The effect of eCG or estradiol at or after norgestomet removal on follicular dynamics, estrus and ovulation in early post-partum beef cows nursing calves

In post-partum anestrous beef cows suckling calves, neither the choice of hormonal regime to ensure the presence of a healthy dominant follicle at the end of a progestagen treatment nor the optimum hormone to induce estrus and ovulation is clear. Twenty-eight beef cows, in good body condition, 25-30 days post-partum, were assigned to one of four treatments: (i) 3mg norgestomet (N) implant with 5mg estradiol valerate (EDV) and 3mg N injection at the time of insertion (Crestar) for 5 days followed by 600 IU eCG at the time of implant removal; (ii) Crestar for 5 days as in (i) followed by 0.75 mg estradiol benzoate (EDB) 24h later; (iii) Crestar for 9 days followed by 600 IU eCG at the time of implant removal; and (iv) Crestar for 9 days followed by 0.75 mg EDB 24h later. Ovarian scanning was preformed from 4 days before implant insertion until ovulation and 4 days postovulation to detect the CL. Daily blood samples were collected from day 20 post-partum until second ovulation for FSH and E(2) assay. Data were analyzed using analysis of variance. There was no effect of the stage of follicle wave at the time of implant insertion on interval to new follicle wave emergence (range 1-7 days; mean 4.7 days). FSH concentrations were decreased to 5.9+/-2.0 and 7.7+/-1.1 ng/ml for pre- and post-selection cows 1 day after start of treatment; thereafter, they increased on Day 2 to 7.9+/-2.0 and 11.0+/-1.1 ng/ml and on Day 3 to 10.3+/-2.7 and 11.4+/-1.7 ng/ml for pre- and post-selection cows, respectively, despite high-estradiol concentrations at that time. There was no effect of treatment on the interval from implant removal to ovulation (3.2-4.0 days) or on the number of cows detected in estrus (26 of 27 cows). The size of the ovulatory follicle in cows given 0.75 mg EDB 24h post implant removal was decreased in animals at the pre-selection stage (12.2+/-0.1mm) of the follicle wave compared with those at the post-selection stage (15.3+/-0.9 mm) at implant removal. Cows given 600 IU eCG at the pre-selection phase of follicular growth had multiple ovulations (4.0+/-1.1). Cows given EDV at the start of a 5-day implant period had higher estradiol concentrations before and on the day of implant removal than those given EDV at the start of a 9-day implant period. The injection of 0.75 mg EDB 1 day after implant removal tended to increase concentrations of estradiol one day later. In conclusion, 5mg EDV and 3mg N at insertion of a 3mg N implant resulted in variable new follicle wave emergence 1-7 days later in post-partum beef cows nursing calves (22 of 27); both eCG and EDB were equally effective at inducing estrus after implant removal in cows in good BCS, but eCG resulted in a significant increase in ovulation rate in cows treated before dominant follicle selection.     

Effect of estrous cycle stage at Syncro-Mate B treatment on conception and time to estrus in cattle

Two trials involving 85 heifers and 67 cows were conducted to determine the effect of estrous cycle stage at the time of Syncro-Mate-B((R)) (SMB) treatment on interval to estrus following implant removal and on conception rate at the synchronized estrus. In Trial 1, 57 beef and 28 dairy heifers were treated with SMB on each representative day of a 22-d estrous cycle (estrus = Day 0). Beef heifers were artificially inseminated approximately 48 h after implant removal, whereas dairy heifers were inseminated 0 to 12 h after detection of estrus. Inseminations were scored by the inseminator according to their difficulty. Interval to the onset of estrus was not different between heifers treated early ( Day 11) in the cycle (35.2 +/- 7.2 h). Conception rate at the synchronized estrus was slightly higher in early-cycle heifers (22 47 = 47% ) compared to late-cycle heifers (14 38 = 37% , P = 0.2). Heifers that were difficult to inseminate had lower (P < 0.01) conception rates (2 11 = 18% ) at the synchronized estrus than heifers considered normal (21 51 = 41% ) or easier than normal to inseminate (13 23 = 57% ). In Trial 2, of the 131 beef cows synchronized, 67 that were estimated to be either early or late in the estrous cycle by progesterone analysis were utilized. Cows were treated with SMB and inseminated without regard to estrus 48-h after implant removal. Inseminations were scored as in Trial 1. Calves were separated from cows from the time of implant removal to insemination. Conception rate was higher (P < 0.05) in cows treated with SMB early ( Day 11, 16 35 = 46% ). Cows that were difficult to inseminate had a lower (P < 0.01) conception rate (0 8 = 0% ) than cows that were normal (43 94 = 46% ) or easier than normal to inseminate (13 29 = 45% ).     

