Spectral Doppler assessment of middle uterine artery hemodynamics across the estrous cycle in Tharparkar cows

Introduction: Breed-specific characterization of uterine hemodynamics is essential for improving reproductive monitoring and management in indigenous Bos indicus cattle; however, systematic Doppler reference data across the estrous cycle remain limited. This study aimed to evaluate cyclic changes in middle uterine artery (MUA) blood flow and its association with luteal function in Tharparkar cows using transrectal spectral Doppler ultrasonography.

Methods: Ten clinically normal, cyclic Tharparkar cows (parity 1–3) were examined at three-day intervals from estrus (D0) to the subsequent estrus (D21). Doppler parameters including resistance index (RI), pulsatility index (PI), time-averaged maximum velocity (TMAX), vessel diameter, and calculated blood flow volume (BFV) were recorded. Follicular dynamics, corpus luteum (CL) size, and serum progesterone (P4) concentrations were assessed concurrently.

Results: Reproductive characteristics were consistent with Bos indicus physiology, with a mean preovulatory follicle diameter of 13.14 ± 0.31 mm, luteal phase length of 16.10 ± 0.23 days, and estrous cycle length of 20.50 ± 0.30 days. CL size showed a strong positive correlation with serum P4 concentrations (r = 0.86, P < 0.05), confirming functional luteal competence. All Doppler indices exhibited significant cyclic variation across the estrous cycle (P < 0.05). RI and PI were lowest at estrus, increased during early luteal development, peaked during the mid-luteal phase (D9–D12), and declined toward the subsequent estrus. RI showed positive correlations with serum P4 (r = 0.73, P < 0.05) and CL size (r = 0.52, P < 0.05). In contrast, TMAX, BFV, and MUA diameter displayed reciprocal trends, with maximum values at estrus and minimum values during the mid-luteal phase. BFV correlated positively with TMAX (r = 0.84, P < 0.05) and vessel diameter (r = 0.70, P < 0.05), and negatively with RI (r = −0.58, P < 0.05) and P4 (r = −0.65, P < 0.05).

Discussion: This study establishes novel, breed-specific uterine artery Doppler reference profiles for Tharparkar cows, demonstrating distinctive cyclic vascular regulation closely linked to luteal function. These findings support Doppler ultrasonography as a functional tool for estrus confirmation, luteal assessment, and fertility monitoring in indigenous cattle.

GRAPHICAL ABSTRACT

Graphical Abstract.

Introduction

Indigenous cattle breeds play a vital role in sustaining rural livelihoods in developing regions by contributing to milk production, draft power, employment, and household income security. These breeds display superior thermotolerance and adaptation to tropical climates (Cooke et al., 2020; Asmarasari et al., 2023), however, they generally produce lower milk yields than Bos taurus breeds (Al Kalaldeh et al., 2023). The sustained genetic selection and extensive use of artificial insemination have substantially enhanced productivity in Bos taurus (Miglior et al., 2017). However, these gains in Bos taurus have often come with diminished fertility due to physiological trade-offs (Lucy, 2001; Walsh et al., 2011). Understanding follicular dynamics, corpus luteum (CL) function, and endocrine changes across the estrous cycle is fundamental to improving fertility. Such understanding provides the basis for developing synchronization and fixed-time ovulation protocols, that rely on hormonal interventions. Furthermore, pre-synchronization and resynchronization strategies have demonstrated satisfactory fertility outcomes in Bos taurus (Stevenson et al., 1999; Wiltbank and Pursley, 2014). However, their effectiveness in indigenous cattle remains inconsistent, largely due to differences in follicular wave patterns, CL development kinetics, and endocrine sensitivity (Pinheiro et al., 1998; Sartori et al., 2016; Jiamjariyatam et al., 2024; Widyastuti et al., 2025). Therefore, systematic characterization of estrous cycle physiology in indigenous breeds is essential for the development of breed-specific reproductive management protocols.

