Effect of gonadotropin-releasing hormone at estrus on subsequent luteal function and fertility in lactating Holsteins during heat stress

Gonadotropin-Releasing Hormone (GnRH) and Its Agonists in Bovine Reproduction II: Diverse Applications during Insemination, Post-Insemination, Pregnancy, and Postpartum Periods

The administration of GnRH and its agonists benefits various aspects of bovine reproductive programs, encompassing physiological stages such as estrous synchronization, post-insemination, pregnancy, and the postpartum period. The positive impact of GnRH administration in overcoming challenges like repeat breeder cows, early embryonic loss prevention, and the management of cystic ovarian disease (COD) is thoroughly surveyed. Furthermore, this review focuses on the significance of GnRH administration during the postpartum period, its role in ovulation induction, and how it enhances the productivity of embryo transfer (ET) programs. An emerging feature of this field is introduced, focusing on nano-drug delivery systems for GnRH agonists, and the potential benefits that may arise from such advancements are highlighted. While this review offers valuable insights into various applications of GnRH in bovine reproduction, it emphasizes the crucial need for further research and development in this field to advance reproductive efficiency and health management in dairy cattle.

Interval from Oestrus to Ovulation in Dairy Cows-A Key Factor for Insemination Time: A Review

This review describes the oestrus-to-ovulation interval, the possibility of predicting the time of ovulation, and the optimum time for insemination relative to oestrus in dairy cows. The duration of oestrus in dairy cows is approximately 8-20 h, with differences possibly related to the methods of oestrus detection and the frequency of observations. Most cows ovulate approximately 24-33 h after the onset of oestrus and 15-22 h after the end of oestrus. The interval from the preovulatory luteinising hormone (LH) surge to ovulation is approximately 4-30 h. Ovulation occurs when follicle diameter averages 18-20 mm. When it is possible to correctly determine the beginning of oestrus, artificial insemination can be performed utilizing the "a.m.-p.m. rule", and only one insemination may be applied. In cows with too long or too short oestrus-to-ovulation intervals, fertility can be compromised. One important factor that can alter the oestrus-to-ovulation interval is acute or chronic heat stress during the warm season. When there is a risk that insemination may occur too early or too late with respect to the time of ovulation, GnRH administration can be considered.

The Effect of Reduced GnRH Dose on Ovulation and Follicular Dynamics in Ovsynch Programme of Pure and Bunaji-Crossbred Cows

Our objective was to compare the efficacy of reducing GnRH dose from 100 µg to 50 µg on the formation of ovulation and sizes of ovarian structures following Ovsynch in apparently healthy Bunaji and Friesian × Bunaji Cows. Thirty female multiparous-apparently-healthy adult [Bunaji (n = 15) and Friesian × Bunaji (n = 15)] breeds of cattle were used. Five cows each were allocated randomly to three groups [control; full dose (FD), and half dose (HD)]. Cows in the control group were treated with 2 ml normal saline while FD-group received 100 µg lecirelin on day 0, with 500 µg clorprostenol on day 7 and with 100 µg lecirelin two days later. Furthermore, HD-group received the same treatment as FD-cows but the dose of lecirelin was reduced to 50 µg at both times of GnRH administration. Ovarian structures were monitored by ultrasound with a 5-MHZ linear transrectal probe on days - 1 to 12. The ovarian responses of the various groups to first GnRH administration showed (0%, 40% and 60%) ovulation rate for C, HD and FD groups respectively in Bunaji breeds while in Friesian × Bunaji, it was (0%, 60%, 60%). Following second GnRH administration ovulation rate for Bunaji was (20%, 60%, and 60%) for Control, HD and FD-groups, respectively, while for Friesian × Bunaji cows it was (20%, 60%, and 80%). There was no significant difference (p > 0.05) in the days of new follicular wave emergence following the first GnRH administration. It was concluded that 50 µg Lecirelin reduced the cost of drug without affecting the efficiency of Ovsynch protocol.

