volume. Colostrum IgG(1) mass was related to IgG(1) concentration (R(2)=0.58). Using DHIA records for 196 animals, we established milk production for these animals to a 15-d equivalent. An established milk secretion relationship to mammary parenchyma tissue (secretory tissue) was calculated and showed no relationship of IgG(1) mass with mammary parenchyma tissue. In addition, we show that approximately 10% of the sampled animals had IgG(1) mass greater than 1 standard deviation above the mean (high mass transfer) and represented all parities tested (1-7). Whereas first-lactation animals showed less overall calculated parenchyma tissue when compared with other parities, approximately 10% of the first-lactation group animals were capable of high mass transfer, with one transporting 2,029 g into first colostrum. Concentration variance of IgG(1) can be attributed to water inclusion, whereas mass transfer provides a clear indication of animal IgG(1) transfer capacity. The specific mechanism of bovine mammary IgG(1) transfer is not clear, but secretory tissue mass does not explain the variation observed. We hypothesize that the animal variation is attributable to endocrine regulation or genetic variation of the transporter(s).