Enhancing reproductive efficiency: Antiluteolytic strategies for luteal support in livestock

Introduction

India's livestock sector forms a vital component of the national agricultural economy, playing a key role in sustaining rural livelihoods and contributing significantly to dairy production. According to the 2019 Livestock Census, the country's livestock population reached 535.78 million, an increase of 4.6% from the previous census-with cattle alone accounting for 192.49 million (Press Information Bureau, Government of India, 2019) [42] . Despite this substantial growth, reproductive inefficiencies remain a critical challenge, adversely impacting productivity and economic sustainability. Reproduction in cattle is a tightly regulated biological process that relies on precise communication between the embryo and the maternal system for successful pregnancy recognition, implantation, and placentation (Guillomot, 1995; Spencer et al., 2007) [14,58] . In normal polycyclic animals, luteolysis occurs and the animal returns to estrus if the oocyte is not fertilized while if fertilization does occur, luteolysis must be inhibited-typically around Day 16 post-estrus-to ensure the establishment and maintenance of pregnancy (Binelli et al., 2001) [6] . This suppression depends on the conceptus's ability to produce strong antiluteolytic signals and the endometrium's capacity to respond by inhibiting the secretion of prostaglandin F 2α (PGF 2α ) (Binelli et al., 2001) [6] . The fertilization rates can reach nearly 100% in heifers (Santos et al., 2004) [46] and about 77-90% in dairy cows, conception rates measured between days 27 and 31 post-AI are significantly lower, ranging from only 35 to 45% (Diskin and Sreenan, 1980, Thatcher et al., 1994, Sreenan et al., 2001, Sartori et al., 2002) [9,62,59,48] . Hence, in spite of the high fertilization rate, the conception rate is very low because of failure in the effective suppression of the luteolysis mechanism. However, fertilization failure accounts for only about 10% of overall reproductive failure while embryo death accounts for more than 30% and major portion of this embryo loss would seem to occur between days 8 and16 (Diskin and Sreenan, 1980) [9] . These reproductive challenges underline the necessity for effective antiluteolytic strategies aimed at supporting corpus luteum (CL) function and minimizing early pregnancy loss. Consequently, various interventions have been developed to extend CL lifespan and enhance embryo viability in farm animals.

GnRH

The gonadotrophin releasing hormone (GnRH) modulates the secretion of luteinizing hormone (LH), and the endogenous surge of LH during estrus which is responsible for ovulation and luteinization of granulosa and theca cells for progesterone secretion (Henderson, 1979) [17] . All this happens via the hypertrophy and increased blood flow to the CL (Farin et al., 1988; Niswender et al., 1976) [10,38] . However the effect of a single injection (day 12) of GnRH or hCG reported no improvement in the fertility of the ewes (Ishida et al., 1999) [20] . Early treatment with GnRH (Howard et al., 2006) [19] has proved to be more effective compared with late treatment, GnRH injections given on day 5 of the estrous cycle consistently induced ovulation of the dominant follicle giving accessory CL (Schmitt et al., 1996) [51] however other study reported only 60% cows with accessory CL administered with GnRH between day 4 to 9 following AI (Howard et al., 2006; Santos et al., 2001; Stevenson et al., 2007) [19,47,60] . Others have reported that animals administered GnRH after day 10 of AI had higher pregnancy per AI as compared to GnRH treatment before day 10 of AI. The opinion of Giordano et al. (2012) [13] suggests that formation of accessory CL during the period of the first follicular wave resulted in a significant increase in pregnancy rate as it creates a favourable uterine environment for embryonic development by increasing the levels of progesterone during the early phases of embryonic development.

