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.     

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.     

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.     

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.     

Effects of restriction of dietary energy intake during the prepubertal period on secretion of luteinizing hormone and responsiveness of the pituitary to luteinizing hormone-releasing hormone in heifers

The working hypothesis that a low plane of nutrition during the prepubertal period delays puberty in heifers by retarding the prepubertal increase in secretion of luteinizing hormone (LH) was investigated. Secretion of LH and the responsiveness of the pituitary to LH-releasing hormone (LHRH) were compared in heifers fed a growing diet (which allowed spontaneous occurrence of puberty; n = 12; control) or an energy deficient diet (which delayed puberty; n = 11; delayed) during the prepubertal period. The dietary treatments were initiated when the heifers were 299 +/- 14 (mean +/- SD) d of age (d 0 of the experiment) and continued until d 175 of the experiment (474 +/- 14 d of age). Weight gains were .79 +/- .05 (mean +/- SE) and .21 +/- .03 kg X head-1 X d-1 for control and delayed heifers, respectively. Puberty occurred on d 120 +/- 14 of the experiment (428 +/- 13 d of age) in control heifers, whereas none of the delayed heifers attained puberty during the feeding period. Serum concentration of LH and the frequency of LH pulses increased rapidly during the 175-d feeding period in control heifers. In delayed heifers, serum LH concentration increased less rapidly and no increase in pulse frequency was detected during the experimental period. Amplitude of LH pulses tended to be higher in control than delayed heifers. Responsiveness of LH secretion to LHRH was lower in delayed than control heifers. It is speculated that failure of secretion of LH to increase is the causative factor for delayed puberty when dietary energy is limited during the prepubertal period in heifers.     

Luteinizing hormone,growth hormone,insulin-like growth factor-i,insulin and metabolites before puberty in heifers fed to gain at two rates

Fall born Angus x Hereford heifers were allotted to treatments at 9 mo of age to achieve the following growth rates: 1) fed to gain 1.36 kg/d (n = 10; HGAIN); and 2) fed to gain 0.23 kg/d for 16 wk, then fed to gain 1.36 kg/d (n = 9; LHGAIN). Growth hormone (GH), insulin-like growth factor-1 (IGF-I), insulin, glucose, nonesterified fatty acids (NEFA), and progesterone were quantified in twice weekly blood samples until onset of puberty. Body weight, hip height, and pelvic area were recorded every 28 d. Frequent blood samples (n = 8 heifers/treatment) were collected every 14 d, commencing on day 29 of treatment until onset of puberty to evaluate secretion of luteinizing hormone (LH) and GH. The HGAIN heifers were younger (369 d; P < 0.001), were shorter at the hip (115 cm; P < 0.05) and had smaller pelvic area (140 cm²; P < 0.10), but body weight (321 kg) did not differ at puberty compared with LHGAIN heifers (460 d; 119 cm; 155 cm²; 347 kg, respectively). The HGAIN heifers had greater (P < 0.05) concentrations of LH, IGF-I, and insulin in serum and glucose in plasma during the first 84 d of treatment than LHGAIN heifers, whereas LHGAIN heifers had greater (P < 0.05) concentrations of GH in serum and NEFA in plasma than HGAIN heifers. On Day 68 of treatment, HGAIN heifers had less mean GH (P < 0.01) and greater (P < 0.05) LH pulse frequency than LHGAIN heifers, whereas LH pulse amplitude and mean LH did not differ (P > 0.10) between treatments. Treatment did not influence secretion of LH and GH at 1 and 3 wk before puberty. Mean GH concentrations in serum and GH pulse amplitude in all heifers were greater (P < 0.05) 2 to 9 d (12.9 and 40.7 ng/ml, respectively) than 16 to 23 d (10.4 and 20.0 ng/ml, respectively) before puberty. Nutrient restriction decreased LH pulse frequency and delayed puberty in beef heifers. Furthermore, dramatic changes in mean concentration and amplitude of GH pulses just before puberty in beef heifers may have a role in pubertal development.     

