GnRH dose at initiation of a 5-day CO-Synch + P4 for fixed time artificial insemination in suckled beef cows

The objective of the present study was to evaluate the effect of GnRH dose administered at initiation (GnRH-1) of a 5-day CO-Synch + P4 protocol on ovulatory response, expression of estrus, and fertility in suckled beef cows. Suckled beef cows (n = 1101) at four locations were randomized to receive either 100 or 200 µg of gonadorelin acetate at initiation (D-8) of a 5-day CO-Synch + P4 protocol concurrently with insertion of an intravaginal progesterone (P4) device. On D-3 the P4 device was removed, two doses of prostaglandin F2α were administered concurrently and a patch was applied to evaluate expression of estrus. Artificial insemination was performed 72 h after P4 device removal (D0) simultaneously with the administration of 100 µg of gonadorelin acetate (GnRH-2). Increasing GnRH dose at initiation of a 5-day CO-Synch + P4 did not enhance ovulatory response (P = 0.57) to GnRH-1, expression of estrus (P = 0.79), nor pregnancies per AI (P/AI; P = 0.91). Both follicle size (quadratic) and circulating P4 (linear) affected (P < 0.01) ovulatory response to GnRH-1 independent of dose. Cows that had ovulation to GnRH-1 had smaller (P < 0.001) follicle size on D-3 and reduced (P = 0.05) expression of estrus compared to cows that did not have ovulation to GnRH-1, however, P/AI did not differ (P = 0.75). In conclusion, increasing the dose of GnRH-1 in the 5-day CO-Synch + P4 protocol did not enhance ovulatory response, expression of estrus, or P/AI in suckled beef cows.

48 Timing of luteolysis and conceptus expulsion after induced pregnancy loss at three different timepoints after maternal recognition of pregnancy in cattle

Results from previous research indicate that induced pregnancy loss (IPL) on Day 36 of gestation in cattle resulted in luteolysis by gestation Day ∼45. The aim of this study was to test the hypothesis that IPL at 25, 35, or 55 days of gestation will result in luteolysis and whether the interval between IPL and luteolysis is positively associated with days of gestation at time of treatment. Pregnant non-lactating beef cows were randomly assigned to undergo IPL, consisting of an intrauterine infusion of 120mL of hypertonic saline (7.2%), at Day 25 (IPL25, n=15), Day 35 (IPL35, n=14), or Day 55 (IPL55, n=14). Corpus luteum (CL) volume and embryo/fetal viability were evaluated by transrectal B-mode ultrasonography daily for 19 days following treatment and every other day subsequently until luteolysis was identified or gestation Day 91. Blood flow through the CL was evaluated using colour-Doppler ultrasonography, and luteolysis was defined as the day when &lt;25% of the CL was represented with colour pixels. Conceptus expulsion was defined as the absence of a conceptus and fluid accumulation in the uterus was &lt;25%. Data were analysed using Proc Mixed in SAS (v9.4; SAS Institute Inc.). Cessation of conceptus heartbeat occurred in all animals within 24h of administration of treatment. Day of luteolysis occurred earlier (P&lt;0.01) in IPL25 (5.3±0.4d) than IPL55 (8.9±1.4d) cows, and luteolysis tended (P=0.07) to occur earlier in IPL35 (6.6±0.6d) than IPL55 cows; however, values for this variable in IPL25 and IPL35 cows were similar (P&gt;0.2). The variance in timing of luteolysis was greater for IPL55 than for IPL35 cows, whereas variance for IPL35 was greater than that for IPL25 (P&lt;0.05) cows. Conceptus expulsion was detected earlier (P&lt;0.01) in IPL25 (Day 4.2±0.4) and IPL35 (Day 5.8±0.5) than IPL55 (Day 10.4±1.4) cows; however, IPL25 and IPL35 cows had similar (P&gt;0.1) values for this variable. Values for luteolysis and conceptus expulsion were correlated in IPL35 (r=0.84; P&lt;0.01) and IPL55 (r=0.88; P&lt;0.01) cows, whereas there was no correlation among values for these variables in IPL25 (r=0.31; P=0.3) cows. There were effects of treatment (P&lt;0.01), time (P&lt;0.01), and treatment by time interaction (P&lt;0.01) on CL volume. Volume of the CL was less (P&lt;0.04) in IPL25-treated cows by treatment day 5 compared with that in IPL55-treated cows. Additionally, CL volume was less (P&lt;0.03) in IPL25 by treatment day 6 than in IPL35 cows, and IPL35 cows had a lesser (P&lt;0.01) CL volume starting on treatment day 10 than IPL55 cows. Induction of pregnancy loss at different times of gestation resulted in luteolysis; however, the interval between treatment and luteolysis tended to increase as gestation days increased. Furthermore, the variability in the timing of luteolysis increased as gestation days increased. Results from the present study support the working hypothesis of presence of a continuous mechanism for CL maintenance during pregnancy beyond the classical maternal recognition period in cattle.

