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Abstracts from VI International Symposium on Avian EndocrinologyMarch 31 - April 5, 1996 Chateau Lake Louise, Alberta Continued Seasonal Cycles of Sex Steroid Plasma Levels in Captive Great Bustard, Otis tardaP. Péczely, P. Mödlinger1 Department of Reproductive Biology, University of Agriculture,
Gödöllö 2103 Páter K.u.1., Hungary; 1Zoo of Budapest, Hungary
Monthly testosterone (T), 17-ß-oestradiol (E2), and progesterone (P4) plasma levels were determined in 5 male and 3 female Great Bustards (kept in a 70 x 50 m volier) by RIA and correlated with their behaviour, determined by video recording. The results of the RIA analyses were as follows: T levels were high in both sexes in March and April. T levels were three times higher in males than in females and showed a positive correlation with the courting display. E2 levels were high in females in the egg laying period. The peaks of E2/T ratio (the relative dominance of E2) coincided in both sexes with the beginning and the end of postnuptial moulting (long day photorefractoriness) and in December with a "sexual stillness" and winter flocking. Plasma levels of P4 were low in spring and summer and then continuously increased in autumn and winter. The increase of P4 in August (females), and September (males) may be connected with the end of postnuptial moulting, autumnal fattening and the flocking behaviour of Great Bustard. This autumnal - winter increase of P4 levels is possibly related with an enhanced adrenocortical function. Experiments to Develop a Captive Breeding System for Great Bustard, Otis tarda, on the Basis of Semen Collection and Artificial InseminationP. Mödlinger, P. Péczely1 Zoo of Budapest, Hungary; 1Department of Reproductive Biology,
University of Agriculture, Gödöllö 2103, Páter K.1., Hungary Great Bustard is one of the most endangered bird species in Europe. For the protection of this species different methods were developed in Hungary in the last two decades. In recent years we have utilized a new technology: the breeding system of captive birds based on the artificial insemination. A large volier system was constructed, and the husbandry and feeding technology of the captive Great Bustards was developed. In the first year of the project the annual life-cycle (courting display of males, nest producing, egg laying, egg sitting behaviour of females, and elements of summer, autumnal and winter behaviour) of 5 males and 3 females was studied using video records. In the first year, the annual cycle of sexual steroids was also studied on the basis of monthly blood samples. Body weight was also systematically measured. In the second year abdominal and back massage and the prepared skin of a female Bustard ("phantome") were used to stimulate semen collection from the males. Two of the five males were perfectly adapted to the manipulations, and we could collect several samples of semen. The semen samples were analyzed and diluted with physiological saline and two females were inseminated with 5 million spermatozoa with the help of an automatic pipette. The efficacy of the technique is not yet known, since the females did not lay eggs last year (1995). We hope, however, that the first chicks will be hatched next spring on our Great Bustard Farm. The Effects of Nerve Growth Factor and Anti-Nerve Growth Factor Antibody on the Neuroendocrine Reproductive System in the European Starling, Sturnus vulgarisG.E. Bentley, a.R. Goldsmith, T.S. Juss, a. Dawson1 School of Biological Sciences, University of Bristol, Woodland
Road, Bristol BS8 1UG; 1ITE, Monks Wood, Abbots Ripton,
Huntingdon, Cambs. PE17 2LS UK Thyroidectomy of starlings causes them to remain in breeding condition indefinitely; deactivation of GnRH neurons that is so characteristic of photorefractoriness does not occur. We hypothesise that a neurotrophin, whose presence or ability to function is dependent upon thyroid hormones, is somehow involved in this termination of GnRH release. Nerve growth factor (NGF) is one such candidate. We therefore infused 2 µg of mouse 7S-NGF (Product No. N-0513, Sigma) dissolved in 2 µl artificial CSF (aCSF) four times daily for 21 days into the lateral ventricle of thyroidectomized male starlings held on long days, to see if photorefractoriness would occur. The result was significant gonadal regression in the treatment group during the infusion period (P=0.0292), with no change in testicular volume in the control group. Testicular recrudescence did, however, occur after the end of the treatment period. Is such a result a non-specific effect, or is it progression towards photorefractoriness per se? To test this, we used castrated, photorefractory starlings held on long days. Anti-NGF antibody (Product No. N-5142, Sigma) was infused daily into the lateral ventricle at a concentration of 1 µg/2 µl aCSF for 61 days.There was a significant rise in circulating LH levels (P=0.0336) in the treatment group as compared to controls. However, the rise in LH was not as large as the "normal" short-day increase in LH in castrates, so it is not yet clear as to whether the result is indeed due to acquisition of photosensitivity. Seasonal Patterns of LH Secretion and Hypothalamic GNRH Levels in White-Winged Crossbills, Loxia leucopteraT.P. Hahn, P. Deviche, G.F. Ball Department of Psychology, Johns Hopkins University, Baltimore,
