Endocrine changes during restoration of estrous cycles following induction of anestrus by restricted nutrient intake in beef heifers

The working hypotheses in this experiment were: that ovarian estradiol would inhibit luteinizing hormone (LH) secretion in heifers that were anestrus as a result of restricted dietary energy intake and the responsiveness of LH secretion to estradiol negative feedback would decrease during the period when restoration of estrous cycles occurred following feeding of diets adequate in energy. Fifteen heifers weighing 341 +/- 12 (mean +/- SE) kg were fed a diet containing 50% of the energy required for maintenance until 40 to 50 d following cessation of estrous cycles. Heifers were assigned to intact control (C, n = 5), ovariectomized (OVX, n = 5) or ovariectomized-estradiol-17 beta-implanted (OVX + E2, n = 5) treatments. Heifers were subsequently provided a high-energy (HE) diet until termination of the study. Progesterone concentrations indicating cessation of corpus luteum function were detected after heifers had lost 71 +/- 8 kg body weight over 186 +/- 28 d. Control heifers re-initiated estrous cycles as indicated by increased progesterone concentrations in serum at 49 +/- 9 d after initiation of feeding the HE diet (360 +/- 18 kg body weight). Initiation of pulsatile LH secretion was observed in heifers by d 12 following OVX. Estradiol suppressed LH secretion in OVX + E2 heifers during the period of nutritional anestrus in C heifers. Suppressive effects of E2 on LH secretion continued in OVX heifers after C heifers had initiated corpus luteum function. Therefore, the working hypothesis that LH secretion is inhibited by E2 in the nutritionally anestrous heifer is accepted but responsiveness to estradiol does not subside with re-initiation of estrous cycles, thus this working hypothesis is rejected.     

Pregnancy rate in beef heifers after synchronization to either random or programmed estrous cycles

We hypothesized that heifers in diestrus at the beginning of a Syncro-Mate-B (SMB) regimen would have higher pregnancy rates to AI than heifers not in diestrus and that administration of a PGF2alpha analogue 11 d before a SMB regimen would increase pregnancy rates to AI. In both replicate years of Exp. 1, heifers (n = 150) were classified by stage of the estrous cycle at the beginning of a SMB regimen (d 0). Following implant removal (d 9), heifers were artificially inseminated 12 h after the onset of estrus (95.5% in estrus by 72 h). Blood samples were collected for progesterone (P4) analysis on d 0, 9, and 20. Pregnancy rates did not differ between yr 1 and 2. Pregnancy rate for heifers classified in diestrus (53.6%; n = 69) was higher (P = 0.06) than for heifers in metestrus (43.7%; n = 48). Pregnancy rate for proestrus (44.4%; n = 18) heifers was not different from that for heifers in the metestrus or diestrus groups. Mean plasma P4 concentration was affected by both treatment and day. Pregnancy rate was higher (P < 0.01) for heifers with P4 > 1 ng/mL plasma (51.6%; n = 120) than for heifers with P4 < or = 1 ng/mL plasma (23.3%; n = 30) on d 0. In Exp. 2, beef heifers (Santa Cruz; n = 195) were allotted to two treatments. Heifers (n = 98) in the control group were administered a conventional SMB treatment. Heifers (n = 97) in the PGF group were injected with PGF2alpha 11 d (d -11) before a SMB regimen. Progesterone concentration was determined from blood samples collected on d -11, -2, 0, and 9. All heifers were artificially inseminated 48 to 50 h after implant removal. At the beginning of the SMB regimen (d 0), a greater (P < 0.05) percentage of PGF (74.2%) than of control heifers (59.2%) were in diestrus (P4 > 1 ng/mL). Mean P4 concentration was not affected by treatment or day x treatment but differed (P < 0.05) among days. Pregnancy rate of cycling heifers was similar for PGF (36%) and control heifers (35.9%). Pregnancy rate was higher (P < 0.01) for heifers with P4 > 1 ng/mL plasma (37.6%) than for heifers with P4 < or = 1 ng/mL plasma (18.5%) on d 0. These results support the hypothesis that fertility is enhanced when a progestin synchrony regimen is initiated during diestrus, but methods to program estrous cycles to increase fertility warrant investigation.     