109 Pre-synchronization and reutilization of progesterone devices during a 6-day CO-Synch protocol for fixed-time artificial insemination in beef heifers

Ovulatory response to the initial gonadotrophin-releasing hormone (GnRH) of the CO-Synch protocol is affected by circulating progesterone (P4) and follicle size. In addition, heifers that ovulate to the initial GnRH treatment have greater fertility after AI. Thus, this study determined the effect of (1) presynchronization (Presynch) before a 6-day CO-Synch protocol and (2) circulating [RCE1] (P4) on ovulatory response, oestrus expression, and pregnancies per AI (P/AI) in beef heifers. Yearling beef heifers (n=233) at three locations were randomly assigned in a 2×2 factorial design to the following treatments: (1) Presynch+6-day CO-Synch with a new P4 device; (2) Presynch+6-day CO-Synch with a once-used P4 device; (3) 6-day CO-Synch with a new P4 device; and (4) 6-day CO-Synch with a once-used P4 device. Presynch consisted of insertion of a new P4 intravaginal device (1.38g of P4) on Day −17 and removal of P4 device on Day −11 concurrently with 500µg of cloprostenol sodium (PGF). On Day −9, all heifers received either a new (New) or once-used (Used) CIDR and 100µg of gonadorelin acetate (GnRH). Six days later (Day −3), CIDRs were removed, 1000µg of PGF was administered and an oestrous detection patch applied (Estrotect). At 72h after CIDR removal, AI was performed concurrently with administration of 100µg of GnRH. Pregnancy was determined by transrectal ultrasonography 31 days after AI. A subset of heifers (n=155) was examined on Day −9 and Day −3 by ultrasonography to determine ovulation to Day −9 GnRH. Data were analysed using generalized linear mixed models (SAS 9.4; SAS Institute Inc.). Presynch heifers had larger follicle diameter on Day −9 (12.7±0.3 vs. 10.1±0.3 mm; P&lt;0.001), greater ovulatory response to Day −9 GnRH (82.5%; 66/80 vs. 56%; 42/75; P&lt;0.001), greater expression of oestrus (90.6%; 106/117 vs. 78.4%; 91/116; P&lt;0.02), and earlier oestrus (49.8±1 vs. 53.1±1 h; P&lt;0.01) compared with controls. There was a treatment×CIDR interaction on oestrous expression, such that a lesser (P&lt;0.05) percentage of control heifers with new CIDR expressed oestrus compared with all other groups (Table 1). Heifers with a used CIDR during the 6-day CO-Synch tended (P=0.08) to have greater P/AI (52.1%; 61/117) than those with a new CIDR (40.5%; 47/116). In conclusion, presynchronization before initiation of a 6-day CO-Synch increased follicle diameter, enhanced ovulatory response and oestrous expression, but did not affect fertility. The earlier onset of oestrus in presynchronized heifers suggests that the timing of AI may need to be modified. Table 1. Oestrous expression and pregnancy per AI (P/AI) in beef heifers with or without presynchronization and treated with a new or used CIDR during a 6-day CO-Synch Treatment CIDR Oestrus (%; n/n) Time of oestrus (h) P/AI (%; n/n) Control New 67.8a (40/59) 53.7±1.5a 33.9 (20/59) Used 89.5b (51/57) 52.7±1.6a 50.9 (29/57) Presynch New 94.7b (54/57) 50.9±1.4b 47.4 (27/57) Used 86.7b (52/60) 48.7±1.3b 53.3 (32/60) P-value Treatment 0.03 0.01 0.21 CIDR 0.62 0.19 0.08 Interaction 0.003 0.75 0.38 a,bValues with different superscripts differ (P&lt;0.05).

The requirement of GnRH at the beginning of the five-day CO-Synch + controlled internal drug release protocol in beef heifers

The objective of this study was to determine if the omission of GnRH at controlled internal drug release device (CIDR) insertion would impact pregnancy rates to timed AI (TAI) in beef heifers enrolled in a 5-d CO-Synch + CIDR protocol that used 1 PGF2α dose given at CIDR removal. Yearling beef heifers in Ohio in 2 consecutive breeding seasons (2011, n = 151, and 2012, n = 143; Angus × Simmental), Utah (2012, n = 265; Angus × Hereford), Idaho (2012, n = 127; Charolais), and Wyoming (2012, n = 137; Angus) were enrolled in the 5-d CO-Synch + CIDR protocol. At CIDR insertion (d -5), heifers were randomly assigned either to receive 100 μg GnRH (GnRH+; n = 408) or not to receive GnRH (GnRH-; n = 415). At CIDR removal (d 0 of the experiment), 25 mg PGF2α was administered to all heifers. All heifers were inseminated by TAI and given 100 μg GnRH 72 h after PGF2α (d 3). In heifers at the Ohio locations (n = 294), presence of a corpus luteum (CL) at CIDR insertion (d -5) was determined via assessment of progesterone concentrations (2011) and ovarian ultrasonography (2012). Subsequently, in both years, ovarian ultrasound was conducted on d 0 to determine the presence of a new CL. In this same subgroup of heifers, blood samples for progesterone analysis were collected on d 3 to assess luteal regression. Pregnancy diagnosis was performed between 32 and 38 d after TAI. At CIDR withdrawal, presence of a new CL was greater (P < 0.05) in the GnRH+ (55.8%, 82/147) than GnRH- (26.5%, 39/147) treatment. Incidence of failed luteal regression did not differ between the GnRH+ (3.4%) and GnRH- (0.7%) treatments. Pregnancy rate to TAI did not differ between the GnRH+ (50.5%) and GnRH- (54.9%) treatments. In conclusion, although the incidence of a new CL at CIDR removal was increased in the GnRH+ treatment, omission of the initial GnRH treatment in the 5-d CO-Synch + CIDR protocol did not influence TAI pregnancy rate in yearling beef heifers. In addition, a single dose of PGF2α at CIDR removal was effective at inducing luteolysis in yearling beef heifers enrolled in the 5-d CO-Synch + CIDR protocol, regardless of whether or not the initial GnRH treatment was given.

