Effects of sequential or group exposure to unrestrained estrual females on expression of sexual behavior in sexually experienced beef bulls.

We tested the hypothesis that expression of sexual behavior in bulls is affected by the manner in which they are exposed to unrestrained, sexually receptive females. Twelve Angus bulls were used in a crossover design involving two treatments, each tested four times for a total of eight tests for each bull. Sexual interactions were quantified for each of four, 30-min periods under the following treatments: 1) exposure to each of four estrual females in sequence (SEQ); or 2) exposure to four estrual females as a group (GRP). Bulls were blocked into three testing groups, the order of which was stratified across eight test days. The order in which bulls were tested on a particular day had no effect on bulls' expression of mount interactions, or flehmen responses, suggesting that each group of bulls had similar sexual motivation at the beginning of each test. However, the bull testing order x treatment x time interaction influenced mounting interval (P = 0.08), copulation frequency (P <0.05), and copulation success ratio (P <0.05). When bulls were in GRP and tested first on test days, more (P <0.05) copulations were distributed to the first three females encountered compared with either the fourth female (P <0.05) or to each of the other females in SEQ (P <0.05). During later tests, other bulls in GRP were not able to copulate as frequently (P <0.05) with each female, displayed lower (P <0.05) copulation success ratios, and were allowed copulations by fewer (P <0.05) females during each 30-min test. When bulls were in SEQ, they displayed similar numbers of copulations regardless of the order in which they were tested, and had stable mounting intervals; however, copulation success ratio decreased (P <0.05) more rapidly during subsequent tests. Flehmen responses were initially displayed more frequently (P <0.05) when bulls were in GRP, but this effect diminished during subsequent 30-min tests. In conclusion, exposure of bulls to GRP induced greater sexual responsiveness than SEQ; however, this effect was due to enhanced sexual activity during the early stages of sexual encounters and with females that were not recently mated. Interestingly, bulls seem to repeatedly copulate with each individual female until, apparently, female sexual receptivity became attenuated. Thereafter, recently mated females allowed fewer episodes of repeated copulations, but they did not completely cease copulating with novel bulls.

Effects of novel females and stage of the estrous cycle on sexual behavior in mature beef bulls.

The objective of this experiment was to determine the effects of unrestrained females on sexual behavior of bulls. Twelve Angus bulls were used in three Latin square replicates where sexual interactions between one bull and one female were quantified for each of four 60-min tests (T1, T2, T3, and T4, respectively). All bulls received the following treatments: 1) exposure to four estrual females in sequence (A-B-C-D); 2) exposure to two estrual females in alternating sequence (E-F-E-F); 3) exposure repeatedly to one estrual female (G-G-G-G); and 4) exposure repeatedly to one diestrous female (CON). During T1, mount interactions, mounts with intromission and mounting intervals were similar when bulls were in A-B-C-D, E-F-E-F, or G-G-G-G. Fewer mount interactions, no mounts with intromission, and increased mounting intervals (P < 0.05) occurred in CON. During T2, there were more mount interactions, more mounts with intromission, and decreased mounting intervals (P < 0.05) when bulls were in A-B-C-D or E-F-E-F compared with when they were in G-G-G-G or CON. More mount interactions (P < 0.05) occurred in G-G-G-G compared with CON, but mounts with intromission and mounting intervals did not differ. During T3, more mount interactions (P < 0.05) occurred in G-G-G-G than in CON; otherwise, sexual behaviors were similar among treatments. Mounting intervals during T3 were similar among A-B-C-D, E-F-E-F, and G-G-G-G, but were all decreased (P < 0.05) compared with CON. During T4, more mount interactions, more mounts with intromission, and decreased mounting intervals (P < 0.05) occurred when bulls were in A-B-C-D compared with other treatments. Mount interactions were similar when bulls were in E-F-E-F, G-G-G-G, or CON; however, more (P < 0.05) mounts with intromission occurred when bulls were in E-F-E-F compared with G-G-G-G or CON. Mounting intervals during T4 were decreased (P < 0.05) in E-F-E-F compared with the CON treatment, whereas in G-G-G-G, they were intermediate. Mounts without intromission were not affected by female novelty or receptivity, but novel females induced more flehmen responses. In conclusion, novel, females, overall, enhanced sexual activity of bulls; however, bull sexual responses diminished after 2 h, even when a novel female was presented. Estrual females that were repeatedly paired with bulls displayed diminished sexual receptivity, but if mated females were rested for 60 min, they allowed further copulation from familiar bulls that were not sexually sated.