Influence of prepubertal ovariectomy and estradiol replacement therapy on secretion of luteinizing hormone before and after pubertal age in heifers

Secretion of luteinizing hormone (LH) and effects of estradiol were evaluated during and after the prepubertal decline in negative feedback of estradiol on secretion of LH. Prepubertal heifers (269 ± 4 days of age; n=10) were ovariectomized on February 6, 1981 (Day 0). Five ovariectomized heifers were administered a subcutaneous implant on Day 0 which provided physiological serum concentrations of estradiol (OVX-E₂). The remaining 5 heifers were not implanted (OVX). A second estradiol implant was administered to OVX-E₂ heifers on Day 164 (n=3) or Day 206 (n=2) of the study. Blood samples were collected sequentially (every 12 min for 8 hr) at approximately two week intervals from Days 0 to 232 of the experiment. The experimental period spanned from approximately 100 days before (269 days of age) to 100 days after (501 days of age) the expected age at puberty. Mean serum concentration of LH and frequency of LH pulses increased rapidly from Days 0 to 36 in OVX heifers and were followed by a further gradual rise in pulse frequency (Day 50 to 232) and a reciprocal decline in mean LH and pulse amplitude. The rapid post-ovariectomy increase in secretion of LH was blocked by estradiol in OVX-E₂ heifers. All characteristics (mean, frequency and amplitude) of secretion of LH increased gradually during the experimental period in OVX-E₂ heifers (Days 0 to 232). Mean concentration and amplitude of pulses were higher in OVX-E₂ than in OVX heifers by Days 148 and 134, respectively. These differences were maintained for the remainder of the experimental period. No acute effects of the second estradiol implant on secretion of LH were observed in OVX-E₂ heifers. Results of this study indicate that long-term changes in secretion of LH occur following prepubertal ovariectomy in heifers and suggest that the previously documented prepubertal decline in negative feedback of estradiol on secretion of LH is followed by a period of positive feedback after pubertal age is surpassed.     

Incidence of puberty in beef heifers fed high- or low-starch diets for different periods before breeding

Spring-born Hereford x Angus heifers (n = 206) were used to determine effects of energy supplementation programs and amount of starch in the diet on incidence of puberty. In Exp. 1, heifers (205 +/- 5 kg; n = 68) grazing dormant native pasture were fed 0.9 kg/d (as-fed basis) of a 42% CP supplement from November until February 14. Heifers were stratified by weaning weight and allotted randomly to treatment before breeding (May to July). Treatments were 1) 0.9 kg (as-fed basis) of a 42% CP supplement/d and pasture (control); 2) a high-starch (HS) diet (73% corn; 53% starch) fed in a drylot for 60 d (HS-60); 3) a HS diet fed in drylot for 30 d (HS-30); or 4) a low-starch (LS) diet (49% corn; 37% starch) self-fed on pasture for 30 d (LS-30). The HS-60 and HS-30 heifers were limited-fed to gain 0.9 kg/d, and the LS-30 heifers had ad libitum access to the diet. High-starch-60 and LS-30 heifers were heavier (P < 0.05) than control and HS-30 heifers at the beginning of the breeding season. Thirty-one, 25, and 26% more HS-60 heifers were pubertal (P < 0.05) on May 1 compared with LS-30, HS-30, and control heifers, respectively. At puberty, HS-60 heifers were 24 and 22 d younger (P < 0.05) than LS-30 and control heifers, and 31 kg lighter (P < 0.01) than LS-30 heifers. In Exp. 2, heifers grazed dormant pasture and were fed 0.9 kg (as-fed basis) of a 42% CP supplement/d from weaning in October to late February; then heifers were assigned randomly to treatments for 60 d before the breeding season. In two years, control heifers (n = 46) grazed pasture and received 0.9 kg of SBM supplement/d; LS (n = 46) heifers were self-fed a distiller's grain and soybean hull-based diet in drylot; and HS heifers (n = 46) were limited-fed a corn-based diet in drylot. During treatment, HS and LS heifers had greater weight gains than control heifers. Pubertal BW (313 +/- 6 kg) was not influenced by treatment, but HS and LS heifers were younger (P < 0.03) than control heifers at puberty. During a 60-d breeding period, the incidence of puberty was greater (P < 0.05) for HS and LS heifers than for control heifers and was greater (P < 0.05) in HS than in LS heifers in Year 1. Feeding a LS or a HS diet for 30 d before breeding may be inadequate to stimulate puberty in beef heifers, but feeding a diet with a greater amount of starch for 60 d before breeding may increase the incidence of puberty during breeding of heifers that have inadequate yearling weight.     