Testicular development of Zebu bulls after chronic treatment with a gonadotropin-releasing hormone agonist

The objective was to compare testis characteristics of Zebu bulls treated with the GnRH agonist, deslorelin, at different times and for different durations during their development. An additional objective was to determine the usefulness of a stain for the transcription factor GATA-binding protein 4 (GATA-4) as a specific marker for Sertoli cell nuclei in cattle. Bulls (54) were allocated to nine groups (n = 6) and received s.c. deslorelin implants as follows: G1 = from birth to 3 mo of age; G2 = from 3 to 6 mo; G3 = from 6 to 9 mo; G4 = from 9 to 12 mo; G5 = from birth to 15 mo; G6 = from 3 to 15 mo; G7 = from 6 to 15 mo; G8 = from 12 to 15 mo; and G9 (control) = no implant. Bulls were castrated at 19 mo of age. Paraffin sections (10 microm) were subjected to quantitative morphometry and GATA-4 immunohistochemistry. At castration, all bulls in the control group (6/6) had attained puberty (scrotal circumference > or = 28 cm), whereas a smaller proportion (P < 0.05) had reached puberty in G2 (2/5) and G6 (1/6). Bulls in G2 and G6 also had a lesser (P < 0.05) testis weight compared with the control group. Total volume of seminiferous epithelium and total daily sperm production in G2 and G6 were only half that observed in the control group. Spermatids were observed in less than 50% of seminiferous tubules in G2, G6, and G7 compared with 82% in the control group (P < 0.05). Staining for GATA-4 was specific for and abundant in the Sertoli cell nucleus in both pre- and postpubertal bulls, and no other cell nucleus inside the seminiferous tubule was positive for GATA-4. Total number of Sertoli cells was not affected by treatment (P = 0.45), but nuclear volume was smaller in G2 and G6 (P < 0.05) compared with the control group. In conclusion, treatment of Zebu bulls with deslorelin had no apparent beneficial effect on testis development and delayed puberty when treatment was initiated at 3 mo of age. Staining for GATA-4 was a useful method for identifying and quantifying Sertoli cell nuclei in both pre- and postpubertal bulls.

Steroidogenic changes and steady state amount of messenger RNA encoding steroidogenic enzymes, gonadotropin receptors and cell-death signalling in the dominant ovarian follicle during estradiol-induced atresia in cattle

Changes in steroidogenic function and associated gene expression were characterized in dominant ovarian follicles (DF) of cattle where follicles were induced to become atretic by systemic administration of estradiol benzoate (EB). In experiment 1, follicular fluid (FF) steroid concentrations in the DF were measured at 12-hourly time points for 48 h in heifers treated with 1 mg EB i.m./500 kg body weight (EB; n=20) as compared with untreated controls (C; n=19). Treatment with EB promoted a transient reduction in circulating FSH, a rapid (12 h) and sustained reduction in FF estradiol, a rapid (12 h) but transient reduction in FF progesterone and a delayed (36 h) increase in FF testosterone concentrations. In experiment 2, whole follicular wall tissue was collected from DF of mature non-lactating cows allocated to a 0 h control group (0 HC: n=7), a 24h control group (24 HC; n=7) or an EB-treated group where tissue was collected 24 h after administration of 1 mg EB i.m./500 kg body weight (EB; n=8). As for experiment 1, EB promoted a transient reduction in circulating FSH, a pronounced reduction in FF estradiol and a smaller but significant reduction in FF progesterone concentrations. Semi-quantitative RT-PCR on follicular wall tissue revealed that the loss in estrogen activity at 24 h after EB was associated with two-fold reduction in aromatase mRNA, with an apparent acceleration in loss of 17alpha-hydroxylase mRNA. Expression of genes for gonadotropin receptors (LHR and FSHR) and a cell-death signalling pathway (Fas antigen and Fas ligand) were unchanged during the initial 24h of EB-induced atresia. These results suggest that EB initiates atresia in dominant ovarian follicles through a rapid suppression of follicular estradiol synthesis, an effect associated with down-regulation of the aromatase gene. A transient suppression in circulating FSH following administration of EB appears to have initiated these events, and it is suggested that subsequent processes involved in atresia follow this loss in estrogenic function.