Postmodernism for animal scientists.

Many scientists regard the term "postmodernism" as controversial. Because postmodern theorists question whether science can be objective, some scientists view postmodernism as anti-scientific. In this paper, we argue that traditional accounts of science developed during the modern era (16th, 17th, and 18th centuries) are still influential in animal science, but are no longer plausible. In particular, the view that science automatically leads to human betterment seems to be disingenuous. A postmodern view that portrays science as a political activity seems more plausible, and offers a means to better understand contentious policy issues that involve science. Although most animal scientists accept the view that theory selection, experimental designs, and technology development require value-laden judgments, most fail to recognize that such values may be politically motivated and embrace prevailing political structures. Postmodernists such as Michel Foucault argue that through the generation of knowledge, scientific disciplines create a discourse that serves to maintain a particular social structure that has political implications. Viewed in this way, it becomes clear how various interest groups can be critical of certain scientific programs. For example, groups that oppose research dealing with cloning, genetically modified organisms, and intensive livestock production may not be as much opposed to science as they are to the political interests served by this science. In other words, such groups view these research agendas as promoting policies that place them at risk. Such a postmodern account of science, may help animal scientists better understand the nature of contentious issues, and provide a basis for reforming the animal science discipline in ways that make it more responsive to the diverse interests of a pluralistic society.

Critical perspectives of animal agriculture: introduction.

The American Society of Animal Science has recently focused its attention on a variety of contentious issues in animal agriculture. This paper deals with critique, a philosophical approach to analyzing and understanding issues. This method has been employed by various contemporary philosophers. For example, feminist theorists have used this approach to critically analyze sexual harassment. Critique involves a critical analysis of the discourse (ideas or language) and practices that define the social reality in which we live. How we think about the world and how we behave in it determines how we humans interact with each other as well as with the rest of nature. This social structure is associated with power structures that benefit some individuals and harm others. In this paper, I demonstrate how critique can be used to better understand the social reality of animal agriculture. By analyzing certain popular texts in this field, I show that a "mechanical view of nature" is dominant in animal agriculture and argue that such a view contributes to a social reality that can be harmful to some humans and other animals. I conclude that various contentious issues can be better addressed when we engage in a critical analysis of this conceptual framework and base our analysis on the experiences of many different people, including those who have been harmed by our current system of animal agriculture.

Castration increases pulsatile luteinizing hormone release, but fails to diminish mounting behavior in sexually experienced bulls.

We tested the hypothesis that mounting and chemoinvestigatory behaviors are testosterone-dependent in bulls. Eighteen bulls were divided into three treatment groups: intact (I), castrated (C) and castrated+testosterone (T). Sexual behaviors of all bulls were tested with an unrestrained receptive female 1 week prior to and weekly for 4 weeks after castration. Mounts with intromissions, aborted mounts and flehmen responses were quantified for each test period. In addition, patterns of LH and testosterone secretion were assessed at these times. Neither mounts with intromissions nor aborted mounts were affected by treatment. In contrast, numbers of flehmen responses were lower in C bulls than in the other groups following castration. Before castration, concentrations of LH were not different among groups and LH pulse frequency was approximately one pulse per hour. Castration resulted in a 2-fold increase in mean concentrations of LH and a 6-fold increase in LH pulse frequency. Neither mean concentration of LH nor LH pulse frequency changed in I or T bulls. The data fail to support the hypothesis that mounting behavior is T-dependent, but supports the hypothesis that this steroid hormone regulates flehmen behavior in sexually experienced bulls.

Melengestrol acetate blocks the preovulatory surge of luteinizing hormone, the expression of behavioral estrus, and ovulation in beef heifers.