Relationship between endogenous estradiol-17 beta and estrous behavior in heifers

Thirty-five Holstein heifers were used to examine the relationship between endogenous estradiol-17 beta and estrous traits. During a non-superovulation period (NSP), estrous cycles were synchronized and during the periovulatory stage blood samples were collected every 6 h for 120 h for subsequent determination of estradiol-17 beta and progesterone. In addition, continuous observation for estrous behavior was performed for 98 h. A gonadotropin-induced superovulation period (SP) was begun 12 d after estrus was detected during NSP. Heifers were injected with FSH twice daily for 4 d and single injections of prostaglandin were given on d 14 and 15. Beginning at d 14, blood samples were collected every 6 h for 120 h for subsequent determination of estradiol-17 beta and progesterone. Continuous observation for estrous behavior was performed for 98 h. Peak estradiol-17 beta was greater during SP than during NSP (49.0 +/- 3.1 vs 12.9 +/- 3.0 pg/ml serum). Thirty-three and 31 of the 35 heifers were in estrus during NSP and SP, respectively; duration of estrus was 2.3 h longer during SP than during NSP. However, number of behavioral interactions during estrus did not differ between NSP and SP. In conclusion, estrous traits were similar, whereas peak estradiol-17 beta concentrations were markedly different between NSP and SP.     

Effect of increasing levels of undegradable intake protein on metabolic and endocrine factors in estrous cycling beef heifers

To determine the influence of three levels of undegradable intake protein (UIP) supplementation on metabolic and endocrine factors that influence reproduction, 23 yearling crossbred heifers (body condition score = 4.5 +/- 0.5; initial BW = 362 +/- 12 kg) were stratified by BW and assigned randomly to one of three supplements: 1) low UIP (1,135 g x heifer(-1) x d(-1); 30% CP, 115 g UIP, n = 7); 2) mid UIP (1,135 g x heifer(-1) x d(-1); 38% CP, 216 g UIP, n = 8); or 3) high UIP (1,135 g x heifer(-1) x d(-1); 46% CP, 321 g UIP, n = 8). Heifers were estrually synchronized before initiation of supplementation. Supplement was individually fed daily for 30 to 32 d, at which time heifers were slaughtered (d 12 to 14 of the estrous cycle) and tissues collected. Heifers were fed a basal diet of sudan grass hay (6.0% CP) ad libitum. On d 28 of supplementation (d 10 of the estrous cycle), no differences were observed (P > 0.10) in serum insulin or IGF-I among treatments. At slaughter (d 10 to 12 of the estrous cycle), treatments did not influence corpus luteum weight, cerebral spinal fluid leptin, or IGFBP; serum estradiol-17beta, progesterone, leptin, IGF-I, and IGFBP; or anterior pituitary content of IGFBP (P > 0.10). Follicular fluid IGFBP-2 and IGFBP-4 were greater in high-UIP heifers than low- or mid-UIP heifers on d 12 to 14 of the estrous cycle (P < 0.05). Basal serum LH concentrations and LH area under the curve (every 15 min for 240 min) did not differ (P > 0.10) following 28 d of supplementation (d 10 of the estrous cycle); however, basal serum FSH concentrations were greater (P = 0.06) in low- and mid- vs. high-UIP heifers (5.2 and 5.2 vs. 4.6 ng/mL, respectively), and FSH area under the curve was greater (P = 0.03) in low- vs. high-UIP heifers. At slaughter (d 12 to 14 of the estrous cycle), anterior pituitary LH and FSH content and steady-state mRNA encoding alpha, LHbeta, and GnRH receptor did not differ (P > 0.10) among treatments. However, FSHbeta mRNA was increased approximately twofold (P = 0.03) in mid vs. low UIP. In summary, low and mid levels of UIP supplements fed to estrous cycling beef heifers seemed to enhance pituitary expression and/or secretion of FSH relative to high levels of UIP. Moreover, high-UIP supplementation was associated with increased low-molecular-weight IGFBP compared with supplementation of low and mid levels of UIP. These data suggest that differing levels of UIP supplementation may alter pituitary and ovarian function, thereby influencing reproductive performance in beef heifers.     