Comparative response of rams and bulls to long-term treatment with gonadotropin-releasing hormone analogs

The objective was to compare the relative response between rams and bulls in characteristics of LH, FSH and testosterone (T) secretion, during and after long-term treatment with GnRH analogs. Animals were treated with GnRH agonist, GnRH antagonist, or vehicle (Control) for 28 days. Serial blood samples were collected on day 21 of treatment, and at several intervals after treatment. Injections of natural sequence GnRH were used to evaluate the capacity of the pituitary to release gonadotropins during and after treatment. Treatment with GnRH agonist increased basal LH and T concentrations in both rams and bulls, with a greater relative increase in bulls. Endogenous LH pulses and LH release after administration of GnRH were suppressed during treatment with GnRH agonist. Treatment with GnRH antagonist decreased mean hormone concentrations, LH and T pulse frequency, and the release of LH and T after exogenous GnRH, with greater relative effects in bulls. Rams previously treated with antagonist had a greater release of LH after administration of GnRH compared with control rams, while rams previously treated with agonist showed a reduced LH response. Bulls previously treated with agonist had reduced FSH concentrations and LH pulse amplitudes compared with control bulls while bulls previously treated with antagonist had greater T concentrations and pulse frequency. The present study was the first direct comparison between domestic species of the response in males to treatment with GnRH analogs. The findings demonstrated that differences do occur between rams and bulls in LH, FSH and testosterone secretion during and after treatment. Also, the consequences of treatment with either GnRH analog can persist for a considerable time after discontinuation of treatment.

Influence of premature induction of a luteinizing hormone surge with gonadotropin-releasing hormone on ovulation, luteal function, and fertility in cattle

We tested the hypothesis that luteal function and fertility would be reduced in cattle induced to ovulate prematurely compared with those ovulating spontaneously. Estrus was synchronized in 56 beef cows (24 that were nonlactating and 32 that were nursing calves). At 6.4 +/- 0.1 d after estrus, all follicles > or = 5 mm were aspirated (day of aspiration = d 0) with a 17-gauge needle using the ultrasound-guided transvaginal approach. On d 1.5 and 2, cows were administered 2 luteolytic doses of PGF2alpha. Ovarian structures were monitored by transrectal ultrasonography from d -2 to 12, or ovulation. Emergence of a new follicular wave occurred on d 1.7 +/- 0.1. When the largest follicle of the newly emerged wave was 10 mm in diameter (d 4.8 +/- 0.1), cows were assigned on an alternating basis to receive 100 microg of GnRH (GnRH-10; n = 29) to induce ovulation or, upon detection of spontaneous estrus, to the spontaneous (SPON) treatment (n = 24). Cows were bred by AI at 12 h after GnRH (GnRH-10) or 12 h after the onset of estrus (SPON) as detected using an electronic surveillance system. Blood samples were collected every other day beginning 2 d after ovulation until pregnancy diagnosis 30 d after AI. Ovulation and AI occurred in 29/29 cows in the GnRH-10 and in 24/24 cows in the SPON treatment. Ovulation occurred later (P < 0.05) in the SPON (d 7.7 +/- 0.1) than GnRH-10 (d 6.8 +/- 0.1) treatment. Double ovulations were detected in 47% of cows, resulting in 1.5 +/- 0.1 ovulations per cow. Diameters of the ovulatory and the second ovulatory (in cows with 2 ovulations) follicles were greater (P < 0.05) in the SPON (12.0 +/- 0.3 mm and 10.5 +/- 0.4 mm, respectively) than in the GnRH-10 (10.7 +/- 0.1 mm and 9.2 +/- 0.3 mm) treatment. Cross-sectional areas of luteal tissue and plasma concentrations of progesterone during the midluteal phase were greater (P < 0.05) in the SPON (3.62 +/- 0.2 cm2 and 6.4 +/- 0.3 ng/mL) than in the GnRH-10 (3.0 +/- 0.2 cm2 and 5.4 +/- 0.2 ng/mL) treatment. The conception rate to AI in the SPON (100%) treatment was greater (P < 0.05) than in the GnRH-10 (76%) treatment. The animal model used in this study resulted in unusually high conception rates and double ovulations. In conclusion, premature induction of the LH surge reduced the diameter of ovulatory follicle(s), the luteal function, and the conception rate to AI.

Induction of precocious puberty in heifers III: Hastened reduction of estradiol negative feedback on secretion of luteinizing hormone1

Precocious puberty (<300 d of age) can be induced in beef heifers by early weaning and feeding a high-concentrate diet. The objective of this experiment was to determine whether precocious puberty occurs as a result of a hastened reduction of estradiol negative feedback on secretion of LH. Thirty crossbred Angus and Simmental heifers were weaned at 83 +/- 2 d of age and 114 +/- 3 kg of BW, blocked by BW, and randomly assigned to receive a high-concentrate (60% corn; H) or control (30% corn; C) diet and to receive ovariectomy (OVX), OVX plus an estradiol implant (OVXE), or to remain intact (INT). Residual ovarian tissue after OVX necessitated withdrawal of 6 heifers during the course of the experiment, resulting in the following treatment groups: OVX-C, n = 3; OVX-H, n = 5; OVXE-C, n = 4; OVXE-H, n = 2; INT-C, n = 5; INT-H, n = 5. To determine concentrations of progesterone and estradiol, blood samples were collected weekly beginning at a mean age of 160 d. To characterize LH concentrations, serial blood samples were collected at 12-min intervals for 12 h at mean ages of 119, 149, 188, 217, 246, 281, 323, 365, 407, and 449 d. By a mean age of 202 d, heifers fed the H diet were heavier (P < 0.05) than those fed the C diet. Heifers in the INT-H treatment attained puberty earlier (P < 0.05) than in the INT-C treatment (275 +/- 30 vs. 385 +/- 14 d of age, respectively). Overall mean concentrations of estradiol did not differ between OVXE-H and OVXE-C, between INT-H and INT-C, or between OVXE and INT treatments. The OVX treatments exhibited greater LH pulse frequency than the OVXE and INT treatments by the first serial blood collection (treatment x age, P < 0.05). The frequency of LH pulses was greater (P < 0.05) in the INT-H than the INT-C treatment by a mean age of 246 d and was greater (P < 0.05) in the OVXE-H than the OVXE-C treatment by a mean age of 281 d. In the OVXE-H treatment, LH secretion increased and subsequently "escaped" from estradiol negative feedback (detection of > or = 1 LH pulse/h) earlier (P < 0.05) than in the OVXE-C treatment (307 +/- 30 and 420 +/- 21 d of age, respectively). It is concluded that advancing the reduction of estradiol negative feedback on secretion of LH is the mechanism by which early weaning and feeding a high-concentrate diet results in precocious puberty in heifers.