Administration of GnRH at Onset of Estrus, Determined by Automatic Activity Monitoring, to Improve Dairy Cow Fertility during the Summer and Autumn

We examined gonadotropin-releasing hormone (GnRH) administration at onset of estrus (OE), determined by automatic activity monitoring (AAM), to improve fertility of dairy cows during the summer and autumn. The study was performed on two dairy farms in Israel. The OE was determined by AAM recorded every 2 h, and a single im dose of GnRH analogue was administered shortly after OE. Pregnancy was determined by transrectal palpation, 40 to 45 d after artificial insemination (AI). Conception risk was analyzed by the GLIMMIX procedure of SAS. Brief visual observation of behavioral estrus indicated that about three-quarters of the events (n = 40) of visually detected OE occurred within 6 h of AAM-detected OE. Accordingly, the GnRH analogue was administered within 5 h of AAM-detected OE, to overlap with the expected endogenous preovulatory LH surge. Overall, pregnancy per AI (P/AI) was monitored over the entire experimental period (summer and autumn) in 233 first, second or third AI (116 and 117 AI for treated and control groups, respectively). Least square means of P/AI for treated (45.8%) and control (39.4%) groups did not differ, but group-by-season interaction tended to differ (p = 0.07), indicating no effect of treatment in the summer and a marked effect of GnRH treatment (n = 58 AI) compared to controls (n = 59 AI) on P/AI in the autumn (56.6% vs. 28.5%, p < 0.03). During the autumn, GnRH-treated mature cows (second or more lactations), and postpartum cows exhibiting metabolic and uterine diseases, tended to have much larger P/AI than their control counterparts (p = 0.07-0.08). No effect of treatment was recorded in the autumn in first parity cows or in uninfected, healthy cows. In conclusion, administration of GnRH within 5 h of AAM-determined OE improved conception risk in cows during the autumn, particularly in those exhibiting uterine or metabolic diseases postpartum and in mature cows. Incorporation of the proposed GnRH treatment shortly after AAM-detected OE into a synchronization program is suggested, to improve fertility of positively responding subpopulations of cows.

Cooling is the predominant strategy to alleviate the effects of heat stress on dairy cows

Reduced reproductive performance of lactating cows during the summer is associated mainly with intensive genetic selection for high milk production, which places a great load on the thermoregulatory mechanism. In the last decades, a big effort has been made to explore the mechanism by which heat stress compromises fertility. The data gained so far revealed that the effect of thermal stress on the female reproductive tract is multifactorial in nature. Based on this understanding, new strategies to mitigate the effect of heat stress have been developed. The review summarizes some of the physiological responses of the cow to elevated temperature and discusses its limitations to maintain normothermia. The review emphasizes that cooling is the predominant strategy used today to alleviate the effects of heat stress. Findings from the Israel dairy herd indicate that efficient cooling management can improve milk production during the summer to a similar level of the winter, expressed by summer to winter ratio of 0.98. However, cooling as a singular approach cannot eliminate the decline in reproduction. Nonetheless, an efficient cooling system is a prerequisite for any other strategy. The review suggests additional hormonal treatments to improve reproductive performance during the summer. Given the complexity of heat stress effects on reproduction, comprehensive reproductive management during the summer is suggested, that is combining two or more strategies in a programme, might be more beneficial.

Invited review: Management strategies capable of improving the reproductive performance of heat-stressed dairy cattle

Impaired fertility during periods of heat stress is the culmination of numerous physiological responses to heat stress, ranging from reduced estrus expression and altered follicular function to early embryonic death. Furthermore, heat-stressed dairy cattle exhibit a unique metabolic status that likely contributes to the observed reduction in fertility. An understanding of this unique physiological response can be used as a basis for improving cow management strategies, thereby reducing the negative effects of heat stress on reproduction. Potential opportunities for improving the management of dairy cattle during heat stress vary greatly and include feed additives, targeted cooling, genetic selection, embryo transfer and, potentially, crossbreeding. Previous studies indicate that dietary interventions such as melatonin and chromium supplementation could alleviate some of the detrimental effects of heat stress on fertility, and that factors involved in the methionine cycle would likely do the same. These supplements, particularly chromium, may improve reproductive performance during heat stress by alleviating insulin-mediated damage to the follicle and its enclosed cumulus-oocyte complex. Beyond feed additives, some of the simplest, yet most effective strategies involve altering the timing of feeding and cooling to take advantage of comparatively low nighttime temperatures. Likewise, expansion of cooling systems to include breeding-age heifers and dry cows has significant benefits for dams and their offspring. More complicated but promising strategies involve the calculation of breeding values for thermotolerance, the identification of genomic markers for heat tolerance, and the development of bedding-based conductive cooling systems. Unfortunately, no single approach can completely rescue the fertility of lactating dairy cows during heat stress. That said, region-appropriate combinations of strategies can improve reproductive measures to reasonable levels.

