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a Teagasc, Research Centre, Athenry, Co. Galway, Ireland
b Faculty of Veterinary Medicine, National University of Ireland, Dublin, Ireland
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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(PGF2) to induce luteolysis, estrus, and ovulation of the first dominant follicle (DF). Animals were inseminated and returned to a diet of 1.2 Mn. Pregnancy diagnosis was performed 30 days later. The maximum diameter subsequently attained by the DF present at progesterone withdrawal was smaller (P < 0.01) in heifers fed 0.4 Mn. Two heifers fed 0.4 Mn failed to ovulate this DF (P > 0.10). Growth rate (P < 0.01) and maximum diameter (P < 0.001) of the DF in the first follicular wave of the next estrous cycle was also reduced in heifers fed 0.4 Mn. After prostaglandin administration, a further 10 heifers fed 0.4 Mn failed to ovulate the first DF of this cycle, and it regressed (P < 0.001), causing anovulation in 12 of 20 heifers within 13–15 days (P < 0.001). Anovulation of the DF present at progesterone withdrawal was preceded by a proestrous estradiol increase but absence of a gonadotropin surge (2 of 2 heifers), while neither endocrine event was detected before anovulation of the DF of the first new follicular wave (2 of 2 heifers). In cases in which ovulation of the first DF of the new cycle occurred, fertility was similar (P > 0.10) in heifers fed either 0.4 (n = 7) or 1.2 Mn (n = 20). In conclusion, acute nutritional restriction of cyclic heifers from 1.2 to 0.4 Mn decreased the growth rate and maximum diameter of DFs and induced failure of the DF to ovulate in 60% of heifers, but, within the confines of limited animal numbers, did not compromise fertility in heifers that ovulated.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Detrimental effects of nutritional restriction on reproduction may not only induce anovulation in some animals, but it may also compromise fertility in those animals that do express estrus and ovulate. Pregnancy rates from first inseminations in Holstein cows has declined over the past 30 years in both the U.S. [6] and in Europe [7]. This has been partly attributable to severe NEB in the early postpartum period [8], especially in high-genetic-merit cows in which the feed requirements for milk production are greater than the dry-matter intake capacity of the cow. Severe NEB in the early postpartum period may affect follicular development, oocyte competence, and ultimately pregnancy rate. Therefore, a further aim of this study was to examine the effect of acute nutritional restriction on pregnancy rate after ovulation of growth-restricted follicles.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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This study was conducted in two replicates involving 41 beef heifers exhibiting regular estrous cycles, and in accordance with the European Commission (EC) directive, 86–609-EC. Before commencement of dietary treatments, heifers were individually fed a grass silage and concentrate diet (fed 1:1 on a dry-matter basis) supplying the energy for 1.2 times maintenance (1.2 Mn), in which the dry-matter intake for maintenance was calculated as live weight x 0.011. The energy density of each feed, as defined by the Agricultural Research Council [9] was silage 9.3 MJ metabolizable energy (ME)/kg dry matter (DM); concentrate 11.0 MJ ME/kg DM. The crude protein content of each feed was silage 115 g/kg DM; concentrate 160 g/kg DM. At the end of the adjustment period preceding diet allocation, heifers were weighed before feeding on two consecutive mornings, and their average live weight was calculated. Their mean live weight (mean ± SEM) was 406 ± 5 kg, and body condition score (BCS), as defined by Lowman et al. [10], was 2.93 ± 0.05. A summary of the experimental design is presented in Figure 1.
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Synchronization of Estrus
The estrous cycles of all heifers were synchronized by insertion of an intravaginal controlled internal drug-releasing device (CIDR) containing 1.9 g progesterone and a 10-mg estradiol benzoate capsule (EAZI-breed CIDR-B; InterAg Ltd, Hamilton, New Zealand) for 8 days. One day before CIDR withdrawal, heifers were given a 500-µg injection of synthetic prostaglandin F2 (PGF2; Estrumate, Coopers Animal Health Ltd, Berkhamsted, UK) to cause luteolysis of any existing corpora lutea. Six days after ovulation of the DF present at CIDR removal (synchronized DF), luteolysis was induced using PGF2 to allow ovulation of the first DF of the subsequent estrous cycle (first DF) [11]. All heifers observed in estrus were inseminated 12–18 h after its onset by one technician, using frozen-thawed semen from the same ejaculate of one bull. Pregnancy diagnosis was initially performed at 30 days postinsemination using ultrasonography and was confirmed 20 days later.