Induction of precocious puberty in heifers II: Advanced ovarian follicular development1

Precocious puberty can be induced in a majority of heifers weaned early and fed a high-concentrate diet. The objective of this experiment was to determine whether induction of precocious puberty is associated with an acceleration of ovarian maturation in heifers. Crossbred Angus and Simmental heifer calves were weaned at 104 +/- 2 (n = 18; early weaned) or 208 +/- 3 (n = 10; normal-weaned, NW) d of age. The early weaned heifers were fed a high-concentrate (60% corn; EWH, n = 9) or control diet (30% corn; EWC, n = 9). The NW heifers were also fed the control diet after weaning. Daily transrectal ultrasonography was performed to characterize a complete follicular wave beginning at a mean age of 126, 161, 196, 224, and 252 (EWH and EWC), or 224 and 252 (NW) d. Blood samples were collected daily during periods of ultrasonography to determine estradiol concentrations and weekly beginning at mean ages of 153 (EWH and EWC) or 216 (NW) d to be analyzed for progesterone concentrations. Heifers in the EWH treatment were heavier (P < 0.01) than EWC heifers from a mean age of 175 d through the end of the study (treatment x age; P < 0.05). Body weights did not differ between EWC and NW. At mean ages of 196 and 224 d, the maximum diameter of the dominant follicle (MaxDF) was greater (P < 0.05) in EWH than EWC heifers. At a mean age of 224 d, MaxDF was greater (P < 0.05) in EWC than NW heifers but was not different by a mean age of 252 d. All EWH, 5 of 9 EWC, and 5 of 10 NW heifers attained puberty at less than 300 d of age (precocious puberty). Age at puberty was less (P < 0.05) in EWH (252 +/- 9 d) than in EWC and NW (308 +/- 26 and 330 +/- 25 d, respectively) treatments. Across all heifers, MaxDF and duration of follicular waves increased with age (P < 0.05), mean number of follicles during follicular waves decreased with age (P < 0.05), and peak concentrations of estradiol during follicular waves increased until a mean age of 224 d. To further characterize aspects of precocious puberty, heifers were compared across treatments between those that experienced precocious puberty and those that did not. In heifers that experienced precocious puberty, BW at puberty was less (P < 0.01) and MaxDF, follicular wave duration, and peak estradiol concentrations were greater (P < 0.05) compared with heifers that did not experience precocious puberty. Ovarian maturation was accelerated in heifers that were weaned early and fed a high-concentrate diet and was associated with precocious onset of puberty.

Induction of precocious puberty in heifers I: Enhanced secretion of luteinizing hormone1

In beef heifers weaned between 3 and 4 mo of age and fed a high-concentrate diet, approximately 50% reach puberty before 300 d of age (precocious puberty). The objectives of this experiment were 1) to determine whether precocious puberty could be induced experimentally by weaning heifers early and feeding a high-concentrate diet, and 2) to determine the dynamics of secretion of LH associated with precocious puberty. Crossbred Angus and Simmental heifer calves were weaned at 73 +/- 3 d of age and 115 +/- 3 kg of BW and fed a high-concentrate (60% corn; HI, n = 9) or control diet (30% corn; CONT, n = 9). Heifers were fed individually, and target BW gains were 1.50 and 0.75 kg/d for the HI and CONT treatments, respectively. Heifers were weighed every 2 wk. Blood samples were collected weekly and assayed for progesterone concentration to determine age at puberty. Serial blood samples were collected at 20-min intervals for 24 h at mean ages of 102, 130, 158, 172, 190, 203, 217, 231, and 259 d and assayed for LH concentration to evaluate the dynamics of secretion of LH. Heifers fed the HI diet exhibited greater BW gain (P < 0.01) than CONT heifers (1.27 +/- 0.05 vs. 0.85 +/- 0.05 kg/d, respectively). As a result, BW in the HI treatment was greater (P < 0.01) than in the CONT treatment by 188 d of age and remained different through the end of the experiment. Precocious puberty occurred in 8 of 9 heifers fed the HI diet and 0 of 9 heifers fed the CONT diet. Age at puberty was reduced in the HI (P < 0.01) compared with the CONT heifers (262 +/- 10 vs. 368 +/- 10 d of age, respectively). Body weight at puberty was also reduced in the HI (P < 0.05) compared with the CONT treatment (327 +/- 17 vs. 403 +/- 23 kg, respectively). Heifers attaining puberty during the experiment continued with subsequent luteal phases as evidenced by cyclic patterns of progesterone concentrations. Frequency of pulses of LH (pulses/24 h) increased with age (P < 0.01) for both treatments. Heifers in the HI treatment exhibited a greater number of pulses of LH (P < 0.01) than those in the CONT treatment by 190 d of age and in all subsequent collection periods (treatment x age, P < 0.05). Mean LH concentrations also increased with age (P < 0.01) for both treatments but did not differ between treatments. In conclusion, precocious puberty induced by early weaning and feeding of a high-concentrate diet is preceded by increasing frequency of pulses of LH.

