Progesterone levels in bovine plasma as determined by competitive protein-binding assay

A competitive protein binding assay procedure with certain modifications was used to measure progesterone in peripheral blood of cows for a 21 day period following insemination. This assay procedure is based upon the ability of progesterone to displace corticosterone-1,2-³H which is bound to the protein, corticosteroid binding globulin.

A total of 19 cows was used in the experiment. The cows were divided into the following groups, according to the day on which they cycled: (a) 19-27 days, (b) 42-43 days, (c) 73-101 days and (D) pregnant. Blood samples were drawn from the Jugular vein every third day beginning on the day of heat and continuing for 21 days. The progesterone determinations were made on 0.5 ml. aliquots of plasma.

Since the lower limit of sensitivity of this assay procedure appears to be 0.30 ng., it is fairly reliable in measuring progesterone from Day 3 through Day 21. The coefficient of variation for recovery rates and pooled plasma was 6.1 percent and 22.6 percent, respectively.

Cow differences on a within group basis were non significant (P<0.05). However, there was a significant difference in progesterone concentration across days in two of the groups; Group A (P<0.05) and Group D (P<0.05). There was also a significant difference (P<0.05) across groups on Day 21. The cows in the pregnant group exhibited significantly (P<0.05) higher progesterone levels than did the other groups on Day 21.