MD, USA; University of Alaska, Fairbanks, AK, USA Crossbills (Loxia sp.) are reproductive opportunists, breeding at almost any time of year if conifer seed supplies are sufficient. However, recent data on red crossbills (Loxia curvirostra) suggest that fall breeding is rare, possibly due to development of a refractory period. We studied seasonal changes in the reproductive system of whitewinged crossbills (Loxia leucoptera) near Fairbanks, AK. In both sexes, plasma luteinizing hormone (LH) levels were elevated MarchAugust, and low OctoberJanuary. Females lacked incubation patches OctoberJanuary; a few females had incubation patches by February, and all did so by July, when new white spruce cones became available for foraging. These data match those for several populations of red crossbills, where LH levels were low and gonads were regressed during autumn even when seed availability remained high. This is consistent with the crossbills being refractory to environmental stimulation (photoperiod and/or other cues) during autumn, as are many other temperate zone birds. In most seasonal species, refractoriness coincides with low hypothalamic content of gonadotropin releasing hormone (GnRH). We collected brains from captive crossbills held on a natural seasonal photocycle for the latitude of Fairbanks, AK, in spring, autumn, and winter when the birds were presumed to be photostimulated, photorefractory and photosensitive, respectively. Preliminary analysis of some of these brains stained by immunocytochemistry for GnRH (antibody detects both avian GnRHI and II, provided by H. Urbanski) shows no decline in GnRH staining during the putative refractory period in autumn, such as would be apparent in several other species. Thus, the autumnal hiatus in crossbill breeding and pituitary secretion of LH may be mediated through downregulation of secretion, with no change in brain content, of GnRH. Persistence of hypothalamic GnRH may reflect a difference between "relative refractoriness," where responsiveness to environmental stimuli is maintained, and "absolute refractoriness," where responsiveness to all manner of cues is lost. Refractoriness in crossbills may only be "relative," and not "absolute." Relationship between Body Condition and Adrenal Stress Response in Captive American Kestrel JuvenilesJ. Heath, a. Dufty Biology Department, Boise State University, Boise, ID 83725,
USA The primary hormone involved in avian stress response is corticosterone (B), an adrenal steroid hormone. Previous studies show that environmental stressors (e.g. handling) increase corticosterone over time. In this study adrenal response patterns have been determined in birds of different body condition. Juvenile, female American kestrels (Falco sparverius) were maintained for six weeks on one of three different diets, including a control diet (fed ad libitum) and two calorically-restricted diets. To invoke a stress response the birds were captured and held in a cloth sack. Blood samples were collected 1, 5, 10, 30 and 60 minutes after capture. All birds responded similarly to handling stress with an increase in plasma B, but control birds (good body condition) showed a more rapid increase to a maximum B level, followed by a decrease in B after 30 minutes. Food-restricted birds had a slower rate of increase to maximum B and then maintained high corticosterone levels. These results suggest that birds in good physical condition respond quicker to stressors and adapt physiologically to stressful situations more rapidly than birds in poor physical condition. This difference may be the result of birds in good condition having the ability to mobilize fat for energy, whereas birds in poor condition must mobilize protein (i.e. muscle). Ecological Factors Underlying the Adrenocortical Response to Capture Stress in Arctic Breeding ShorebirdsK.M. O'Reilly, J.C. Wingfield1 Colby College, Waterville, ME 04901, USA; 1University of