Estradiol benzoate delays new follicular wave emergence in a dose-dependent manner after ablation of the dominant ovarian follicle in cattle

Administration of estradiol benzoate (EB) induces atresia of the dominant follicle (DF) in the ovaries of cattle within 36 h but emergence of a new wave of follicular development is delayed by 3-5 days. The present study investigated the role of EB in determining timing of emergence of a new follicular wave after removing the influence of the DF. At 6.4+/-0.2 days after ovulation in Angus and Angus/Simmental cattle (n=26), aged 4.9+/-0.6 years and weighing 634+/-20 kg, all ovarian follicles > or =5mm in diameter were aspirated with a 17-gauge needle using an ultrasound-guided transvaginal approach (Day 0 or Hour 0) and animals immediately received 0 (0EB), 1 (1EB), 2 (2EB) or 4 (4EB) mg EB i.m./500 kg body weight (n=6 or 7 per treatment). Ovarian structures were monitored by ultrasonography on a daily basis until emergence of a new wave of follicular development. Concentrations of estradiol (E2) were different among all treatments between Hours 24 and 72, increasing (P<0.01) with greater doses of EB administered. Hour of peak follicle-stimulating hormone (FSH) was 29.3+/-4.0, 53.3+/-4.5, 81.1+/-15.5, and 91.4+/-8.2 for the 0EB, 1EB, 2EB, and 4EB treatments, respectively, and emergence of a new wave of follicular development occurred on Days 1.5+/-0.2, 3.3+/-0.3, 4.0+/-0.6 and 4.4+/-0.4, respectively. Timing of peak FSH and emergence of a new wave of follicular development was earliest (P<0.05) in the 0EB treatment, similar (P>0.1) among the 1EB and 2EB treatments, and most delayed (P<0.05) in the 4EB treatment when compared to the 0EB or 1EB treatments. The overall mean interval from peak FSH to emergence of a new wave of follicular development was 15.7+/-3.3 h and was not affected by treatment. Concentrations of E2 at 24 h before new emergence were not different among EB-treated animals (20.2+/-5.5 pg/ml), but lower (P<0.01) in the 0EB treatment (1.6+/-0.2 pg/ml). In a dose-dependent manner, EB delayed the pre-emergence surge in FSH that stimulates new follicular development after the DF has ceased to be functional. The importance of using an 'optimal' dose of EB in hormonal regimens using this agent to strategically regulate follicular development is emphasized by the outcomes of this study.

Chronic treatment with an agonist of gonadotropin-releasing hormone enhances luteal function in cattle

Our hypothesis was that luteal function, as determined by plasma progesterone concentrations, and corpus luteum (CL) size is enhanced in cattle administered an agonist of GnRH when the CL is developing as compared with administration of an agonist when the CL is fully functional. Cattle were chronically administered a GnRH agonist, azagly-nafarelin, from Day 3 to Day 21 (D3) or Day 12 to Day 21 (D12) or served as untreated control females (Day 0 = behavioral estrus). Blood samples were serially collected on Days 7 and 14 to evaluate LH secretory patterns and twice daily to measure plasma progesterone. Ultrasonographic examinations were conducted daily to record the area of the CL. CL size and plasma progesterone concentrations were both enhanced in the D3 group as compared with the control group. Progesterone was increased in the D12 group on Days 16 and 17 as compared with the control females. Treatment with GnRH agonist increased basal and mean LH concentrations in both D3 and D12 groups as compared with the controls. We rejected our hypothesis because chronic administration of a GnRH agonist increased plasma progesterone when administered both when the CL was developing and when it was fully functional. The enhanced luteal function was likely due to increased basal LH.

Effect of chronic treatment with the gonadotrophin-releasing hormone agonist azagly-nafarelin on basal concentrations of LH in prepubertal bulls

Administration of GnRH agonist for an extended period inhibits pulsatile LH release but enhances testicular function of bulls. The mechanism whereby long-term administration of GnRH agonist enhances testosterone concentration in the blood of bulls has not been determined. The aim of this study was to determine whether chronic treatment with the GnRH agonist, azagly-nafarelin, increases blood concentrations of LH and FSH in prepubertal bulls. Two different doses of the GnRH agonist were administered via Alzet mini-osmotic pumps for 28 days. Blood samples were collected at 20 min intervals for 24 h at days 2, 13 and 25 of treatment. Agonist-treated groups had reduced testosterone pulse frequency (P < 0.05) and increased mean and basal concentrations of testosterone (P < 0.05) compared with untreated control bulls. Basal LH concentrations were higher in agonist-treated bulls during all three periods (P < 0.05) and overall (1 ng ml(-1) higher, compared with control bulls; P < 0.001). Frequency of LH pulses in the agonist-treated groups was reduced to less than one pulse in 24 h. Agonist-treated bulls tended to have (P < 0.10) or had (P < 0.05) a slight but significant increase in blood FSH concentration. In conclusion, the higher blood testosterone concentration in bulls after prolonged treatment with GnRH agonist may result, at least in part, from changes in the testes induced by enhanced basal concentration of LH.

