Relationship between serum insulin-like growth factor-I and genotype during the postpartum interval in beef cows

The objective of this study was to determine the effect of genotype and week postpartum on serum concentrations of IGF-I, body condition score (BCS), BW, and ovarian function in beef cows. Cows from the following genotypes were utilized in two consecutive years: Angus (A x A; n = 9), Brahman (B x B; n = 10), Charolais (C x C; n = 12), Angus x Brahman (A x B; n = 22), Brahman x Charolais (B x C; n = 19) and Angus x Charolais (A x C; n = 24). Serum concentrations of IGF-I, BCS, and BW were determined between wk 2 and 9 postpartum. Rectal ultrasound was used to determine days postpartum to first medium (6 to 9 mm) and first large (> or = 10 mm) follicle. Averaged across genotype, BCS decreased (P < 0.05) from 5.0 +/- 0.1 on wk 3 to 4.8 +/- 0.1 on wk 6 postpartum, and BW decreased (P < 0.05) between wk 2 and 3 and again between wk 4 and 9 postpartum. Averaged over year and week postpartum, serum IGF-I concentrations were greatest (P < 0.05) in B x B cows (46 +/- 5 ng/mL) compared with all other genotypes; lowest in A x A (12 +/- 4 ng/mL), C x C (13 +/- 4 ng/mL), and A x C cows (18 +/- 3 ng/mL); and intermediate (P < 0.05) in A x B (28 +/- 3 ng/mL) and B x C (26 +/- 3 ng/mL) cows compared with all other genotypes. Serum IGF-I concentrations did not change (P > 0.10) with week postpartum in C x C, A x A, and A x C cows, but increased (P < 0.05) between wk 2 and 7 postpartum in B x C, A x B, and B x B cows. Average interval to first medium (16 +/- 2 d) and first large (35 +/- 2 d) follicle did not differ (P > 0.10) among genotypes. Serum IGF-I concentrations correlated with BCS (r = 0.53 to 0.72, P < 0.001) but not with days to first large follicle (r = -0.19 to -0.22, P > 0.10). Averaged across genotypes, cows that lost BCS postpartum had lower (P < 0.01) serum IGF-I concentrations. Cows that calved with adequate BCS (i.e., > or = 5) had greater (P < 0.01) serum IGF-I concentrations postpartum than cows that calved with inadequate BCS (i.e., < 5) but days to first large and medium follicle did not differ (P > 0.10). In conclusion, concentrations of IGF-I in serum differed among genotypes and were associated with BCS but not days to first large or medium follicle in postpartum beef cows.

Norgestomet implants prevent pregnancy in beef heifers on pasture

The efficacy of erodible norgestomet implants for preventing pregnancy in postpubertal heifers was evaluated in two experiments at five locations each. Heifers (n = 896) within each study location were stratified by weight and allotted randomly to receive an ear implant containing either 0, 24, 36, or 48 mg of norgestomet (d 0). Heifers were exposed to fertile bulls immediately after implantation for 75 d (d 0 to 74) in Exp. 1 (n = 476) or for 80 d (d 75 to 154) in Exp. 2 (n = 420). Weights were recorded on d 0 and 74 (Exp. 1 and 2) and d 154 (Exp. 2). Each heifer was palpated rectally for pregnancy at the end of each experiment. Pregnancy rates were higher (P < .01) for control heifers (0 mg implant) than for heifers that received 24, 36, or 48 mg of norgestomet. In Exp. 1, pregnancy rates were 96, 29, 6, and 4% for heifers that received 0, 24, 36, and 48 mg implants of norgestomet, respectively. In Exp. 2, pregnancy rates were 85, 36, 19, and 9% for heifers that received 0, 24, 36, and 48 mg implants of norgestomet, respectively. Estrous activity during the first 3 wk of bull exposure was reduced (P < .05) among heifers that received norgestomet implants compared to control heifers but was not completely abolished at any dosage in Exp. 1. During the first 75 d of Exp. 1 and 2, heifers treated with 36 or 48 mg norgestomet implants gained weight faster (P < .05) than control heifers. Combined across both experiments, ADG during the first 74 d were .53, .56, .59, and .60 kg/d for heifers treated with 0, 24, 36, and 48 mg implants of norgestomet, respectively. These data indicate that norgestomet implants increased rate of weight gain, reduced estrous activity, and reduced the occurrence of pregnancy in heifers on pasture.