Effect of prostaglandin F2α and GnRH administration at the time of artificial insemination on reproductive performance of dairy cows

The present study aimed to determine the effect of administrating prostaglandin F2α (PGF2α) and GnRH at the time of artificial insemination (AI) on the pregnancy per artificial insemination (P/AI) and the pregnancy survival rate of dairy cows. A number of 830 lactating Holstein cows were randomly divided into four groups. Cows in group 1 (n=200) treated with 150 µg d-cloprostenol. In group 2 (n=212), cows received 10 µg buserelin acetate, and group 3 (n=205) was treated with both 150 µg d-cloprostenol and 10 µg buserelin acetate. In addition, 213 cows were assigned as control group which received normal saline as placebo (group 4). To measure progesterone, milk samples were collected at the insemination day and five days later. Pregnancy diagnosis was performed 28 and 60 days after the insemination, and the size and number of corpus luteum (CL) and twin pregnancies were recorded. Hormone therapies had no effect on the P/AI, pregnancy survival rate, and the size and number of CL. The P/AI ratio in groups 1, 2, 3 and 4 were 38.50%, 42.92%, 41.46% and 40.84%, and the pregnancy survival rates in groups 1, 2, 3 and 4 were 84.42%, 86.81%, 88.23% and 83.91%, respectively. The probability of a twin pregnancy was significantly higher in group 1 (15.58%) than other groups. There was no significant difference between groups in terms of the offspring gender. In conclusion, the administration of d-cloprostenol or buserelin acetate at the time of AI had no effect on P/AI and pregnancy survival rate in dairy cattle under no heat stress condition, while the administration of d-cloprostenol increased the probability of twin pregnancies.

OvSynch Protocol and its Modifications in the Reproduction Management of Dairy Cattle Herds - an Update

Current knowledge about the function of the reproductive tract and appropriate use of hormonal drugs affords control of the oestrus cycle of cows. One of the hormonal protocols is OvSynch, which enables artificial insemination (AI) to be performed at the precise optimum time without control of the ovaries and uterus. Use of such protocols in reproductive management allows oestrus cycles to be synchronised and cows to be effectively inseminated without oestrous detection, which is time-consuming and difficult in farms with numerous cows. Therefore, OvSynch has become the first management tool for AI and is an alternative method to heat detection. Over the 20 years since its first implementation, OvSynch has been modified many times to improve its reproduction outcomes and widen its use. Besides its original use for heat synchronisation, it is also used in many ovarian disorders as a therapeutic method. This review article describes the possibilities which OvSynch provides, its current modifications, various applications, and the advantages and disadvantages of its use in practice.

Impact of Buserelin Acetate or hCG Administration on the Day of First Artificial Insemination on Subsequent Luteal Profile and Conception Rate in Murrah Buffalo (Bubalus bubalis)

This study was designed to investigate the impact of buserelin acetate (BA) or human chorionic gonadotropin (hCG) administration on the day of first artificial insemination (AI) on subsequent luteal profile (diameter of corpus luteum (CL) and plasma progesterone) and conception rate in Murrah buffalo. The present experiment was carried out at two locations in 117 buffalo that were oestrus-synchronized using cloprostenol (500 μg) administered (i.m.) 11 days apart followed by AI during standing oestrus. Based on treatment (i.m.) at the time of AI, buffalo were randomly categorized (n = 39 in each group) into control (isotonic saline solution, 5 ml), dAI-BA (buserelin acetate, 20 μg) and dAI-hCG (hCG, 3000 IU) group. Out of these, 14 buffalo of each group were subjected to ovarian ultrasonography on the day of oestrus to monitor the preovulatory follicle and on days 5, 12, 16 and 21 post-ovulation to monitor CL diameter. On the day of each sonography, jugular vein blood samples were collected for the estimation of progesterone concentrations. All the buffalo (n = 117) were confirmed for pregnancy on day 40 post-ovulation. The conception rate was better (p < 0.05) in dAI-BA (51.3%) and dAI-hCG (66.7%) groups as compared to their control counterparts (30.8%). Furthermore, the buffalo of dAI-hCG group had improved (p < 0.05) luteal profile, whereas the buffalo of dAI-BA group failed (p > 0.05) to exhibit stimulatory impact of treatment on luteal profile when compared to control group. In brief, buserelin acetate or hCG treatment on the day of first AI leads to an increase in conception rate; however, an appreciable impact on post-ovulation luteal profile was observed only in hCG-treated Murrah buffalo.