Diets
Heifers were allocated randomly to one of two differing allowances of the same diet on the day before CIDR removal, and continued to be individually fed. These allowances provided either a restricted or a control ration supplying the nutritional requirements for 0.4 Mn (n = 20) or 1.2 Mn (n = 21), respectively. Diets were allocated at this time to help the rumen to adjust to the change in nutritional supply before emergence of the first follicular wave after ovulation of the synchronized DF. All heifers were returned to a diet supplying the nutritional requirement for 1.2 Mn after emergence of the second follicular wave following CIDR withdrawal, after approximately 2 wk, and were kept on this diet until pregnancy diagnosis at 30 days postinsemination. At 2 wk after dietary treatment began, and before return to a diet of 1.2 Mn, heifers were weighed and had their BCS assessed before feeding on two consecutive mornings.
Ovarian Ultrasonography
Ovarian structures were evaluated daily per rectum on all heifers, using an Aloka SSD-500V ultrasound scanner fitted with a 7.5-MHz transducer (Aloka Co. Ltd, Tokyo, Japan). During each periovulatory period, ovarian scanning was performed on each heifer twice daily to determine time of ovulation and new follicular wave emergence. During the first periovulatory period, twice-daily ovarian scanning began 48 h after CIDR withdrawal and continued for 96 h, or until ovulation or regression of the DF, whichever was longer. During the second periovulatory period, twice-daily ovarian scanning began 24 h after PGF2 administration and continued for 96 h, or until ovulation or regression of the DF, whichever was longer. Follicles of 
3-mm diameter were measured and recorded. Emergence of a follicular wave was defined by the appearance of follicles 4 mm in size, and the number of follicles emerging was defined as the number of follicles 3 mm present on both ovaries on the day of emergence. Day of ovulation was defined as the day on which a large DF, present on either ovary the previous day, had disappeared. Follicular diameter was obtained by averaging the "height" and "width" of each follicle as observed under ovarian ultrasound. The growth rate (mm/day) of the first DF after ovulation of the synchronized DF was defined as the mean daily increase in follicular diameter between first appearance as a 4-mm follicle and the day of maximum diameter.
Blood Sampling
Blood samples were taken twice daily (0800 and 2000 h) from a subsample of heifers (replicate 1: 0.4 Mn, n = 9; 1.2 Mn, n = 10) from the day preceding diet allocation until emergence of the second follicular wave after the synchronized DF. During each periovulatory period, heifers were blood-sampled at 4-h intervals for 96 h or until ovulation or regression of the DF, whichever was longer, to detect the proestrous estradiol increase, gonadotropin surge, and periovulatory pre-emergence FSH increase. During the first periovulatory period, blood sampling at 4-h intervals began 24 h after CIDR withdrawal, and during the second periovulatory period, blood sampling began 24 h after PGF2 administration. Heifers within this subsample that failed to ovulate were blood-sampled twice weekly for a further 5 wk to confirm anestrus. All samples were placed in an iced water bath until centrifugation at 1000 x g at 4°C for 15 min. After being decanted, plasma was stored at -20°C until assayed for LH, FSH, estradiol, and progesterone.
Hormone Assays
LH assays were performed using a double-antibody RIA [12]. The first antibody was LH antibody (518B7, supplied by Dr. Jan Roser, University of California, Davis, CA), and the second antibody was goat anti-mouse (Sac-Cel AA-SAC4; IDS, Boldon, Tyne and Wear, UK), but modifications included use of USDA-bLH-12 as both standard and 125I-labeled tracer. The sensitivity of the assay was 0.19 ng/ml. Intra- and interassay coefficients of variation (CVs) for the assays performed (n = 11) were 7.3% and 10.7%, respectively, for a pool of plasma containing a mean LH concentration of 0.97 ng/ml.
