J.A. Vizcarra

Nutritionally Induced Anovulation in Beef Heifers: Ovarian and Endocrine Function During Realimentation and Resumption of Ovulation

Abstract Nutritionally induced anovulatory and cyclic Angus × Hereford heifers were used to evaluate follicular growth and concentrations of hormones and metabolites during anovulation and resumption of ovulation. Anovulatory heifers were fed to gain 0.6 (LGAIN) or 1.5 (HGAIN) kg/day until resumption of ovulation, and heifers with normal estrous cycles were fed a maintenance diet (M). Follicles ≥ 4 mm in diameter were measured by daily ultrasonography in HGAIN and LGAIN heifers during one follicular wave before realimentation (Wan) and in two waves (W-2, W-1) immediately before the wave resulting in first ovulation or luteinization (W0). Ovaries of M heifers were evaluated to determine the day of ovulation of the second-wave dominant follicle (DF). Resumption of ovulation after realimentation occurred 23 days earlier in HGAIN than in LGAIN. Maximum diameter, growth rate, and persistence of dominant follicles increased, while persistence of first subordinate follicles decreased between anovulation and resumption of ovulation in anovulatory heifers. Concentrations of LH in serum were similar for HGAIN and LGAIN and gradually increased during realimentation. The increase in estradiol before the first ovulation was less in realimented heifers compared with cyclic heifers. Concentrations of insulin-like growth factor-I (IGF-I) in HGAIN and LGAIN gradually increased during realimentation but were lower than concentrations of IGF-I in cyclic heifers at ovulation. Increased diameter, growth rate, and persistence of the DF were associated with increased concentrations of LH, estradiol, and IGF-I during the transition from nutritionally induced anovulation to resumption of ovulatory cycles.

Reproduction in Male Birds

Reproduction is a process that can be organized into distinct developmental and functional phases. In the case of the male, these include fertilization, formation of a patent reproductive tract, production of sperm, manifestation of male-specific behavioral patterns, and expulsion of sperm from the body. This perspective can provide insight that may be missed if reproduction is viewed primarily as an isolated act. For example, although the reproductive tract is fully functional only in the adult, it is formed, for the most part, prior to hatching. Furthermore, although spermatogenesis is associated with puberty, spermatogenesis is not constrained by chronological age but rather by the extent to which testicular cells proliferate and differentiate, which in turn is coupled to the developmental limitations of gonadotropin secretion. Finally, androgens essential to the function of the reproductive tract, the appearance of secondary sexual attributes, and male behavior may have detrimental effects upon the development of immune and connective tissue if the hormonal signal appears during the period of rapid prepubertal growth and differentiation. Thus, this chapter will discuss the process of reproductive system development and function in the male bird.

Molecular cloning and functional characterization of a vasotocin receptor subtype expressed in the pituitary gland of the domestic chicken (Gallus domesticus): avian homolog of the mammalian V1b-vasopressin receptor.

The neurohypophysial hormone arginine vasotocin (AVT) stimulates adrenocorticotropin hormone (ACTH) secretion from the avian anterior pituitary gland resulting in increased adrenal secretion of corticosterone in response to stress. Here, we report molecular cloning and functional characterization of a gene encoding an AVT receptor subtype, designated the VT2 receptor, that may mediate the stimulatory effect of AVT on ACTH secretion in birds. The open reading frame predicts a 425 amino acid polypeptide that includes seven segments of 19 to 24 hydrophobic amino acids, typical of guanine nucleotide-protein coupled receptors. Phylogenetic analysis revealed that the VT2 receptor shares highest identity with the mammalian V1b-vasopressin receptor subtype. Expressed VT2 receptors in COS7 cells mediate AVT-induced phosphatidylinositol turnover and Ca(2+) mobilization. In the domestic chicken, expression of VT2 receptor gene transcripts is limited to the pituitary gland. Based on similarities in sequence, site of expression and coupled signal transduction pathways, we conclude that the VT2 receptor is the avian homolog of the mammalian V1b-vasopressin receptor, and therefore may play an important role in the avian stress response.