HCG

The administration of Human Chorionic Gonadotropin (HCG) has been widely studied as an antiluteolytic strategy to improve conception rates in cattle, particularly in repeat breeders. Karsavuranoğlu et al. (2022) [21] reported that conception rates were similar in groups treated with GnRH alone and those treated with a combination of GnRH and HCG, suggesting no added benefit from combining the two. In contrast, Thakur et al. (2018) [61] found that HCG administration, whether at the time of artificial insemination (AI) or on the 5th or 12th day post-AI, resulted in improved conception rates in cattle. Similarly, Lim et al. (2018) [25] observed increased conception rates in dairy cows when HCG was administered on the 5th day post-AI. Miura et al. (2018) [32] reported enhanced conception success specifically in cattle possessing a first-wave dominant follicle (DF) in the ovary ipsilateral to the corpus luteum (CL), when HCG was administered on day 5 after AI. Rahman et al. (2021) [43] also found a beneficial effect of HCG administration either at the time of AI or within a few days post-AI in repeat-breeding cows. Parikh et al. (2018) [40] observed improved conception rates with both HCG and GnRH administration. Additionally, Santos et al. (2001) [47] reported that the use of HCG enhanced conception rates in dairy cattle, which was attributed to the formation of accessory corpora lutea, thereby supporting luteal function and early pregnancy maintenance.

Progesterone

Several studies have investigated the role of progesterone supplementation in enhancing fertility in cattle, particularly among animals with reproductive challenges. Friedman et al. (2012) [11] reported that Controlled Internal Drug Release (CIDR) devices containing progesterone improved fertility in cows with low body condition scores (BCS) and postpartum disorders, especially during the summer months. Awasthi et al. (2002) [5] also observed improved conception rates in crossbred cows when progesterone was administered during the mid-luteal phase of the estrous cycle. Similarly, Singh, et al. (2008) [52] demonstrated that progesterone treatment significantly enhanced conception rates in repeat-breeding cattle. Supporting this, Ababneh et al. (2007) [1] found that administering progesterone after insemination led to higher conception success. Kumar et al.

(2012) [23] noted that progesterone administration on the 3rd or 5th day post-insemination had a positive effect on conception rates in repeat breeders. However, not all findings were uniformly positive. Parikh et al. (2018) [40] suggested that while progesterone supplementation does have beneficial effects, it is generally less effective than treatments involving HCG or GnRH. More recently, Modaresi et al. (2024) [33] proposed that the use of sustained-release progesterone formulations could be especially useful in managing fertility in repeat-breeding cows. On the other hand, Kumar et al. (2025) [24] reported that progesterone supplementation showed no significant improvement in conception rates in buffaloes, indicating that species-specific differences and management practices may influence outcomes. These varying results underscore the need for further research to better understand the optimal conditions and protocols for progesterone use in different breeds and production systems.

Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)

PGF 2α is synthesized from arachidonic acid through a cascade of enzymatic reactions involving cyclooxygenase-2 (COX-2). Various COX-2 inhibitors such as meloxicam, aspirin, and flunixin meglumine have been studied by researchers for their potential to suppress prostaglandin production and thereby enhance conception rates in livestock. Singh et al. (2021) [53] noted that invasive reproductive procedures can trigger stress-induced prostaglandin release, negatively impacting progesterone levels and pregnancy maintenance. NSAID administration in such cases helps counteract this effect by inhibiting COX activity and reducing inflammatory responses. Merrill et al. (2007) [31] reported that administration of flunixin meglumine (FM) improved conception rates in cattle. Similarly, Skidmore et al. (2002) [55] observed a reduction in PGFM concentrations in camels treated with FM prior to embryo transfer (ET). Supporting these findings, Scenna et al. (2005) [49] demonstrated an increased pregnancy rate following FM administration at the time of ET in cattle. Damarany and Ghanem (2020) [8] also confirmed the beneficial effects of FM in improving conception rates in Egyptian Baladi cows during the hot season. Alkan et al. (2021) [4] concluded that carprofen administration post-artificial insemination (AI) enhanced conception rates in cattle that had experienced prolonged infertility postpartum. Regarding meloxicam, Rajkumar et al. (2010) [44] reported a 20% improvement in conception rates in buffaloes treated with meloxicam compared to the control group. Similarly, Aguiar et al. (2013) [2] found that meloxicam treatment improved conception rates after ET in heifers. Tolfenamic acid has also shown promising results. Schlapp et al. (2015) [50] reported that its administration during ET in mice improved pregnancy rates and pup survival. Maithani et al. (2023) [28] further demonstrated that tolfenamic acid enhanced conception rates after ET by reducing PGFM levels and increasing progesterone concentrations. Additionally, Singh et al. (2020) [54] found that tolfenamic acid administration post-AI improved conception rates in repeat breeder cattle. Kasimanickam et al. (2018) [22] reported improved conception rates in temperamentally sensitive beef cows following embryo transfer when flunixin meglumine (FM), an NSAID, was used. Schlapp et al. 2015 [50] reported administration of Tolfenamic Acid at the time of ET improve both maintenance of pregnancy and embryo survival. Spencer et al. 2020 [56] concluded that NSAID in form of FM or ibuprofen improve the conception rate if administered at the time of ET. Collectively, these findings highlight the potential of NSAID-based interventions in improving conception outcomes, minimizing early embryonic loss, and supporting overall reproductive efficiency in both livestock and laboratory animal models.