We tested the hypothesis that melengestrol acetate (MGA), an orally active progestin, blocks estrus and the preovulatory surge of luteinizing hormone (LH) in beef heifers. Cycling yearling Angus heifers were divided randomly into two groups: MGA-treated (n = 6) and control (n = 5). All heifers received injections of prostaglandin F2alpha (PGF) on d -25, -11, and 0 to synchronize estrus. Following the last PGF injection on d 0, heifers were fed either 0.5 mg MGA in a carrier or the MGA carrier each day for 8 d. At 4-h intervals on d 1 through 6, all heifers were observed for expression of estrous behavior, and blood samples were collected and assayed for LH. Daily blood samples were collected at 0800 on d 1 through 10 and assayed for circulating progesterone concentrations. All control heifers exhibited estrus and a preovulatory surge of LH. In each case, this was followed by increases in circulating concentrations of progesterone indicative of ovulation and normal luteal function. In contrast, none of the MGA-treated heifers exhibited estrus, LH surges, or evidence of ovulation. The results of this experiment show that MGA prevents ovulation in cattle by inhibiting the preovulatory surge of LH.

Effects of melengestrol acetate on reproductive behavior and concentrations of LH and testosterone in bulls.

We investigated the use of an orally active progestin (melengestrol acetate; MGA) to suppress reproductive activity in yearling beef bulls. Twenty-four crossbred bull calves were given a daily dose of 0, 0.5, 1.0, or 2.0 mg MGA for 99 d. Pulsatile patterns of LH and concentrations of testosterone and MGA were characterized on d 8, 36, 63, and 92 of the experiment. Numbers of aborted mounts, mounts with intromission, total mounts, and flehmen responses were assessed on d 15, 43, 71, and 99. Plasma concentrations of MGA were proportional to dose of MGA. Melengestrol acetate did not consistently affect mounting behavior in a dose-related manner, but, on d 99, number of total mounts for MGA-treated bulls was lower (P = 0.07) than that for control bulls. On d 15, MGA suppressed (P = 0.07) numbers of flehmen responses in a dose-dependent manner, but this effect was not sustained throughout the experiment. On d 8, concentrations of testosterone in control bulls were higher (P = 0.02) than in MGA-treated bulls, but this effect was not observed at other time periods. Overall, MGA caused slight decreases in mean concentrations of LH (P = 0.09) and LH pulse frequency (P = 0.06). Scrotal circumference was not affected by MGA. None of the behavioral traits was correlated with mean concentrations of LH or LH pulse frequency. Mounting activity was not correlated with testosterone concentrations, but number of flehmen responses was positively correlated with testosterone concentrations (P = 0.01). These results fail to support the hypothesis that progestins impair male sexual behavior or fertility in males.

An appropriate role for ethics in teaching contemporary issues.

There seems to be general agreement among animal scientists that training in contemporary issues is an appropriate goal of animal science curricula. One approach is based on the view that public policy issues embody moral dilemmas that arise when the moral perspectives (values) of opposing groups are incompatible. Based on this view, an understanding of such issues requires some training in ethics. More specifically, knowledge of major moral theories and the nature of values is necessary to identify and analyze the moral components of issues. An appreciation for ethics also provides insight into addressing issues in a morally responsible manner. The plurality of values and the contextual nature of policy issues precludes solutions based on application of universal moral principles. Nevertheless, informed judgments, based on the collective wisdom of individuals, frequently offer solutions that address opposing values. Such judgments can be attained when those affected by issues form communities, attempt to achieve a collective understanding of the problem, and, if possible, construct a common set of values that facilitates consensus. Based on this analysis, issues courses should facilitate the development of learning communities wherein students and teachers critically analyze (i.e., deconstruct) and reconstruct issues in ways that enhance understanding and instill a respect for diverse moral perspectives. An expression of such understanding and respect is the ability to distinguish between situations when consensus offers fair solutions and when it subjects minority groups to the tyranny of the majority.

Toward a pluralistic animal science: postliberal feminist perspectives.

Women have not had the authority to define the conditions of animal agriculture or to develop strategies for addressing its problems because the field of animal science has been and remains dominated by men. Therefore, criteria for assessing the professional performance of animal scientists reflect the views of men and the experiences of masculinity. Recent attempts to improve the retention rates of women in the animal science profession have focused on socializing female protégés to the norms and expectations of the discipline. This approach is based on traditional liberal beliefs that all humans are essentially alike and that embodiment and(or) social location are irrelevant in understanding the world. Such socialization assumes that problems of gender inequity can be solved by providing opportunities for women to succeed in the existing system, with its existing norms. This approach ignores the possibility that the structure of women's lives and the normative commitments arising from women's experiences may be quite different from those of men. Professional reward structures that favor the life experiences and perspectives of men and ignore those of women coerce women into adopting values that may be incompatible with their identities. This not only devalues their perspectives and contributions, it forces them to commit self-betrayal. This is both harmful and antithetical to the idea of an empirically relevant science that blends multiple perspectives and understands diverse values. Substantive diversity in the animal science community requires standards that reflect and accommodate the pluralism in which we live.

