Concentrations of progesterone and conception rates in Holstein Concentrations of progesterone and conception rates in Holstein heifers after hCG administration during the first 3 weeks after heifers after hCG administration during the first 3 weeks after estrus estrus

Summary We conducted two experiments to determine the effects of administering human chorionic gonadotropin (hCG) on day 4 or on days 15, 16, and 17 after estrus on conception rates and progesterone secretion by the corpus luteum in Holstein heifers. In the first experiment, 60 heifers received hCG or saline on day 4 after estrus and AI. Conception rates were similar between groups. Concentrations of progesterone were increased in treated pregnant and nonpregnant heifers 7 and 14 days after treatment compared with pregnant and nonpregnant controls. In the second experiment, 62 heifers


Introduction
Human chorionic gonadotropin (heG) is luteotrophic (sustains the function of the corpus luteum) in cows and increases progesterone synthesis by corpora lutea.Since some studies have shown that concentration of progesterone after estrus and AI tend to be higher in cows that remain pregnant than in those that keep cycling, stimulation of the corpus luteum with hCG should have a beneficial effect on conception rates.Early administration of hCG (3 days after estrus and AI) in one study increased progesterone without affecting conception rates.
Other experiments involving the administration of hCG 15 days after estrus have reported increased progesterone, prolonged cycle dmation and increased pregnancy rate in a low-fertility herd, but decreased pregnancy rate in a high-fertility herd.The objectives of our study were: (1) to determine if production of progesterone from a developing or mature corpus luteum is altered by administering hCG to bred heif ers, (2) to determine if increased progesterone in bred heifers influences conception, and (3) to ascertain if production of progesterone from a corpus luteum is affected by the stage of luteal development and(or) the presence of an embryo.

Results and Discussion
Exp. 1. Conception rates are summarized in Table 1.Conception of treated heifers (55%) tended (P=.16) to be lower than that of controls (72%).Concentrations of progesterone in treated and control pregnant heifers were similar in blood samples collected before treatment on day 4 after estrus.However, treated pregnant heifers had higher (P<.Ol) progesterone than pregnant controls 7 days (6.3 + .5 vs 2.6 + .4nglml) and 14 days (6.2 + .5 vs 5.1 + .4ng/ml; P<.10) after treatment.Concentrations of progesterone mserum were similar in both groups 21 days after treatment.Nonpregnant treated and control heifers also had similar concentrations of progesterone prior to treatment on day 4. Progesterone in treated nonpregnant heifers was higher (P<.Ol) than in nonpregnant controls 7 days (7.0 + .5 vs 3.8 + .6 nglml) and 14 days (6.2 + .5 vs 3.7 + .6 nglml) after treatment.Progesterone concentrations were similar between treated pregnant and nonpregnant heifers on the day of treatment and 7 and 14 days later.By 21 days after treatment with hCG, pregnant heifers had higher (P<.OOl) progesterone than nonpregnant heifers (5.7 + .5 vs 1.6 + .5 nglml) because of maintenance of pregnancy.*Tended to be less than saline-treated heifers (P=.16).
Exp. 2. Table 2 summarizes the conception rates in this study.Conception rates were similar between hCG-treated heifers (67%) and controls (59%).Concentrations of progesterone were similar between pregnant treated and control heifers before first day of treatment (day 15 after estrus).On the second and third f days of treatment (days 16 and 17), progesterone was higher (P<.05) in treated pregnant heifers than in pregnant controls (4.5 + .2vs 3.7 + .3ng/ml and 5.4 + .23.8 + .3ng/ml).-Onday 21 (4 days after the end of treatment), treated pregnant heifersstill had higher (P<.OI) progesterone than pregnant controls (6.1 ~.2 vs 3.7 ~.3 ng/m}).Concentrations of progesterone were similar in nonpregnant treated and control heifers prior to initiation of treatment on day 15 and on the second day of treatment (day 16).On days 17 and 21, concentrations of progesterone were higher (P<.Ol) in treated nonpregnant heifers than in nonpregnant controls (5.5 .! .3vs 3.4 + .3ng/ml and 3.8 + .3vs 1.3 + .3ng/mI).Treatment with hCG also apparently delayed luteolyslS in nonpr~nant heifers.Treated pregnant and nonpregnant heifers had similar concentrations of progesterone on each day of treatment, but on day 21 (4 days after the end of treatment) pregnant heifers had higher (P<.OOI) progesterone than nonpregnant heifers (6.1 + .2vs 3.8 + .3ng/m}) because of maintenance of pregnancy.
--One-time stimulation of the newly formed (day 4) corpus luteum by hCG (5,000 IU) increased production of progesterone but did not appear to affect conception.Repeated daily stimulation of the mature corpus luteum by hCG (1,000 lU on days 15, 16, and 17) prior to luteolysis increased concentrations of progesterone (and delayed luteolysis) without affecting conception rates of heifers.

Table 1 .
Conception rates of heifers treated with hCG or saline 15, 16, and 17 days postestrus