Plasma FSH concentrations were quantified using a heterologous RIA [13] using NIDDK anti-ovine (o) FSH antibody (AFP-C 5288113), 125I-labeled oFSH-I-SI-AFP-19 tracer, and a bovine FSH standard (USDA bFSH-B1). The second antibody was donkey anti-rabbit (Sac-Cel AA-SAC1; IDS). The sensitivity of the assay was 3.4 ng/ml. The intra- and interassay CVs for the assays (n = 10) were 12.9% and 12.8%, respectively, for a pool of plasma containing a mean FSH concentration of 9.8 ng/ml.
After extraction with diethyl ether, plasma concentrations of estradiol were quantified using a validated modification [14] of the Biodata Estradiol Maia kit assay (Code 12264; Bio-Stat Ltd., Stockport, Cheshire, UK). The sensitivity of the assay was 0.42 pg/ml. Intra- and interassay CVs for the assays (n = 11) averaged 10.1% and 12.2%, respectively, for a pool of plasma containing a mean estradiol concentration of 3.1 pg/ml.
Plasma progesterone concentrations were quantified using a 125I-labeled progesterone double-antibody RIA [15]. This nonextraction assay was modified to enable the use of rabbit anti-progesterone antibody (Batch No. 7044X; Scottish Antibody Production Unit, Carluke, Lanarkshire, UK) and a cellulose-coated second antibody, donkey anti-rabbit (Sac-Cel AA-SAC1; IDS). The assay employed a commercially available tracer (progesterone-11a-glucoronide-[125I] iodotyramine; Amersham International PLC, Little Chalfont, Buckinghamshire, UK). Hormone standards were obtained from Sigma Chemical Company Ltd. (Poole, Dorset, UK). The sensitivity of this assay was 0.14 ng/ml. Intraassay CV for the assay (n = 1) was 14.3% for a pool of plasma containing a mean progesterone concentration of 0.92 ng/ml.
Statistical Analysis
Heifer performance data was analyzed by ANOVA using the PROC GLM procedure of the Statistical Analysis Systems (SAS; [16]). All follicular and hormone data were initially aligned to the day of diet allocation (Day 0) to compare replicates. Subsequently, the effects of diet on follicular dynamics were analyzed using the PROC GLM procedure [16], while the effects of diet on the frequency of ovulation versus anovulation and pregnancy rate were tested using Fisher's exact test output of the PROC FREQ procedure [16]. Follicular dynamics among heifers fed 0.4 Mn were analyzed using the PROC GLM procedure [16] to compare the effects before ovulation versus anovulation.
The gonadotropin profiles of individual heifers were assessed for presence or absence of a coincident LH and FSH surge during both periovulatory periods, and for identification of the increase in concentration of FSH preceding emergence of a new follicular wave (pre-emergence FSH increase). The effects of diet on the frequency of a gonadotropin surge were tested using Fisher's exact test [16]. The FSH profiles in each periovulatory period, because of their complex wavelike nature, were divided into three phases as follows: 1) the mean of the 2 lowest samples preceding the gonadotropin surge (pre-increase nadir), 2) the mean of the 4 highest samples comprising the pre-emergence FSH increase following the gonadotropin surge (increase), and 3) the mean of the 2 lowest samples following the pre-emergence FSH increase (postincrease nadir). The FSH concentrations of the above phases were aligned chronologically for the pre-emergence period of both waves, and classified as phases 1–6 for statistical analysis using repeated-measures procedures (PROC GLM [16]), in which the effects of diet, phase, and diet x phase were tested against the appropriate error term. Further analyses were performed by grouping the above phases by type (FSH increase, phases 2 and 5; nadir, phases 1, 3, 4, and 6) and analyzed using repeated-measures procedures (PROC GLM [16]) in which the effects of diet, type, and diet x type were tested against the appropriate error term. All FSH data were log-transformed before statistical analysis, and results are presented as back-transformed means.
