Variable progesterone response and estradiol secretion in prepubertal beef heifers following treatment with norgestomet implants

Two experiments were conducted to determine the effects of norgestomet ear implants on progesterone response and estradiol secretion in prepubertal beef heifers. In the first experiment, 47 beef heifers were treated with norgestomet. The implants were implanted subcutaneously for 9 d. After implant removal, blood samples were taken from heifers 2 to 4 d per week for 40 d. Following progesterone determination in jugular venous plasma, heifers were classified according to their progesterone response: 1) no response (Group 1); no rise in progesterone above 1 ng/ml throughout the sampling period; 2) one cycle (Group 2); one increase in progesterone above 1 ng/ml for at least 2 d followed by no further increase in progesterone during the sampling period; and 3) two cycles (Group 3); a rise in progesterone above 1 ng/ml for at least 2 d followed by another cycle of normal duration. Heifers treated with norgestomet were classified as 23 with no response, 9 with 1 cycle and 15 with 2 cycles. Concentrations of estradiol were measured in jugular venous samples on Day 2 after implant removal. Mean concentrations of estradiol were greater in Group 3 than in Group 1 (P < or = 0.01). In Experiment 2, 29 prepubertal beef heifers were assigned randomly to either a 9-d treatment with norgestomet (n = 14) or to serve as untreated controls (n = 15). Blood plasma samples were collected daily from Days 0 to 44 after implant removal. After progesterone determination, heifers were classified as 8 with no response, 4 with 1 cycle and 3 with 2 cycles in the control group, and 5 with no response, 3 with 1 cycle and 6 with 2 cycles in the norgestomet group (frequencies did not differ; P > 0.1). Jugular venous blood plasma was also collected at 4-h intervals from 0 h to 96 h after implant removal and concentrations of estradiol were measured. Patterns of estradiol secretion differed (P < or = 0.05) and overall mean concentrations of estradiol over the first 96 h following implant removal were greater (P < or = 0.01) in norgestomet-treated heifers versus the controls. We conclude that norgestomet can produce a variable progesterone response with heifers with 2 cycles secreting more estradiol. Implants of norgestomet also causes more acute secretion of estradiol in prepubertal beef heifers.     

Improvement of reproductive performance in crossbred zebu anestrous primiparous cows by treatment with norgestomet implants or 96 h calf removal

The aim of the present study was to compare the effect of norgestomet implants (NI) and calf removal (CR) for 96 h on estrus induction and first service fertility in crossbred zebu primiparous anestrous cows after 120 days postpartum. A total of 152 cows in a tropical environment were allotted to three experimental groups: CR for 96 h (n = 51), NI for 9 days plus 500 IU of PMSG at implant removal (n = 51) and a control group (CG) without treatment (n = 50). Estrous rate (%) and mean interval to first estrus (days) were 50.9 and 26.3+/-6.8; 60.7 and 13.8+/-6.8; 16.0% and 61.8+/-5.9 days for groups CR, NI and CG, respectively. CR and NI were statistically similar, however, both groups were different (P < 0.01) when compared with CG. Calving interval to first service and to conception were: CR: 151.2+/-8.4 and 157.8+/-21.4; NI: 145.2+/-8.5 and 150.9+/-21.3; CG: 186.8+/-7.3 and 201.0+/-18.5. A difference (P < 0.01) was found in both intervals when comparing the CR and NI with CG, with longer intervals for cows in the CG group. First service conception rates were 61.5, 67.7 and 62.5% for groups CR, NI and CG, respectively, and were not different (P > 0.05). These results demonstrate the efficacy of NI + PMSG or temporary CR for 96 h in inducing a fertile estrus and shortening the interval to estrus and to conception in primiparous crossbred anestrous zebu cows in the tropics.     