Washington, Seattle, WA 98195, USA Sandpipers breeding in the Arctic exhibited marked variation in the adrenocortical response to stress, as measured by the maximum corticosterone secreted in response to capture and handling. During breeding, this variation was correlated with two factors: degree of parental care and breeding environment. Parents offering a high degree of parental care had a reduced stress response relative to parents offering little or no parental care. This pattern was observed in polygynous, monogamous, and polyandrous sandpiper species (Scolopacidae). Breeding environment, when categorized as low Arctic or high Arctic, was also negatively correlated with the stress response in Semipalmated Sandpipers (Calidris pusilla), with high Arctic breeders suppressing the stress response relative to low Arctic breeders. Migrating Western Sandpipers (Calidris mauri) exhibited variation in the adrenocortical response to capture stress, with spring migrants showing a lower response than autumn migrants. The common thread present in each of these correlations is the antagonism between the selective pressure to breed, whether by travel to the breeding grounds or incubation of eggs once there, and the presence of potentially stressful conditions, such as low food availability or severe weather. Individuals responsible for parental care and individuals breeding or migrating north in severe conditions can least afford to exhibit a typical stress response because they may abandon their reproductive effort for the year. Suppression of the adrenocortical response to stress is an important endocrine phenomenon enabling sandpipers to breed in the Arctic. Plasma LH in Juvenile American Kestrels Exposed to Short DaysM.M. Muir, R.J. Etches1, D.M. Bird2 Department of Pharmacology and Toxicology, Medical College of
Wisconsin, 8701 Watertown Plank Rd., Milwaukee, WI 53211, USA;
1Department of Animal and Poultry Science, University of Guelph,
Guelph, ON N1G 2W1 Canada; 2Avian Science and Conservation
Centre, McGill University, 21,111 Lakeshore Rd., Ste. Anne de
Bellevue, PQ H9X 3V9 Canada Juvenile female and male American kestrels, Falco sparverius, were exposed to short days (SD) (8L:16D) for 6, 9, or 12 weeks in the autumn after their hatch, then paired and shifted to long days (LD) (16L:8D) for 12 weeks. After pairing, birds were unpaired, returned to flight pens and exposed to SD. After 6, 9, or 12 weeks of SD, birds were returned to breeding pens, paired and exposed to LD for a second breeding cycle. Prior to pairing in both breeding cycles, luteinizing hormone (LH) concentrations in females and males were low, not significantly different between groups, and did not change significantly while birds were unpaired. In the first photoperiodically driven breeding cycle, after birds were paired and exposed to LD, LH concentrations increased significantly in all females and males. Maximum LH concentrations in females occurred prior to oviposition, during pair formation/courtship. Maximum LH concentrations in males occurred one week earlier than in females. Fertile eggs were produced in the groups exposed to 9 and 12 weeks of SD. During the second photoperiodically induced breeding cycle, only pairs exposed to 12 weeks of SD demonstrated significant increases in mean LH concentrations. Mean LH concentrations of females exposed to 12 weeks of SD were not significantly different between the two breeding cycles, while mean LH concentrations of males exposed to 12 weeks of SD were significantly lower in the second breeding cycle compared to the first. In both breeding cycles, LH concentrations for females and males declined within 1 week after maximum concentrations were reached. Only pairs exposed to 12 weeks of SD produced fertile eggs in both breeding cycles. These results demonstrate that reproductive maturation can be induced prematurely in juvenile kestrels through exposure to SD. Steroid Hormones in the Yolk of Red-Winged Blackbird EggsJ.L. Lipar, E.D. Ketterson, V. Nolan Jr Department of Biology, Indiana University, Bloomington, IN
47405, USA Egg yolks of redwinged blackbirds (Agelaius phoeniceus) were analyzed with radioimmunoassay for the presence of testosterone, 5adihydrotestosterone, estradiol, and corticosterone. Testosterone levels were greater than that of the other three measured hormones, and also showed the greatest degree of variation. Regression analysis of hormone level versus egg number yielded a significant increase in testosterone concentration with laying order. Testosterone levels increased between the first and last eggs in fifteen out of sixteen clutches. No significant relationships between laying order and the concentrations of 5adihydrotestosterone, estradiol, and corticosterone were found, although 5adihydrotestosterone and testosterone levels were significantly correlated. Although a positive correlation of yolk testosterone concentration with laying order has been previously found in the canary (H. Schwabl, 1993, PNAS 90: 1144611450), this study is the first to demonstrate the same pattern in a noncaptive species. Future studies to examine the evolutionary significance of variable yolk testosterone levels may