Doppler ultrasonography enables non-invasive assessment of reproductive hemodynamics and functional vascular changes, complementing anatomical imaging (Herzog and Bollwein, 2007). Earlier invasive methods showed rhythmic uterine and ovarian blood flow changes across the cycle (Ford and Christenson, 1979). In addition to that, Doppler assessment of reproductive hemodynamics, including vascular adaptations during pregnancy (Honnens et al., 2008), placental separation (Mo and Rogers, 2008), puerperium (Heppelmann et al., 2013), incomplete cervical dilatation (Chaudhari et al., 2023) and endometritis (Sharma et al., 2021) has been carried out. Doppler ultrasonography has facilitated assessment of vascular perfusion in the follicle (Acosta et al., 2003), super ovulatory response (Chandra et al., 2023), CL function (Lüttgenau and Bollwein, 2014), ovulatory structures (Aslan et al., 2011) and more recently has been applied to predict pregnancy at embryo transfer (Kanazawa et al., 2016). Despite these advances, breed-specific uterine artery hemodynamic patterns across the estrous cycle in indigenous cattle remain poorly defined.

To date, no study has systematically established Doppler reference values for middle uterine artery perfusion throughout the estrous cycle in Tharparkar (Bos indicus) cows. Generating such baseline data is critical for developing reproductive management protocols that align with the physiological characteristics of indigenous breeds. This is particularly relevant for the Tharparkar cow, a resilient dual-purpose breed from the South Asian region, globally recognized for its remarkable heat tolerance, drought resilience, and consistent milk production under challenging climatic conditions (Patel et al., 2021). Their genetic traits make them a vital resource for improving dairy productivity and enhancing climate adaptability in tropical and arid regions worldwide (Patel et al., 2022), although they differ in economic importance and physiological characteristics from Bos taurus breeds (Patel et al., 2022). Therefore, this study aimed to characterize cyclic changes in uterine blood perfusion in Tharparkar cows and examine their relationship with luteal function using spectral Doppler ultrasonography.

Materials and methods

Location and climatic parameters

The study was conducted at the Cattle and Buffalo Farm, ICAR-IVRI, Izatnagar (lat. 28°N, long. 79°E; altitude 564 m) during the months of March to June 2024. Mean ambient temperature during the study period was 30.7°C (range 15.1–40.9°C) and mean relative humidity was 53.2%. Ambient environmental conditions were monitored at the farm level; however, individual animal body temperatures were not recorded during Doppler examinations.

Animals, feed management and experimental design

Ten clinically normal, cyclic Tharparkar cows (parity 1–3; age 4.56 ± 0.24 years; body weight 386.3 ± 3.65 kg; daily milk yield 4.11 ± 0.98 kg) were enrolled. Cows were maintained under semi-intensive management with a standardized feeding regimen, receiving 1.5–2.5 kg/day concentrate (approximately 20% digestible crude protein and 70% total digestible nutrients), ad libitum wheat straw, and green fodder offered twice daily. All animals were housed in well-ventilated sheds with access to open paddocks and had free access to clean drinking water.

Inclusion criteria

Clinically healthy Tharparkar cows with regular estrous cycles, parity 1–3, and no history of reproductive disorders.

Exclusion criteria

Clinical or ultrasonographic evidence of uterine infection, retained placenta, dystocia within the previous 3 months, systemic illness, or treatment with reproductive hormones within 60 days. Reproductive soundness was confirmed by vulvar inspection, transrectal palpation and B-mode ultrasonography; presence of clear vaginal mucus and absence of intrauterine fluid were required for inclusion (Saleem et al., 2023).