Effects of melengestrol acetate on onset of puberty, follicular growth, and patterns of luteinizing hormone secretion in beef heifers.

Our objective was to test the hypothesis that short-term (8 days) treatment of prepubertal heifers with melengestrol acetate (MGA) and subsequent steroid withdrawal would stimulate LH secretion and follicular growth. Angus heifers were divided randomly into two groups; MGA-treated (n = 8) or control (CON; n = 9). Puberty was determined by monitoring circulating concentrations of progesterone and ovarian morphology during a 14-day period following MGA withdrawal. LH secretory patterns were assessed upon initiation of MGA (Day 0), during MGA (Day 7), and 1 day after withdrawal of MGA (Day 9). All MGA-treated heifers, versus four CON heifers, exhibited corpora lutea and luteal phase concentrations of progesterone within 10 days after treatment (p = 0.01). Mean LH and LH pulse frequency increased (p = 0.005 and 0.0001, respectively) between Days 0 and 9 in MGA-treated heifers. In CON heifers, mean LH concentrations and pulse frequencies did not change. During the same period, diameter of the largest follicle increased in MGA-treated animals (p = 0.003) but did not change in the CON heifers. On the basis of these results, we suggest that MGA withdrawal enhances onset of puberty by stimulating pulsatile LH secretion that accelerates follicle growth to the preovulatory stage.

Teaching animal science: education or indoctrination?

Traditional animal science curricula ignore sociological aspects of scientific research and therefore portray scientific knowledge as value-free. This view gives rise to a teaching method that involves imparting lists of scientific facts that are to be accepted by students without critical evaluation. This amounts to little more than indoctrination and misrepresents science as a system of knowledge. An alternative approach is based on the view that science is a creative human activity that reflects the values and biases of its practitioners. The goal of this approach is to teach students to think analytically and to make independent judgments about scientific claims. This requires a scientific literacy: an understanding of principal scientific theories, the nature of scientific research, and the relationship between science and society. To achieve this goal, a teacher must become less of an authority figure, whose role is to simply pass on information, and more of a facilitator, whose role is to promote questioning, exploration, and synthesis. This requires a learning community in which students feel comfortable taking risks and develop the courage to make and defend judgments. This teaching approach enhances the intellectual and ethical development of students, allowing them to serve themselves and society in responsible ways.

Estrus synchronization with an oral progestogen prior to superovulation of postpartum beef cows.

Ovarian follicular dynamics and steroid secretion patterns were monitored in postpartum beef cows that were synchronized for estrus with melengestrol acetate (MGA) or prostaglandin F(2alpha) (PGF) prior to superovulation. Twenty-four muhiparous Angus cows were stratified by number of days postpartum to an MGA or PGF treatment prior to superovulation. Cows in the MGA group were fed 0.5 mg MGA/d for 14 d in a grain carrier. Superstitnulatory treatments began 14 d after withdrawal of MGA from feed or 11 d after administering a single injection of 500 microg cloprostenol (PGF). Supersthnulatory treatments (FSH) were administered twice daily in decreasing doses (7.5, 5, 5, 2.5 mg) over 4 d. Sixty and 72 h after initiating the superstimulatory treatments, all cows were treated with 750 microg and 500 microg PGF, respectively Cows were inseminated at 0, 12, and 24 h from the onset of standing estrus with semen from 2 proven sires. Cows within treatment were inseminated with 1, 2 and 1 (single) or 2, 4 and 2 units (double) of semen at the designated insemination times. Blood sampling and transrectal ultrasonography of ovaries were performed daily beginning 2 d prior to the initiation of FSH treatment and were continued through embryo recovery. Ovaries were examined daily to determine the number and size of follicles. Plasma samples were analyzed for progesterone and estradiol. Follicles were counted and categorized based on a 5 to 9 mm range or >/= 10 mm. At the end of superovulatory treatment there were more (P /= 10 mm among cows that were estrus synchronized with MGA (75 +/- 1.2) than with PGF (3.9 +/- 1.2) These differences were reflected in higher (P

Change in morphology of corpora lutea, central luteal cavities and steroid secretion patterns of postpartum suckled beef cows after melengestrol acetate with or without prostaglandin F2alpha.