Synchronization of ovulation with human chorionic gonadotropin in lactating dairy cows with ovarian cysts during heat stress

A study was conducted during hot season to determine the effect of synchronization of ovulation with human chorionic gonadotropin (hCG) on fertility of lactating dairy cows with ovarian cysts. Non cyclic Holstein dairy cows (n = 80) were stratified by parity and diagnosed as having an ovarian cyst. The cows were further identified as follicular or luteal cysts according to the plasma progesterone (P4) concentration and the cystic image of ultrasonography. Cystic cows were randomly assigned to receive treatments (Ovsynch as the control or Ovsynch plus 3000 IU hCG). All cows were artificially inseminated at 16-18 h after the second gonadotropin releasing hormone injection. Cows supplemented with hCG had a greater number of corpus luteum (1.8 ± 0.2 and 0.8 ± 0.3; P < 0.05) and had greater P4 concentration on day 12 than those control cows (6.3 ± 0.3 and 3.9 ± 0.4 ng/ml; P < 0.05). Concentration of cortisol did not differ between groups of cystic cows. No significant differences were found in overall conception rates between the treatments; however, significantly greater conception rate (P = 0.03) was observed in cows with luteal cysts receiving Ovsynch plus hCG. This study highlights that administration of hCG following the Ovsynch-based timed artificial insemination (AI) is more effective than the control Ovsynch by which the hCG affects corpus luteum (CL) development, P4 concentration, and thus improves conception rate in dairy cows with luteal cysts.

The effect of buserelin injection 12 days after insemination on selected reproductive characteristics in cows

The aim of this study was to evaluate the effect of buserelin injection on day 12 postinsemination on fertility in lactating dairy cattle. A total of 57 cows were assigned to two groups and four subgroups. In the treatment group, the cows were synchronized with PGF2α-PGF2α (group A) or GnRH-PGF2α (group B) protocol, and buserelin was injected on day 12 postinsemination. Cows in the control group were synchronized with PGF2α-PGF2α (group C) or GnRH-PGF2α (group D) protocol, saline solution was injected on day 12, and served as controls. Pregnancy rates on day 21 and 45 and embryonic death rates were 85.7%, 71.4% and 16.7%, 85.7%, 85.7% and 0.0%, 73.3%, 62.1% and 27.3% and 85.7%, 71.4% and 16.7% in groups A, B, C and D, respectively. There was no significant difference between synchronization protocols for pregnancy rates, and among groups A, B, C and D for pregnancy rates and embryonic death rates. Mean progesterone concentrations in pregnant cows in groups A and B were higher than that in groups C and D, respectively, on day 18 and 21 (p < 0.05). In conclusion, GnRH injection on day 12 postinsemination increased the plasma progesterone concentrations on day 18 and 21 postinsemination. However, it did not alter the pregnancy rates and prevent embryonic deaths.

The effect of administration of different levels of GnRH on the day 0, 5 and 12 post-insemination on progesterone concentration in dairy heifers

This study was carried out to evaluate the effects of different levels of GnRH in different days of reproductive cycle on progesterone concentration in dairy heifers. Two hundred heifers were divided into ten experimental treatment: control group with no injection (1) administration of 2.5 mL Gonadorelin (a GnRH analogue) in day of insemination (2) administration of 5 mL GnRH in day of insemination (3) administration of 10 mL GnRH in day of AI insemination (4) administration of 2.5 mL GnRH in day 5 post-insemination (5) administration of 5 mL GnRH in day5 post-insemination (6) administration of 10 mL GnRH in day5 post-insemination (7) administration of 2.5 mL GnRH in day 12 post-insemination (8) administration of 5 mL GnRH in day 12 post-insemination (9) administration of 10 mL GnRH in day 12 post-insemination. Blood samples were collected in days 0, 5, 12 and 19 post insemination (AI = Day 0) for analysis of serum P4 concentration. There was no significant difference among experimental groups on day of insemination. Evaluation of P4 concentration on day 19 illustrated differences in progesterone concentration between groups on day 19 post-insemination. P4 concentration of serum in the day 19 post-insemination significantly increased in groups either by 5 or 10 mL injection of GnRH whether in day 5 or 12 post-insemination versus control group (9.24 +/- 2.2, 8.6 +/- 1.96, 9.43 +/- 2.15 and 9.42 +/- 2.14 versus 5.5 +/- 0.8, respectively; p < 0.05). GnRH administration in the day 5 and 12 post-AI significantly increased progesterone concentration that may decline early embryonic death and improve pregnancy rate.