Conception rates and serum progesterone concentration in dairy cattle administered gonadotropin releasing hormone 5 days after artificial insemination

The objectives of this study were to determine the effect of administration of exogenous GnRH 5days after artificial insemination (AI) on ovarian structures, serum progesterone concentration, and conception rates in lactating dairy cows. In experiment 1, 23 Holstein cows were synchronized using the Ovsynch protocol. Five days after AI (day 0) cows were assigned randomly to receive either saline (saline; n=11) or 100microg GnRH (GnRH; n=12). To examine ovarian structures, ultrasonography was performed on day 1 and every other day beginning on day 5 until day 13. On days 5 and 13 blood samples were obtained to measure serum progesterone concentrations. All cows in the GnRH-treated group developed an accessory corpus luteum (CL), whereas cows in the saline group did not. Mean serum progesterone concentrations did not differ between GnRH and saline groups on day 5 (1.64+/-0.46ng/ml versus 2.04+/-0.48ng/ml). On day 13 serum progesterone concentrations were greater (P<0.05) in the GnRH group compared with saline (5.22+/-0.46ng/ml versus 3.36+/-0.48ng/ml). In experiment 2, 542 lactating cows, at two different commercial dairies, were used to test the effect of administering GnRH 5 days after AI on conception rates. Cows were synchronized and detected for estrus according to tail chalk removal. Cows detected in estrus received AI within 1h after detection of estrus. Five days after AI, cows were assigned randomly to receive either GnRH (n=266) or saline (n=276). Pregnancy status was determined by palpation per rectum of uterine contents approximately 40 days after AI. There was no effect of farm on conception rate. There was no effect of treatment as conception rates did not differ between GnRH and saline groups (26.7% GnRH versus 24.3% saline). Regardless of treatment, days in milk, parity, milk yield, and number of services had no effect on the odds ratio of pregnancy. In summary, the results of this study indicated that GnRH administered 5 days after AI increased serum progesterone by developing an accessory CL but did not improve conception rates in dairy cattle.     

Estrus and pregnancy after synchrony with lutalyse in conjunction with Syncro-Mate-B.

Estrous response and pregnancy rates are decreased for cows given Syncro-Mate-B (SMB) during metestrus (Day 1 to 5 of an estrous cycle). Data indicate these decreases are due, in part, to retention of a functional corpus luteum (CL). Our objective was to determine whether PGF2alpha administered in conjunction with SMB would improve estrous response and pregnancy rates in metestrous cows with no detrimental effects to cows in other stages of the estrous cycle. Three hundred seventy-three suckled beef cows were observed for estrus for 21 d before SMB administration to determine stage of an estrous cycle. Blood samples were collected 14 and 7 d before treatment and at SMB administration. Serum was assayed for concentration of progesterone to verify stage of estrous cycle or noncyclicity. All cows received the standard SMB regime and were allotted by age and stage of cycle to one of two groups. Cows denoted SMB + L received 25 mg of PGF2alpha 8 d after implantation, whereas cows denoted SMB served as controls. On Day 10, SMB implants were removed and females were observed for subsequent estrus. At this time, calves were removed from their dams for 48 h. Artificial insemination was performed 12 hr after observation of a standing estrus. Timed insemination was performed at 48 hr after implant removal for cows not inseminated at 24 or 36 hr after implant removal. Interval to synchronized estrus (within 5 d of implant removal) was lengthened for metestrous cows compared to cows in other stages of the cycle irrespective of treatment (P < 0.001). Cows receiving PGF2alpha had a greater pregnancy rate at 5 d compared to controls (P = .0672). Interval to estrus, estrous response, and pregnancy rate to A1 at d 28 or end of breeding season were not affected by administration of PGF2alpha in conjunction with SMB when compared to the standard SMB protocol.     

Influence of Synchro-Mate-B and breed composition on estrous response and pregnancy rate in spring- and fall-bred Brahman crossbred beef cows

Estrous response, pregnancy rate, and the relationship between palpable corpora lutea (CL) and plasma progesterone concentrations were studied during the spring (n = 149) and fall (n = 146) breeding seasons using postpartum, crossbred beef cows consisting of 0-Brahman, 1 4 - Brahman , or 1 2 Brahman breeding. At the start of each breeding season, a jugular blood sample was collected for progesterone analysis; each cow was palpated for the presence of a CL and randomly alloted within breed to a non-treated control or Synchro-Mate-B (SMB) treatment. Fewer (P<0.01) fall than spring bred cows exhibited estrus within the first 21 d of breeding. Within 48 h of implant removal, 84 and 64% (spring and fall, respectively) of synchronized cows exhibited estrus. In both seasons, more SMB than control cows became pregnant by 7 d of breeding. Percentage of Brahman breeding did not influence estrous response or pregnancy rate. On the basis of 292 evaluations, palpation per rectum correctly assessed ovarian status 74% of the time. These data indicate that although season can influence response. SMB can be used effectively to synchronize Brahman crossbred cows. In addition, it was found that there is a close relationship between palpation per rectum and plasma concentrations of progesterone.     