Changes in metabolites, metabolic hormones, and luteinizing hormone before puberty in Angus, Braford, Charolais, and Simmental heifers

We determined changes in insulin, glucose, free fatty acids (FFA), growth hormone (GH), insulin-like growth factor I (IGF-I) and LH before puberty in Angus, Braford, Charolais, and Simmental heifers. Our primary objective was to identify metabolites and metabolic hormones that serve as metabolic cues for onset of puberty. Angus (n = 12). Braford (n = 7), Charolais (n = 9), and Simmental (n = 7) heifers were assigned at weaning (289 +/- 25 d of age; 264 +/- 23 kg) to open-sided pens with slotted floors, and they were fed a corn silage-concentrate diet formulated to provide gains of .91 kg/d. Puberty was defined as the 1st d (d 0) that serum progesterone (determined in blood samples collected at weekly intervals) exceeded 1 ng/ml. Blood samples were collected before and after feeding at 15-min intervals for 8 h at 21-d intervals before puberty in a subsample of heifers (at least five per breed). Angus and Simmental heifers weighed less and were younger (P less than .05) at puberty than Charolais and Braford heifers. Serum FFA before feeding and frequency of LH release increased (P less than .05) from d-40 +/- 3 to d-17 +/- 3 in all breeds. Conversely, concentrations of insulin were greater (P less than .05) at -40 than at -17 d from puberty in Angus, but not in Braford, Charolais, or Simmental heifers. Frequency of GH release was greater at d -40 than at d -17 in Angus heifers; however, in Braford and Charolais heifers frequency of GH release was greater at d -17 than at d -40. Concentrations of IGF-I (measured every 2 wk) increased linearly (P less than .07) from d -56 to 0 from puberty in Angus but not in other breeds. In conclusion, frequency of LH release and concentrations of FFA increased before puberty in all breeds; however, consistent changes in other metabolites and hormones were observed only in Angus heifers.     

Negative feedback control of luteinizing hormone secretion in prepubertal beef heifers at 60 and 200 days of age

Prepubertal beef heifers at 60 and 200 d of age, born in the fall or spring, were assigned randomly to one of three treatment groups: (1) intact = 1; (2) bilateral ovariectomy (OVX); or (3) OVX plus estradiol-17 beta(E2) administered in silastic implants (OVX + E2). Luteinizing hormone (LH) was measured in serum samples collected at 20-min intervals for 4 h from heifers on -1, +7, +21, +35 and +49 d after OVX. Luteinizing hormone concentrations increased in the serum by 7 d after OVX in heifers at both 60 and 200 d of age (P less than .001; time X treatment). Prior to OVX, the LH patterns were characterized by low levels and infrequent episodic pulses. By 49 d after OVX, the mean LH concentrations increased and the pattern changed to one of rhythmic LH pulses with a periodicity of 1 h (P less than .001; time X treatment). Estradiol-treated OVX heifers did not exhibit a postovariectomy rise in serum LH concentrations. Serum E2 concentration 49 d after OVX in OVX heifers was threefold greater than in 1 or OVX heifers, thus demonstrating that E2 exerted negative feedback on pituitary LH secretion in prepubertal heifers. There was no measurable difference in serum E2 concentrations between I and OVX heifers; however, the contrast in the concentration and pattern of serum LH between the two groups was dramatic and suggested gonadal factors in addition to E2 are involved in controlling LH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Suckling inhibits release of luteinizing hormone-releasing hormone from the bovine median eminence following ovariectomy