Factors affecting reproductive performance of Holstein heifers

The objectives were to evaluate factors affecting reproductive performance of dairy heifers. Holstein heifers (6389) were housed in a feed lot located in Parma, ID. Each week heifers weighing > or =290 kg were initiated in the reproductive program, which consisted of one injection of PGF(2alpha) and AI on detection of estrus. Heifers not inseminated by 11 days after the initiation of the breeding program received a second injection of PGF(2alpha). Pregnancy was diagnosed at 40+/-3 and 90+/-3 days after AI. Average daily minimum temperature (ADMnT), average daily maximum temperature (ADMxT), and average daily rainfall (ARF) were recorded between 15 days prior to and 15 days after the day of AI or the day of initiation of the breeding program. Exposure to air temperature was classified as: cold stress (CS=ADMnT< or =4 degrees C), no stress (NS=ADMnT>4 degrees C and ADMxT<29 degrees C), and heat stress (HS=ADMxT> or =29 degrees C). Exposure to rainfall was classified as above (HRF) or below (LRF) the mean for the period in question. Heifers were classified according to body weight at initiation of the breeding program as thin (TH<340 kg); moderate (MD=340-365 kg); and heavy (HY>365 kg). Service sire was associated with conception rate at 40 and 90 days after first AI. Although exposure to air temperature was not correlated with conception rate at 40 days after first AI, heifers exposed to cold stress had smaller conception rates at 90 days after first AI because they were more likely to lose pregnancy between 40 and 90 days of gestation. The proportion of heifers inseminated after initiation of the breeding program was correlated with body weight and exposure to cold stress. Exposure to cold stress was also correlated with the proportion of heifers conceiving within 11 and 22 days after initiation of the breeding program. From this study a correlation was established between body weight and rate of insemination and between the exposure to cold stress and reproductive efficiency of Holstein heifers.     

Induction of a new follicular wave in holstein heifers synchronized with norgestomet

Treatments with progestin to synchronize the bovine estrous cycle in the absence of the corpus luteum, induces persistence of a dominant follicle and a reduction of fertility at doses commonly utilized. The objective of the present research was to induce a new wave of ovarian follicular development in heifers in which stage of the estrous cycle was synchronized with norgestomet. Holstein heifers (n=30) were used, in which estrus was synchronized using two doses of PGF2alpha i.m. (25 mg each) 11 days apart. Six days after estrus (day 0=day of estrus) heifers received a norgestomet implant (6 mg of norgestomet). On day 12, heifers were injected with 25 mg of PGF2alpha i.m. and assigned to treatments (T1 to T4) as follows: treatment 1, heifers received a second norgestomet implant (T1: N+N, n=6), treatment 2, received 100 microg of GnRH i.m. (T2: N+GnRH, n=6), treatment 3, 200 mg of progesterone i.m. (T3: N+P4, n=6), treatment 4, control treatment with saline solution i.m. (T4: N+SS); in the four treatments (T1 to T4) implants were removed on day 14. For treatment 5, heifers received 100 microg of GnRH i.m. on day 9 and 25 mg of PGF2alpha i.m. (T5: N+GnRH+PGF2alpha) at the time of implant removal (day 16). Ovarian evaluations using ultrasonographic techniques were performed every 48 h from days 3 to 11 and every 24 h from days 11 to 21. Blood samples were collected every 48 h to analyze for progesterone concentration. A new wave of ovarian follicular development was induced in 3/6, 6/6, 3/6, 1/6 and 6/6, and onset of estrus in 6/6, 0/6, 6/6, 6/6 and 6/6 for T1, T2, T3, T4 and T5, respectively. Heifers from T1, T3 and T4 that ovulated from a persistent follicle, showed estrus 37.5 +/- 12.10 h after implant removal and heifers that developed a new wave of ovarian follicular development showed it at 120.28 +/- 22.81 h (P<0.01). Ovulation occurred at 5.92 +/- 1.72 and 2.22 +/- 1.00 days (P<0.01), respectively. Progesterone concentration was <1 ng/ml from days 7 to 15 in T1, T2 and T4; for T3 progesterone concentration was 2.25 +/- 0.50 ng/ml on day 13 and decreased on day 15 to 0.34 +/- 0.12 ng/ml (P<0.01). For T5, progesterone concentration was 1.66 +/- 0.58 ng/ml on day 15. The more desirable results were obtained with T5, in which 100% of heifers had a new wave of ovarian follicular development induced, with onset of estrus and ovulation synchronized in a short time period.     

Synchronization of estrus and fertility in goats with norgestomet ear implants

Estrus was synchronized in 64 dairy goats in July with norgestomet ear implants. Half the does received ear implants that contained 6 mg norgestomet and the remaining does received implants that contained 3 mg. Implants were left in place for 11 days. Each doe received i.m. injections of 400 IU PMSG and 50 mug cloprostenol 24 hours prior to implant removal. Twenty-eight of 32 does (87.5%) that received 6 mg or 3 mg norgestomet exhibited onset of estrus within 24 hours of implant removal. All does had exhibited onset of standing estrus by 43 hours after implant removal. Does were hand-mated to fertile bucks twice daily while in standing estrus. There were no differences between does implanted with 6 mg or 3 mg in fertility to the induced estrus (74.2% vs 75% kidding), mean length of gestation (151.0 +/- 3.2 vs 151.6 +/- 2.0 days), mean number of kids per doe (2.1 +/- 0.8 vs 2.3 +/- 0.7) or in mean kid weights (3.10 +/- 0.80 vs 3.06 +/- 0.86 kg) (6 mg vs 3 mg, respectively). It was concluded that ear implants that contained 3 mg of norgestomet were equally as effective as implants that contained 6 mg for synchronization of estrus in dairy goats.     