Effects of a growth implant and age of dam on growth and fertility in heifer calves

A field trial was conducted to determine the effects of a growth implant on growth, pregnancy rate, calving difficulty and calf weaning weight in replacement beef heifers. Heifers were assigned equally to either a control (n = 43; no implant), Ralgro (n = 43) or Synovex-C (n = 42) treatment group based on age of dam and age and weight at the start of the trial (implantation at 45–90 d of age). Neither Ralgro nor Synovex-C enhanced heifer weight or average daily gain (ADG) from the start of the trial to weaning (approximately 7 mo of age), breeding, or pregnancy determination. At breeding, Synovex-C implanted heifers had greater pelvic area than controls while Ralgro implanted heifers were intermediate (control = 149.7, Ralgro = 154.7 and Synovex-C = 157.4 pooled SE of 2.2 cm2; P = 0.05). Pelvic dimensions were similar among treatments at pregnancy determination and there were no treatment effects on calving difficulty. Age of dam affected heifer weight (P = 0.0005) and ADG (P = 0.001) to weaning, with heifers from mature cows weighing more at weaning and having a higher ADG from the start of the study to weaning than heifers from either first- or second-calf cows. Post-weaning performance of heifers from first-calf cows was similar to heifers from mature cows while heifers from second-calf cows were lighter at breeding (P = 0.04) and had a lower ADG from weaning to breeding (P = 0.04). Age of dam did not influence pregnancy rate (P = 0.10), but more heifers from second-calf cows calved late in the calving season (≥ 64 d) compared with heifers from either first-calf or mature cows (P = 0.04). It appears from this field trial that use of a growth implant between 45 and 90 d of age neither enhanced growth nor impaired fertility in beef heifers bred as yearlings. It also appears that second-calf 3-yr-olds may need to be managed separately from the mature cow herd in order to optimize replacement heifer development. Key words: Bovine, replacement heifers, growth implant, age of dam, weight, pregnancy rate

The efficacy of predicting dystocia in yearling beef heifers: I. Using ratios of pelvic area to birth weight or pelvic area to heifer weight

Three methods for predicting difficult births were tested on 4,140 yearling heifers measured before breeding and(or) at pregnancy check approximately 6 mo later. These heifers were from 115 beef herds in Alberta and British Columbia. The overall incidence of dystocia for normal presentations was 26.5%, which included 17.2% easy assists, 7.7% hard pulls, and 1.6% Caesarean sections. In Method 1, heifers were predicted as difficult (hard pull and Caesarean section) or easy (unassisted and easy pull) calvers by dividing their pelvic area (PA) by previously calculated PA to calf birth weight (PA/BWT) ratios. The ratio used depended on heifer weight and age. Method 1 predicted 63.7% to be difficult calvers. Of these only 10.4% were actually difficult calvers. The accuracy of this method was 40.0% and was not a useful on-farm method for predicting difficult births in first-calf, 2-yr-old heifers. In Method 2, 3,278 heifers measured before breeding and 1,125 heifers measured at pregnancy check were predicted as difficult or easy calvers by dividing their PA by 4.19 at prebreeding or 5.51 at pregnancy check. These values were PA/calf BWT ratios previously determined to be threshold levels. The accuracy of Method 2 was 78.5% and culling by this method would have reduced difficult birth rate by 9.6%. However, of the 738 heifers (16.8%) predicted to be difficult calvers, 86.0% actually calved easily. Heifers predicted to be easy calvers by Method 2 were heavier (P < .001), had a larger PA (P < .001), had more PA per kilogram of BW (P < .001), and had heavier (1.0 kg) calves at birth (P = .05) than heifers predicted to be difficult calvers. In Method 3, 3,269 heifers measured before breeding and 1,087 heifers measured at pregnancy check were predicted as difficult or easy calvers by dividing their PA by their BW. Heifers having ratios that were among the lowest 16% of the herd were predicted to be difficult calvers, and the rest were predicted to be easy calvers. The accuracy of Method 3 was 79.4% and culling by this method would have reduced difficult birth rate by 9.5%. However, of the 677 (15.5%) heifers predicted to be difficult calvers, 85.7% actually calved easily. Heifers predicted to be easy calvers by Method 3 were lighter (P < .001), had a larger PA (P < .001), had more PA per kilogram of BW (P < .001), and had lighter (1.2 kg) calves at birth (P = .04) than heifers predicted to be difficult calvers.(ABSTRACT TRUNCATED AT 400 WORDS)