include an investigation of the origin of variation in yolk testosterone levels and an exploration of the effects of testosterone on hatchling development, physiology, behavior, and mortality as they relate to hatching asynchrony in the redwinged blackbird. Maternal Thyroid Hormones in Quail EggsC.M. Wilson, F.M.a. McNabb Biology Department, VA Tech, Blacksburg, VA 24061, USA Methods were developed and validated for a methanolchloroform extraction of thyroid hormones (THs) from the yolk of Japanese quail eggs and measurement of extract THs by RIA. Untreated quail produce eggs with yolk [T4] that is low relative to plasma [T4] but comparable for plasma [T3]. Hens deposit yolk in eggs for five days prior to laying. To determine the effects of changes in thyroid status on yolk TH deposition, hens were dosed orally, twice daily with T4, with different trials using doses from 1 and 3X the daily thyroid secretion rate of T4. Maternal TH deposition in egg yolk varies with the thyroid status of hens. However, although each of the "levels" of hen hyperthyroidism resulted in higher yolk THs, the relationships between hen plasma [THs] and yolk [THs] were not simple. Within each level of hyperthyroidism, hens appeared to regulate yolk TH content. Hens made hypothyroid by oral dosing with the goitrogen methimazole ceased laying eggs. This suggests that THdeficient eggs may not be produced when hens by thyroid deficient hens. Current studies are assessing the effects of alterations in yolk [TH] concentrations on the early development of embryos prior to the time when the thyroid gland is producing and secreting appreciable amounts of THs. Percent hatchability, body weight and general morphological development are not altered significantly in eggs with high [TH] content compared to those from controls. We are assessing histological and biochemical development of pelvic cartilages, in which tissue differentiation is triggered by THs, to evaluate the influence of maternal alterations in yolk TH on developmental events in early embryonic development. (Supported by grants from Sigma Xi and Graduate Research Development Projects, VA Tech) The Effects of Equimolar Doses of Thyroxine (T4) or Triiodothyronine (T3) on Organizational-like Programming of Testicular Growth, Photorefractoriness, and Postnuptial Molt in Thyroidectomized Male American Tree SparrowsB.D. Reinert Division of Biology, Kansas State University, Ackert Hall,
Manhattan, KS 66506-4901, USA Photosensitive male American tree sparrows (Spizella arborea) were radiothyroidectomized, transferred to long days, and administered 14 daily injections (sc) of 0.1, 1, or 10 µg T4 or T3 in 1 mmol·l-1 NaOH (V). Thyroidectomized (THX) and thyroid-intact (THI) controls received only V. Testis length was measured to monitor reproductive status, and primary remiges were scored to chart postnuptial molt. After 7 weeks of photostimulation, birds were transferred to constant light and given T4(1) [1 mg T4 in V] in lieu of drinking water to test for absolute photorefractoriness. At week 12 of photostimulation, hypothalami were collected for cGnRH-I (chicken gonadotropin-releasing hormone I) assay. THX birds injected with 1 or 10 µg T4 underwent robust testicular growth and then regression, initiated postnuptial molt, and had low hypothalamic cGnRH-I content, as did THI controls, indicating that they were programmed for testicular growth, photorefractoriness, and molt. THX birds injected with 0.1 µg T4, or 1 or 10 µg T3, also were programmed for testicular growth (but showed recrudescence during the photorefractory assay), did not initiate molt, and had high hypothalamic cGnRH-I content, thereby signifying photosensitivity (i.e. they were not programmed for photorefractoriness and molt). Testicular growth, photorefractoriness, and molt were not programmed in THX birds injected with 0.1 µg T3 or THX controls injected only with V. The latter two groups exhibited only thyroid-hormone independent testicular growth. (This study was supported in part by a grant-in-aid of research from Sigma Xi) Photoperiod and Puberty in the Male Emu, Dromaius novaehollandiaeI.a. Malecki1,2, G.B. Martin1,2 1Faculty of Agriculture, University of Western Australia,
Australia 6907; 2CSIRO Division of Animal Production, Australia