Spectral Doppler acquisition

The MUA imaging was performed using a transrectal linear transducer (Exago ECM, France)] with a frequency of 7.5 MHz using color and spectral Doppler. The abdominal aorta was traced caudally to the internal/external iliac bifurcation and the MUA localized within the mesometrium using color Doppler. Spectral Doppler was applied with the sample gate centered within the lumen and gate length set approximately to vessel diameter. The insonation angle was kept ≤60° (mean used: 50°), and color gain was standardized (~20 dB). Pulse repetition frequency (PRF) was adjusted (typically 3000–5000 Hz; in this study 4000 Hz) to avoid aliasing; when aliasing occurred PRF or scale was adjusted and angle reconfirmed. For each MUA, recordings were accepted only when at least three consecutive, high-quality, consistent cardiac cycles were visualized; three such waveforms were frozen and used to calculate indices, and the mean of these three measurements per side was used for analysis (Bollwein et al., 2000; Hassan et al., 2017). Prior to averaging bilateral measurements, an initial comparison between left and right middle uterine artery Doppler parameters was performed and did not reveal a significant side effect across cycle days; therefore, bilateral values were averaged for subsequent analyses to account for potential asymmetry related to ovarian activity. All scans were performed by a single experienced operator to reduce interoperator variability. Each session required ~30–45 min per cow and was performed every 3 days from estrus (D0) through the next estrus (D21). Doppler examinations were conducted during the morning hours (08:00–11:00 h) to minimize diurnal variation and were performed in a shaded, calm handling area under farm conditions. Animals were processed individually in a restraining chute, and the order of examination was not based on parity or milk yield.

Operator training and quality control

All Doppler scans were performed by one operator experienced in bovine reproductive ultrasonography. Inter- and intra-observer variation were minimized by standardizing machine presets and acquisition protocol. Representative waveforms were reviewed and quality-checked; any recordings with motion artifact or aliasing were discarded and reacquired.

Measurement of spectral Doppler attributes of the MUA

Resistance index (RI) and pulsatility index (PI) were calculated automatically by the ultrasound system using standard formulas: (Gosling and King, 1974, 1974)

$$RI=(PSV-EDV)/PSV$$

$$PI=(PSV-EDV)/meanvelocity$$

The author Gosling and King (1974) commonly used convention; see Ginther and Utt (2004) for Doppler principles. Timed-averaged maximum velocity (TMAX; units cm s⁻¹) was taken as the system’s time-averaged peak velocity across the cardiac cycle. Blood flow volume (BFV, mL min⁻¹) was estimated using: (Varughese et al., 2013)

$$BFV=TMAX\times \pi \times {(D/2)}^2\times 60,$$

where D is the internal vessel diameter (cm) measured intima-to-intima, and the factor 60 converts seconds to minutes. Reported BFV thus represents an approximation assuming laminar flow and circular cross-section; limitations of this calculation are acknowledged.

Measurement of diameter of MUA

Internal vessel diameter (intima-to-intima) was measured on three cardiac-matched frozen B-mode frames at the Doppler sampling site; the mean of three measurements per side was used (Hassan et al., 2017).

Blood sampling and progesterone estimation

After each Doppler exam, 4 mL blood was collected by jugular venipuncture into serum clot-activator tubes (BD Vacutainer^®, silica-based clot activator), allowed to clot for 30 min at room temperature, and centrifuged at approximately 1500 × g for 10 min. Serum was aliquoted and stored at −20°C. Progesterone (P4) was measured by DRG Progesterone ELISA (EIA-1561); assay sensitivity = 0.045 ng mL⁻¹, intra-assay CV = 6.4%, inter-assay CV = 6.6%. Samples were analyzed in duplicate. To minimize handling stress, blood collection was performed immediately after Doppler examination under calm conditions; nevertheless, the potential influence of procedural stress on circulating progesterone concentrations is acknowledged as a limitation of the study.

Statistical analysis

Data were analyzed using GraphPad Prism 10.6.0 (GraphPad Software, LLC, San Diego, CA, USA). Data normality was assessed using the Shapiro–Wilk test. Repeated-measures one-way ANOVA with Geisser–Greenhouse correction was used to test the effect of cycle day on mean values of RI, PI, TMAX, diameter, BFV, and P4. The model included animal as a repeated subject. When ANOVA indicated significance, pairwise comparisons were performed using Tukey’s post-hoc test. Associations among Doppler indices, TMAX, BFV, CL size, and serum P4 were tested using Pearson’s correlation for normally distributed variables; Spearman correlation was used if data were non-normal. Results are presented as mean ± SEM, and significance was set at P < 0.05. Sample size was constrained by the availability of cyclic Tharparkar cows meeting inclusion criteria (n = 10). A post-hoc power analysis (G*Power v3.1) indicated the study had ≈100% power (Cohen’s d = 4.26) to detect the observed mean difference in RI (0.415 ± 0.097) between estrus (D0) and mid-luteal (D12) at α = 0.05. Likewise, a repeated-measures ANOVA model (six time points, within-subject correlation ρ ≈ 0.5) demonstrated near-complete power (> 0.99) for detecting the overall time effect. Accordingly, the primary findings regarding cyclic variation in RI are robust, though replication in larger herds remains recommended to confirm generalizability.