The efficacy of predicting dystocia in yearling beef heifers: II. Using discriminant analysis

Discriminant analysis was used to develop equations for predicting dystocia in 2-yr-old, first-calf beef heifers. Data collected on 893 yearling heifers from 34 farms were used to derive four sets of prediction equations (Data Set 1). There was a set of equations for British x British (BRBR) heifers at prebreeding, BRBR heifers at pregnancy check, Continental x British-Continental x Continental (CNBR-CNCN) heifers at prebreeding, and CNBR-CNCN heifers at pregnancy check. A second set of data collected on 3,728 heifers at prebreeding and 875 heifers at pregnancy check were used to test these equations (Data Set 2). Measurements taken prebreeding and at pregnancy check included heifer age, weight, body condition score, pelvic height, pelvic width, pelvic area, hip height, and shoulder height. A precalving estimate of average calf birth weight (AVGBWT) and actual calf birth weight (BWT) were also recorded. When actual calf BWT was used, the prediction equations developed for BRBR heifers at prebreeding and at pregnancy check correctly classified 84.6 and 88.5% of the heifers in Data Set 1, respectively. The prediction equations developed for CNBR-CNCN heifers correctly classified 88.1% of the heifers at prebreeding and 87.9% at pregnancy check. When AVGBWT was used, the accuracy of these four sets of prediction equations ranged from 77.1 to 83.1% for heifers in Data Set 1. In Data Set 2, the prebreeding equations correctly classified 78.5% of BRBR and 79.0% of CNBR-CNCN heifers when AVGBWT was used. Culling by these prebreeding equations would have reduced difficult birth rate in BRBR and CNBR- CNCN heifers by 8.5 and 15.7%, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of lasalocid on the GnRH-induced LH and testosterone release during puberal development in the Brahman bull

To determine the effect of lasalocid on endocrine patterns associated with puberty, 12 half-sib prepuberal Brahman bulls were allotted by age and weight (174 to 256 d of age; 141 to 243 kg) to control or lasalocid treatments. Bulls in the control treatment were fed a 4:1 corn:cottonseed meal concentrate plus Coastal bermudagrass hay to which the bulls were given ad libitum access. The lasalocid treatment was identical except for the addition of 200 mg of lasalocid.animal-1.d-1. Blood samples were collected frequently before and after GnRH (200 micrograms, i.m.) on d 7, at 28-d intervals thereafter, and within 14 d after puberty (defined as 50 x 10(6) sperm cells with 10% motility). By d 7, bulls fed lasalocid released more LH (P less than .05), but not testosterone (T;P greater than .10), in response to GnRH than controls. At the time that the first sperm cells were observed in an electroejaculate (FS), lasalocid-fed bulls released more (P less than .05) LH and T than controls. At puberty, there was no difference (P greater than .10) between treatments in amount of T released, although lasalocid-fed bulls released more LH (P less than .05). Before puberty, concentrations of LH were positively correlated with concentrations of T in samples collected 1 and 2 h later. Both groups of bulls exhibited a linear increase in T response with advancing age (P less than .005). Release of LH decreased with age in the control bulls (P less than .10) but was unaffected by age in lasalocid-fed bulls. Both groups showed a decreased (P less than .001) LH:T response ratio with advancing age. Results of this study with bulls confirm previous reports in heifers of the enhancing effect of an ionophore on reproductive function.

Insulin-like growth factor I in follicular development and function in postpartum beef cows