Effect of beta-carotene supplementation on reproductive performance of dairy heifers

Three trials were conducted to determine the effect of feeding supplemental beta-carotene on reproductive performance of Holstein heifers. In each trial, the animals were randomly assigned to either a control or treatment group. Animals in the treatment groups received 300 mg supplemental beta-carotene daily for the test periods which varied from 6 to 8 weeks in length. To facilitate sample collections and observations, estrus was synchronized with two injections of 25 mg PGF(2alpha) at 11 day intervals. The first injection was after 2 weeks of feeding supplemental beta-carotene. Blood serum beta-carotene concentrations were higher in the heifers fed supplemental beta-carotene as compared to concentrations in control heifers after 1 week of feeding and this difference increased throughout the test periods. The interval from the second injection of PGF(2alpha) to onset of estrus was shorter in the control heifers as compared to that interval in heifers supplemented with beta-carotene (trial 1,44.0 vs 56.0 hr; trial 2, 51.3 vs 70.8 hr; trial 3, 40.7 vs 62.5 hr, respectively). The intervals from PGF(2alpha) administration to the preovulatory LH peak (43.3 vs 61.5 hr) and ovulation (69.3 vs 85.9 hr) were also shorter in the control heifers in trial 3. No significant differences were found among treatments in the number of heifers that expressed estrus, the blood serum progesterone concentrations or the conception rates in any of the three trials.     

Synchronization of estrus in beef heifers with a norgestomet implant and prostaglandin F2alpha

In a 5-year study (1973-1977), 281 cycling beef heifers were treated with a 7-day norgestomet (SC21009) ear implant and an intramuscular injection of prostaglandin F(2alpha) (PGF(2alpha)) at the time of implant removal or 24 hr before implant removal. Percentages of heifers in estrus by 36, 48, 60, 72, and 120 hr after implant removal were 32.4, 52.7, 71.6, 80.1, and 93.2, respectively. Onset of estrus occurred an average of 49.8 +/- 4.7 hr after treatment. Percentages of heifers in estrus 36 hr after treatment were 5.7 and 51.7 for those with a corpus luteum and those without a corpus luteum (or determined regressing by palpation) at implant removal, respectively. When PGF(2alpha) was injected 24 hr before implant removal, 55% of the heifers were in estrus by 36 hr after implant removal compared to 30% when PGF(2alpha) was injected at the time of implant removal; however, by 60 hr after implant removal the difference was 76% vs. 71%. First-service conception rates for synchronized and nonsynchronized heifers were 62.2% and 59.6%, respectively. During 1976 and 1977 heifers were checked for estrus every 4 hr and inseminated 2, 6, 10, 14, 18, 22, 26, or 30 hr after first detected to be in standing estrus. Conception rate was not significantly affected by time of insemination but tended to be higher for heifers bred 26 and 30 hr after first being detected in standing estrus (78.9% and 70.0% vs. average 59.2%). Treatment with a 7-day norgestomet implant plus a single injection of PGF(2alpha) 24 hr before or at implant removal appears to be a practical technique for synchronizing estrus in cycling heifers without affecting conception.     

Effects of repetitive norgestomet treatments on pregnancy rates in cyclic and anestrous beef heifers

Sixty-eight 12- to 14-month-old crossbred beef heifers averaging 285 kg were assigned at random to treated (n = 35) and control (n = 33) groups to evaluate the use of repetitive norgestomet treatments. Treated heifers received an ear implant containing 6 mg norgestomet on two occasions 16 days apart. Injections of 3 mg norgestomet and 5 mg estradiol valerate (EV) were given intramuscularly the same day as first implantation (Syncro-Mate-B). Implants were removed after eight days. Four bulls were then placed in each of two pastures containing half of the treated and half of the control heifers for 24 days after the time of the first implant removal. Progesterone concentrations from blood samples collected prior to the first treatment were used to determine reproductive status. The overall pregnancy rate 64 days after first implant removal for treated anestrous heifers (61%; 14 23 ) was similar (p > 0.25) to untreated (73%; 11 15 ) and treated (75%; 9 12 ) cyclic heifers, but higher (p < 0.1) than for untreated anestrous heifers (33%; 6 18 ). This treatment advantage resulted from an increased (p < 0.01) pregnancy rate after the second implant removal. In summary, repetitive norgestomet treatments enhanced pregnancy rate in anestrous heifers within a 24-day breeding season.     