Results

Follicular and luteal dynamics

Follicular dynamics were consistent with Bos indicus physiology: the number of small follicles (<5 mm) averaged 15.36 ± 0.84 per cycle, while medium (5–9 mm) and large (≥9 mm) follicles averaged 1.81 ± 0.33 and 0.68 ± 0.10, respectively. The preovulatory follicle diameter at ovulation was 13.14 ± 0.31 mm. Mean luteal phase duration was 16.10 ± 0.23 days and mean cycle length 20.50 ± 0.30 days. CL size correlated strongly with serum P4 (r = 0.86, P < 0.05), confirming functional luteal activity across the estrous cycle in the studied cows.

Uterine artery Doppler indices

There was an inverse relationship between vascular resistance (RI/PI) and perfusion. RI varied significantly with cycle stage (P < 0.05; Figures 1, 2A): it was lowest at estrus (D0), rose during the early luteal phase, peaked at mid-luteal (D9–D12), and declined toward the subsequent estrus (D21). Both, the day of cycle (within-subject effect) and animal significantly influenced RI (P < 0.05). RI correlated positively with serum P4 (r = 0.73, P < 0.05; Figure 3A) and CL size (r = 0.52, P < 0.05; Figure 3B). PI followed a similar pattern (P < 0.05; Figures 1, 2B) and correlated strongly with RI (r = 0.76, P < 0.05; Figure 3C) and moderately with P4 (r = 0.61, P < 0.05). The coefficient of variation across animals was 16.97% for RI and 25.73% for PI.

Figure 1

Figure 1. Representative spectral Doppler sonograms of the middle uterine artery (MUA) during (A) follicular (estrus) and (B) luteal phases. PSV, peak systolic velocity; EDV, end-diastolic velocity. Three consecutive, high-quality cardiac cycles were used for measurements.

Figure 2

Figure 2. Doppler attributes of the MUA across the estrous cycle: (A) Resistance index (RI), (B) Pulsatility index (PI), (C) Timed-averaged maximum velocity (TMAX), (D) Blood flow volume (BFV), (E) Diameter. Data are mean ± SEM. Different superscript letters indicate significant differences (P < 0.05) between days.

Figure 3

Figure 3. Representative correlation plots showing relationships among uterine artery Doppler indices and luteal parameters: (A) resistance index (RI) vs serum progesterone (P4), (B) corpus luteum (CL) size vs RI, (C) RI vs pulsatility index (PI), (D) RI vs time-averaged maximum velocity (TMAX), (E) TMAX vs blood flow volume (BFV), (F) BFV vs middle uterine artery (MUA) diameter, (G) RI vs BFV, (H) serum P4 vs BFV, and (I) RI vs MUA diameter. Pearson’s correlation coefficients and significance levels are shown.

Blood flow velocity and volume

The mean value of TMAX and BFV showed reciprocal patterns to RI and PI (P < 0.05; Figures 2C, D): both were maximal at estrus (D0), declined during luteal development (nadir D9–D12) and increased by D21. TMAX correlated negatively with RI (r = −0.63, P < 0.05; Figure 3D), while BFV correlated positively with TMAX (r = 0.84, P < 0.05; Figure 3E) and vessel diameter (r = 0.70, P < 0.05; Figure 3F), and negatively with RI (r = −0.58, P < 0.05; Figure 3G) and serum P4 (r = −0.65, P < 0.05; Figure 3H). Coefficients of variation were 20.56% (TMAX) and 26.86% (BFV).