Change in morphology of the corpus luteum (CL) and patterns of progesterone and estradiol secretion after treatment with melengestrol acetate (MGA) were monitored in postpartum beef cows. Twenty Angus cows were randomly assigned to MGA or MGA + prostaglandin F(2alpha) (PGF) treatments. All cows were fed 0.5 mg of MGA per cow per day for 14 d. The MGA-treated cows (n = 10) were allowed to return to estrus spontaneously at the second estrus after withdrawal of MGA from the feed. The MGA + PGF-treated cows (n = 10) received an injection containing 25 mg of PGF(2alpha) 17 d after the last feeding of MGA. Cycle 1 was defined as the first luteal phase after MGA feeding and Cycle 2 represented the subsequent cycle or luteal phase after PGF. Blood sampling and transrectal ultrasonography of the ovaries was done daily through the completion of 2 estrous cycles upon removal of MGA from the feed. Blood samples were analyzed for plasma progesterone and estradiol concentrations. Area of CL and fluid-filled cavities within each CL were determined by ultrasonography. Concentrations of progesterone and area of CL were similar between cycles and treatments. Estradiol concentrations were higher (P < 0.05) in Cycle 2 than in Cycle 1. Fluid-filled cavities were larger (P < 0.001) in Cycle 1 than in Cycle 2 for both mid-luteal (Days 5 to 9) and late-luteal (Days 10 to 14) phases. Multiple CL (2 or more during 1 cycle) were observed in 5 cows. Progesterone concentrations and total area of luteal tissue did not change with respect to treatment or cycle, but CL morphology was altered in the first cycle after MGA treatment. Of the 19 cows that ovulated after withdrawal of MGA, 3 experienced a short luteal phase. These data characterize changes that occur among cows that are fed melengestrol acetate during the postpartum period and enhance observations from prior studies regarding MGA use.

Metabolic interfaces between growth and reproduction. V. Pulsatile luteinizing hormone secretion is dependent on glucose availability.

To test the hypothesis that mechanisms controlling the secretion of LH are modulated by glucose availability, the acute effects of glucoprivation were studied. The model was the gonadectomized male lamb raised on a limited diet of artificial milk. The approach was to monitor LH secretion before and after the administration of a competitive antagonist of glucose metabolism, 2-deoxyglucose (2DG). We first determined whether LH secretion was influenced by glucose availability by administering 2DG at several doses. Peripheral administration of the glucose antagonist (240 and 480 mg/kg 2DG, single iv injection) transiently decreased LH pulse frequency, but not LH pulse amplitude. By contrast, LH secretion (frequency or amplitude) was not affected by lower doses (60 or 120 mg/kg) of the glucose antagonist. A second study was conducted to determine whether either the pituitary gland or the GnRH neurosecretory system per se is directly affected by short term glucoprivation. The competency of the pituitary was assessed by administering GnRH during the time when LH secretion is suppressed by pharmacological glucose blockade. Similarly, the function of the GnRH neurosecretory system was assessed by administering a GnRH secretagogue (N-methyl-D,L-aspartate) under the same glucoprivic conditions. In response to an optimized iv dose of 2DG, LH pulse frequency decreased. However, in lambs that received either GnRH or N-methyl-D,L-aspartate during the period of glucoprivation, LH pulse frequency was sustained at levels comparable to those before 2DG was given. To determine whether the effect of glucoprivation was central in origin, the glucose antagonist was administered into the lateral cerebral ventricle at 1/100th the doses used peripherally. Central administration of 2DG, independent of dose, transiently decreased LH pulse frequency, but not pulse amplitude. However, unlike the case with peripheral injection, plasma glucose values did not change after the administration of any dose of 2DG tested centrally. These findings indicate that glucose availability in the developing sheep influences LH secretion. Moreover, based upon analysis of LH pulse frequency, glucoprivation does not directly impair either the pituitary gland or the GnRH neurosecretory system. Collectively, these results suggest that glucose availability affects LH secretion by acting within the central nervous system at a detection site(s) peripheral to the GnRH neuron.