Effects of Dietary Neutral Detergent Fiber on Intakes of Dry Matter and Net Energy by Dairy and Beef Cattle: Analysis of Published Data

Meta-analysis was applied to evaluate the relationships between dietary NDF and voluntary intakes of DM and net energy by cattle from 2 databases. The databases were configured from literature publications for dairy (18 experiments) and beef cattle (11 experiments) in which dietary NDF concentration varied as a result of changes in proportions of forage and concentrate, and NDF was not contributed from by-product feeds. In dairy cattle, the relationship with dietary NDF (% of DM) was studied for DMI (% of animal BW and kg/d per animal), NEl intake above maintenance (kcal/kg of BW0.75), NEl intake above maintenance relative to DMI (Mcal/kg of DMI), and milk fat content (%), whereas for beef cattle, relationships were evaluated with DMI (% of animal BW and kg/d per animal), NEg intake (kcal/kg of BW0.75), and NEg intake relative to DMI (Mcal/kg of DMI). Variables were weighted for different pooled SE of treatment DMI (or milk fat) means across trials. Initial models were fit to account for random slope and intercept terms to yield trial-adjusted data, and trial-adjusted dependent variables were regressed on dietary NDF. In dairy cattle, for dietary NDF concentrations ranging from 22.5 to 45.8%, DMI (% of BW and kg DM/d per animal) decreased (r2 = 0.595 and 0.672, respectively) with increasing NDF. The NEl intake above maintenance decreased (r2 = 0.815) more markedly than DMI; however, milk fat increased linearly (r2 = 0.617) with increasing NDF. In beef cattle, dietary NDF (range = 7.5 to 35.3%) was positively related to DMI (r2 = 0.954 for % of BW and 0.965 for kg DM/d per animal) and NEg intake (kcal/kg of BW0.75, r2 = 0.859), but differences in NDF did not account for variation in NEg intake per unit of DMI (r2 = 0.001). Total NDF concentration of diets should be an effective basis of exchanging fiber sources in feedlot diets to achieve equal DMI, and small increases in dietary NDF in feedlot diets might increase NEg intake without major changes in feed efficiency. Conversely, with dairy cattle, total DM and NEL intakes decreased with increasing dietary NDF. Increased NDF resulted in greater percentages of milk fat, which would need to be balanced with potentially lower milk production associated with decreased NEl intake.

Episodic Gonadotropin Secretion in the Mature Fowl: Serial Blood Sampling from Unrestrained Male Broiler Breeders (Gallus domesticus)

Abstract Forty-week-old male broiler breeders were used in two experiments. Males were reared as recommended by the breeder, housed in individual cages, and cannulated to facilitate blood sampling. In experiment 1, blood samples were collected at 10- min intervals for 4 h commencing the day of cannulation (Day 0) and for 12 h on each of Days 1 and 2. In experiment 2, blood samples were collected at 10-min intervals for 8 h on Day 1. After centrifugation, plasma was stored at −20°C until LH, FSH (experiment 1 and 2), testosterone, and corticosterone (experiment 1) concentrations were determined by RIA. Different statistical methods used to identify hormone secretion profiles revealed a characteristic pulsatile pattern of LH and FSH in plasma. However, LH pulses were more frequent and had greater amplitude than FSH pulses. Less than 32% of the FSH pulses were associated with LH episodes. Conversely, the association between LH and testosterone pulses averaged 83% in birds with testis weight greater than 10 g. Concentrations of corticosterone tended to increase after cannulation and remained elevated for only 3–4 h. Our data indicate that LH, FSH, and testosterone secretion is pulsatile in male broiler breeders. Additionally, LH pulses are associated with testosterone episodes but not with FSH pulses. The pulsatile pattern of FSH secretion, which is unique from those of LH, in adult males suggests that FSH secretion is independently regulated in the adult male fowl.