Comparison of progestin-based estrus synchronization protocols before fixed-time artificial insemination on pregnancy rate in beef heifers

The objective of the experiment was to compare pregnancy rates resulting from fixed-time AI after administration of either 1 of 2 controlled internal drug release (CIDR)-based protocols. Heifers at 3 locations (location 1, n = 78; location 2, n = 61; and location 3, n = 78) were assigned to 1 of 2 treatments within reproductive tract scores (1 = immature to 5 = cycling) by age and BW. Heifers assigned to CIDR Select received a CIDR insert (1.38 g of progesterone) from d 0 to 14 followed by GnRH (100 mug, i.m.) 9 d after CIDR removal (d 23) and PGF2alpha (PG, 25 mg, i.m.) 7 d after GnRH treatment (d 30). Heifers assigned to CO-Synch + CIDR were administered GnRH and received a CIDR insert on d 23 and PG and CIDR removal on d 30. Heifers at location 1 were fitted with a HeatWatch estrus detection system transmitter from the time of PG until 24 d after fixed-time AI to allow for continuous estrus detection. Artificial insemination was performed at predetermined fixed times for heifers in both treatments at 72 or 54 h after PG for the CIDR Select and CO-Synch + CIDR groups, respectively. All heifers were administered GnRH at the time of AI. Blood samples were collected 10 d before and immediately before treatment initiation (d 0) to determine pretreatment estrous cyclicity (progesterone > or = 0.5 ng/mL). At location 1, the estrous response during the synchronized period was greater (P = 0.06; 87 vs. 69%, respectively), and the variance for interval to estrus after PG was reduced among CIDR Select- (P < 0.01) compared with CO-Synch + CIDR-treated heifers. Fixed-time AI pregnancy rates were significantly greater (P = 0.02) after the CIDR Select protocol (62%) compared with the CO-Synch + CIDR protocol (47%). In summary, the CIDR Select protocol resulted in a greater and more synchronous estrous response and significantly greater fixed-time AI pregnancy rates compared with the CO-Synch + CIDR protocol.     

Influence of ambient temperature on prolactin concentrations in serum of Holstein and Brahman x Hereford heifers

Four Holstein and four Brahman x Hereford heifers about 8 mo of age were used in a study to determine whether breed influences the effects of ambient temperature on concentrations' of prolactin in serum. Two heifers of each breed were stanchioned in each of two environmental chambers at 21 C for 7 d, after which chamber temperatures were changed to 7 or 31 C during 6 h. After 5 d at 7, 21 or 31 C, heifers were injected with 60 micrograms thyrotropin-releasing hormone (TRH). A switch-back design was used and each heifer was exposed to all treatments. Concentrations of prolactin in serum of heifers during exposure to 7, 21 or 31 C for 5 d were related to ambient temperature (9.0, 20.9 and 29.5 ng/ml, respectively; P less than .001), but the response was not influenced by breed. Heifers of both breeds responded similarly to treatment with TRH, and prolactin in serum increased (P less than .001) within 5 min from 7.0 +/- 3.2 to 45.7 +/- 8.2 ng/ml in heifers at 7 C, from 13.1 +/- 1.6 to 97.2 +/- 9.6 ng/ml in heifers at 21 C and from 18.2 +/- 3.5 to 96.2 +/- 11.3 ng/ml in heifers at 31 C. We conclude that concentrations of prolactin in serum of heifers are positively associated with ambient temperature and that the effects of temperature on basal and TRH-stimulated concentrations of prolactin do not differ significantly between Holstein and Brahman x Hereford heifers. Thus, differences in tolerance to heat were not related to differences in prolactin secretion.     

Systemic progesterone concentration following human chorionic gonadotropin administration at various times during the estrous cycle in beef heifers

In Trial 1, 26 heifers were allotted randomly to a control group or one of four groups to receive a single injection of 3,000 IU hCG on d 1, 4, 7 or 10 of an estrous cycle. Heifers next completed a nontreated cycle, and at their third estrus were reallotted to one of the five groups described previously. Estrous cycle length was extended in cycle 1 but was not altered during the nontreated cycle or in cycle 3. Administration of hCG on d 4 or 7 increased (P less than .05) mean serum progesterone (P4) over the first 16 d of the estrous cycle by .9 and .8 ng/ml, respectively. In Trial 2, 23 heifers were allotted randomly to one of two groups to receive a placebo or a single injection of 3,000 IU hCG on d 4 of an estrous cycle. Heifers were inseminated artificially at subsequent estrus. On d 4 postbreeding, treatments were repeated. Administration of hCG on d 4 of the prebreeding estrous cycle increased (P less than .05) mean P4 over the first 16 d by .9 ng/ml, whereas mean P4 over the first 16 d postbreeding was not affected by a second injection of hCG on d 4 postbreeding. Administration of hCG increased (P less than .05) first-service pregnancy rate (92 vs 55%). In conclusion, progesterone concentrations were enhanced by hCG given on d 4 or 7 of the estrous cycle, and pregnancy rate was increased when hCG was administered both on d 4 of the prebreeding cycle and d 4 postmating.     