Ovarian function in crossbred beef heifers treated with dexamethasone

Each of 32 crossbred beef heifers was brought from pasture on day 16 of its estrous cycle and assigned randomly to one of four treatment groups as follows: Field Control (FC), Field Dexamethasone (FD), Pen Control (PC), and Pen Dexamethasone (PD). Field groups were kept in a 0.8-ha field and pen groups were kept in 4.6-x 9.8-m pens in a pole barn during the trial. Dexamethasone (DEX) groups received 20 mg of DEX on cycle day 16 and 30 mg daily on days 17 through 20. Control heifers received an equal volume of physiological saline solution on corresponding cycle days. Average treatment cycle lengths (±SD) for control heifers in FC and PC groups were 21.1 ± 2.8 and 21.6 ± 1.8 days, respectively, and were not significantly different. Average time from progesterone decline (<1 ng/ml) to estrus was two days for each of the control groups. Four DEX-injected heifers had not shown estrus by day 44 of the treatment cycle. Progesterone had declined for two of these heifers by cycle day 18 and remained below 1 ng/ml past cycle day 48. The other two showed a decline in plasma progesterone by cycle days 18 and 32, respectively, and a progesterone rise by day 42 without having been detected in estrus. The remaining 12 DEX heifers had an average cycle length of 29.4 days. Four extended cycles resulted from extended CL function, five from an extended period from progesterone decline to estrus and three from a combination of these factors. These observations suggest that the administration of pharmacological doses of glucocorticoid during the late diestrus or early proestrus may result in altered ovarian function.     

Methods of identifying and inseminating nonpregnant beef females after synchronization of second estrus with norgestomet implants

Beef females (547) were included in three experiments to evaluate methods of identifying and inseminating nonpregnant beef females after synchronization of second estrus with norgestomet implants. In the first experiment, heifers not pregnant to the first insemination were identified for insemination via estrus (inseminated via the a.m./p.m. rule or 48 h after implant removal). In the second experiment, females not pregnant to the first insemination were identified for insemination via estrus (inseminated via the a.m./p.m. rule) or progesterone concentrations < 1.5 ng/mL at implant removal (inseminated 48 h after implant removal). In the third experiment, heifers not pregnant to the first insemination were identified for insemination via progesterone concentrations (as in experiment 2) or anterior vagina electrical resistance values < 81 ohm resistance 48 h after implant removal (inseminated after resistance measured). All methods of identifying and inseminating nonpregnant females were equally effective (P > 0.10) and did not effect (P > 0.10) calving rates from the first and second AI.     

Ovarian cysts and their consequences on the reproductive performance of swine herds

The aim of this work was to determine the incidence of ovarian cysts in the breeding herd and their consequences in the reproductive performance of the herd. Data from 1990 cyclic sows from two farms, with 0-12 parities, lactation length between 6 and 47 days and weaning-estrus interval between 0 and 32 days were evaluated by ultrasound examination for cyst incidence. As cyst was considered an anaechoic structure with smooth and thin walls with a diameter larger than 2 cm that remained visible for at least 5 days after estrus onset. Cyst incidence was found to be 2.4%. Sows with ovarian cysts have a greater return to estrus rate (34.0 x 7.7%, P<0.01), and cysts were associated with around 10% of regular and irregular return to estrus patterns on both farms. The adjusted farrowing rate (52.2 x 90.0%, P<0.01) and anestrual sows that were not pregnant (10.6 x 0.6%, P<0.01) were also influenced by the appearance of ovarian cysts, but they did not influence litter size (P>0.05). The incidence of cysts was not influenced by parity (P>0.05). Sows with shorter lactation had a greater incidence of cysts (P<0.05). Sows with a weaning-estrus interval shorter than 3 days had a greater incidence of ovarian cysts (P<0.05). The time of the year had no influence on the incidence of ovarian cysts (P<0.05).     