The effect of suckling on depletion of hypothalamic LHRH from the median eminence (ME) following ovariectomy (OVX) was determined in cattle. Multiparous, postpartum Holstein cows were assigned randomly to three groups: intact, nonsuckled (INT, n = 4); ovariectomized (3 to 5 d after parturition), nonsuckled (OVX, n = 4); and ovariectomized (3 to 5 d after parturition) and suckled by three calves (OVX-S, n = 5). Blood samples were collected at three periods (1 to 7 d before parturition and 3 to 5 d and 31 to 37 d after parturition) to determine plasma LH concentration. At 31 to 37 d after parturition, all cows were slaughtered and each ME was collected and mid-sagitally sectioned. The left half of each ME was used to determine content and concentration of LHRH. Concentrations of LH and LHRH were determined by RIA. Plasma LH concentration was similar among the three groups at 1 to 7 d before parturition and 3 to 5 d after parturition; however, at 31 to 37 d after parturition, OVX cows had a greater (P less than .05) concentration of LH (2.25 +/- .64 ng/ml) than either INT (.47 +/- .10 ng/ml) or OVX-S (.92 +/- .14 ng/ml) cows. Content of LHRH in the ME of INT (80.12 +/- 15.0 ng) and OVX-S 109.8 +/- 16.4 ng) cows was similar but was greater (P less than .05) than that in OVX cows (48.95 +/- 5.9 ng).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of prebreeding body weight or progestin exposure before breeding on beef heifer performance through the second breeding season

Two experiments evaluated prebreeding target BW or progestin exposure for heifers developed lighter than traditional recommendations. Experiment 1 evaluated the effects of the system on heifer performance through subsequent calving and rebreeding over 3 yr. Heifers (229 kg) were assigned randomly to be developed to 55% of mature BW (299 kg) before a 45-d breeding season (intensive, INT; n = 119) or 50% of mature BW (272 kg) before a 60-d breeding season (relaxed, RLX; n = 142). Prebreeding and pregnancy diagnosis BW were greater (P 0.15) between systems. Cost per pregnant 2-yr-old cow was less for the RLX than the INT heifer development system. Of heifers that failed to become pregnant, a greater proportion (P = 0.07) of heifers in the RLX than in the INT system were prepubertal when the breeding season began. Therefore, a second 2-yr experiment evaluated melengestrol acetate (MGA, 0.5 mg/d) as a means of hastening puberty in heifers developed to 50% of mature BW. Heifers were assigned randomly to the control (n = 103) or MGA (n = 81) treatment for 14 d and were placed with bulls 13 d later for 45 d. Prebreeding and pregnancy diagnosis BW were similar (280 and 380 kg, respectively; P > 0.10) for heifers in the control and MGA treatments. The proportion of heifers pubertal before breeding (74%), pregnancy rate (90%), calving date, calf weaning weight, and second breeding season pregnancy rate (92%) were similar (P > 0.10) between treatments. Developing heifers to 50 or 55% of mature BW resulted in similar overall pregnancy rates, and supplementing the diets of heifers developed to 50% of mature BW with MGA before breeding did not improve reproductive performance.     

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.     