The objective of this study was to determine whether nutrition affects follicular growth and(or) steroid and insulin-like growth factor I (IGF-I) concentrations in follicular fluid. Beginning 6 d after calving, Hereford-cross cows (n = 28) were fed either 14 (ad libitum) or 7 (restricted) kg.animal1.d-1 of chopped alfalfa-brome hay. Half the cows in each treatment were ovariectomized on d 20 (OVX-20) and the remaining half on d 35 (OVX-35) postpartum. Cow weight and condition score were recorded weekly, and blood was collected thrice weekly for determination of insulin, IGF-I, glucose, and free fatty acid (FFA) concentrations. At ovariectomy, follicular fluid from each follicle greater than or equal to 4 mm in diameter was aspirated for determination of IGF-I, progesterone (P4), and estradiol-17 beta (E2) concentrations. Restricted cows lost more weight after calving than did ad libitum cows (P less than .0001), although all cows lost similar amounts of body condition (time postpartum, P = .008). Concentrations of FFA were elevated (P less than .0001) in restricted cows from wk 2 through 5 after calving but did not change with time in ad libitum cows. Plasma concentrations of glucose were lower in restricted than in ad libitum cows (59.6 +/- .4 vs 61.8 +/- .4 mg/dl; P = .05), but insulin and IGF-I were similar (P greater than .10) between dietary treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum concentrations of IGF-I in postpartum beef cows

Four experiments assessed changes in serum IGF-I under various physiologic conditions in postpartum cows. In Exp. 1, anestrous suckled cows (n = 25) were infused for 6 d with either saline or glucose at two different infusion rates. In Exp. 2, anestrous cows (n = 29) received either a saline (weaned and suckled controls) or 3 g/d phlorizin (weaned phlorizin) infusion for 3 d. Calves from the weaned groups were removed from 15 h before and throughout infusions. In Exp. 3, cycling suckled cows (n = 20) received prostaglandin F2 alpha (PGF2 alpha) when the 5-d saline or phlorizin infusion began. In Exp. 4, suckled cows (n = 20) had ad libitum access to feed or received 50% of control feed consumption from 30 to 40 d postpartum. Increasing glucose availability (Exp. 1) increased (P less than .05) serum IGF-I by 30 to 35%. IGF-I remained stable after weaning (Exp. 2) in phlorizin-infused cows (128.8 +/- 12.7 ng/ml), but increased (P less than .05) by 3 d after calf removal in weaned control cows (152.2 +/- 7.5 ng/ml). IGF-I also remained stable in phlorizin-infused cows following PGF2 alpha injection (Exp. 3), but increased in control cows by 2 d after PGF2 alpha (156.8 +/- 18.3 on d 2 vs. 133.7 +/- 9.8 ng/ml pre-injection; P less than .05) and remained elevated (P less than .05) during the periovulatory period. In cows receiving restricted feed intake (Exp. 4), IGF-I decreased by approximately 50% within 4 d of feed restriction (71.3 +/- 9.4 vs 137.4 +/- 16.6 ng/ml; P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)

Follicular phase gonadotropin secretion in cyclic postpartum beef cows with phlorizin-induced hypoglycemia

Twenty mature, lactating Hereford-cross cows were used to determine the effect of phlorizin-induced hypoglycemia on gonadotropin secretion following prostaglandin-induced luteolysis. Cows were 43 to 108 d postpartum and had a functional corpus luteum (CL) at the start of infusion treatment (d 1). Infusions consisted of either saline (control) or 3 g/d of phlorizin infused continuously from the time of prostaglandin injection at 1000 on d 1 until 0800 on d 5. Blood samples were collected for determination of plasma concentrations of insulin, glucose and free fatty acids (FFA) and for serum concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and progesterone. Plasma concentrations of insulin (P less than .05) and glucose (P less than .05) were lower, whereas FFA concentrations increased (day X treatment, P less than .05) over the infusion period in phlorizin-treated cows compared with saline-infused controls. Mean serum concentrations of LH (1.17 +/- .10 vs 1.53 +/- .20 ng/ml; P less than .05) and LH pulse amplitude (1.69 +/- .14 vs 2.47 +/- .37 ng/ml; P less than .10) were lower in phlorizin-infused compared with saline-infused cows during the 0 to 24-h period immediately preceding the ovulatory gonadotropin surge. The FSH pulse frequency increased (.33 +/- .11 to .55 +/- .12 pulses/h) in saline-infused cows, but decreased (.61 +/- .10 to .41 +/- .11 pulses/h) in phlorizin-infused cows before the gonadotropin surge. Other characteristics of gonadotropin secretion were similar among phlorizin-infused and saline-infused cows. All but one phlorizin-infused cow ovulated and formed functional CL similar to controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Corpus luteum function following spontaneous or prostaglandin-induced estrus in Brahman cows and heifers