Conception rate in Bos taurus and Bos indicus crossbred heifers after postweaning energy manipulation and synchronization of estrus with melengestrol acetate and fenprostalene

Conception rate in heifers after synchronization of estrus with melengestrol acetate (MGA) and fenprostalene (a prostaglandin F2 alpha analogue; PGF) was determined in pubertal Bos taurus and Bos indicus crossbred yearling heifers. Angus x Hereford (AH, n = 137) and Brahman x Hereford (BH, n = 97) heifers were sorted by body weight after weaning into light (LW) and heavy (HW) weight blocks. Heifers were assigned by age to diets to reach a target weight of 55% (LE) or 65% (HE) of their projected mature weight by the start of breeding. Heifers that exhibited estrus and had serum progesterone greater than or equal to 1 ng/ml (0 or 10 d before estrous synchronization treatment) were assigned randomly within breed and nutritional groups to either an estrous synchronization (S) or control (C) group. Heifers in the S group were fed .5 mg of MGA for 7 d and injected s.c. with 2 mg PGF on d 7 of MGA. All heifers were inseminated 12 h after first detected estrus. A greater proportion of AH (P less than .01) than of BH heifers were in estrus within 6 d after PGF, and more S heifers than C heifers (P less than .01) were in estrus. Conception rate at first service was proportionately higher (P less than .001) in AH than in BH heifers and lower (P less than .02) in S than in C heifers. There was a breed x energy level interaction (P less than .01) for conception rate at first service. Stage of the estrous cycle at the time treatment with MGA was initiated influenced (P less than .05) conception rate at first service in the S, AH heifers, with lower conception rates among heifers beginning treatment late in their estrous cycles (greater than or equal to d 12). Pregnancy rates after 21 d were higher (P less than .01) in AH than in BH heifers and higher (P less than .01) in HW than in LW heifers. More HE than LE heifers (P less than .02), and more AH than BH heifers were pregnant after 45 d. Pregnancy rates at the end of 21 d were higher among HE, BH heifers than among LE contemporaries. A higher (P less than .02) percentage of HE, HW, BH heifers were pregnant at the end of 45 d compared with other BH groups. Results indicated that a 7-d MGA-PGF treatment reduced conception rates at first service in pubertal yearling heifers. Pregnancy rate was affected by prebreeding nutrition in BH yearling heifers at the end of 45 d.     

Insemination of Holstein heifers at a preset time after estrous cycle synchronization using progesterone and prostaglandin

Two experiments were conducted to study estrous cycle control regimens that combine progesterone administration via an intravaginal device ( PRID ) with a single injection of prostaglandin F2 alpha (PG). In Exp. I, 242 Holstein heifers were assigned randomly to one of three treatment groups at 14 to 18 mo of age. Treatments were: 1) control, 2) PRID -6 + PG-6 ( PRID in place for 6 d plus PG on the day of PRID removal) and 3) PRID -7 + PG-6 ( PRID in place for 7 d plus PG on the day before PRID removal). Heifers were observed for estrous activity and were inseminated at 8 to 20 h after estrus was detected. Estrus and ovulation were effectively synchronized after both PRID + PG treatments. Ninety-nine percent of the heifers in each group were in estrus within 168 h after PG injection. However, the interval from PG administration to the onset of estrus was longer after PRID -7 + PG-6 (75 +/- 2 h) than after PRID -6 + PG-6 (66 +/- 2 h). A lower variance in the interval from PG treatment to estrus was observed after PRID -7 + PG-6, suggesting that the 24 h delay in PRID withdrawal improved the synchrony of the onset of estrus. Pregnancy rates (72 to 82%) did not vary across treatment groups. Two-hundred seventy-four heifers were assigned to Exp. II. Treatments were 1) control, 2) 2 X PG (two injections of PG at an 11 d interval) and 3) PRID -7 + PG-6.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chronic administration of recombinant ovine leptin in growing beef heifers: effects on secretion of LH, metabolic hormones, and timing of puberty