Serum progesterone concentrations in pregnant and nonpregnant heifers and after gonadotropin releasing hormone in luteal phase heifers

Serum progesterone (P₄) concentrations were quantitated in 18 Holstein heifers from days 5 to 16 after estrus in an effort to ascertain the effects of pregnancy on circulating levels of this hormone. The P₄ concentration rose faster between days 5 and 10 in the pregnant heifers compared to P₄ levels in both the non-pregnant heifers and the killed sperm inseminated group. It was found that serum P₄ levels were significantly (P<.002) elevated across the entire interval in the pregnant heifers compared to the other experimental groups. It was concluded that pregnancy had a stimulatory effect on P₄ levels and that pregnancy failure is not simply due to the inability of corpora lutea to secrete sufficient P₄.The administration of 250μg gonadotropin releasing hormone (GnRH) to 10 Holstein heifers on day 7 after estrus resulted in a significant (P<.025) elevation of serum luteinizing hormone concentrations from 15 min to 3 h postinjection. Similarly this treatment resulted in a significant (P<.05) elevation of serum P₄ levels from .5 to 4 h post-injection. It is concluded that GnRH at this dosage has a transient effect on P₄ levels in luteal phase heifers.     

Ovarian activity and reproductive potential in a natural population of Drosophila melanogaster

Summary The ovarian state of Drosophila melanogaster females from a wild population in the south of France was studied by immediate dissection, in the field, during two different periods in the year: early in June and at the end of September. It was compared with either the reproductive capacities of aliquot females kept in laboratory conditions or the reproductive potential of the first laboratory generation.Among the wild flies, about 20% were young (less than 24 h). 85 to 90% of all females were inseminated and virgins were found among the youngest ones.In the field, the vitellogenetic activity appeared to be very low and noncontinuous. Females with active vitellogenesis were fairly rare but about 60% of the females presented eggs in retention in the ovaries and 50% had a fertilized egg waiting in the uterus. All these observations showed that fecundity in the field was certainly low.When wild females were studied in the laboratory, their daily egg production was relatively high, more than 50 eggs per day per female. This production was stimulated by insemination and decreased with adult population density.Daughter females reared in the laboratory were bigger, had more numerous ovarian tubes than their mothers (an average of 50 instead of 32), and their daily egg production was very high. However the daily overiole production in these two groups was very similar.The genetic reproductive potential of this natural French population seems to be very high but, under natural life conditions, this potential is not expressed. Wild flies do not seem to give priority to their reproductive effort but to individual survival and activity.     

Dose determination and efficacy of remotely delivered norgestomet implants on contraception of white-tailed deer

Management of overabundant wildlife populations using contraceptives is being considered with increasing frequency in many localities. A wide array of effective contraceptives is needed to meet a variety of management objectives. Therefore, we evaluated the synthetic progestin norgestomet for its efficacy and its minimum effective dose in free-ranging white-tailed deer (Odocoileus virginianus). We evaluated two doses of norgestomet implants (14 and 42 mg) at a site in southern Connecticut during 1992–1995. Four doses (14, 21, 28, 42 mg) of norgestomet implants were tested at a site in northern Indiana during 1993–1996. The effectiveness of norgestomet implants in preventing pregnancy was similar for the 42 mg (92%), 28 mg (100%), and 21 mg (100%) doses. There was a significant decline in efficacy using the 14 mg (48%) dose. It appears that 21 mg is approximately the lowest dose that consistently prevents reproduction in adult white-tailed deer of various sizes and ages. Norgestomet implants show promise as a highly effective contraceptive agent that is safe to treated animals and secondary consumers and simple to deliver remotely. Zoo Biol 16:31–37, 1997. © 1997 Wiley-Liss, Inc.     

Influences of diet during gestation on potential postpartum reproductive performance and milk production of beef heifers