Corpus luteum (CL) function following spontaneous or prostaglandin F2 alpha-induced estrus was studied in 27 Brahman cows and 16 Brahman heifers. Females received one injection of 0, .38, .75 or 2.25 mg alfaprostol (PGF)/100 kg body weight at 12 +/- .1 d post-estrus. Serum progesterone (P4) during CL formation, following the subsequent estrus, increased over time (P less than .001, 1 to 13 d post-estrus) and was influenced by PGF (P less than .06) and a PGF X time interaction (P less than .02). Low serum P4 concentrations were observed during CL formation (d 3, 4, 10, and 12) in cattle that received 2.25 mg PGF. On d 13 post-estrus, CL were removed. No differences in weight, P4 content or number of luteinizing hormone (LH) receptors was evident between CL derived 13 d following spontaneous vs PGF-induced estrus. The CL formed following PGF-induced estrus had fewer large (P less than .002) and small (P less than .09) luteal cells in heifers, a lower (P less than .008) percentage composition of histological type I and II luteal cells in cows and a lower (P less than .0001) in vitro P4 response to LH in both cows and heifers when compared with the CL formed following spontaneous estrus. Cows had heavier (P less than .002) CL with a higher (P less than .05) number of large cells, a higher (P less than .09) percentage composition of histological cell type I and II luteal cells, and a higher (P less than .03) in vitro basal and stimulated P4 secretion. It is concluded that 1) the events associated with artificially shortening the estrous cycle with PGF altered subsequent CL function in Brahman females; 2) cows had heavier CL, which were composed of more steroidogenically functional luteal cells, than heifers and 3) age of animal interacted with PGF to alter subsequent CL function in Brahman females.

Effects of progesterone and weaning on LH and FSH responses to naloxone in postpartum beef cows

Two experiments were conducted to evaluate the effects of naloxone, an endogenous opioid receptor antagonist, on LH and FSH secretion in postpartum beef cows. In Experiment 1, 24 cows were divided into three equal groups. On day 15 postpartum, all cows were bled for 8 hr at 10 min intervals to evaluate LH secretory parameters. On day 18 postpartum, three treatments were administered: (a) saline at 0730 and 1130 hr; (b) 275 mg naloxone at 0730 and 1130 hr; (c) naloxone as in (b) above, plus this group was also treated with 50 mg progesterone (P4) twice daily from day 16 to day 19. In each treatment, jugular vein samples were collected at 10 min intervals from 0800 to 1600 hr. On day 19 the same treatments were administered at the same times, however, all cows were given 25 micrograms GnRH at 1200 hr to evaluate the LH secretory response. Naloxone increased mean LH concentration (P less than .05) and tended to increase pulse amplitude and frequency compared to controls. However, the most dramatic difference was due to P4 treatment which suppressed mean LH, pulse amplitude and frequency. Treatments had no effect on LH secretion in response to a 25 micrograms dose of GnRH. In Experiment 2, the effects of suckling on the naloxone response were examined in 16 postpartum cows. On day 21 postpartum, blood was collected at 10 min intervals for 8 hr and then calves were removed from half the cows. After 3 days of calf removal, all cows were sampled at 10 min intervals for 4 hr; then naloxone was injected after each 10 min sample at a dose rate of 200 mg/hr (33 mg per injection). Naloxone treatment and sampling continued for an additional 8 hr. Calf removal alone had very little effect on LH pulsatility. However, naloxone resulted in increased pulse frequency and mean LH compared to the control period. We conclude that LH release in the early postpartum cow is partially regulated by endogenous opioid peptides. We were unable to detect any effects on FSH secretion nor on pituitary sensitivity to exogenous GnRH.

Hypoglycemia alters pulsatile luteinizing hormone secretion in the postpartum beef cow