Serum concentrations of leptin increase linearly from approximately 16 wk before until the week of pubertal ovulation in beef heifers. To test the hypothesis that exogenous leptin can hasten the onset of puberty in heifers, we examined the effects of chronic administration of recombinant ovine leptin (oleptin) on timing of puberty, pulsatile and GnRH-mediated release of LH, and plasma concentrations of GH, IGF-I, and insulin. Fourteen fall-born, prepubertal heifers (Brahman x Hereford, 12 to 13 mo; 304.7+/-4.12 kg) were used. Heifers were stratified by age and BW and assigned randomly to one of two groups (seven animals per group): 1) Control; heifers received s.c. injections of saline twice daily (0700 and 1900) for 40 d; and 2) Leptin; heifers received s.c. injections of oleptin (19.2 microg/kg) twice daily at 0700 and 1900 for 40 d. Blood samples were collected at 10-min intervals for 5 h on. d 0, 5, 10, 20, 30, and 40, and twice daily, just before each treatment injection, throughout the study. On d 41, heifers received i.v. injections of GnRH at 0 (0.0011 microg/kg) and 90 min (0.22 microg/kg), with additional sampling for 5.5 h to examine releasable pools of LH. Diets promoted a gain of 0.32+/-0.09 kg/d, which did not differ between groups. Plasma concentrations of leptin increased markedly in leptin-treated heifers and were greater (P < 0.001) than controls throughout (27.8+/-0.8 vs. 4.9+/-0.12 ng/mL). None of the heifers reached puberty during the experiment, but did so within 45 d of its termination. Mean concentrations of plasma LH, GH, IGF-I, and insulin were not affected by treatment, nor was there an overall effect on the frequency of LH pulses. However, a treatment x day interaction (P = 0.02) revealed that the frequency of LH pulses (pulses/ 5 h) was greater (P = 0.03) in controls (3.6+/-0.36) than in leptin-treated heifers (1.7+/- 0.28) on d 10. Characteristics of GnRH-induced release of LH were not affected by treatment. In summary, chronically administered leptin failed to induce puberty or alter endocrine characteristics in beef heifers nearing the time of expected puberty.     

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.     

Influence of insulin and energy intake on ovulation rate, luteinizing hormone and progesterone in beef heifers

Ovarian and gonadotropin responses to insulin and energy restriction were investigated in a 2 X 2 factorial experiment using 2-yr-old Brangus heifers. Thirty heifers were paired by weight and body condition, then assigned to treatment groups receiving 75 (LE) or 180% (HE) of NRC recommendations for dietary energy for maintenance. Diets were adjusted weekly to maintain daily .25 to .5 kg weight loss or 0 to .25 kg weight gain, respectively. On d 10 of the first estrous cycle subsequent to the initial 45 d of feeding, heifers within each dietary group were allocated to receive twice daily infusions of either 40 U insulin (I) or saline (C). Infusions began at 5 and 10 h postprandial and were given in six boluses, 20 min apart. Infusions continued daily until d 20 or estrus, whichever occurred first. On d 11, blood samples were collected at 15-min intervals for 12 h to determine luteinizing hormone (LH) and insulin concentrations. On d 16 to 20, twice daily im injections of 1 mg follicle stimulating hormone (FSH) were administered. Heifers were ovariectomized on d 11 after estrus. Number of corpora lutea (CL) in LE-I heifers was greater (P less than .05) in LE-C, HE-C or HE-I. Total CL weight (g) per heifer was greater (P less than .05) in HE-C and LE-I heifers than in LE-C. Individual CL wt was heavier in HE than in LE heifers (P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of obesity on insulin and glucose metabolism in cyclic heifers