Changes in ovarian function in mature beef cows grazing endophyte infected tall fescue

The objective was to examine follicular and luteal development and function in mature, lactating beef cows grazing endophyte free (E-) or endophyte infected (E+) tall fescue during the early postpartum period. Angus, Hereford, and Angus x Hereford cows were exposed to pasture for 37-39 days before synchronized estrus. Serum concentrations of prolactin were evaluated during the luteal phase before the synchronized estrus. Every Monday, Wednesday, and Friday for one estrous cycle ovaries were monitored by transrectal ultrasonography and blood was collected for determination of serum concentrations of progesterone and estradiol in cows that responded to synchronization. Signs of fescue toxicosis in E+ cows included decreased serum concentrations of prolactin (84.9+/-13.6 pg/ml versus 32.3+/-12.0 pg/ml; P < 0.009) measured during the luteal phase (day 37 of grazing) and decreased body condition of cows and weight of cows and calves (P < 0.001). Neither serum concentrations of progesterone or estradiol, nor diameter of the CL differed between treatments. Diameter of the largest follicle tended to be smaller for cows grazing E+ fescue, especially between days 8 and 12 of the estrous cycle (P < 0.08). Numbers of class 1 (3-5 mm) and class 3 (>10 mm) follicles were similar (P > 0.05) between treatments, but number of class 2 (6-9 mm) follicles was reduced in E+ cows for most of the cycle (days 10 through 20; P < 0.03). Length of synchronized estrous cycle, days open, calving interval, and pregnancy rate at 30, 45, 60, and 90 days post-breeding was similar (P > 0.05) among treatment groups. Even though follicular dynamics (diameter of the largest follicle and number of class 2 follicles) were altered in cows grazing E+ tall fescue, follicular function was apparently not affected by ergot alkaloids.     

Comparison of oocyte developmental competence and follicular steroid content of nulliparous heifers and cows at different stages of lactation

Reduced reproductive performance and lower conception rates of lactating cows are closely associated with genetic progress for high milk production. In contrast, the fertility of nulliparous Holstein heifers has remained fairly stable over the years and appears to be markedly higher than that of mature lactating cows. Possible differences in oocyte quality and follicular steroid levels, which could be associated with the low fertility of high-lactating cows, were examined in 13-month-old heifers, cows around the time of first AI (60-95 d post-partum, yielding 49+/-2.4 kg/d) and cows at mid-lactation (120-225 d post-partum, yielding 37+/-2.1 kg/d). Estrus was synchronized by two doses of PGF2alpha and follicles (5-8 mm) were aspirated on days 4, 8, 11 and 15 of the cycle by an ultrasound-guided procedure. Oocytes were morphologically examined, matured in vitro, chemically activated and cultured for 8d. Cleavage rate and the proportion of developing parthenogenetic blastocysts were determined on days 3 and 8 post-activation, respectively. On day 17, heifers and cows received additional PGF2alpha and follicular fluids from preovulatory follicles were collected on day 19 perior to the expected estrus. Follicular-fluid volumes were similar in cows and heifers, as were estradiol, progesterone and androstenedione concentrations in the follicular fluid. Percentages of high-grade oocytes, proportions of cleaved oocytes and developed blastocysts did not differ between the groups. Results suggest that the fertility gap between nulliparous heifers and high-lactating cows is not directly related to steroid content in the preovulatory follicular fluid or oocyte developmental competence.     