This study tested the hypothesis that the increased glucose requirement of lactation had effects that were independent of the suckling-dependent inhibition of postpartum endocrine function in beef cows. Mature Hereford cows were either suckled ad libitum and infused with saline iv (n = 9) from d 2 through 4 (d 0 = jugular catherization on d 32 +/- 3 postpartum); were nonsuckled and infused with saline from d 2 through 4 (n = 10); or were nonsuckled and infused with phlorizin (3 g/d) from d 2 through 4 (n = 10). Nonsuckled cows infused with phlorizin had lower (P less than .05) plasma concentrations of glucose and amino acid nitrogen (AAN) on d 2 compared with pre-infusion levels (d 1), but their metabolic profile returned to levels similar to the suckled cows by d 3 and 4. Nonsuckled cows infused with saline had elevated glucose and insulin and lower AAN and free fatty acids (FFA) on d 3 and 4 compared with pre-weaning (d 1) levels (P less than .05). Nonsuckled cows infused with phlorizin did not show this weaning-induced elevation in glucose and insulin. The number of luteinizing hormone (LH) pulses was not affected by treatment. However, in contrast to the large LH pulses observed in the nonsuckled cows infused with saline, both the suckled cows and the nonsuckled cows treated with phlorizin had more small and fewer large amplitude pulses (P less than .01). Treatment did not affect serum concentrations of follicle stimulating hormone, gonadotropin release in response to gonadotropin releasing hormone (25 micrograms) or the number of cows ovulating by 55 d after calving. We conclude that the increased glucose clearance caused by phlorizin infusion or lactation results in depression of LH pulse amplitude in suckled postpartum beef cows.

Failure of human chorionic gonadotropin injections to sustain gonadotropin-releasing hormone-induced corpora lutea in postpartum beef cows

Anestrous postpartum (PP) Hereford cows (n =20) were used to determine the effects of repeated injections of human chorionic gonadotropin (hCG) on the progesterone (P4) secretion and functional lifespan of gonadotropin-releasing hormone (GnRH)-induced corpora lutea (CL). Suckling was reduced to once a day from Day 21 to Day 25 PP, and all cows received injections of 200 micrograms GnRH at 1500 h on Day 24 PP to induce ovulation. Treated cows (HCG, n = 10) received 200 IU hCG b.i.d. from 1900 h on Day 27 PP to 1900 h on Day 33 PP; control cows (CTRL, n=10) were not injected. Blood was collected on Days 21, 23, 25, and 27 to 33, 35, 37, and 39 PP. Serum P4 concentration was measured by radioimmunoassay and used to classify luteal lifespan and the associated estrous cycle as short (SHORT) or normal (NORM) in duration. Treatment with hCG resulted in more (p less than 0.01) cows with SHORT cycles (7 of 9 vs. 4 of 9). Serum P4 concentrations were similar (p greater than 0.20) between groups from 4 days before until 6 days after GnRH injection. Cows with NORM cycles (n = 7) had greater serum P4 concentrations (p less than 0.05) on Days 7 to 11 after GnRH than cows with SHORT cycles (n = 11). By Day 39 PP, all cows with SHORT cycles appeared to have undergone a second ovulation. Charcoal-stripped serum pools from before (PRE) and during hCG injection (INJ) were assayed for total luteinizing hormone-like bioactivity (LH-BA) using a dispersed mouse-Leydig cell bioassay.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Changes in metabolic and reproductive characteristics associated with lactation and glucose infusion in the postpartum ewe

This study examined mechanisms whereby the metabolic environment interacts with basic reproductive function. Ewes lambing during the breeding season were fed to maintain (MAINT, n = 10) or gain (GAIN, n = 11) body weight during the last 4 mo of gestation. From d 7 to 22 postpartum, ewes were infused iv with saline (n = 10) or glucose at a rate calculated to increase normal glucose entry rate by 75% (n = 11). Blood samples were collected daily to determine plasma concentrations of nutritive metabolites and insulin and at frequent intervals on d 14 and 21 to determine serum gonadotropin concentrations. Hypothalami and pituitaries were collected on d 22 to determine hormone content and receptor concentrations. Plasma concentrations of nutritive metabolites and insulin indicated that MAINT ewes mobilized more (P less than .01) body fat and protein reserves during gestation and early lactation than ewes in the GAIN group. Glucose infusion elevated plasma concentrations of glucose (P less than .05) and insulin (P less than .07) and reduced (P less than .05) fat and protein mobilization, even though it depressed feed intake (P less than .001), compared with saline infusion. Hypothalamic gonadotropin-releasing hormone (GnRH), pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH) content and pituitary GnRH receptor concentration were similar between treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of lasalocid on growth and puberal development in Brahman bulls