The influences of nutritional protein and energy during early and mid pregnancy on milk production and postpartum reproductive parameters were determined in 70 beef heifers of two composite breeds (Bos indicus X Bos taurus). At artificial insemination (AI), heifers were divided into four dietary treatment groups identified by the level of protein, and to a lesser extent energy, fed during the first and second trimesters: high/high (HH), high/low (HL), low/high (LH), and low/low (LL). Milk production was lower in the heifers receiving high treatment in first trimester than that in heifers receiving the low treatment (P = 0.01). Milk production was negatively associated with dam body condition score (BCS; P = 0.01), nonesterified fatty acids (P = 0.001), and leptin (P = 0.02) and positively associated with urea (P < 0.001) concentrations during lactation. Increased dietary protein in the first trimester increased or decreased concentrations of colostral protein dependent upon genotype (P = 0.03). Colostral protein was positively associated with bovine pregnancy associated glycoprotein from late gestation (P = 0.007). Milk fat was negatively associated with BCS (P = 0.007) and influenced by genotype (P = 0.003). Dietary treatment did not affect the postpartum reproductive performance of beef heifers. Gestation length (P < 0.001) and the postpartum interval to first estrus (PPI; P = 0.02) were positively associated with calf size. Placental size was negatively associated with placental expulsion time (P < 0.01). Prepartum BCS of the heifers was negatively associated with PPI (P = 0.01). Overall, high levels of nutrition during early gestation are detrimental to milk production in beef heifers.     

Ultrasonic characterization of ovulatory follicular evacuation and luteal development in heifers.

The characteristics of follicle evacuation during ovulation and the development of the corpus luteum until day 5 (day 0 = ovulation) were studied in seven nulliparous Holstein heifers using real-time ultrasonography. Ovulation was induced and synchronized with a single injection of PGF2 alpha followed in 36 h by GnRH. Continuous scanning and videotaping was performed from apparent stigma formation until antral fluid was no longer detected. The beginning of follicular evacuation (second 0) was defined, retrospectively, after the antral area decreased 10% or more in 1 s. The completion of evacuation was defined as the inability to detect the antrum (the beginning of luteal development, 0 h). Corpora lutea development was monitored at 0, 4 and 20 h, and every 24 h thereafter until day 5. Changes in the maximal cross-sectional area of the antrum, luteal tissue, and central luteal cavities and in the pixel intensity of luteal tissue were determined using a computerized image program. The initial antral fluid evacuation occurred in two patterns that could be readily separated: (1) rapid, means of 58 and 89% evacuation in 1 and 4 s, respectively (four heifers); and (2) slow, means of 17 and 35% in 1 and 4 s, respectively (three heifers). The initial loss that distinguished the two patterns involved about 4 and 20 s for rapid and slow evacuation, respectively. Thereafter, the loss patterns were similar for the two types. The time from the beginning to the completion of evacuation ranged from 6 s to 14.5 min. Mean luteal tissue area increased (P < 0.05) between completion of evacuation (91.2 +/- 6.5 mm2) and day 3 (164.4 +/- 13.7 mm2) and between day 3 and day 4 (263.4 +/- 26.6 mm2). The growth rate of the luteal tissue area between day 3 and day 4 (103.2 +/- 16.0 mm2 day-1) was greater (P < 0.05) than that between day 2 and day 3 (41.9 +/- 12.4 mm2 day-1) and between day 4 and day 5 (49.7 +/- 22.0 mm2 day-1). In contrast to increasing luteal tissue area, mean pixel intensity decreased (P < 0.05) progressively between the completion of evacuation (78.4 +/- 6.3) and day 2 (60.4 +/- 2.5) and did not change significantly thereafter. In conclusion, initial follicular fluid loss during ovulation occurred in two patterns, involving about 4 and 20 s, respectively. The most intensive luteal tissue growth occurred between day 3 and day 4, and the echogenicity of the luteal tissue decreased between day 0 and day 2.     

Effects of testosterone and 5alpha-dihydrotestosterone on luteal lifespan in dairy heifers

Endogenous concentrations of testosterone increase approximately 7 d prior to estrus in cattle and goats. Inhibition of testosterone synthesis results in a delay of luteal regression in both species. The purpose of this experiment was to determine if treatment with testosterone or 5alpha-dihydrotestosterone (DHT), 2 to 6 d prior to the endogenous rise in testosterone, would result in premature luteal regression. Sixteen heifers were randomly assigned to one of three treatment groups: 1) Control (n = 6); 2) testosterone (100 mug, n = 5); or 3) DHT (100 mug, n = 5). Each heifer received a single injection of the appropriate steriod on Day 8, 9, 10, 11 or 12 post estrus. Jugular venous blood samples were collected at frequent intervals for 24 h to quantify testosterone, and then daily for 14 d to quantify progesterone. Concentrations of testosterone increased within 15 min of injection of testosterone, and reached a maximum at 30 min. Concentrations were maintained at > 2 ng/ml throughout the first 24 h after injection. Based on concentrations of progesterone, neither androgen had any effect on the lifespan of the corpus luteum or the level of luteal function.