The estradiol profiles of heifers, associated with development of both the synchronized DF and the first DF, were assessed for presence or absence of a proestrous increase, as defined by the presence of two consecutive samples with concentrations 2 pg/ml. Where a proestrous increase in estradiol concentrations occurred, the mean estradiol concentration during the increase was calculated as the mean of the highest 4 samples and was used for statistical analysis. This was performed using repeated-measures procedures (PROC GLM [16]) in which the effects of diet, follicle (synchronized DF or first DF), and diet x follicle were tested against the appropriate error term. During the second periovulatory period, the period of time during which plasma estradiol concentrations were 1 pg/ml was calculated for each heifer inseminated (n = 11). Subsequently, the rate of decline in the plasma concentration of estradiol was calculated as the mean hourly decline from the single highest concentration to the nadir (defined as the first sample after the peak that had a concentration 
assay sensitivity). The single highest concentration of the proestrous estradiol increase and the rate of decline were then tested for the effects of diet, pregnancy status (pregnant versus nonpregnant), and diet x pregnancy using PROC GLM [16].
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Follicular Data
The size of the DF present on the ovaries of heifers allocated to either diet was similar on the day of diet allocation (P > 0.10), but the maximum diameter attained by this follicle was smaller in heifers fed 0.4 Mn than those fed 1.2 Mn (P < 0.01; Table 1). Subsequently, in two heifers fed 0.4 Mn, the DF failed to ovulate (P > 0.10). One of these heifers failed to exhibit estrus. The DF regressed in both heifers, and a new wave emerged and a new DF was selected, but this also regressed, despite the absence of a corpus luteum. The growth rate (P < 0.01) and maximum diameter (P < 0.001) of the first DF to develop after the synchronized DF were reduced in heifers fed 0.4 Mn compared with those fed 1.2 Mn (Table 1). In two heifers, one allocated to each diet, emergence of the second new follicular wave began before administration of PGF2 on the sixth day after ovulation of the synchronized DF. Subsequently, the first DF regressed and the second DF ovulated, but neither heifer was inseminated. In the remaining heifers, despite the occurrence of luteolysis, the first DF failed to ovulate in 10 of 17 heifers fed 0.4 Mn, while all but 2 of the 20 fed 1.2 Mn ovulated the first DF (P < 0.01). Although the 2 heifers fed 1.2 Mn demonstrated estrus at this time, continued ovarian ultrasonography revealed that the first DF of the new cycle persisted and ovulated approximately 1 wk later. Both heifers remained cyclic, as determined by recurrent increases in plasma progesterone, while the heifers fed 0.4 Mn became anestrous, as determined by the absence of systemic progesterone. Therefore, after 13–15 days of feed restriction to 0.4 Mn, 12 of 20 heifers were anovulatory (P < 0.0001) compared to none of the heifers fed 1.2 Mn. In heifers fed 0.4 Mn, the maximum diameter of the first DF of the new estrous cycle was larger in heifers that ovulated than in those that failed to ovulate (11.7 ± 0.5 versus 10.0 ± 0.3 mm, respectively; P < 0.01). This was not due to difference in growth rate (P > 0.10), but was due to a difference in the number of days growing (8.5 ± 0.3 versus 6.8 ± 0.6 days, respectively; P < 0.10).