6014 The laying season of the emu typically begins in late autumn, suggesting that this species is a short day breeder. Juvenile birds, however, rarely experience their first breeding season before they are about 70-80 weeks old. In this study, we tested whether progress to puberty depends on the photoperiod that emus experience in their first year of life. Male emus (n=24) were raised from hatching till 14 weeks of age on increasing natural daylength, out of doors. Around the summer solstice (daylength 14 h), birds were transferred to light control rooms and treated with either constant long days (14L10D; n=8) or constant short days (10L14D; n=8) or natural changes in daylength, supplied through large windows(14L10D to 10L14D and back to 14L10D; n=8). These treatments continued for 52 weeks. Plasma LH concentrations were higher throughout the experiment in birds kept on long days, compared to birds on natural photoperiod or short days, and this difference was significant between weeks 18-44 of the experiment (weeks 32-58 of age). Plasma concentrations of testosterone did not change in birds on natural daylength and short days. Compared with the two other groups, there was a short but significant peak in plasma testosterone (0.21±.07 ng/mL; P<0.05) at week 36 of experiment (week 50 of age) in the emus treated with long days. This group subsequently began to display courtship behaviour and territorial aggressive behaviour between 52-56 weeks of age. None of these behaviours were observed in the other groups before the experiment ended. In conclusion, emus are photoperiodic and the preliminary stages of their pubertal development can be advanced by exposure to continuous long days. The Influence of Rainfall on the Timing of the Breeding Season of the Emu Dromaius novahollandiaeK.M. Williams, G.B. Martin, E. Potter1 Faculty of Agriculture, University of WA, Australia 6907;
1Dromaius Australia Ltd, P.O. Box 142, Cloverdale, WA 6105, USA The egglaying season of the emu generally begins in late autumn (April in Australia), suggesting that this ratite is a shortday breeder. However, for a given location, there is considerable variation between seasons in the start of the season so the effects of photoperiod are probably modulated by other factors. One possibility is rainfall, which exerts a major influence over the timing of breeding in other Western Australian birds (Halse and Jaensch, 1989; Emu 89, 232). This may apply to the emu because, in their natural range, the yearly pattern of rainfall can vary markedly due to the advent of summer cyclones. To test this hypothesis, we studied the egg and rainfall records over 4 years (1992 to 1995) from a large emu farm (1500 breeders) that is located in the normal habitat of the emu. We correlated the date of start of lay with the amount of rainfall between January 1 and the start of the laying season. Data from more years are needed, but the strong negative correlation suggests that rainfall influences the onset of the laying season of the emu. Fig. 1: relationship between start of lay and rainfall leading up to start of lay. Endocrine Correlates of Delayed Breeding in the Mountain White-Crowned SparrowM.L. Morton Biology Department, Occidental College, Los Angeles, CA 90041,
USA Mountain White-Crowned Sparrows (Zonotrichia leucophrys oriantha) usually arrive on their mountain-meadow breeding areas in early May and commence nesting in late May or early June. In 1995 a heavy snowpack occurred and late-lying snows prevented nesting until early July, offering an opportunity to see how delayed breeding affected gonadal development, energy balance, and behaviors. Testicular growth proceeded the same in 1995 as in normal years (1984, for example) but cloacal protuberance development was delayed in 1995 suggesting that plasma T levels were depressed while the birds were waiting to breed. Ovarian follicles were 1-2 mm at arrival and remained at that stage for nearly two months until nesting sites became available. Female body condition was poor; clutch sizes were normal but egg sizes were reduced. Prior to onset of breeding, both sexes periodically left their snow-bound territories and retreated to lower elevations, probably in order to obtain food. Differences in Vernal Reproductive System Development between Second-Year and Older Free-Living Male Dark-Eyed JuncosP. Deviche Institute of Arctic Biology, University of Alaska Fairbanks,
Fairbanks, AK 99775-7000, USA In most temperate and high latitude avian species, increasing photoperiods in the spring induce gonadal recrudescence. Many studies have reported a modulation of vernal gonadal development by proximate environmental factors, but little is known regarding the age-dependency of this development. To address this question, we measured plasma luteinizing hormone (LH) (courtesy of Dr. J. Wingfield, University of Washington) concentrations, testicular masses, and cloacal protuberance sizes (CPs, an androgen-dependent secondary sexual characteristic) throughout spring and summer in second-year (SY, first-time breeders) and older (>SY) free-living male dark-eyed juncos (Junco hyemalis). We found that SY and >SY males had similar plasma LH concentrations throughout the breeding season. SY males had smaller testes than older males during the period of rapid gonadal development, but not thereafter. Finally, SY males had smaller CPs than >SY males