Vessel diameter

The mean MUA diameter varied across the cycle (P < 0.05), peaking at estrus and reaching a nadir at mid-luteal (D12) before increasing at D21 (Figure 2E). Diameter correlated weakly and negatively with RI (r = −0.24, P < 0.05; Figure 3I), indicating relatively stable structural caliber compared with functional changes in vascular resistance. The within-animal coefficient of variation was 9.66%.

Discussion

This study provides the first detailed characterization of uterine artery hemodynamics across the estrous cycle in Tharparkar cows using spectral Doppler ultrasonography. Distinct, cyclical changes were observed in resistance (RI, PI), velocity (TMAX), volume (BFV), and vessel diameter, demonstrating that uterine perfusion is dynamically regulated throughout the cycle. These findings align with previous studies in Bos taurus and Sahiwal cows (Hassan et al., 2017; Abdelnaby et al., 2018), but importantly extend existing knowledge by establishing breed-specific perfusion patterns in an indigenous Bos indicus breed that differs physiologically from intensively selected dairy cattle. In our study, the RI and PI were lowest at estrus, increased progressively during the early luteal phase, peaked mid-luteal, and declined before the subsequent estrus. This pattern corresponds with earlier invasive and Doppler-based studies, which demonstrated maximal uterine blood flow around estrus in cows and ewes (Anderson et al., 1977; Ford and Christenson, 1979; Abdelnaby et al., 2020). The resistance indices peaked at D12 of the estrous cycle in the present study, suggesting that Tharparkar cows may exhibit a relatively pronounced luteal-phase uterine vascular resistance compared with Sahiwal cow, in which peak resistance is often observed slightly earlier on D10 (Hassan et al., 2017). This pattern likely reflects sustained progesterone dominance during mid-luteal development, leading to vasoconstriction and reduced uterine perfusion. The positive correlations of RI and PI with P4 and CL size further support the role of luteal activity in regulating uterine blood flow, consistent with prior reports (Lüttgenau and Bollwein, 2014; Hassan et al., 2017; Sharma et al., 2021).

In contrast, time-averaged maximum velocity (TMAX), blood flow volume (BFV), and middle uterine artery diameter exhibited reciprocal cyclic patterns, with maximal uterine perfusion at estrus, a pronounced decline during the luteal phase, and recovery toward the subsequent estrus. This estrus-associated increase in uterine perfusion is consistent with estradiol-mediated vasodilation and hyperemia of the reproductive tract during follicular dominance, a phenomenon well documented across the bovine estrous cycle (Ando et al., 2007; Ford and Christenson, 1979). Elevated uterine blood flow during estrus coincides with prominent behavioral and anatomical signs of estrus, including opening of the external uterine orifice and changes in cervical mucus viscosity that facilitate sperm transport and fertilization (Chen et al., 2004; Sumiyoshi et al., 2014). At the cellular level, estradiol is known to induce rapid vasodilatory responses in uterine arteries through non-genomic mechanisms involving G-protein–coupled estrogen receptor (GPER) signaling, activation of calcium- and ERK1/2-dependent pathways, and enhanced endothelial nitric oxide synthase (eNOS) activity, collectively promoting smooth muscle relaxation and increased uterine perfusion (Stice et al., 1987; Acosta et al., 2003; Li et al., 2022; Tropea et al., 2022). Consistent with these mechanisms, Doppler studies have repeatedly demonstrated that uterine blood flow velocity indices peak around estrus when circulating estradiol concentrations are high and decline during progesterone-dominated luteal phases (Ando et al., 2007; Abdelnaby et al., 2023, 2024). Importantly, deviations from these normal cyclic perfusion patterns have been associated with compromised luteal function, embryonic loss, and reduced fertility, underscoring that uterine hemodynamic changes are not merely physiological phenomena but are directly linked to reproductive efficiency and management outcomes in cattle (Abdelnaby et al., 2024; Reddy et al., 2025; Sahu et al., 2025). The strong correlations of BFV with TMAX and vessel diameter, and its inverse association with RI and P4, indicate that uterine perfusion is jointly regulated by functional vascular tone and blood flow velocity rather than large cyclic changes in vessel caliber alone. The relatively low variability observed in uterine artery diameter compared with velocity and resistance indices suggests that dynamic modulation of vascular resistance is the primary driver of cyclic perfusion changes. Similar observations have been reported in both veterinary and human reproductive hemodynamic studies, where Doppler velocity indices are more sensitive indicators of functional blood flow regulation than arterial diameter measurements (Bernstein, 2002; Browne et al., 2015).