Effects of degree of obesity on basal concentrations of insulin, glucose, thyroxine (T4), triiodothyronine (T3), estradiol-17 beta (E) and progesterone (P) were measured in serum from 50 estrous and 73 diestrous Holstein heifers and the insulin response to glucose infusion was assessed in diestrous obese (n = 7) and lean (n = 7) heifers. Basal concentrations of glucose, T4, T3, E and P were not correlated with degree of obesity, although concentrations of glucose, T4 and T3 were higher (P less than .05) at estrus than diestrus. Basal concentrations of insulin at estrus and diestrus were positively correlated (r = .6; P less than .001) with degree of obesity but this relationship was different (P less than .001) between estrus and diestrus. Furthermore, there was interaction (P less than .001) between body condition and stage of the estrous cycle only for basal concentrations (mean +/- SE) of insulin, with the difference in insulin levels (microU/ml) between 12 obese and 12 lean heifers at diestrus (11.7 +/- 1.3 vs 6.7 +/- .6; P less than .05) increasing during estrus (21.9 +/- 2.4 vs 10.8 +/- 1.3; P less than .001). Insulin response to glucose infusion was greater in obese than in lean heifers, whether determined as actual concentration (P less than .01) or as insulin response areas (P less than .05) above base-line concentrations. Obese heifers were less responsive to insulin since hyperinsulinemia and euglycemia coexisted, and because glucose fractional removal rates were similar in both groups after glucose infusion in spite of greater concentrations of insulin in obese heifers. Thus, obesity in heifers was associated with insulin resistance, basal hyperinsulinemia and greater glucose-induced secretion of insulin.     

Nonpuberal estrus in beef heifers

The frequency of occurrence of behavioral estrus without subsequent development of functional luteal tissue (termed nonpuberal estrus, NPE), was determined in 43 Simmental X Hereford-Brahman heifers. Blood samples were collected weekly from the start of the study to first behavioral estrus and then daily from d 1 (d 0 = estrus) through d 14 following first and subsequently observed estrous behaviors. All blood samples were analyzed for serum progesterone (P4) concentrations by radioimmunoassay. More heifers (62.8%) exhibited NPE than had luteal development after their first behavioral estrus (37.2%). There was a tendency for fewer light-weight heifers (less than or equal to 240 kg at the start of the experiment) to exhibit a puberal first estrus compared with the heavy-weight (greater than 240 kg at the start of the experiment) heifers (31.2% vs 68.8%, respectively; P = .12). Heifers that had a puberal first estrus were older (376 +/- 12 d vs 334 +/- 9 d, P less than .05) compared with heifers that had NPE. Weight at first behavioral estrus was similar between heifers that had a puberal first estrus and those that had NPE (298 +/- 8 kg and 289 +/- 6 kg, respectively). More heifers that had a puberal first estrus also had an elevation in serum P4 concentrations before that first estrus (64.3% vs 20.0%, P less than .05), and the serum P4 elevation was greater (2.5 +/- .4 ng vs 1.2 +/- .1 ng, P less than .05) than heifers that had NPE. We have concluded from these results that NPE is a common occurrence in heifers approaching puberty.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of termination of pregnancy or long-term progestogen exposure on subsequent estrous cycle length and concentration of progesterone in plasma of heifers

An experiment was conducted to determine whether short estrous cycles following abortion of heifers between 70 and 75 d of gestation are due to factors associated with the previous presence of a conceptus or long-term exposure of the uterus and(or) ovaries to a progestogen. Fifty crossbred heifers were randomly allotted at estrus (d 0) to five groups: control (n = 10), pregnant (Preg.; n = 14), progestogen (norgestomet) implant (Norg.; n = 9), progesterone-releasing intravaginal device (PRID; n = 9), or hysterectomy (Hyst.; n = 8). Control heifers were injected during the mid-luteal phase of an estrous cycle with 25 mg prostaglandin F2 alpha (PGF2 alpha) and length of the subsequent estrous cycle was determined. Beginning 6 to 8 d after estrus, heifers in the Norg. or PRID groups were given norgestomet ear implants or intravaginal coils, respectively, every 10 d for 70 d. Heifers were hysterectomized 5 to 8 d after estrus. Seventy to 75 d after conception, progestogen treatment or hysterectomy, heifers were injected (i.m.) with 25 mg PGF2 alpha and the last norgestomet ear implants or PRIDs were removed. Interval from PGF2 alpha injection to first estrus (means +/- SE) ranged from 2.5 +/- .2 to 4.4 +/- .7 d (P greater than .05). Length of the first estrous cycle means +/- SE) following PGF2 alpha-induced luteolysis or progestogen withdrawal was shorter (P less than .01) for the Preg. group (8.2 +/- .4 d) than for the control, Norg. and PRID groups (21.5 +/- .6 d; 19.3 +/- 1.4 d; and 18.2 +/- 1.3 d, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of 17 beta-estradiol and diets varying in energy on secretion of luteinizing hormone in beef heifers