Exposure to a Physiologically Relevant Elevated Temperature Hastens In Vitro Maturation in Bovine Oocytes

The objective of this study was to evaluate nuclear (progression to metaphase II) and cytoplasmic (translocation of cortical granules to the oolemma) maturation in control (38.5 degrees C) and heat-stressed (41.0 degrees C) oocytes. Hoechst staining indicated that a similar proportion of control and heat-stressed oocytes progressed to meta-phase II. More heat-stressed oocytes had type III cortical granule distribution suggesting that heat stress accelerated cytoplasmic maturation. The kinetics of nuclear maturation was examined in a second experiment in which a higher proportion of heat-stressed oocytes progressed to metaphase I by 8 h and arrested at meta-phase II at 16 and 18 h after placement into maturation medium. However, differences related to maturation temperature were no longer apparent by 21 h. Heat-induced alterations in kinetics of nuclear and cytoplasmic maturation prompted a third experiment to evaluate if earlier insemination of heat-stressed oocytes ameliorates heat-induced reductions in development. A significant temperature x insemination time interaction was noted when evaluating blastocyst development. Blastocyst development was reduced when heat-stressed oocytes were inseminated with sperm 24 h after placement into maturation medium compared with controls. In contrast, blastocyst development was similar to controls when heat-stressed oocytes were inseminated at 19 h. Based on this interaction, earlier insemination in vitro prevented heat-induced reductions in oocyte development. Collectively, these studies suggest a cumulative effect of heat stress to hasten in vitro maturation in bovine oocytes.

GnRH in non-hypothalamic reproductive tissues

Gonadotropin releasing hormone (GnRH) is a hypothalamic neuronal secretory decapeptide that plays a pivotal role in mammalian reproduction. GnRH and its analogues are used extensively in the treatment of hormone dependent diseases and assisted reproductive technology. Fourteen structural variants and three different forms of GnRH, named as hypothalamic GnRH or GnRH-I, mid brain GnRH or GnRH-II and GnRH-III across various species of protochordates and vertebrates have been recognised. The hormone acts by binding to cell surface transmembrane G protein coupled receptors (GPCRs) and activates Gq/11 subfamily of G proteins. Although hypothalamus and pituitary are the principal source and target sites for GnRH, several reports have recently suggested extra-hypothalamic GnRH and GnRH receptors in various reproductive tissues such as ovaries, placenta, endometrium, oviducts, testes, prostrate, and mammary glands. GnRH-II appears to be predominantly expressed in extra pituitary reproductive tissues where it produces its effect by PLC, PKA2, PLD, and AC cell signalling pathways. In these tissues, GnRH is considered to act by autocrine or paracrine manner and regulate ovarian steroidogenesis by having stimulatory as well as inhibitory effect on the production of steroid hormones and apoptosis in ovarian follicle and corpus luteum. In male gonads, GnRH has been shown to cause a direct stimulatory effect on basal steroidogenesis and an inhibitory effect on gonadotropin-stimulated androgen biosynthesis. Recent studies have shown that GnRH is more abundantly present in ovarian, endometrial and prostrate carcinomas. The presence of type-II GnRH receptors in reproductive tissues (e.g. gonads, prostrate, endometrium, oviduct, placenta, and mammary glands) suggests existence of distinct role(s) for type-II GnRH molecule in these tissues. The existence of different GnRH forms indicates the presence of distinctive cognate receptors types in vertebrates and is a productive area of research and may contribute to the development of new generation of GnRH analogues with highly selective and controlled action on different reproductive tissues and the target-specific GnRH analogues could be developed.