Chronic stress, hormone profiles and estrus intensity in dairy cattle

The objectives of the present study were to determine if lameness, a model for a natural chronic stressor, affects hormone concentrations in milk prior to estrus and/or the subsequent expression of estrus in the postpartum period. Dairy cows>20 days postpartum were scored for lameness and observed for estrus intensity using a weighted scoring system (>100 points=estrus=Day 0). Increasing lameness score was not associated with daily profiles of milk progesterone (throughout Days -18 to 0), estradiol (Days -6 to 2) or cortisol (Days -18 to 2) around estrus, maximum estradiol values or estradiol concentrations on Day 0. However, post hoc pair wise comparisons revealed that prior to estrus, severely lame cows had lower maximum progesterone concentrations compared to nonlame cows (1.3+/-0.1, 1.2+/-0.2, 0.7+/-0.1 ng/ml milk; P=0.042). Furthermore, severely lame cows expressed behavioral estrus with lower intensity (284+/-128 points, n=9) compared to moderately lame (662+/-310 points, n=9) or nonlame animals (583+/-275 points, n=18; P=0.05 and P=0.02, respectively). Resting concentrations of cortisol (Days 20-80 postpartum) did not vary between days postpartum or lameness score. The incidence of behavioral estrus was not affected by increasing lameness score, as 54.2%, 56.2% and 50.0% periods with low progesterone were associated with spontaneous estrus expression, respectively. Concluding, in this biological model of chronic stress, lameness did not affect the incidence of behavioral estrus but did reduce estrus intensity once ovarian cyclicity had resumed after calving. This reduced intensity of estrus was associated with lower maximum progesterone values prior to estrus but not abnormal daily cortisol or estradiol values in milk.     

Concentrations of progesterone in milk as a monitor of early pregnancy diagnosis in dairy cows

The objectives of the experiment were to evaluate the efficacy of using progesterone concentrations in milk and palpation per rectum on days 21 or 22 postbreeding to estimate pregnancy and evaluate management practices; and to investigate physiological occurrences leading to incorrect diagnosis of pregnancy when serial samples of milk were collected. Of particular interest were indications of early embyronic death and insemination of cows not in estrus. Milk samples were collected at the afternoon milking of days 0 or 1 (day 0 = day of estrus), 9 or 10, 21 or 22 and 27 or 28 following breeding in 200 lactating dairy cows. Tentative diagnosis of pregnancy was made based on concentrations of progesterone in milk on days 21 and 22 alone and on days 21 or 22 and 27 or 28. In addition all cows were palpated per rectum on days 21 or 22 postbreeding and a tentative pregnancy diagnosis was made. Pregnancy was confirmed by examination of the genital tract per rectum between 35 and 50 days after breeding. Values of 4 ng/ml or greater and/or the presence of a mature corpus luteum were considered positive signs of pregnancy. Progesterone in milk ranged from 0.1 to 18 ng/ml. On days 0 or 1, 9 or 10, 21 or 22 and 27 or 28 concentrations of progesterone in milk averaged 1.5 +/- 0.3, 11.1 +/- 0.5, 12.0 +/- 0.4 12.5 +/- 0.5 ng/ml for pregnant cows. Corresponding samples from nonpregnant cows averaged 1.2 +/- 0.2, 10.3 +/- 0.4, 3.0 +/- 0.4, 6.8 +/- 0.6 ng/ml, respectively. Ninety-six and 104 cows were classified as pregnant and nonpregnant on days 21 or 22 as compared to 78 and 118 cows diagnosed as pregnant and nonpregnant on days 21 or 22 and 27 or 28 combined. Pregnancy detection by progesterone in milk on days 21 or 22 with pregnancy determined via rectal palpation 35 to 50 days postbreeding was 77 and 100% accurate for positive and negative diagnosis, respectively. The percent agreement using progesterone in milk on days 21 or 22 and 27 or 28 combined was 95 and 100%, respectively, for positive and negative diagnosis. Diagnosis based on rectal palpation 21 or 22 days postbreeding was 63 92 (69%) and 76 88 (87%) for pregnant and nonpregnant cows, respectively. Ten of the 200 cows had progesterone concentratins in milk of > 4 ng/ml at the time of breeding. Six of these cows were pregnant from a previous insemination. The other four cows were nonpregnant and were inseminated during the luteal phase of the cycle. In conclusion, measurement of progesterone in milk is a useful tool in early detection of pregnant and nonpregnant cows and may be useful in detecting reproductive problems in a dairy herd. It will probably be most useful when used in combination with later pregnancy diagnosis per rectum .     

Influence of GnRH infusion on endocrine parameters and duration of postpartum anestrus in beef cows