Testis development and gonadotropin secretion in broiler breeder males

One-day-old chicks were used to evaluate testis development and concentrations of luteinizing hormone, follicle-stimulating hormone (FSH), and testosterone during ontogenesis. Males on a conventional breeder program (control) were reared on a 15L:9D photoperiod and ad libitum food and water intake for 2 wk. On the third week, males were placed on a restricted diet and the photoperiod was reduced to 8L:16D. Males on a pedigree breeder program (broilerized) were reared on a 23L:1D photoperiod and unrestricted food and water intake for 6 wk. At 7 wk, males were placed on a restricted diet and the photoperiod was reduced to 8L:16D. On wk 18, both treatment groups were photostimulated (16L:8D) until the end of the experiment (50 wk). After photostimulation, there was an exponential increase in testis weight (TW), FSH, and testosterone concentrations. At 28 wk, TW from broilerized males were significantly heavier than those from control birds, and concentrations of luteinizing hormone, FSH, and testosterone were maximal at that time. After 28 wk, there was a significant decrease in FSH and testosterone concentrations that were associated with reduced TW. No correlation was observed between BW and TW. However, TW was highly correlated with FSH concentrations and daily sperm production. Our data suggest that management and photoperiod had a profound effect on testicular function that was associated with FSH concentrations in male broiler breeders.

The Effect of Feed Restriction on Plasma Ghrelin, Growth Hormone, Insulin, and Glucose Tolerance in Pigs

Twelve crossbred barrows (82.1 ± 1.1 kg BW) were assigned randomly to 2 diets to characterize ghrelin concentration and the metabolic profile of pigs undergoing dietary restriction. Pigs in one treatment group were allowed unrestricted access to feed (FF), whereas pigs in the other treatment group (RES) were fed 50% of the quantity of feed per unit of BW given to FF pigs. Animals in both treatments were fed daily (1 meal) beginning at 0900 h. Plasma samples were obtained twice weekly for 7 consecutive weeks to evaluate concentrations of ghrelin, growth hormone (GH), glucose, and insulin. Within each week, blood samples were taken before and 1.5 h after feeding. Three barrows in each dietary treatment were selected randomly for a glucose tolerance test before and after treatments were applied. The RES treatment decreased ADG and was associated with increased GH and decreased insulin concentrations. Glucose and insulin concentrations were decreased during fasting compared with concentrations 1.5 h after feeding. Results of the glucose tolerance test demonstrated increased glucose clearance rate, decreased glucose half-life, and decreased area under the curve in RES vs. FF pigs, reflecting the inability of FF animals to dispose of a test dose of glucose with the same efficiency as RES pigs. Restricting feed intake significantly altered glucose clearance rate, increased circulating concentrations of GH, and decreased insulin concentrations. Although restriction did not significantly affect plasma ghrelin, preprandial concentrations of ghrelin were increased compared with concentrations 1.5 h after feeding.

Changes in Plasma Concentrations of Luteinizing Hormone, Progesterone, and Estradiol-17β in Peripubertal Turkey Hens under Constant or Diurnal Lighting

Abstract Possible circadian fluctuations and long-term changes in concentrations of reproductive hormones in peripubertal female birds is poorly documented in comparison with mammalian species. Our objective was to document changes in concentrations of several reproductive hormones the several days before and after initial pubertal preovulatory surges of LH in turkey hens photostimulated with either constant (24L:0D) or diurnal (14L:10D) lighting. The hens were cannulated for hourly blood sampling, starting 10 days after photostimulation and continuing until all hens had laid at least two eggs. First eggs were oviposited between 16 and 24 days after photostimulation, and egg production ranged from two to nine eggs/hen during the experimental period. With both lighting treatments, concentrations of LH declined slightly, concentrations of progesterone (P4) increased, and concentrations of estradiol-17β (E2) were constant the 3–4 days prior to initial LH surges with no circadian fluctuations in hormone concentrations. Most (10 of 13) initial preovulatory surges of LH were coupled with ovulations, and all LH surges were coupled with P4 surges. Those LH and P4 surges not coupled with ovulations (blind surges) occurred with both lighting treatments, but the incidence of blind surges was higher with diurnal lighting. The interval between LH and P4 surges was longer between the first and second surges than between subsequent surges, when the interval was approximately 26 h. The duration of LH surges (7.4 ± 3.0 h) was shorter than that of P4 surges (10.0 ± 2.0 h). We conclude that, in the peripubertal female turkey, 1) prior to puberty (first LH-P4 surges), there are no circadian fluctuations in concentrations of LH, P4, and E2, 2) 3 days prior to initial LH surges, E2 concentrations are stable, LH concentrations decline slightly, and P4 concentrations increase, and 3) surges of LH are coupled to surges of P4 but LH-P4 surges are not always coupled to ovipositions (blind surges), possibly because of internal ovulations.