Heat stress and seasonal effects on reproduction in the dairy cow--a review

In dairy cows inseminated during the hot months of the year, there is a decrease in fertility. Different factors contribute to this situation; the most important are a consequence of increased temperature and humidity that result in a decreased expression of overt estrus and a reduction in appetite and dry matter intake. Heat stress reduces the degree of dominance of the selected follicle and this can be seen as reduced steroidogenic capacity of its theca and granulosa cells and a fall in blood estradiol concentrations. Plasma progesterone levels can be increased or decreased depending on whether the heat stress is acute or chronic, and on the metabolic state of the animal. These endocrine changes reduce follicular activity and alter the ovulatory mechanism, leading to a decrease in oocyte and embryo quality. The uterine environment is also modified, reducing the likelihood of embryo implantation. Appetite and dry matter intake are both reduced by heat stress thus prolonging the postpartum period of negative energy balance and increasing the calving-conception interval, particularly in high producing dairy cows. The utilization of cooling systems may have a beneficial effect on fertility but dairy cows cooled in this way are still unable to match the fertility achieved in winter. Recent studies suggest that the use of gonadotropins to induce follicular development and ovulation can decrease the severity of seasonal postpartum infertility in dairy cows.

Effects of GnRH administered to cows at the onset of estrus on timing of ovulation, endocrine responses, and conception

Two experiments examined effects of GnRH administered within 3 h after onset of estrus (OE) on ovulation and conception in dairy cows. In experiment 1, 46 cows received either saline, 250 microg of GnRH, or 10 microg of the GnRH analogue, Buserelin. Cows were observed for estrus, blood samples were collected, and ovulations were monitored by ultrasound. In controls, 76% of cows had intervals from estrus to ovulation of < or = 30 h and 24% had intervals > 30 h. Treatment with either GnRH or GnRH analogue (data combined) increased magnitude of LH surges and decreased intervals from estrus to LH surge or to ovulation. Treated cows all ovulated < or = 30 h after OE. Among control cows, plasma estradiol concentrations before estrus correlated positively with amplitudes of LH surges. Higher plasma progesterone was observed in the subsequent estrous cycle in GnRH-treated cows compared to control cows with delayed ovulations. Experiment 2 included 152 primiparous and 211 multiparous cows in summer and winter. Injection of GnRH analogue at OE increased conception rates (CR) from 41.3 to 55.5% across seasons. In summer, GnRH treatment increased CR from 35.1 to 51.6%. Across seasons, GnRH increased CR from 36.0 to 61.5% in cows with lower body condition at insemination and GnRH increased CR (63.2 vs. 42.2%) in primiparous cows compared to controls. Use of GnRH eliminated differences in CR for cows inseminated early or late relative to OE and increased CR in cows having postpartum reproductive disorders. In conclusion, GnRH at onset of estrus increased LH surges, prevented delayed ovulation, and may increase subsequent progesterone concentrations. Treatments with GnRH increased conception in primiparous cows, during summer, and in cows with lower body condition.

The effects of GnRH administration postinsemination on serum concentrations of progesterone and pregnancy rates in dairy cattle exposed to mild summer heat stress

The objective of this study was to evaluate whether administration of GnRH postinsemination would improve reproductive performance in heat-stressed dairy cattle. Estrous cycles of Holstein cows were synchronized using the OvSynch protocol and cows were artificially inseminated. Cows were then administered the following treatments: control (no GnRH; n=37), GnRH (100 microg) on Day 5 (GnRH-D5; n=34), or GnRH (100 microg) on Day 11 (GnRH-D11; n=34) postinsemination. Cows were provided access to both fans and sprinklers, and environmental data was collected hourly. Rectal temperatures and blood samples were obtained from cows on Days -9, -2, 0 (AI) and on alternate days from Day 5 to Day 19 postinsemination. Blood serum was collected for the analysis of progesterone (P(4)) by RIA. In a subset of cows (n=6/treatment) ultrasonography was performed on alternate days from Day 5 to Day 19 postinsemination to assess numbers of corpora lutea (CL) and CL cross-sectional areas. Pregnancy status of cows was confirmed at Day 30 postinsemination. Environmental data indicated that cows experienced mild heat stress during the trials (mean daily THI=73-77). Serum P(4) was greater (P<0.05) after Day 9 for GnRH-D5 cows and after Day 15 for GnRH-D11 cows through Day 19 postinsemination. The number of CLs present for GnRH-D5 cows was greater (P<0.05) on Day 17 than in either the control or GnRH-D11 treatment groups. On Day 17 postinsemination, both the GnRH-D5 and GnRH-D11 cows were observed to have greater (P<0.05) total CL tissue area than control cows. The interval from insemination to when serum P(4) returned to <1 ng/ml (i.e. luteolysis and return to estrus) did not differ (P>0.10) among treatment groups. Control cows (19%) tended to exhibit lower pregnancy rates (P<0.08) compared to the GnRH-D5 and GnRH-D11 treatment groups combined (35%). In summary, the treatment of heat-stressed dairy cows with GnRH postinsemination (Day 5 or 11) results in the appearance of more CL tissue, increased serum concentrations of P(4) and a tendency toward greater pregnancy rates.