Effects of dietary fat and sire breed on puberty, weight, and reproductive traits of F1 beef heifers

Prepubertal F1 heifers (n = 246; from crossbred dams bred to either Hereford [H], Limousin [L], or Piedmontese [P] sires) were fed 1.9% (LF) or 4.4% (HF) dietary fat from 254+/-4 d of age until they reached puberty or the breeding season started. Safflower seeds (37% oil with 79% linoleic acid) were the added fat source. Blood samples and backfat thickness measurements were obtained from 60 randomly selected heifers representing the sire breeds and diets studied. In addition, five H-sired heifers from both diets were serially bled at 28-d intervals. Total gain, ADG, body condition score, and backfat thickness were affected by sire breed (P < 0.001) but not diet. Backfat thickness was affected (P < 0.01) by the diet x time on feed interaction. Diet did not affect pubertal age (P > 0.10) but tended (P = 0.08) to affect the percentage of heifers pubertal by the beginning of breeding (June 4). Sire breed effects on puberty age at beginning of breeding, percentage pubertal at the beginning of breeding, and puberty age during the entire study were all highly significant. The effect of the diet x sire breed interaction on percentage of heifers pubertal at beginning of breeding (P < 0.05) was 74.4 vs 76.3% in H-sired, 69.8 vs 60.5% in L-sired, and 76.2 vs 97.6% in P-sired heifers (LF vs HF, respectively). Number of AI services per pregnancy and final pregnancy percentage were not affected by diet or the diet x sire breed interaction. Diet affected progesterone (P < 0.05) and cholesterol (P < 0.001) concentrations, and sire breed tended to affect (P = 0.06) cholesterol concentrations. The effect of the diet x time on feed interaction on cholesterol concentrations was highly significant. There were no effects of diet or sample period on insulin or growth hormone concentrations in serially collected blood samples. We conclude that effects of supplemental dietary fat may be breed-dependent and hypothesize that a feeding period of approximately 60 d duration may be more appropriate than the 162 d used in this study.     

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.     

The effect of acute nutritional change on follicle wave turnover, gonadotropin, and steroid concentration in beef heifers

The effects of acute nutritional change on endocrine and ovarian characteristics were studied in cyclic (intact; n = 20) and long-term ovariectomized (ovx; n = 18) heifers being fed 1.2 x maintenance (1.2M). On d 7 of an 8-d progesterone and estradiol treatment, intact and ovx heifers were randomly allocated to diets providing .4, 1.2, or 2.0M until emergence of the second follicular wave after ovulation in intact heifers. In intact heifers, two of eight fed .4M failed to ovulate. In the other six, growth rate and maximum diameter (1.1+/-.09 mm/d and 10.1+/-.7 mm, respectively) of the first dominant follicle (DF) postovulation were less (P<.05) than in heifers fed either 1.2 (1.6+/-.18 mm/d; 12.9+/-.44 mm) or 2.0M (1.6+/-.08 mm/d; 12.7+/-.7 mm). In intact heifers, LH pulse frequency and amplitude were not affected by diet (P>.10). In ovx heifers, the frequency of LH pulses was unaffected by diet (P>.10), but heifers fed .4M had a greater pulse amplitude (P<.05) and mean concentration of LH (P<.001) than those fed 1.2 or 2.0M. Plasma concentrations of FSH were greater (P<.05) in ovx heifers fed .4M than in those fed 1.2 or 2.0M and increased linearly with time (P<.01). The FSH concentrations in heifers fed 1.2 and 2.0M were similar (P>.10) and decreased linearly with time (P<.001). In intact heifers, concentrations of FSH preceding follicle wave emergence were greater in heifers fed .4M (P<.001), but basal concentrations were not affected (P>.10). Concentrations of progesterone and estradiol were unaffected by diet (P>.10). Significant diet x ovarian status interactions in plasma IGF-I concentrations existed. Plasma concentrations of insulin increased as the level of nutrition increased, whereas concentrations of NEFA decreased. In conclusion, growth rate and maximum diameter of the DF were decreased by acute nutritional restriction, without affecting the concentration of LH. The magnitude of the FSH increase preceding new follicle wave emergence increased following dietary restriction, but concentrations of FSH were unaffected during the other stages of DF growth. The results of this study may have important implications for the feeding strategies adapted for high-yielding dairy cows in the early postpartum period when feed intake is often physiologically restricted.     