Effects of cysteamine on pulsatile growth hormone release and plasma insulin concentrations in sheep.

The effects of cysteamine (CSH; 0, 50, or 100 mg/kg BW), a somatostatin depleting agent, on growth hormone (GH) and insulin (INS) secretion were studied in sheep (Ovis aries). Cysteamine was administered as a single intragastric bolus on day 0 (0900). Jugular blood samples were collected at 15-min (GH) and 2-hr (INS) intervals over an 8-hr period (1100-1900) on day 0, 3, and 7. Intragastric administration of CSH at 50 mg/kg BW augmented (quadratic, P = .04) mean plasma GH concentration, with the greatest response occurring on day 3. Baseline GH concentrations were elevated in wethers dosed with 50 mg/kg BW CSH on day 3, whereas wethers dosed with 100 mg/kg BW CSH had lower baseline GH concentrations on day 0 (CSH x day interaction, P = .02). Cysteamine administration increased GH pulse amplitude (quadratic, P = .15), with the greatest magnigtude of change occurring with 50 mg/kg BW CSH on day 0 and 3. Frequency of GH pulses was increased (quadratic, P = .10) following CSH treatment. Administration of 100 mg/kg BW CSH augmented plasma INS on day 0 (CSH x day interaction, P = .09). These findings indicate that CSH alters GH and INS secretion in a dose-dependent and temporal manner. The observed changes in mean and baseline plasma GH concentrations associated with 50 mg/kg BW CSH are consistent with somatostatin depletion; however, higher doses of CSH appear to disrupt GH secretion by an alternative mechanism.

Improved synchrony, conception rate, and fecundity in postpartum suckled beef cows fed melengestrol acetate prior to prostaglandin F2 alpha.

Occurrence of estrus, conception rate, and fecundity were compared between postpartum suckled beef cows fed or not fed melengestrol acetate (MGA) before synchronizing estrus with prostaglandin F2 alpha (PG). One hundred thirty-three purebred Angus cows were stratified by age and days postpartum to one of two treatments. All cows received an injection containing 25 mg of PG; however, one group (MGA+PG) was fed .5 mg of MGA.cow-1.d-1 for 14 d then received PG 17 d after the last feeding of MGA. The other group (PG) received only PG. Cows in both treatments that failed to exhibit estrus within 6 d after PG received a second injection of PG 11 d after the first injection was administered. Cows were checked for estrus from the first PG injection until 6 d after the second PG injection. Cows were inseminated with semen from a 3/4 Brahman bull 12 h after observed estrus. Cows were exposed to Angus bulls after the AI period. Conception date was determined by palpation per rectum and verified by calving date and calf phenotype. Differences in response variables were analyzed by chi-square. A greater proportion of cows (P < .04) that were pretreated with MGA exhibited estrus after the first injection of PG (76%) than of cows that received only PG (60%). Total response did not differ between treatments after both injections of PG. Conception rate among cows that received a first service was higher (P < .007) for the MGA+PG (88%) group than for the PG (67%) group, although the total number of cows that received a first service was not different between groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Cysteamine-induced depletion of somatostatin in sheep: time course of depletion and changes in plasma metabolites, insulin, and growth hormone.