Twenty-four half-sib Brahman bulls (171 to 276 d of age; 140.9 to 243.2 kg) were paired by age and weight and randomly allotted to one of two treatment groups (12 bulls/treatment) with two replicates (six bulls/replicate). Control treatment (C) was a 4:1 corn (IFN 4-02-931):cotton-seed meal (IFN 3-01-621) diet fed at 1% of the average body weight plus Coastal bermudagrass hay (IFN 1-00-716) ad libitum. Lasalocid treatment (L) was the control diet with the addition of 200 mg lasalocid X head-1 X d-1. Lasalocid resulted in lower acetate and butyrate with higher propionate and no change in total ruminal volatile fatty acids. Lasalocid-fed bulls tended to gain more weight (L = 153.6 +/- 8.5 kg, C = 138.4 +/- 7.9 kg; P less than .08), tended to have a greater average daily gain (ADG; L = .78 +/- .05 kg/d, C = .71 +/- .04 kg/d; P less than .09) and had greater feed conversion (11.5% less feed/kg gain) than did the control bulls. Lasalocid did not affect bull height (HT), weight:height ratio, length (LE) or heart girth (HG) at any time tested during puberal development. Lasalocid bulls exhibited a greater increase in HG through 175 d on treatment (L = 27.6 +/- 1.0 cm, C = 24.3 +/- 1.8 cm; P less than .05) and through first detected sperm (FS) in an ejaculate (L = 30.1 +/- 2.2 cm, C = 26.1 +/- 1.4 cm; P less than .05), but the increase in HG from the start of the trial through puberty was not different between treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

The postpartum induced corpus luteum: functional differences from that of cycling cows and the effects of progesterone pretreatment

Anestrous postpartum (PP) Hereford cows (n = 41) were used to compare corpora lutea (CL) from gonadotropin-releasing hormone (GnRH)-induced ovulation with CL from cycling cows. Postpartum cows were injected i.m. daily with 100 mg progesterone (P4) or oil on Days 25 through 28 PP and then given 200 micrograms GnRH i.m. on Day 30 PP. Corpora lutea were removed from one-half of the PP cows in the oil- and P4-treated groups 6.5 days after GnRH injection, and from the cycling cows 7 days after estrus. Intact PP cows were used to evaluate cycle length. Blood was collected daily from all PP cows from Day 25 PP through luteectomy and on Days 9, 11, and 13 post-GnRH from the oil- and P4-intact cows to determine short (SHORT) versus normal (NORM) luteal phases. Cycling cows were bled daily from estrus until CL removal NORM PP cows had higher (P less than 0.001) P4 levels than did SHORT PP cows from Day 7 through Day 13 post-GnRH, and more (P less than 0.05) P4-intact cows were NORM compared with oil-intact cows (45.5% vs. 14.3%, respectively). Corpora lutea from cycling cows were heavier (P less than 0.05) and had a higher luteinizing hormone (LH) receptor concentration (P less than 0.05), but CL P4 concentration did not differ from PP cows. Corpora lutea weight, LH receptor and P4 concentration, and in vitro P4 production were similar in the oil-and P4-treated PP cows. NORM cows had heavier CL (P less than 0.05) than SHORT cows, although P4 content and LH receptor concentration did not differ.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Postpartum nutrient intake and body condition: effect on pituitary function and onset of estrus in beef cattle

From calving through first estrus, 30 Brangus females were assigned equally to one of three diets to study the effect of postpartum nutrient intake and body condition on the ability of the pituitary to release luteinizing hormone (LH) and on the postpartum interval to estrus (PPI). The postpartum diets were calculated to achieve a 1) low [90% of the National Research Council (NRC) recommendations], 2) maintenance (100% of the NRC recommendations), or 3) high (110% of the NRC recommendations) level of nutrient intake. The females were group-fed within a treatment and calves were allowed to suckle ad libitum. Cow weight, body condition score and calf weight were recorded 24 h aftercalving, d 20 postpartum and at first behavioral estrus. On d 21 postpartum, blood samples were collected via jugular cannulae at 15-min intervals for 4 h, followed by a 100-micrograms im injection of gonadotropin-releasing hormone (GnRH) and continued sampling at 15-min intervals for an additional 6 h to determine serum LH. Although there was a significant decrease in PPI with increasing levels of nutrient intake (low = 57.5 +/- 8.8 d; maintenance = 40.3 +/- 6.6 d; high = 34.7 +/- 5.1 d), there were no differences in any of the observed LH characteristics (or variables) due to treatment. There were, however, marked differences in both the PPI and LH characteristics when data were analyzed on the basis of ability to maintain body condition from calving through 20 d postpartum, regardless of calculated dietary treatments. Cows that maintained body condition (MBC) had a shorter PPI [MBC, 31.7 +/- 2.8 vs lost (LBC) 60.0 +/- 7.5 d; P less than .01], higher basal levels of endogenous LH (MBC, .83 +/- .09 vs LBC, .61 +/- .04 ng/ml; P less than .025), higher GnRH-induced peak LH concentration (MBC, 58.99 +/- 11.15 vs LBC, 38.86 +/- 8.37 ng/ml. P less than .10), higher LH levels throughout the GnRH-induced LH surge (P less than .001), and greater release curve areas for the endogenous (MBC, 124.6 +/- 13.3 vs LBC, 91.7 +/- 5.6 units; P less than .025), GnRH-induced (MBC, 4370.8 +/- 699.5 vs LBC, 3039.7 +/- 683.3 units; P less than .10) and total (MBC, 4510.7 +/- 706.7 vs LBC, 3141.9 +/- 684.7 units; P less than .10) LH release.(ABSTRACT TRUNCATED AT 400 WORDS)