Effects of suckling and low doses of estradiol on luteinizing hormone secretion during the postpartum period in beef cows

An experiment was conducted to test if suckling acutely suppressed circulating levels of LH during the postpartum period in beef cows. In addition, the influence of exogenous administration of low concentrations of estradiol on LH secretion during the postpartum period was evaluated. Twelve mature cows were randomly assigned before parturition to one of three treatments. Four intact cows were used as controls (INT). Eight cows were ovariectomized within the first 7 days following parturition. Four of these cows received a silastic 17β-estradiol implant subcutaneously at the time of ovariectomy (OVX-E); the remaining four cows received no further treatment (OVX). All cows were allowed to nurse one calf for 30 min daily between 1200 and 1230 hours for the duration of the experiment. Blood samples were collected at 12 min intervals for 6 hr before and 6 hr after suckling on days 9, 30, 44 and 58 postpartum. Mean interval (mean ± SE) to the first increase in peripheral progesterone concentrations indicative of the onset of ovarian luteal activity was detected in INT cows 37 ± 4.9 days postpartum. An acute effect of suckling on LH secretion did not occur in INT and OVX cows but mean LH concentrations were reduced in OVX-E cows following suckling on days 44 and 58. Mean LH concentrations remained low in INT cows; whereas, in OVX and OVX-E cows LH concentrations increased linearly (P<0.05) as the interval from time of ovariectomy increased. Cows in the OVX-E group had a higher mean concentration of LH than cows in the OVX group at 30, 44 and 58 days postpartum (P<0.05). Frequency of LH pulses did not differ between cows in the OVX and OVX-E groups at any period. Data from this experiment support the concept that suckling is acting in a chronic fashion to inhibit LH secretion during the postpartum period. In the absence of ovaries, chronic administration of exogenous estradiol in low concentrations has a positive effect on secretion of LH in the postpartum cow.     

Age at puberty,total fat and conjugated linoleic acid content of carcass,and circulating metabolic hormones in beef heifers fed a diet high in linoleic acid beginning at four months of age

In the current study, we hypothesized that diets high in linoleic acid would increase conjugated linoleic acid (CLA) tissue content, reduce adiposity and leptin production, and result in an increase in the age at puberty in heifers. Heifers were weaned and blocked by body weight (heavy, n = 10, and light, n = 10) and allocated randomly within block to receive isocaloric and isonitrogenous diets with either added fat (HF, n = 10) or no added fat (C, n = 10) from 4 mo of age until post-pubertal slaughter. Whole sunflower seed (55% oil; 70% linoleic acid) was used as the fat source in HF diets and provided 5% added fat from the start of the study until heifers weighed 250 +/- 8 kg, at which time added fat was increased to 7% of dry matter until slaughter. Body weights were recorded weekly, and blood samples were collected weekly for total cholesterol and hormone analyses. Puberty was confirmed based on serum concentrations of progesterone and ultrasonographic confirmation of corpora lutea. Heifers were slaughtered at 325 +/- 10 d of age, and longissimus muscle between the 9th and 11th rib was collected and analyzed to estimate carcass composition. Subcutaneous and kidney, pelvic, and heart fat were collected at slaughter for fatty acid analyses. The HF heavy group tended (P < 0.10) to reach puberty later than all other groups, and one HF light heifer did not reach puberty during the study. Linoleic acid and cis-9, trans-11 CLA tissue contents were higher (P < 0.03) in HF heifers than controls, but neither total carcass fat nor percentage of dry matter differed by dietary group, although the percentage of protein tended (P < 0.10) to be lower in HF heifers. Mean serum concentrations of leptin did not differ due to diet; however, leptin increased (P < 0.01) linearly as puberty approached. Circulating concentrations of growth hormone and insulin-like growth factor I increased or remained relatively constant between wk 2 to 10 of feeding, and then declined (P < 0.01) until the onset of puberty. Serum IGF-I was lower (P < 0.01) in heifers receiving the HF diet. Mean serum concentrations of insulin and total cholesterol increased (P < 0.01) with time in both groups, but only total cholesterol was increased by the HF diet (P < 0.05). Results indicate that diets high in linoleic acid fed to growing beef heifers beginning early in life have little or no effect on total carcass fat, circulating leptin, or age at puberty despite measurable increases in CLA accumulation.     

