Joint Alpharma-Beef Species Symposium: considerations on puberty in replacement beef heifers.

Many important changes occur throughout reproductive development in beef heifers, including during the prenatal, early postnatal, and peripubertal periods. Wave-like patterns of follicular development have been observed in heifer calves as early as 2 wk of age. Some dramatic changes occur from about 2 to 5 mo of age, most notably the transient increase in LH secretion. Most components of the hypothalamic-pituitary-ovarian axis are fully competent by approximately 5 to 6 mo of age. Peripubertal changes include increases in LH secretion, estradiol production, follicular development, and reproductive tract size. Eventually, the process reaches the point that the initial ovulation is achieved. Heifers that reach puberty and experience multiple estrous cycles before the onset of their initial breeding season have a greater probability for early conception and optimal lifetime productivity. Attainment of puberty typically occurs at around 12 to 14 mo of age in beef heifers but varies greatly. Genetic differences and environmental factors contribute to this variation. In typical U.S. cow-calf operations, calves are generally weaned at approximately 200 d of age. The impact of postweaning management on age at puberty in heifers has been demonstrated, and there is considerable flexibility in the timing of gain from weaning to breeding. However, even when heifers are grown to the desired BW before the start of breeding, there remains a pronounced variation in the timing of puberty, which impacts pregnancy rates. Less attention has been focused on the impact of preweaning management on age at puberty. Heifer calves with increased growth rates from birth to weaning have reached puberty at earlier ages. Precocious puberty has also been induced in a majority of heifers with early weaning and feeding a high-concentrate diet. Nutritional control during early maturation in heifers exerts a substantial influence on the timing of puberty. Understanding the mechanisms involved in reproductive development increases our ability to effectively manage replacement beef heifers for optimal reproductive performance.

Decreasing the interval between GnRH and PGF2alpha from 7 to 5 days and lengthening proestrus increases timed-AI pregnancy rates in beef cows.

Four experiments were conducted in postpartum beef cows to evaluate the influence of reducing the interval from GnRH to PGF(2alpha) from 7 to 5d in a Select-Synch + CIDR or CO-Synch + CIDR estrous synchronization program. In Expt 1, cows (n=156) were treated with either a 7 or 5d Select-Synch + CIDR program. A second PGF(2alpha) treatment was given to all cows in all experiments at 12h after the initial PGF(2alpha) (to ensure that luteolysis occurred with the 5d program). Estrous response, interval to estrus, conception rate, and first service AI pregnancy rates were similar between treatments. In Expt 2, cows (n=223) were treated with either a 7 or 5d CO-Synch + CIDR program, with timed-AI concomitant with GnRH at 60 h after PGF(2alpha). Timed-AI pregnancy rates were similar between treatments. In Expt 3 (n=223) and 4 (n=400) cows were treated with either a 7 or 5d CO-Synch + CIDR program with timed-AI concurrent with GnRH at either 60 h (7d) or 72 h (5d) after CIDR withdrawal. Timed-AI pregnancy rates were 13.3% (P<0.05; Expt 3) and 9.1% (P<0.05; Expt 4) greater for the 5 than 7d program. In conclusion, timed-AI pregnancy rates were improved with a 5d CO-Synch + CIDR program with timed-AI at 72 h after CIDR withdrawal, compared to a 7d CO-Synch + CIDR program with timed-AI at 60 h after CIDR withdrawal.

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.

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.

Effect of timing of feeding a high-concentrate diet on growth and attainment of puberty in early-weaned heifers.

Precocious puberty (<300 d of age) can be successfully induced in a majority of heifers with early weaning and continuous feeding of a high-concentrate diet. The objective of this experiment was to determine the relative effects of timing of feeding a high-concentrate diet on age at puberty in early-weaned heifers. Sixty crossbred Angus and Simmental heifer calves were weaned at 112 +/- 2 d of age and 155 +/- 3 kg of BW and were fed a receiving diet for 2 wk. Heifers were blocked by age and BW, and assigned randomly to receive a high-concentrate (60% corn; H) or control (30% corn; C) diet during phase 1 (mean age 126 to 196 d) and H or C during phase 2 (mean age 196 to 402 d), resulting in 4 treatments (HH, n = 15; HC, n = 15; CH, n = 15; and CC, n = 15). Blood samples were collected weekly beginning at a mean age of 175 d and assayed for progesterone concentration to determine age at puberty. After 56 d on the experimental diets, BW of heifers fed the H diet during phase 1 were greater (P < 0.05) than those of heifers fed the C diet (mean age of 182 d; treatment x mean age, P < 0.01). After 70 d on the new diets (mean age of 266 d), heifers fed the H diet during phase 2 reached heavier BW (P < 0.05) than heifers fed the C diet, when compared within phase 1 diet groups (HH > HC; CH > CC). Body weights in HC and CH treatments differed from a mean age of 169 through 238 d, after which BW did not differ between these treatments. The ADG over the entire experimental period was greatest for the HH treatment (1.2 +/- 0.04 kg/d; P < 0.05), followed by the HC and CH treatments (1.0 +/- 0.03 and 1.0 +/- 0.02 kg/d, respectively), which were not different, and the CC treatment gained the least (0.7 +/- 0.04 kg/d; P < 0.05). Precocious puberty occurred in 67, 47, 47, and 20% of heifers in the HH, HC, CH, and CC treatments, respectively (HH > CC; P < 0.05). Mean age at puberty for the HH and HC treatments (271 +/- 17 and 283 +/- 17 d of age, respectively) was earlier (P < 0.05) than for the CC treatment (331 +/- 11 d of age). Age at puberty in the CH treatment (304 +/- 13 d of age) was intermediate to and not different from the other treatments. Heifers fed the H diet during phase 1 attained puberty earlier (P < 0.05) than heifers fed the C diet during phase 1. In conclusion, increasing dietary energy intake in early-weaned heifers, through feeding a high-concentrate diet from 126 to 196 d of age, decreased age at puberty regardless of the diet fed after 196 d of age.

Induction of precocious puberty in heifers I: enhanced secretion of luteinizing hormone.

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.

Induction of precocious puberty in heifers III: hastened reduction of estradiol negative feedback on secretion of luteinizing hormone.

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 development.

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.

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.