throughout the breeding season. The body masses of SY and >SY juncos were similar, indicating that age differences were specific to reproductive tissues. SY males caught during migration, i.e. before they had reached their breeding sites, had smaller testes than migrating >SY males. Therefore, slower gonadal development in SY than in >SY males did not derive from breeding site-associated influences such as territory quality or food availability. The reproductive physiology and morphology of SY and >SY male juncos may differ because younger birds have a higher photoperiodic threshold and/or a lower tissue sensitivity to GnRH (pituitary), gonadotropins (gonads) and/or gonadal steroids (CP) than older birds. (Supported by NSF Award BNS-9121258) A Comparison of Testosterone and Prolactin in Group-Living Mexican Jays (Aphelocoma ultramarina) and Monogamous Scrub Jays (Aphelocoma coerulescens woodhousei)C. Vleck, J. Brown1, E. Brown1, J. Adams Department of Zoology and Genetics, Iowa State University,
Ames IA 50011, USA; 1Department of Biological Sciences, State
University of New York at Albany, Albany, NY 12222, USA Mexican (graybreasted) jays in Arizona live in permanent, social groups of up to 20 birds and defend a stable, yearround territory from other groups. Males compete aggressively for females in the group and successful breeding by two to four pairs in each group is common. Most birds in the group (both breeders and nonbreeders) feed the incubating females and help to raise the young in each of the nest. In contrast, scrub jays in Arizona breed in monogamous pairs without helpers. In the groupliving Mexican jays testosterone is elevated in males, both breeders and many nonbreeders, from midMarch through April, during both the courtship and incubation stages of nesting. This may reflect the role of testosterone in maintaining these males' positions within the group's dominance hierarchy and/or their attempts to mate with other females in the group. In monogamous scrub jays testosterone is elevated in males only in midMarch during territory establishment and pair bonding. Prolactin levels in Mexican Jays increase earlier in females than they do in males, but all birds in the flock have elevated prolactin during the incubation and nestling stages (mean = 17.5±6.8 ng/ml, range 733 ng/ml). The same temporal pattern is seen in scrub jays, however the prolactin values are considerably lower on average (mean = 11.6±2.4 ng/ml, range 5 16 ng/ml) than they are in the Mexican jays at the same time of year. Body Weight and Age at Photostimulation: Effects on Reproductive Morphology in Female Meat Type ChickensP.R. Goerzen, F.E. Robinson, R.T. Hardin, M. Newcombe1, R.I. McKay1 Department of Agricultural, Food and Nutritional Science,
University of Alberta, Edmonton, AB T6G 2P5; 1Shaver Poultry
Breeding Farms Ltd., Cambridge, ON N1R 5V9, Canada Reproductive traits in meat-type hens may be affected by small changes in body weight (BW) targets. Feed restriction is necessary in broiler breeders to have birds achieve reproductive success. Pullets were grown on three growth curves: standard (as recommended by the breeder), low (-7.8% at 20 wk) and high (+7.8% at 20 wk). All birds were grown on the standard curve up to 3 wk, at which time feed allocations were altered to slowly achieve the desired difference in BW. All birds were individually caged at 18 wk. One half of the birds were photostimulated at 19 wk and one half were photostimulated at 21 wk. Experimental design was a 3 x 2 factorial with the resulting treatments being; STAND19, HIGH19, LOW19, STAND21, HIGH21 or LOW21. Twelve birds of each treatment were processed at PS (Group A). Ten birds from each treatment were processed at sexual maturity (first oviposition) (Group B). Group B birds were blood sampled weekly for 17b-estradiol analysis. Photostimulation age had no effect on ovary weight or whole carcass lipid (Group A) whereas 21 wk PS increased whole carcass protein and oviduct weight. Although body weight target had no effect on whole carcass protein or oviduct weight, the HIGH birds had a higher whole body lipid content and a heavier ovary at PS. Al 19 wk B birds reached sexual maturity (SM) at the same BW. The BW of the HIGH21 birds were 7% heavier than the LOW21 birds at SM. The breast muscle, fatpad, oviduct, ovary, and stroma weight and liver lipid content was the same across all treatments at sexual maturity. The HIGH curve birds had more large yellow follicles (LYF) within 1 g of each other than the LOW curve birds. This small increase in BW may impair reproductive performance. The 17b estradiol profiles generated for BW target and for the individual treatments were not different from each other. Profiles for 19 vs 21 wk PS were different in timing. The 17b estradiol levels rose sharply in response to the PS cue. |