Effects of long-term estrogen implants in beef heifers

Crossbred beef heifers (n = 78) were assigned randomly to one of three treatments. Heifers received either no implant, one estradiol-releasing implant (Compudose), or two estradiol-releasing implants. Heifers were implanted at birth and then reimplanted every 150 d. Calves were maintained with the cows until weaning at approximately 200 d of age. Heifers were placed in the feedlot as one group and fed a growing diet for 56 d. Following the growing phase, heifers were segregated into their respective treatment groups and fed until selected by industry buyers for harvest. Beginning at 1 yr of age and continuing every 14 d until puberty or harvest, heifers were palpated per rectum, and blood samples were collected for determination of ovarian activity and attainment of puberty. Serum progesterone of > or =1 ng/mL and(or) palpation of a detectable corpus luteum were criteria of puberty. At weaning and again at harvest, an x-ray was taken of the left front leg of six heifers selected randomly from each group. Metacarpal bones III and IV from the same animals were collected at harvest and transected for determination of epiphyseal plate closure. The x-ray scores and the actual measurements had a correlation of .94. Epiphyseal plate closure occurred in a dose-related manner, with heifers on the higher dose of estrogen having earlier plate closure than heifers on the lower dose. At harvest, reproductive tissues and carcass data were collected for all heifers. Eighteen of 25 untreated control heifers (P < .05), 6 of 26 heifers treated with one implant, and 2 of 26 heifers treated with two implants attained puberty by the end of the experiment. No differences (P > .10) were detected among treatment groups for carcass traits. These data suggest that early and continuous treatment of heifers with estradiol implants can retard reproductive function.     

Nutritionally induced anovulation in beef heifers: ovarian and endocrine function preceding cessation of ovulation.

Angus x Hereford heifers were used to determine endocrine and ovarian function preceding nutritionally induced anovulation. Six heifers were fed to maintain body condition score (M), and 12 heifers were fed a restricted diet (R) until they became anovulatory. Starting on d 13 of an estrous cycle, heifers were given PGF2alpha every 16 d thereafter to synchronize and maintain 16 d estrous cycles. Ovarian structures of M and R heifers were monitored by ultrasonography daily from d 8 to ovulation (d 1 of the subsequent cycle) until R heifers became anovulatory. Concentrations of LH and FSH were quantified in serum samples collected every 10 min for 8 h on d 2 and 15 (48 h after PGF2alpha), and estradiol and IGF-I were quantified in daily plasma samples from d 8 to 16 during the last ovulatory cycle (Cycle -2) and the subsequent anovulatory cycle (Cycle -1). During the last two cycles before anovulation, M heifers had 50% larger (P < .0001) ovulatory follicles than R heifers and 61% greater (P < .0001) growth rate of the ovulatory follicles. There was a treatment x cycle x day effect (P < .001) for concentrations of estradiol. The preovulatory increase in estradiol occurred in the R and M heifers during Cycle -2 but only in M heifers during Cycle -1. A treatment x cycle x day effect (P < .05) influenced LH concentrations. During Cycle -2, LH concentrations were similar for M and R heifers, but during Cycle -1, M heifers had greater LH concentrations than did R heifers. Concentrations of FSH were greater (P < .05) in R than M heifers after induced luteolysis when R heifers failed to ovulate. There was a treatment x cycle interaction (P < .05) for IGF-I concentrations, and M heifers had 4.7- and 8.6-fold greater IGF-I concentrations than did R heifers during Cycle -2 and -1, respectively. We conclude that growth rate and diameter of the ovulatory follicle, and concentrations of LH, estradiol, and IGF-I are reduced before the onset of nutritionally induced anovulation in beef heifers.