An experiment was conducted to test the hypothesis that 17 beta-estradiol (E2) would not suppress secretion of luteinizing hormone (LH) in heifers fed a diet limited in energy during the period before the onset of nutritionally induced anestrus. Sixteen of 20 heifers that had been exhibiting normal estrous cycles (20 mo of age, 409 +/- 6 kg body weight) were ovariectomized, and half of them were assigned at random to receive an E2 implant. The ovariectomized heifers were assigned at random to receive diets that contained low (L; 5.8 Mcal X animal-1 X d-1, n = 8) or high levels of energy (H; 20.0 Mcal X animal-1 X d-1, n = 8) for 100 d. The other four heifers remained intact and were fed the L-diet. The intact heifers were utilized to determine the status of reproductive function in animals fed the L-diet. Heifers lost body weight rapidly after initiation of feeding the L-diet. Heifers fed the L-diet then stabilized at a lighter weight until the latter part of the experiment. One of the four intact heifers fed the L-diet became anestrus near the end of the study. Mean concentrations of LH in blood serum increased linearly (P less than .05) in ovariectomized heifers fed the L- and H-diet. Mean concentration of LH in heifers fed the H-diet that were implanted with E2 was similar to ovariectomized heifers fed the H-diet that received no E2. Mean LH in serum of ovariectomized heifers implanted with E2 fed the L-diet was suppressed and remained low throughout the study. Frequency of pulses of LH in ovariectomized heifers fed the L-diet was less (P less than .01) than that in ovariectomized heifers fed the H-diet. Estradiol decreased the number of pulses of LH in heifers fed the L-diet. We conclude that dietary energy restriction in beef heifers has a direct action on the hypothalamo-pituitary axis to lower the number of pulses of LH in the absence of ovarian steroids. However, ovarian E2 appears to suppress further secretion of LH in heifers fed limited levels of dietary energy before the onset of nutritional anestrus occurs, therefore, our working hypothesis is rejected.     

Pulsatile or continuous infusion of luteinizing hormone-releasing hormone and hormonal concentrations in prepubertal beef heifers

An experiment was conducted to determine if exogenous luteinizing hormone-releasing hormone (LHRH) administered iv intermittently as pulses (P) or by continuous sc infusion (I) using osmotic minipumps could sustain pulsatile LH release and induce estrous cyclicity in prepubertal heifers. Prepubertal heifers were assigned randomly to: 1) receive pulses of LHRH (n = 6; 2.5 micrograms LHRH/2 h for 72 h), 2) be infused with LHRH (n = 11; 1.25 micrograms LHRH/h for 72 h), or 3) serve as controls (n = 16). Blood was collected at 20-min intervals for 8 h (0900 to 1700 h) from six heifers in each group on d 1, 2, 3 (during treatment), and on d 4 (during 8 h after terminating LHRH treatments). Heifers given LHRH had higher (P less than .01) LH concentrations than controls. Preovulatory-like LH surges occurred in three I, two P and no control heifers during treatment. Pulse frequencies of LH (no. LH pulses/8 h) were greater (P less than .001) for P heifers than for I and control heifers due to pulsatile LHRH treatment. Serum estradiol was higher (P less than .01) during treatment for LHRH-treated heifers than for controls. Serum follicle-stimulating hormone, cortisol, and progesterone were unchanged during treatment. High levels of cortisol on d 1 declined (P less than .001) to baseline by d 2. Characteristic progesterone rises or short luteal phases occurred within 10 d of treatment initiation in more (P less than .05) LHRH-treated heifers (I = 45%, P = 33%) than controls (6%), although days to first observed estrus and first ovulation were unaffected by treatments. Although both continuous and pulsatile administration of LHRH successfully induced LH and estradiol release as well as preovulatory-like LH surges in some heifers, earlier initiation of estrous cycles was not achieved. Estrous cycles appeared to be delayed by exposure to continuous LHRH infusions during the peripubertal period.     