Omega-3 Polyunsaturated Fatty Acids (n-3 PUFAs)

Dietary supplementation with omega-3 polyunsaturated fatty acids (n-3 PUFAs), primarily sourced from fish oil and linseed, has demonstrated positive effects on reproductive performance in various farm animal species. These fatty acids are known to modulate prostaglandin synthesis, steroid hormone profiles, and ovarian function, ultimately enhancing fertility. Mahala et al. (2016, 2017) [27,26] reported that supplementation of goats with n-3 PUFA-rich fish oil delayed the onset of PGF₂α-induced estrus, suggesting a potential role in modulating luteolysis and extending the luteal phase. Verma et al. (2018) [65] further concluded that dietary fish oil inhibits oxytocin (OXT)-induced PGF₂α release and reduces estradiol (E₂) concentrations during the late luteal phase in does, thereby supporting maternal recognition of pregnancy. Verma et al. (2017) [64] also found that n-3 PUFA supplementation alters serum biochemical profiles, including a dose-dependent reduction in blood cholesterol, an essential precursor for steroid hormones such as estrogen, potentially optimizing the hormonal environment for reproduction. In buffaloes, Nazir et al. (2013) [36] demonstrated that dietary supplementation with crushed flaxseed improved conception rates by attenuating luteolytic signals and enhancing the post-breeding luteal profile. Similarly, Akhtar et al. (2024) [3] reported increased serum progesterone (P₄) concentrations following linseed supplementation in sheep, indicating improved luteal function. In cattle, Girgo et al. (2020) [12] observed significantly higher conception rates (58.30% vs. 33.33%) and markedly reduced embryo mortality (14.30% vs. 50.00%) in cows receiving n-3 PUFA supplementation compared to controls. Gulliver et al. (2012) [15] found that n-3 PUFA supplementation extended the interval to estrus and parturition, correlating this delay with reduced PGF₂α levels. Mohammed et al. (2024) [34] reported that dietary inclusion of extruded flaxseed (7.0%) or salmate (25 g/head/day) during the transition period until six months postpartum in commercial Holstein cows significantly accelerated ovarian activity and improved conception rates. Moreover, Pereira et al. (2021) [41] highlighted the benefits of n-3 PUFA in improving fertility among high-yielding dairy cows by promoting early postpartum conception. Collectively, these findings support the strategic inclusion of n-3 PUFA-rich feed sources-such as fish oil and flaxseed-in the diets of farm animals as a means to enhance reproductive performance, reduce early embryonic losses, and improve conception outcomes.