Factors affecting and prediction of dystocia in Charolais heifers

Calving difficulty at first and second parturition, using data recorded on 476 purebred Charolais heifers from a ranch in Southeastern Arizona, were analyzed by analysis of variance, step-wise multiple regression and discriminant analysis. Dystocia score (1 = no assistance through 4 = extreme difficulty) was the dependent variable with dam and calf effects independent variables. In this herd, 31.1 and 15.0% of the heifers experienced calving difficulty at first and second parturition, respectively. Birth weight of calf was the most important factor influencing dystocia, accounting for 71 and 61% of the total variation explained by the analysis of variance model when calf effects as well as dam effects were included at first and second parturition. Mean birth weight was 39.0 kg for primiparous heifers and 44.3 kg for second-half cows. A significant increase in dystocia (at both first and second parturition) occurred among male calves with birth weights of 45.5 kg or greater. At first parturition, no significant increase in dystocia occurred among female calves until birth weight exceeded 50 kg. Birth weight was not a significant factor influencing dystocia for female calves at second parturition. Yearling weight of cow and dystocia score of cow's dam were the only significant dam variables, and only at first parturition. Pelvic height measurements did not significantly affect dystocia scores at either parturition. A reasonably accurate prediction of dystocia based on the variables included in this study would be impossible, even if some means were devised to reliably predict birth weight of the calf.

Effect of monensin on the estrogen-induced LH surge in prepuberal heifers

The effect of dietary monensin on the luteinizing hormone (LH) surge following estradiol-17 beta (E2) injection was investigated in prepuberal Simmental X Brahman-Hereford heifers. Ten heifers, weighing approximately 260 kg and approximately 10 mo of age, were equally divided by age and weight into two groups: control (C) heifers each received 1.8 kg/d of a concentrate diet plus Coastal bermudagrass hay ad libitum; monensin (M) heifers each received the same diet plus 200 mg monensin/d. All heifers were maintained in dry lots on their respective diets for 14 d before the E2 challenge. On d 15, all heifers were injected in with 5 mg of E2 in corn oil. Blood samples were collected via tail vessel puncture immediately before E2 injection and at 2-h intervals until 48 h after the E2 injection. The samples were processed for serum and stored at -20 degrees C until LH concentrations were measured by radioimmunoassay. Mean concentrations of LH in serum differed (P less than .005) between C and M heifers and with time after E2 injection. A treatment X sampling period interaction (P less than .10) indicated that maximum serum concentrations of LH (LH surge) were detected earlier (P less than .001) for M (17.2 +/- 1.8 h) than C (27.0 +/- 6.0 h) heifers after the E2 injection. When the data were arrayed relative to the time of the LH surge, treatment (P less than .05) and sampling period (P less than .001) effects were significant, but a treatment X sampling period interaction was not detected. Peak LH concentration was 23.1 +/- 3.0 ng/ml for M heifers and 21.6+/- 4.2 ng/ml for controls (P greater than .10). Duration of the LH surge was 8.0 +/- .9 h in M heifers and 4.8 +/- 1.6 h in C heifers (P less than .001). Area under the LH surge was greater (P less than .001) in M heifers than in control heifers. We conclude that dietary monensin altered the estrogen-induced LH surge in prepuberal heifers.