Impaired reproduction in heat-stressed cattle: basic and applied aspects

Summer heat stress (HS) is a major contributing factor in low fertility in lactating dairy cows in hot environments. Although modern cooling systems are used in dairy farms, fertility remains low. This review summarizes the ways in which the functioning of various parts of the reproductive system of cows exposed to HS is impaired. The dominance of the large follicle is suppressed during HS, and the steroidogenic capacity of theca and granulosa cells is compromised. Progesterone secretion by luteal cells is lowered during summer, and in cows subjected to chronic HS, this is also reflected in lower plasma progesterone concentration. HS has been reported to lower plasma concentration of LH and to increase that of FSH; the latter was associated with a drastic reduction in plasma concentration of inhibin. HS impairs oocyte quality and embryo development, and increases embryo mortality. High temperatures compromise endometrial function and alter its secretory activity, which may lead to termination of pregnancy. In addition to the immediate effects, delayed effects of HS have been detected as well. Among them, altered follicular dynamics, suppressed production of follicular steroids, and low quality of oocytes and developed embryos. These may explain the low fertility of cattle during the cool autumn months. Hormonal treatments improve low summer fertility to some extent but not sufficiently for it to equal winter fertility. A limiting factor is the inability of the high-yielding dairy cow to maintain normothermia. A hormonal manipulation protocol, which induces timed insemination, has been found to improve pregnancy rate and to reduce the number of days open during the summer.

Postpartum subestrus in dairy cows: comparison of treatment with prostaglandin F2 alpha or GnRH + prostaglandin F2 alpha + GnRH

Two experiments (Experiment 1, 185 cows in 1996/97; Experiment 2, 168 cows in 1997/98) were conducted with Prim Holstein dairy cattle in the Mayenne region of France to investigate subestrus. Cows which had not been observed in estrus since calving were allocated alternately to treatment groups between 60 and 90 d post partum as follows: Experiment 1-Group 1: GnRH (Day 0, 100 micrograms i.m.), PGF2 alpha (Day 7, 25 mg i.m.), GnRH (Day 9, 100 micrograms i.m.) and AI (Day 10); Group 2: PGF2 alpha (Day 0, 25 mg i.m.), AI at estrus, or, if estrus was not observed, a second PGF2 alpha injection on Day 13, and AI on Day 16 and Day 17. Treatments in Experiment 2 were as follows: Group 1: as Experiment 1-Group 1 but AI at the observed estrus after Day 0, or at Day 10 if estrus was not observed; Group 2: as Experiment 1--Group 2, however, if a second PGF2 alpha injection was given on Day 13, AI at the observed estrus. Progesterone was measured in serum at Day 0 and in milk at AI. Pregnancy diagnosis was performed by measuring bovine pregnancy-specific protein B (bPSPB; Day 50 +/- 3) and confirmed by ultrasonography when the result was doubtful. In Experiment 1, farmers observed 47/101 (46.9%) Group 1 cows in estrus, 33/91 cows on Day 10 and 10 cows before Day 10. The progesterone concentrations were compatible with estrus in 69/86 (80%) cows on Day 10. In Group 2, 36/83 (43.4%) cows were inseminated after the first PGF2 alpha injection. After the second PGF2 alpha injection, only 29/43 (67%) cows had a low progesterone concentration at AI. Pregnancy rates were 36.1 and 32.5% for Groups 1 and 2, respectively. In Experiment 2, estrus was observed in 31/93 (33.7%) Group 1 cows. In Group 2, 51/75 (66%) cows were inseminated after the first injection of PGF2 alpha, 13/75 (17.3%) cows after the second injection, while 11/75 (14.7%) were not observed in estrus. Pregnancy rates were 53.7 and 53.3% in Groups 1 and 2, respectively. In conclusion, it is recommended that subestrus be treated with PGF2 alpha followed by AI at the observed estrus when estrus detection is good, while the use of GnRH + PGF2 alpha + GnRH is recommended when estrus detection is poor.