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Hormone Data
In replicate 1, all heifers had a proestrous increase in plasma estradiol concentration associated with development of the synchronized DF following CIDR withdrawal, the magnitude of which was unaffected by diet (P > 0.10). Subsequently, all 10 heifers fed 1.2 Mn had a coincident LH and FSH surge, and the DF ovulated, whereas only 7 of 9 heifers fed 0.4 Mn had coincident LH and FSH surges and ovulated (P > 0.10). The two heifers that did not have a gonadotropin surge and failed to ovulate had no further increase in estradiol concentrations or concomitant preovulatory-type LH and FSH surge, but they did have an increase in the plasma concentration of FSH preceding the next new wave emergence (Fig. 2c). After administration of PGF2, luteolysis occurred in all heifers, but two of those fed 0.4 Mn failed to ovulate (P > 0.10), resulting in 4 of 9 anestrous heifers compared to 0 of 10 anestrous heifers fed 1.2 Mn (P < 0.05). Neither of these two anovulatory heifers had a proestrous increase in concentrations of estradiol, nor a gonadotropin surge (P > 0.10; Fig. 2b). In cases in which ovulation did occur, diet had no effect on the magnitude of the proestrous increase in concentration of estradiol (3.7 ± 0.3 pg/ml for heifers fed either diet; P > 0.10).
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There was no interaction between diet and phase on plasma concentrations of FSH throughout the profile (P > 0.10). Phase of the FSH profile had a significant effect (P < 0.0001); concentrations of FSH in samples taken during the pre-emergence increase (phases 2 and 5) were significantly higher than those taken at the FSH nadir (phases 1, 3, 4, and 6). Plasma concentrations of FSH tended to be higher (P = 0.07) in heifers fed 0.4 Mn than those fed 1.2 Mn. During the pre-emergence increase, mean concentrations were 11.7 and 8.5 ng/ml in heifers fed 0.4 and 1.2 Mn, respectively, whereas during the nadir phases mean concentrations were 4.4 and 3.1 ng/ml, respectively.
Estrous Response at First DF and Pregnancy Rate
After administration of PGF2 on the sixth day after ovulation of the synchronized DF, 7 of 18 remaining heifers fed 0.4 Mn and 20 of 21 fed 1.2 Mn were detected in estrus and inseminated. Heifers fed 0.4 Mn that became anestrus at this stage (n = 10) did not exhibit estrous activity. In those cases in which new wave emergence occurred before administration of PGF2 (one heifer from each diet), estrus was exhibited before ovulation of the second DF, but neither heifer was inseminated. The pregnancy rate at 30 days postinsemination, defined as the number of heifers pregnant per number of heifers inseminated, was 4 of 7 (57%) and 11 of 20 (55%) in heifers fed 0.4 Mn and 1.2 Mn, respectively (P > 0.10).
Rate of Estradiol Decline/Pregnancy Rate
There was no diet x pregnancy status interaction (P < 0.05) for any parameter connected to the estradiol increase in the periovulatory period in heifers that were inseminated. However, there was a significant effect of estradiol parameters on pregnancy status. Peak proestrous concentrations of estradiol were higher in heifers that became pregnant than in those that did not (5.0 ± 0.2 versus 3.7 ± 0.4 pg/ml, respectively; P < 0.05), and the rate of decline to nadir was greater (0.62 ± 0.03 versus 0.33 ± 0.06 pg/h, respectively; P < 0.01). However, 2 of 5 heifers that failed to become pregnant contributed largely to these differences.
Duration of Anovulation
In the four heifers of replicate 1 in which anovulation had occurred, anestrus was confirmed by the absence of plasma progesterone. After 5 wk of being fed 1.2 Mn, two heifers were still anestrus and two had resumed cyclicity. In the two that had resumed cyclicity, concentrations of progesterone 1 ng/ml were detected 19 days after return to 1.2 Mn in one heifer and 30 days after return in the other. On resumption of ovulation, both heifers exhibited a short estrous cycle as characterized by concentrations of progesterone exceeding 1 ng/ml for < 12 days.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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While two heifers failed to ovulate the DF present at progesterone withdrawal, frequent blood sampling during the periovulatory period established that there was a proestrous increase in the plasma concentration of estradiol, but only one of these heifers exhibited standing estrus, and neither had a gonadotropin surge. Failure of ovulation of the DF present at progesterone withdrawal in a proportion of heifers is consistent with previous findings from our laboratory by Mackey et al. [5] where, in a similar study, the DF present at progesterone withdrawal failed to ovulate in 2 out of 8 heifers. In the current study, induction of luteolysis by the administration of PGF2 on the sixth day after detected ovulation provided the appropriate endocrine environment for the first DF of the new estrous cycle to ovulate [11]. However, this DF failed to ovulate in a large proportion of heifers fed 0.4 Mn. Although only two of these heifers were blood-sampled during the "periovulatory period," hormone profiles indicated that failure of ovulation was due to the absence of a proestrous increase in the plasma concentration of estradiol.
