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)

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)

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.

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)

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.

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.

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.

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