The effect of an intravenous infusion of gonadotrophin releasing hormone (GnRH) on the duration of postpartum anestrus in suckled beef cows was studied. Twenty-eight, mature, suckled beef cows were assigned in equal numbers to one of four treatment groups which were based on infusion with saline or GnRH (15ug/hour for 12 hours) and stage postpartum (pp) (20 or 35 days). Serum LH and progesterone were determined by radioimmunoassay for the period which began 5 days pre-infusion and ended at 55 days postpartum (ie: 35 or 20 days post-infusion). Serum LH remained below 5ng/ml during infusion in all control cows. Peak serum LH values, times of LH peaks, and duration of LH responses (means +/- SE) during infusion were 49 +/- 12 ng/ml, 162 +/- 42 minutes and 7.8 +/- 1.3 hours for the 20 day group and 44 +/- ng/ml, 144 +/- 6 minutes, and 8.2 +/- 1.1 hours for the 35 day group respectively. Serum progesterone levels indicated that the proportion of cows showing the onset of estrous cycles within 10 days of infusion was greater in the 20 day pp GnRH group (4/7) than the 20 day pp saline group (0/7) (p < .05) but was not significantly different between the 35 day pp GnRH (4/7) and 35 day pp saline (2/6) groups. The incidence of estrus was not affected by GnRH treatment and was 37% in all cows prior to 55 days pp. It was concluded that infusions of GnRH for 12 hours at a rate of 15 ug/hour could induce estrous cycles in suckled beef cows treated at 20 days postpartum.     

Effect of estrus synchronization with Syncro-Mate-B((R)) on serum luteinizing hormone, progesterone and conception rate in Brahman heifers

Twenty-two estrous cyclic, 2-yr-old Brahman heifers were randomly assigned to receive either estrus synchronization with Syncro-Mate-B((R)) (SMB; 11) or no treatment (Control; 11). Blood samples were collected via tail vessel puncture at onset of estrus and daily thereafter until Day 11 after estrus. Blood samples were also collected from five SMB and five Control heifers at 0, 4, 8 and 12 h after the onset of estrus. All samples were processed to yield serum and stored at -20 degrees C until radioimmunoassay. Heifers were inseminated by one technician using semen from a single ejaculate of a Brahman bull 12 h after the onset of estrus. All SMB heifers exhibited estrus within 72 h of implant removal. All heifers had corpora lutea (CL) detected by rectal examination 8 to 12 d following estrus. Serum luteinizing hormone (LH) was not affected by treatment, time (4 - h intervals) or an interaction of treatment by time (P > 0.10). Independent analysis with h indicated that at h 12, SMB (2.2 +/- 0.06 ng/ml) had lower LH than did control heifers (8.9 +/- 2.1 ng/ml). Serum progesterone increased from Day 1 through Day 12 in all heifers, which is indicative of functional CL. Serum progesterone was affected by treatment (P < 0.0001) and time (d intervals; P < 0.10). Progesterone elevation was lower (P < 0.05) and area under the progesterone curve was lower (P < 0.03) in SMB (5.6 +/- 0.5 ng/ml, 32.0 +/- 4.5 units, respectively) when compared with control heifers (7.0 +/- 4 ng/ml, 43.7 +/- 2.4 units, respectively). Conception rate was lower (P < 0.01) in SMB heifers (2 of 11) than in control heifers (8 of 11). The lowered conception rate in SMB treated Brahman heifers may be due to altered timing of LH release following estrus, resulting in an altered time of ovulation.     

Effect of days postpartum and exogenous GnRH on reproductive hormone and ovarian changes in postpartum suckled beef cows

Two experiments were conducted to determine the effect of days postpartum and exogenous gonadotropin releasing hormone (GnRH) on reproductive hormone and ovarian changes in postpartum suckled beef cows. In experiment 1, eight suckled cows were bled at .5 hour intervals for 4 hours on days 7, 14, 21 and 28 postpartum. Although mean concentrations of plasma luteinizing hormone (LH) were positively correlated with days postpartum, mean concentrations did not differ. The mean maximum change and the variance of plasma LH were low on days 7, 14, 21 and 28 postpartum. Although the number of cows with an ovarian follicle and follicular size increased with days postpartum, mean concentrations of estradiol-17beta did not change. The interval from parturition to the first detected ovarian follicle and the first postpartum estrus was 17.5 +/- 2.6 days and 36.0 +/- 2.2 days, respectively. An elevation in plasma progesterone was detected about one week prior to the first postpartum estrus in 6 of the eight cows in the absence of corpora lutea. In experiment 2, gonadotropin releasing hormone (GnRH) induced ovulation in 4 of the 8 cows treated on day 27, 28 or 29 postpartum whereas none of the 8 saline treated cows ovulated to treatment. The interval from parturition to first estrus and conception were similar for both groups (P >.10).     