Anovulation in the current study was always preceded by the absence of a gonadotropin surge, but not always by the absence of a proestrous increase in the concentration of estradiol. However, the presence of a proestrous increase in the concentration of estradiol appears to depend on whether anovulation occurs after 4–5 days, or after a longer period (13–15 days), of restriction to 0.4 Mn, and suggests that the mechanisms leading to anovulation may operate differently, depending on the duration of acute nutritional restriction. Recent studies in both sheep [17–19] and cattle [20] have established that LH is released episodically from the anterior pituitary in response to pulsatile release of GnRH from the hypothalamus. Therefore, the absence of an LH and FSH surge could be mediated at either the hypothalamus or the anterior pituitary. During spontaneous luteolysis, as progesterone concentrations decline, LH pulse frequency increases [21] in response to increased pulses of GnRH [20]. This increase in LH pulse frequency stimulates follicular androgen production [22] and hence increased estradiol secretion from the DF. The exact mechanism whereby increasing estradiol concentrations stimulate a gonadotropin surge has not been fully established, but it is believed to act on specific neuronal pathways impinging on GnRH neurons within the hypothalamus, leading to an increase in the secretion of GnRH [23]. In the current study, when anovulation of the first DF of the next estrous cycle occurred, the absence of a proestrous increase in the concentration of estradiol suggests that there were insufficient LH pulses to stimulate androgen production and hence estradiol secretion. Through lack of positive feedback, low concentrations of estradiol could prevent the follicular-phase increase in GnRH pulsatility and the change in the mode of secretion necessary to induce a gonadotropin surge. However, in this study, blood sampling was not performed at sufficiently regular intervals to determine LH pulsatility during the follicular phase. After luteolysis in the final estrous cycle before anovulation, Bossis et al. [3] observed the absence of a proestrous increase in the concentration of estradiol and a suppression in both the frequency and amplitude of LH pulses. This would suggest that there was insufficient positive feedback during the follicular phase in the current study to stimulate both androgen production and GnRH secretion. However, in the absence of ovarian steroids, there was no effect of acute nutritional restriction on LH pulse frequency in ovariectomized heifers [5]. Recent evidence has shown that nutritional factors may be involved in suppression of reproductive function, and their effects may be mediated at the level of the hypothalamus and pituitary by compounds such as neuropeptide Y [20, 24], or at the level of the hypothalamus, pituitary, and ovaries, by compounds such as leptin [24–26]. Therefore, we speculate that such factors may be involved in the suppression of periovulatory gonadotropin concentrations in the current study.
While the absence of a gonadotropin surge resulted in anovulation, nutritional effects on follicle growth rate were evident much earlier. Nutritional restriction of energy has been well documented to decrease the growth rate and maximum diameter of DFs in both chronically [1–3, 27] and acutely restricted heifers [5]. In the present study, acute restriction from 1.2 Mn to 0.4 Mn caused an immediate suppression of the maximum diameter attained by the DF present on withdrawal of a progesterone synchronizing device. In a similar study applying the same dietary regimen [5], there was no immediate effect on the existing DF recorded. However, in both the initial study [5] and the current study, the growth rate and the maximum diameter of the subsequent DF were similarly decreased. In the initial study, suppression of growth rate and maximum diameter occurred without any effect of diet on LH pulse characteristics and hence was attributable to changes in intraovarian factors, possibly insulin-like growth factor I and its binding proteins. However, in contrast to that study, in the current study, PGF2 was administered to induce luteolysis and reduce the progesterone negative feedback on LH, hence providing the appropriate endocrine environment for the first DF of the new estrous cycle to ovulate. Among the group of heifers allocated to 0.4 Mn, the maximum diameter of the first DF was greater in heifers that ovulated than in those that failed to ovulate, although growth rate was not affected; this effect was due to a shorter period of growth in nonovulatory DFs, suggesting that after luteolysis the LH pulse frequency was probably insufficient to stimulate final growth and maturation of the DF. Evidence from Sartori et al. [28] suggests that DFs < 10 mm do not ovulate in response to exogenous LH, but in the current study DFs as small as 9 mm ovulated.