Composition and morphology of the follicular basal lamina during atresia of bovine antral follicles

The fate of the follicular basal lamina during atresia was investigated using bovine follicles, in which different follicle phenotypes have been observed. These phenotypes include: healthy follicles with rounded basal granulosa cells with an aligned basal lamina or follicles with columnar basal granulosa cells with a basal lamina of many loops (loopy), and atretic follicles in which either the antral granulosa cells (antral atresia) or the basal cells (basal atresia) die first. Loopy lamina and basal atresia occur only in small antral follicles < 5 mm in diameter. Follicles were collected from cattle of unknown reproductive history and processed for immunohistochemistry and electron microscopy, and from animals in which follicle growth had been monitored by daily measurements of follicle diameter by ultrasonography. Electron microscopic observations of dominant follicles during the growth phase, plateau and regression showed that the basal lamina was still visible and intact upon atresia. These follicles had a conventional aligned basal lamina, which they retained, except for some degree of folding, as they progressed into antral atresia. In small follicles (2-5 mm in diameter), the basal cell shape (rounded or columnar) and appearance of the basal lamina (aligned or of many loops) did not appear to be related to the type of atresia. On atresia the follicular basal laminae retained immunoreactive laminin alpha1 and beta2, type IV collagen alpha1 and nidogen. Laminin alpha2, which may come from the theca, was present in the follicular basal lamina of only 22% of healthy follicles, but was expressed very commonly in 71% of the atretic follicles. Laminin alpha2 expression was found in both phenotypes of healthy follicles, antral and basal atretic follicles, and follicles with aligned or loopy basal laminae. It is concluded that the basal lamina is not degraded upon atresia, but does undergo a variety of other changes.

Composition and morphology of the follicular basal lamina during atresia of bovine antral follicles

The fate of the follicular basal lamina during atresia was investigated using bovine follicles, in which different follicle phenotypes have been observed. These phenotypes include: healthy follicles with rounded basal granulosa cells with an aligned basal lamina or follicles with columnar basal granulosa cells with a basal lamina of many loops (loopy), and atretic follicles in which either the antral granulosa cells (antral atresia) or the basal cells (basal atresia) die first. Loopy lamina and basal atresia occur only in small antral follicles < 5 mm in diameter. Follicles were collected from cattle of unknown reproductive history and processed for immunohistochemistry and electron microscopy, and from animals in which follicle growth had been monitored by daily measurements of follicle diameter by ultrasonography. Electron microscopic observations of dominant follicles during the growth phase, plateau and regression showed that the basal lamina was still visible and intact upon atresia. These follicles had a conventional aligned basal lamina, which they retained, except for some degree of folding, as they progressed into antral atresia. In small follicles (2-5 mm in diameter), the basal cell shape (rounded or columnar) and appearance of the basal lamina (aligned or of many loops) did not appear to be related to the type of atresia. On atresia the follicular basal laminae retained immunoreactive laminin alpha1 and beta2, type IV collagen alpha1 and nidogen. Laminin alpha2, which may come from the theca, was present in the follicular basal lamina of only 22% of healthy follicles, but was expressed very commonly in 71% of the atretic follicles. Laminin alpha2 expression was found in both phenotypes of healthy follicles, antral and basal atretic follicles, and follicles with aligned or loopy basal laminae. It is concluded that the basal lamina is not degraded upon atresia, but does undergo a variety of other changes.

Atresia revisited: two basic patterns of atresia of bovine antral follicles

Our observations of bovine follicles indicated that the original histological classifications of atresia were inaccurate. A detailed histological, ultrastructural and immunohistochemical study of antral follicles from bovine ovaries collected from an abattoir and from animals whose large follicles had been monitored by ultrasonography was conducted to investigate this further. Nidogen and CD68 were immunolocalized to observe the follicular basal lamina and macrophages, respectively. In randomly collected ovaries, approximately one quarter of all antral follicles were undergoing antral atresia, as designated in this study. Antral atresia was characterized by early destruction of the layers of the membrana granulosa closest to the antrum, whereas the most basal cells remained intact. Numerous pyknotic nuclei were observed in the most antral layers and in the antrum close to the membrana granulosa. This is the classic description of atretic follicles and was observed at all sizes of follicle development and almost universally in large follicles (> 5 mm in diameter), including dominant follicles. Basal atretic follicles, as designated in this study, were almost as prevalent as the antral atretic follicles, and were characterized by initial destruction of the most basal layer of granulosa cells, whereas the cells in the most antral layers remained associated with each other and were predominantly healthy. Pyknotic nuclei and the nuclei of dying basal cells budded into apoptotic bodies were observed rarely. The basal lamina of basal atretic follicles was often breached by macrophages, which were phagocytosing dying basal granulosa cells. The theca was characterized by an increased deposition of collagen, and the cells were orientated randomly, rather than lying parallel to the membrana granulosa as in healthy follicles. Basal atresia occurred in small (< 5 mm in diameter) follicles only. Importantly, these basal atretic follicles were originally identified incorrectly in the literature. Thus, on the basis of the results of this study and another on the expression of steroidogenic enzymes in atretic follicles, it is suggested that the standard biochemical methods for measuring steroid hormone concentrations in follicular fluids to assess atresia should be re-evaluated.