Comparisons of eight commercial on-farm milk progesterone tests

To evaluate eight commercial on-farm milk progesterone kits, milk samples (50 ml each of foremilk and postmilk strippings) were collected during the estrous cycle from 10 cycling Holstein cows for 24 consecutive days. Relative concentrations of progesterone were classified as low or high by comparison with standard progesterone samples supplied with each kit. The concentration of progesterone in each milk sample was determined by radioimmunoassay (RIA). Accuracy of classification into low or high levels by commercial tests was determined by the percentage of similarity with RIA values using discriminant analysis. Accuracy of the eight tests ranged from 89.0 to 98.9% for low progesterone, 74.8 to 85.6% for high progesterone, and 80.3 to 87.3% for all samples (n = 238). The percentage of fat in milk or an interaction of the percentage of milkfat by day of estrous cycle influenced commercial test results for all tests except Accufirm and Calfcheck. Progesterone levels, estimated by the test-kits, were low from 1.5 +/- 0.5 to 2.8 +/- 0.9 days before estrus (X +/- SEM) and until 4.0 +/- 0.6 to 5.9 +/- 1.3 days after estrus. These data support the principle that a single low progesterone sample cannot be used to determine proper timing of insemination. All eight commercial kits can be used to determine accurately the relative concentrations of progesterone in milk samples.     

Syncro-mate B induces estrus in ovariectomized cows and heifers

Eleven ovariectomized Hereford x Simmental cows and 10 ovariectomized crossbred heifers (primarily Angus and Hereford) were given the Syncro-Mate B (SMB) estrous synchronization treatment. The SMB treatment consisted of a 2 ml i.m. injection containing 5 mg of estradiol valerate and 3 mg of norgestomet plus a hydron ear implant containing 6 mg of norgestomet. The ear implant was removed 9 d later. Cows and heifers were considered in estrus only if they stood for mounting by a herdmate or a bull. Observations for estrus were made four or six times each day for 3 d after implant removal. The 21 animals were used in eight trials. Each trial involved 9 or 11 cows or 5 or 10 heifers. Four days to three weeks elapsed between implant removal and implant insertion for the next trial. No ovariectomized cow or heifer was observed in estrus for 21 d before treatment with SMB. In the eight trials, 3 of 9, 7 of 9 and 6 of 11 cows exhibited estrus, whereas 5 of 10, 1 of 5, 3 of 5, 3 of 5 and 5 of 5 heifers exhibited estrus after treatment. When data were pooled, 16 of 29 (55.2%) cows and 17 of 30 (56.7%) heifers exhibited estrus after treatment. Our data indicate that the SMB treatment can induce estrus in cows and heifers, independently of the ovaries.     

A note on the effects of calf stimuli on the response of Zebu cows to Synchro-mate-B

The effect of Synchro-mate-B (SMB) was measured on the estrous response and the establishment of cyclicity using different calf stimuli. Multiparous Zebu cows, were divided in three groups. In the first, 32 animals were treated with SMB leaving the calves present (SMB+CP). In the second, 33 cows were treated with SMB and calves partially removed (fence-line contact) for 48h (SMB+CPR). In the third group (n=33), cows received SMB and calves were removed for 48h (SMB+CCR) with no visual or olfactory contact. A control group (CG, n=33) involved neither SMB or calf separation. Blood samples for progesterone assessment were obtained at 11 and 4d prior to SMB treatment and on days 7 and 11 after the average return to estrous for each group. All animals were observed continuously for mounting activity during 72h after SMB implant removal. A significant difference (P<0.05) in estrous response was found between SMB-treated and non-treated animals, regardless of calf management (56 versus 8%, respectively). Cows with SMB+CCR and SMB+CPR came into estrous sooner (P<0.05) (26.5+/-2.6 and 18.1+/-4.94h, respectively), than those that remained with their calf present (40.4+/-12.8h). Cows with SMB+CCR displayed longer (P<0.05) periods of mounting behavior (13.0+/-4.4h) in comparison with SMB+CPR and SMB+CP (7.4+/-1.8 and 8.1+/-4.0h), respectively. Furthermore, 84% (P<0.05) of the cows in the SMB+CCR had high concentrations of progesterone after mounting behavior was displayed, in comparison with 68 and 54% in the other two groups, respectively. No difference was found (P>0.05) in the number of mounts per hour in estrous. It was concluded that: (1) SMB increases the number of cows that display estrous; (2) temporary weaning shortens the period from SMB implant withdrawal to mounting activity, and (3) SMB+CCR, increases the length of sexual receptivity and the number of cows that continue to cycle.     