The tendency for increased concentrations of FSH in heifers restricted to 0.4 Mn confirms an observation by Mackey et al. [5] that, during the pre-emergence FSH increase, plasma concentrations were significantly higher in heifers fed 0.4 M than in those fed 1.2 or 2.0 Mn. However, in that study, nadir concentrations were unaffected by diet, in contrast to the current study, in which nadir concentrations in restricted heifers also tended to be higher. This would suggest that the DF of restricted heifers in the present study was not as effective in suppressing the concentration of FSH as the DFs from heifers fed 1.2 Mn. In contrast to LH, there is little pituitary storage of FSH [29], and once synthesized it is constitutively released [30]. This suggests that the increase in concentration of FSH following dietary restriction was due to effects on the anterior pituitary resulting in increased synthesis and release of FSH, perhaps mediated via direct effects on the activin-inhibin-follistatin axis within the gonadotrophs. Greater FSH concentrations following luteolysis in the final estrous cycle before the onset of nutritional anestrus have also been observed in chronically restricted heifers [3, 4]. Beckett et al. [31] found an increase in the anterior pituitary content of both FSH and FSHß mRNA after 7 wk of restriction in orchidectomized rams and attributed this to either a persistent stimulation of adrenal function during nutritive stress or to nutrition-dependent changes in activity of the GnRH pulse generator.
Resumption of normal estrous cycles in heifers that failed to ovulate after dietary restriction was characterized by the presence of a short estrous cycle. This observation is consistent with observations following resumption of ovulation in long-term nutritionally anestrous heifers [4], postpartum anestrous beef cows [32–34], and pubertal heifers [35, 36].
Because of the high incidence (60%) of anovulation among heifers fed 0.4 Mn, which limited animal numbers, the aim of determining the effect of acute nutritional restriction on pregnancy rate was not fulfilled. However, the initial indications are that neither oocyte competence nor ability to establish a pregnancy were affected by acute nutritional restriction in those heifers that exhibited estrous activity and ovulated. Similarly, with greater numbers of heifers, the feeding of 0.8 Mn for 10 days preinsemination was not detrimental to embryo survival compared with survival of embryos of heifers fed 2.0 Mn [37], and short-term feed restriction in the final stages of follicle maturation had no immediate effect on oocyte quality [38]. While this study did not attempt to test the longer-term effects of NEB on fertility, Britt [39] hypothesized that preantral oocyte development might be compromised in high-yielding dairy cows subjected to severe NEB in the early postpartum period, therefore affecting fertility at subsequent estrous periods. Further studies are required to examine the longer-term effects of a short period of acute NEB, in which beef heifers could serve as models, without the confounding effects of lactation.
In summary, this experiment confirmed that acute restriction from 1.2 to 0.4 Mn for a period of 13–15 days suppresses the growth rate and maximum diameter of dominant follicles, and results in the failure of dominant follicles to ovulate in 60% of heifers. We suggest that failure of ovulation is due to the absence of an LH/FSH surge, which in some cases is preceded by the absence of a proestrous increase in the concentration of estradiol. While the high incidence of anovulation limited animal numbers, pregnancy rate in heifers that did ovulate did not appear to be compromised.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 Correspondence. FAX: 353 91 845847; mdiskin{at}athenry.teagasc.ie
Accepted: August 10, 1999.
Received: May 11, 1999.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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