Insulin response to glucose in estrous and diestrous obese and lean heifers

This study determined if the insulin and glucose responses to glucose infusion in obese (n = 4) and lean (n = 4) Holstein heifers were affected by stage of the estrous cycle. Glucose (.35 g/kg) was infused within 2 min into the jugular veins of heifers during diestrus (d 15) and at the subsequent estrus (d 0). Concentrations of insulin and glucose were determined in jugular venous serum obtained from blood samples collected at 60, 45, 30, 15 and 1 min before and at 2.5, 5, 10, 15, 20, 30, 40, 50, 60, 80, 100, 120, 140, 160, 180, 210 and 240 min after glucose. Mean (+/- SE) pretreatment concentrations of glucose (mg/100 ml) in obese (68 +/- 1.9) and lean (71 +/- 2.5) heifers were unaffected by body condition and stage of the cycle. Mean (+/- SE) pretreatment concentrations of insulin (microU/ml) were unaffected by stage of the cycle but were higher (P less than .05) in obese (33 +/- 3.6) than in lean (18 +/- 2.7) heifers. Body condition affected the insulin response with greater absolute concentrations (P less than .01) and total (P less than .005) response areas of insulin in obese than in lean heifers. Kinetics of the injected glucose were unaffected by body condition and stage of the cycle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of direction of body weight change on pattern of gonadotropin secretion in ovariectomized beef heifers of equivalent body weight

Our hypothesis was that alterations in pattern of gonadotropin secretion induced by level of dietary energy intake are dependent on direction of BW change. Beef heifers were assigned to one of two treatments 21 d after ovariectomy: 1) increasing followed by decreasing BW (I-D; n = 9) or 2) decreasing followed by increasing BW (D-I; n = 9). Heifers assigned to the I-D treatment received 15.4 Mcal ME/d (HE) from wk 0 to 8 followed by 4.3 Mcal ME/d (LE) from wk 9 to 16. Heifers assigned to the D-I treatment received the LE diet for the initial 8 wk followed by the HE diet for the subsequent 8 wk. At 2-wk intervals blood samples were collected at 10-min intervals for 11 h to determine the pattern of secretion and(or) concentration of gonadotropins in circulation (LH and FSH) and pituitary responsiveness to administration of 750 ng and 50 micrograms of LHRH. Frequency of LH pulses increased (P less than .05) in a quadratic fashion with increasing ADG in heifers in both treatment groups. Amplitude of LH pulses decreased (P less than .05) in a linear fashion with increasing ADG in heifers in both treatment groups and magnitude of this decline was greater (P less than .05) in heifers receiving the D-I treatment. Amplitude of response to 750 ng of LHRH decreased (P less than .05) in a linear fashion with increasing ADG in heifers receiving the D-I treatment but was unrelated to changes in ADG (P greater than .05) in heifers receiving the I-D treatment. When heifers in both treatment groups attained similar BW (P greater than .05; wk 14), amplitude of LH pulses, responsiveness to 750 ng of LHRH and mean concentration of FSH in serum were higher (P less than .05) in heifers receiving the I-D than in heifers receiving the D-I treatment. Thus, we accept the hypothesis that alterations in pattern of gonadotropin secretion in heifers fed diets with a low energy content are dependent on direction of BW change.     

Influence of biostimulation by mature bulls on occurrence of puberty in beef heifers

The objective of this study was to determine if biostimulation of prepuberal beef heifers by mature bulls would alter proportions of heifers exhibiting puberty, or age or weight at puberty. Angus (A), A X Hereford (H) and Tarentaise X HA heifers (n = 103) were stratified by age and weight within breed-type and location of birth and allotted randomly to the following treatments: 1) heifers exposed to mature bulls (T1; n = 52) or 2) heifers isolated from bulls (T2; n = 51). At the start of the experiment, heifers in T1 and T2 were 287 +/- 2 and 286 +/- 2 d of age, respectively. Male-to-female ratio for T1 was 1:26. Heifers in T1 and T2 were maintained in drylots separated by .5 km. Heifers were observed for estrus twice daily for 152 d. Puberty was characterized by the following criteria: 1) behavioral estrus, 2) presence of a palpable corpus luteum (d 9; estrus = d 0) and 3) a rise in serum progesterone above 1 ng/ml (d 9). Proportions of heifers reaching puberty by 11, 12, 13, 14 and 15 mo of age did not differ (P greater than .10) between treatments. Percentages of heifers reaching puberty by the end of the experiment were 84 and 89% for T1 and T2, respectively. Age and weight at puberty did not differ (P greater than .10) between treatments and averaged 370 +/- 7 d and 293 +/- 4 kg, respectively. Results from this experiment indicated that presence of mature bulls did not alter proportions of beef heifers reaching puberty, or age and weight at puberty.