Eight crossbred wethers (51 +/- 2 kg BW), surgically fitted with abomasal cannulas, were used to determine the extent and time course of cysteamine (CSH)-induced depletion of somatostatin (SRIF) in abomasal tissue and associated changes in plasma metabolites, insulin, and growth hormone (GH). Cysteamine was administered as a single i.v. bolus (50 mg.kg BW-1 x 10 min-1) on d 0. Abomasal biopsies were obtained on d -7, -3, 0, 1, 3, and 10. On d 0, additional biopsies were taken at 2, 4, and 8 h after CSH administration. Jugular blood samples were collected over 8 h at 15-min intervals on d -2, 0, and 1. Cysteamine administration decreased (P < .05) tissue SRIF on d 0 (2, 4, and 8 h), 1, and 3; maximal depletion (42 to 55% of Pre-treatment; Pre-trt) occurred during the initial 24 h, returning to Pre-trt by d 10. Gel chromatography of pooled -7 d abomasal tissue extracts showed five peaks of SRIF immunoreactivity; the predominate peak eluted in the same position as synthetic SRIF-14. Plasma glucose, lactate, and NEFA concentrations increased (P = .001) after CSH administration and reached peak at 2 h after treatment and declined to Pre-trt concentrations by 24 h. Insulin increased (P = .001) to a maximum at h 4 and returned to Pre-trt by 24 h. Mean and baseline GH were higher (P < .07) on day of CSH administration, and pulse amplitude was lower (P < .10) on d 0 and 1. These data show that CSH rapidly reduces SRIF in abomasal tissue in a reversible manner; suggesting that CSH-treated sheep may provide a SRIF-deficient model for studying the physiological role of SRIF in ruminants.

Effects of recombinant bovine somatotropin and dietary energy intake on growth, secretion of luteinizing hormone, follicular development, and onset of puberty in beef heifers.

The effects of dietary energy and recombinant bovine somatotropin (bST) on pattern of LH release, follicular development, and onset of puberty were studied in 40 Angus heifers. At 7 mo of age, heifers were assigned to a 2 x 2 factorial experiment; the main effects were dietary energy (high [HDE]: 14.15 Mcal of ME/d or moderate [MDE]: 10.84 Mcal of ME/d) and somatotropin (bST; 350 mg every 2 wk or vehicle). Beginning at 9 mo of age, heifers were observed twice daily for estrous activity. From 10.5 to 12 mo of age, five heifers from each treatment group were selected for weekly ultrasound examination of ovarian structures and biweekly sequential blood sampling to determine concentrations of LH. Somatotropin treatment altered intermediary metabolism in a manner consistent with enhanced accretion of lean tissue and decreased deposition of fat. The HDE heifers were younger (P < .001) at puberty than the MDE heifers, but BW at puberty was not different among treatment groups. Age and body weight at puberty were not affected by bST. Frequency of LH pulses increased within the 10.5 to 12 mo of age sampling window in HDE-treated heifers but not in MDE heifers (dietary energy x month of age; P < .02). Secretion of LH was unaffected by bST. Ovaries of bST-treated heifers tended (P < .09) to have fewer follicles > 5 mm in diameter than those of vehicle-treated heifers. We conclude that chronic treatment with bST did not alter age at puberty or pattern of LH release in heifers and that even modest differences in energy intake influence the timing of the prepubertal increase in pulsatile LH release.

Effects of acute, intracerebroventricular administration of insulin on serum concentrations of luteinizing hormone, insulin, and glucose in ovariectomized lambs during restricted and ad libitum feed intake.

To test the hypothesis that the increase in pulsatile LH secretion associated with increased feed intake in growth-restricted lambs is due to elevated insulin concentrations, we evaluated the effects of intracerebroventricular (ICV) administration of insulin on patterns of LH in ovariectomized ewe lambs. After weaning (10 wk of age), 12 lambs were fed to maintain a mean body weight of 18.3 kg. At approximately 32 wk of age a permanent cannula was inserted into the lateral ventricle of each lamb. For the first experiment, animals received 3 ICV injections of either 500 ng (n = 6) or 500 micrograms (n = 6) insulin. Blood samples were collected every 10 min for 8 h, with animals receiving injections at 2, 4, and 6 h. Patterns of LH during the 2-h preinjection period were compared to those in the subsequent 2-h periods following each insulin injection. Insulin did not affect mean LH, LH pulse frequency, or LH pulse amplitude. Only the 500-micrograms injections increased (p < 0.0001) peripheral insulin and decreased (p < 0.001) peripheral glucose. The experimental protocol was repeated during a second experiment conducted after 2 wk of ad libitum feeding, when animals weighed an average of 21.4 kg. Increased feed intake was associated with increases in mean LH, LH pulse frequency, and insulin (p < 0.05). Both doses of insulin decreased (p < 0.01) mean LH and LH pulse frequency. The 500-micrograms injections increased (p < 0.0001) peripheral insulin and decreased (p < 0.001) glucose.(ABSTRACT TRUNCATED AT 250 WORDS)