Influence of exogenous steroids, nutrition and calf removal on reproductive performance of anestrous beef cows

Methods for inducing a fertile estrus in anestrous beef cows suckling calves were explored in three studies. Trial 1 . Eighty-two primiparous and pluriparous Brahman-Hereford crossbred and Angus cows suckling calves were divided into four groups: 1) good body condition, high level of nutrition (GH); 2) good body condition, low level of nutrition (GL); 3) poor body condition, high level of nutrition (PH); 4) poor body condition, low level of nutrition (PL). All cows received the Shang Treatment (S). Following treatment, more cows (P<0.05) ovulated in the GH group (95%) than either the GL (61%), PH (58%), or PL (53%) group. Pregnancy rates were low and were not different between groups following the insemination taking place 54 hours after implant removal or after cows had been bred for 21 days. Trial 2 . One hundred fourteen two-year-old Santa Gertrudis-Hereford crossbred cows in poor body condition were divided into four groups. Forty-nine cows received S treatment and 65 cows served as controls (C) with approximately half of each group fed a high (H) or a medium (M) level of nutrition (SH, SM, CH, CM). One cow in the SH group ovulated and was pregnant 21 days after breeding. The remaining cows failed to exhibit estrus, ovulate or become pregnant. Trial 3 . Cows used in this trial were those from Trial 2 which did not exhibit estrus nor had a palpable corpus luteum present 21 days after implant removal. Cows were divided into 1) control (C), 2) wean (W), 3) wean + Syncro-Mate-B (W+SMB) and 4) Syncro-Mate-B + wean (SMB+W). The percentage of cows ovulating by 2 and 40 days after the start of breeding was similar (P>0.05) in the W, W+SMB, and SMB+W groups but all were greater (P<0.05) than the percentage of cows ovulating in the C groups. Pregnancy rates of cows in the W+SMB and SMB+W groups were higher (P<0.05) than pregnancy rates of cows in the W and C groups by two days after the start of breeding. Pregnancy rates 40 days after the start of breeding were higher (P<0.05) for cows in the W, W+SMB) and SMB+W groups than for cows in the C group.     

Estrogen and progesterone concentrations in bovine milk during the estrous cycle

Estrogen and progesterone concentrations in milk during the estrous cycle were estimated in 18 normally cycling Holstein dairy cows. The estrogen and progesterone concentrations in milk during the estrous cycle followed the pattern described for them in blood in the corresponding period. During most of the estrous cycle, estrogen concentration remained at approximately 200 pg/ml and reached a proestrous peak of 360 +/- 127 pg/ml on day 19. The progesterone concentration in milk during the estrous cycle increased to a peak on day 13 (45.5 +/- 6.6 ng/ml) and thereafter declined towards estrus. Estrus detection/prediction based on milk progesterone concentrations appears feasible in view of the significant differences in milk progesterone concentrations between the early luteal (post-ovulatory), luteal and rapid follicular growth periods of the estrous cycle.     

Effect of norgestomet on peripheral levels of progesterone and estradiol-17beta in beef cows

Peripheral levels of progesterone and estradiol 17beta were quantified in 27 cycling cows following administration of a single Hydron ear implant (G. D. Searle and Co.) containing 2, 4 or 6 mg norgestomet or controls which received no implant. Implants were inserted subcutaneously in the ear on day 15 of the estrous cycle (day of estrus = day 0) and removed 9 days later. The 4 mg (seven of seven cows) and 6 mg (six of six cows) implants suppressed estrus; however, three of eight cows in the 2 mg group exhibited estrus prior to implant removal. The 6 mg implant group had a significantly longer interval from implant removal to estrus than either the 2 or 4 mg group. Failure to detect differences in the rate at which progesterone declined indicated norgestomet treatment did not affect normal corpus luteum regression. Estradiol levels rose at a similar rate approaching estrus in all treatments. There was no indication of increased endogenous estradiol levels due to norgestomet treatment.     

Effect of estradiol valerate on ovarian follicle dynamics and superovulatory response in progestin-treated cattle