Atresia revisited: two basic patterns of atresia of bovine antral follicles

Our observations of bovine follicles indicated that the original histological classifications of atresia were inaccurate. A detailed histological, ultrastructural and immunohistochemical study of antral follicles from bovine ovaries collected from an abattoir and from animals whose large follicles had been monitored by ultrasonography was conducted to investigate this further. Nidogen and CD68 were immunolocalized to observe the follicular basal lamina and macrophages, respectively. In randomly collected ovaries, approximately one quarter of all antral follicles were undergoing antral atresia, as designated in this study. Antral atresia was characterized by early destruction of the layers of the membrana granulosa closest to the antrum, whereas the most basal cells remained intact. Numerous pyknotic nuclei were observed in the most antral layers and in the antrum close to the membrana granulosa. This is the classic description of atretic follicles and was observed at all sizes of follicle development and almost universally in large follicles (> 5 mm in diameter), including dominant follicles. Basal atretic follicles, as designated in this study, were almost as prevalent as the antral atretic follicles, and were characterized by initial destruction of the most basal layer of granulosa cells, whereas the cells in the most antral layers remained associated with each other and were predominantly healthy. Pyknotic nuclei and the nuclei of dying basal cells budded into apoptotic bodies were observed rarely. The basal lamina of basal atretic follicles was often breached by macrophages, which were phagocytosing dying basal granulosa cells. The theca was characterized by an increased deposition of collagen, and the cells were orientated randomly, rather than lying parallel to the membrana granulosa as in healthy follicles. Basal atresia occurred in small (< 5 mm in diameter) follicles only. Importantly, these basal atretic follicles were originally identified incorrectly in the literature. Thus, on the basis of the results of this study and another on the expression of steroidogenic enzymes in atretic follicles, it is suggested that the standard biochemical methods for measuring steroid hormone concentrations in follicular fluids to assess atresia should be re-evaluated.

Effects of maturity of the potential ovulatory follicle on induction of oestrus and ovulation in cattle with oestradiol benzoate

The effect of maturity of the dominant follicle (DF) on the capacity of oestradiol benzoate (ODB) to induce oestrus and ovulation was examined in cattle. In experiment 1, 31 prepubertal heifers each received an intravaginal progesterone insert (IPI) and 1mg ODB i.m./500kg BW (ODB1). Daily ovarian ultrasonography detected emergence of a new follicular wave 3.1+/-0.1 days after ODB1. The IPI was removed when newly emerged DF were "young" (1.3+/-0.1 days after emergence; YDF; n=15) or "mature" (4.2+/-0.1 days; MDF; n=16), and 24h later, heifers received 0.75mg ODB/500kg BW (ODB2; n=16) or no further treatment (NoODB2; n=15). Most of the heifers receiving ODB2 were observed in oestrus (15/16) and ovulated (12/16), as compared to 0/15 and 1/15 in the NoODB2 group, respectively (P<0.01). In experiment 2, 32 heifers received ODB1 on day 6 of the oestrous cycle, and new follicular wave emergence was detected 3.2+/-0.1 days later. Heifers received an injection of prostaglandin-F2alpha (PGF) when the DF was young (1.1+/-0.1 days after emergence; YDF; n=16) or mature (4 days; MDF; n=16), and then ODB2 24h later or no further treatment (NoODB2). The interval from PGF to oestrus was greater (P<0.01) in the YDF-NoODB2 (70+/-3.9h) as compared to MDF-NoODB2 group (57+/-1.8h). Inclusion of ODB2 reduced (P<0.01) this interval to 47.0+/-0.7h without regard to the maturity of the DF (maturityxODB2, P<0.05) and also reduced (P<0.05) the interval to ovulation. In experiment 3, 21 suckling anoestrous cows received an IPI and ODB1 at 29.3+/-1.7 days postpartum. The IPI were removed either 1 day (YDF; n=9) or 3.9+/-0.1 days (MDF; n=9) after emergence of a new follicular wave and every cow received ODB2. Oestrus was subsequently detected in all but one animal. Ovulation of the newly emerged DF was detected within 48h of ODB2 in nine of nine cows of the MDF group, and in four of nine of the YDF group (P<0.05). During the subsequent ovulatory cycle, luteal size and plasma concentrations of progesterone were greater (P<0.01) in the MDF group compared to the YDF group. We conclude that behavioural oestrus is readily induced by 0.75mg ODB i.m./500kg BW. Maturity of the DF appeared to have little influence on the ability of the DF to ovulate in heifers. In contrast, young DF in lactating anoestrous cows were less likely to respond to the ovulatory cue provided, and luteal development was compromised in those that did ovulate.