Concentrations of Progesterone in Milk of Cows Administered an Intravaginal Progesterone Insert

Abstract

Milk from pregnant cows contains concentrations of progesterone (P4) considered safe for human consumption. The objective of this study was to determine if concentrations of P4 in milk during administration of an intravaginal progesterone insert (CIDR insert) are less than concentrations of P4 in milk associated with pregnancy. Results have implications for human use of milk from cows receiving CIDR inserts. Holstein cows (N = 64;>40 and<150 d after calving) were administered 25mg of PGF2α i.m. (study d 0) and 20 cows detected in estrus from 2 to 4 d later were assigned randomly to either control (N = 10; no further treatment) or CIDR insert (N = 10; 1.38g of P4) inserted on study d 17 (14±1 d after estrus) and removed 7 d later. Composite milk samples were collected contemporaneously from each of the 20 estrous cycling cows and from 10 pregnant cows (≥ 60 and≤220 d of gestation) twice daily from study d 17 to 27. Concentrations of P4 in de-fatted milk samples were quantified using a validated radioimmunoassay. Mean logs of areas under the curve of concentrations of P4 from the afternoon on study d 17 through the afternoon on study d 27 were 3.05ng day/ml for control, 3.33ng day/ml for CIDR insert, and 3.81ng day/ml for pregnant cows. Therefore, increased P4 due to pregnancy was 0.76ng day/ml (3.81−3.05), whereas the increase in P4 due to CIDR insert was only 0.28ng day/ml (3.33−3.05). Applying a 95% confidence interval to 0.28ng day/ml provided an upper value of 0.70ng day/ml, lower than the increase due to pregnancy. Because milk from pregnant cows is considered safe for human consumption, it follows that milk from cows administered CIDR inserts should also be considered safe, based on concentrations of P4.