Three experiments evaluated the effects of estradiol valerate (EV) on ovarian follicular and CL dynamics, intervals to estrus and ovulation, and superovulatory response in cattle. Experiment 1 compared the efficacy of two norgestomet ear implants (Crestar and Syncro-Mate B; SMB) for 9 d (with PGF at implant removal), combined with either 5 mg estradiol-17beta and 100 mg progesterone (EP) or 5 mg EV and 3mg norgestomet (EN) im at the time of implant insertion on CL diameter and follicular wave dynamics. Ovaries were monitored by ultrasonography. There was no effect of norgestomet implant. Diameter of the CL decreased following EN treatment (P < 0.01). Mean (+/- S.D.) day of follicular wave emergence (FWE) was earlier (P < 0.0001) and less variable (P < 0.0001) in EP- (3.6 +/- 0.5 d) than in EN- (5.7 +/- 1.5 d) treated heifers. Intervals from implant removal to estrus (P < 0.001) and ovulation (P < 0.01) were shorter in EN- (45.7 +/- 11.7 and 74.3 +/- 12.6 h, respectively) than in EP- (56.4 +/- 14.1 and 83.3 +/- 17.0 h, respectively) treated heifers. Experiment 2 compared the efficacy of EP versus EN in synchronizing FWE for superovulation in SMB-implanted cows. At random stages of the estrous cycle, Holstein cows (n = 78) received two SMB implants (Day 0) and were randomly assigned to receive EN on Day 0 or EP on Day 1. Folltropin-V treatments were initiated on the evening of Day 5, with PGF in the morning and evening of Day 8, when SMB were removed. Cows were inseminated after the onset of estrus and embryos were recovered 7 d later. Non-lactating cows had more CL (16.7 +/- 11.3 versus 8.3 +/- 4.9) and total ova/embryos (14.7 +/- 9.5 versus 7.9 +/- 4.6) than lactating cows (P < 0.05). EP-treated cows tended (P = 0.09) to yield more transferable embryos (5.6 +/- 5.2) than EN-treated cows (4.0 +/- 3.7). Experiment 3 compared the effect of dose of EV on ovarian follicle and CL growth profiles and synchrony of estrus and ovulation in CIDR-treated beef cows (n = 43). At random stages of the estrous cycle (Day 0), cows received a CIDR and no further treatment (Control), or an injection of 1, 2, or 5 mg im of EV. On Day 7, CIDR were removed and cows received PGF. Follicular wave emergence occurred within 7 d in 7/10 Control cows and 31/32 EV-treated cows (P < 0.05). In responding cows, interval from treatment to FWE was longer (P < 0.05) in those treated with 5 mg EV (4.8 +/- 1.2 d) than in those treated with 1 mg (3.2 +/- 0.9 d) or 2 mg (3.4 +/- 0.8 d) EV, while Control cows were intermediate (3.8 +/- 2.0 d). Diameter of the dominant follicle was smaller (P < 0.05) at CIDR removal and tended (P = 0.08) to be smaller just prior to ovulation in the 5 mg EV group (8.5 +/- 2.2 and 13.2 +/- 0.6 mm, respectively) than in the Control (11.8 +/- 4.6 and 15.5 +/- 2.9 mm, respectively) or 1mg EV (11.7 +/- 2.5 and 15.1 +/- 2.2 mm, respectively) groups, with the 2mg EV group (10.7 +/- 1.5 and 14.3 +/- 1.7 mm, respectively) intermediate. Diameter of the dominant follicle at CIDR removal was less variable (P < 0.01) in the 2 and 5mg EV groups than in the Control group, and intermediate in the 1mg EV group. In summary, treatment with 5mg EV resulted in a longer and more variable interval to follicular wave emergence than treatment with 5mg estradiol-17beta, which affected preovulatory dominant follicle size following progestin removal, and may have also affected superstimulatory response in Holstein cows. Additionally, 5 mg EV appeared to induce luteolysis in heifers, reducing the interval to ovulation following norgestomet removal. Conversely, intervals to, and synchrony of, follicular wave emergence, estrus and ovulation following treatment with 1 or 2 mg EV suggested that reduced doses of EV may be more useful for the synchronization of follicular wave emergence in progestogen-treated cattle.     

Luteal competency during the resumption of ovarian cyclicity in postpartum Brahman cows

Twenty pluriparous, spring-calving Brahman cows were used to determine luteal competency, as measured by serum progesterone concentrations, during the first and the second postpartum estrous cycles. Prior to and after calving, all cows were maintained in good body condition on Coastal bermudagrass pasture (IFN 1-00-703). The calves were allowed to suckle ad libitum, and sterile marker bulls were maintained with the cow herd as an aid in estrus detection throughout the trial. Cow weight and body condition score were recorded within 24 hours after calving and again at the first behavioral estrus observed. From day 1 through day 14 (day 0 = estrus) of both the first and the second postpartum estrous cycles, blood samples were collected from each cow, processed to yield serum and analyzed by radioimmunoassay for progesterone concentrations. There was a higher incidence of abnormal estrous cycles following the first postpartum estrus (35%) than following the second (5%) postpartum estrus (P<0.05). The abnormal first estrous cycles were characterized by either a short luteal phase (four cows) or by standing estrus behavior without luteal tissue formation (three cows). When serum progesterone concentrations were compared for all cows during the first estrous cycle with those during the second estrous cycle, there was less progesterone released during the cycle (P<0.05) and lower peak progesterone concentrations (P<0.10) during the first estrous cycle. However, if the abnormal cows were excluded from the analyses, there was no difference (P>0.10) in either progesterone concentrations through the 14 days measured or in peak progesterone concentrations between the first and the second postpartum estrous cycles. It can be concluded from this study that the higher incidence of abnormal luteal function following the first postpartum estrus may contribute to the decreased conception rates observed when cows are bred at their first postpartum estrus.