Interferon Tau (IFNT)

Interferon tau (IFNT) plays a crucial role in maternal recognition of pregnancy in ruminants, primarily secreted by trophoblastic cells of the blastocyst during early pregnancy (Hernandez et al., 1992) [18] . Northey and French (1980) [39] showed that intrauterine infusion of homogenized bovine conceptuses from Days 17 and 18 of gestation prolonged the inter-oestrus interval by inhibiting corpus luteum (CL) regression. Their findings led to the conclusion that, between Days 15 and 17 of pregnancy, the bovine conceptus produces a luteoprotective signal-now known as interferontau (IFNT)-which is crucial for sustaining CL function and supporting early pregnancy. Further research using embryo removal and transfer techniques established that ovine embryos influence the luteal lifespan as early as day 13 after mating (Moor & Rowson, 1966) [35] . Rowson and Moor (1967) [45] demonstrated that homogenates of Day 13 and 14 ovine conceptuses contained substances capable of prolonging the luteal lifespan in nonpregnant ewes. Similarly, Martal et al. (1990) [29] showed that homogenates of conceptuses between 14 and 16 days could extend the CL lifespan for over a month when injected into the uteri of cyclic ewes, further supporting the role of conceptus-derived factors in luteal maintenance. In cattle, Northey and French (1980) [39] found that removal of the conceptus at Days 17, 18, or 19 of pregnancy resulted in prolonged luteal lifespans, compared to nonbred controls or conceptus removal at earlier days (13 or 15) of pregnancy. Research involving IFNT infusion into the uterus has further confirmed its role in extending the luteal lifespan. Twice-daily infusions or injections of IFNT (Vallet et al., [63] or recombinant ovine IFNT (Martal et al., 1990) [29] into the uterus of sheep for 2-8 days extended the luteal lifespan to 22-27 days post-estrus. Similarly, uterine vein infusion of recombinant ovine IFNT (roIFNT) has been shown to extend the luteal lifespan in ewes (Bott et al., 2010) [7] , underscoring the critical role of IFNT in pregnancy maintenance. This secretion continues as the blastocyst elongates (Nephew et al., 1991) [37] . IFNT is the key signaling molecule from the fetus that signals its presence to the mother, preventing luteal regression and supporting the corpus luteum (CL) function, thereby sustaining pregnancy. Upon secretion, IFNT interacts with its receptors on the uterine endometrium, modulating the production of Prostaglandin F 2α (PGF₂α), a critical mediator of luteal regression (Vallet et al., 1988; Spencer and Bazer, 1996) [63,57] . This action inhibits PGF₂α synthesis, ensuring that the corpus luteum is maintained and progesterone production continues to support pregnancy. Studies have shown a positive correlation between the levels of Interferon Stimulated Genes (ISG15, MX2) mRNA and the quantity of IFNT administered (Matsuyama et al., 2012) [30] . These studies, which involved uterine infusion of varying amounts of IFNT (0, 500, or 1000 µg), demonstrated that the amount of IFNT infused into the uterine horn ipsilateral to the CL at 16 or 17 days after estrus was correlated with the expression of these genes, confirming the role of IFNT in the regulation of luteal lifespan. In a recent study finding indicate if interferon directly delivered in the CL, then CL has an additional antiluteolytic effect on the endometrium (Guzeloglu et al., 2024) [16] . These findings highlight the importance of IFNT in the regulation of the luteal phase and maternal recognition of pregnancy in ruminants. The ability to manipulate IFNT levels or its signaling pathway holds potential for improving reproductive efficiency in livestock.

Conclusion

In conclusion, enhancing reproductive efficiency in cattle requires a multifaceted approach involving hormonal interventions (GnRH, HCG, Progesterone), NSAIDs, dietary supplementation with omega-3 PUFAs, and understanding of key molecular signals like interferon tau (IFNT). These strategies help improve conception rates, support corpus luteum function, and reduce early embryonic losses. However, variability in outcomes emphasis the need for developing the suitable protocols based on species, physiological status, and management conditions.

Acknowledgement

AcknowledgmentAuthors are thankful to the Dean, College of Veterinary and Animal Sciences, SVPUAT, Meerut (Uttar Pradesh).

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