Influence of dose, frequency, and duration of infused gonadotropin-releasing hormone on secretion of luteinizing hormone and follicle-stimulating hormone in nutritionally anestrous beef cows ☆

Nutritionally induced anovulatory cows were ovariectomized and used to determine the relationships between dose, frequency, and duration of exogenous gonadotropin-releasing hormone (GnRH) pulses and amplitude, frequency, and concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in serum. In Experiment 1, cows were given pulses of saline (control) or 2 micrograms of GnRH infused i.v. during a 0.1-, 1.25-, 5-, 10-, or 20-min period. Concentrations of LH and FSH during 35 min after GnRH infusion were greater than in control cows (P < 0.01), and FSH concentrations were greater when GnRH infusions were for 10 min or less compared with 20 min. In Experiment 2, the effect of GnRH pulse frequency and dose on LH and FSH concentrations, pulse frequency, and pulse amplitude were determined. Exogenous GnRH (0, 2, or 4 micrograms) was infused in 5 min at frequencies of once every hour or once every 4th hr for 3 d. There was a dose of GnRH x frequency x day effect on LH and FSH concentrations (P < 0.01), indicating that gonadotropes are sensitive to changes in pulse frequency, dose, and time of exposure to GnRH. There were more LH pulses when GnRH was infused every hour, compared with an infusion every 4th hr (P < 0.04). Amplitudes of LH pulses were greater with increased GnRH dose (P < 0.05), and there was a frequency x dose x day effect on FSH pulse amplitude (P < 0.0006). We conclude that LH and FSH secretion in the bovine is differentially regulated by frequency and dose of GnRH infusions.

Lamprey GnRH‐III Releases Luteinizing Hormone but not Follicle Stimulating Hormone in Pigs

The effect of exogenous administration of lamprey GnRH-III (IGnRH-III) on gonadotropin secretion was evaluated in pigs. Six crossbred barrows (82.4 ± 3.5 kg body weight) were assigned randomly to a replicated 3 × 3 Latin square design to evaluate the effect of 0.1, 1.0 or 10.0 μg/kg body weight of exogenous IGnRH-III on LH and FSH secretion. To facilitate blood collection and infusion of IGnRH-III, barrows were catheterized in the jugular vein 1 day before initiation of experiments. Blood samples were taken at 10-min intervals for 6 h, starting 2 h before treatments were applied. Relative concentrations of LH and FSH were calculated by obtaining the ratio of the average concentration of each hormone 2 h after infusion divided by the average concentration during the 2 h before infusion. Relative concentrations of FSH after IGnRH-III infusion did not influence mean concentration of FSH at any of the doses; yet 10.0 μg/kg body weight had a significant effect on LH secretion (p < 0.01). Relative concentrations of LH averaged 1.2, 1.0 and 3.0 ng/ml (for doses of 0.1, 1.0 and 10.0 μg/kg body weight of IGnRH-III respectively). Only a dose of 10 μg/kg body weight elicited a significant LH increase that was associated with exogenous IGnRH-III infusion. We conclude that IGnRH-III is a weak GnRH agonist and at high doses, IGnRH-III has the ability to release LH but not FSH in barrows.