The cyclic uterine perfusion patterns identified in this study have strong applied relevance for reproductive management in indigenous Bos indicus cattle, particularly under tropical and low-input production systems. Inefficiency of estrus detection remains a major constraint in Bos indicus breeds due to attenuated behavioral signs, shortened estrus duration, heat stress, and nutritional variability, which together contribute to mistimed artificial insemination and reduced conception rates (Pinheiro et al., 1998; Lucy, 2001; Walsh et al., 2011; Sartori et al., 2016; Cooke et al., 2020). Under such conditions, reliance on behavioral estrus expression or endocrine measurements alone often fails to provide sufficient resolution for precise reproductive decision-making. In this context, Doppler ultrasonography offers a functional, real-time assessment of uterine and luteal perfusion that can complement conventional reproductive examinations, particularly for estrus confirmation in cows with weak or ambiguous estrous signs. Doppler ultrasonography has therefore been increasingly investigated as an applied tool that provides functional information on uterine and luteal blood flow, directly reflecting reproductive status. Multiple studies have demonstrated that cyclic changes in uterine and ovarian perfusion correspond closely with estrous stage, luteal competence, and early pregnancy establishment (Lüttgenau and Bollwein, 2014; Hassan et al., 2017; Chandra et al., 2023). In applied herd settings, deviations from the expected luteal-phase resistance and perfusion patterns may assist in early identification of luteal insufficiency or abnormal luteal regression, enabling timely resynchronization or therapeutic intervention. Moreover, Doppler-based assessment of uterine and luteal blood flow has been shown to improve early identification of non-pregnant cows, support earlier resynchronization, and refine the timing of artificial insemination and embryo transfer programs (Kanazawa et al., 2016; Pugliesi et al., 2018; Fontes and Oosthuizen, 2022; Gawai et al., 2024).

Functionally, elevated uterine perfusion during estrus may support follicular growth, estrus expression, and gamete transport, whereas reduced perfusion during the luteal phase may facilitate endometrial receptivity and luteal maintenance (Ruiz et al., 2022; Sangkate et al., 2025). Accordingly, the reference values generated in the present study may also support donor selection and prediction of super-ovulatory response, as adequate uterine and luteal vascularization is closely linked with follicular recruitment, corpus luteum functionality, and embryo yield. Importantly, most Doppler reference values and reproductive management strategies have been developed in Bos taurus cattle, despite well-documented physiological and endocrine differences from Bos indicus breeds. Comparative studies consistently report differences in follicular dynamics, luteal function, metabolic status, and reproductive hormone sensitivity that influence vascular regulation and fertility outcomes (Sartori et al., 2016; Cooke et al., 2020; Chandra et al., 2023; Gawai et al., 2024). The breed-specific uterine perfusion reference patterns generated in the present study therefore fill a critical applied knowledge gap and provide a practical framework for adapting Doppler-guided estrus confirmation, luteal assessment, donor selection, and fertility monitoring strategies to Tharparkar and other indigenous Bos indicus cattle.

A limitation of the present study is the relatively small sample size (n = 10), which reflects the intensive, longitudinal nature of the experimental design but may restrict extrapolation of the derived Doppler reference values to larger or more heterogeneous cattle populations. Accordingly, validation of these findings in broader herds and under diverse management conditions is warranted. In addition, the absence of concurrent estradiol measurements precluded direct correlation of uterine perfusion parameters with circulating estrogen concentrations, despite the well-established role of estradiol in regulating uterine vascular tone. While cyclic hemodynamic changes were interpreted in the context of existing endocrine knowledge, future studies integrating estradiol profiling would allow more direct confirmation of hormone–perfusion relationships. Doppler examinations and blood sampling involve animal handling, which may introduce mild, transient stress with the potential to influence endocrine measurements; however, standardized handling procedures and consistent examination timing were employed to minimize this effect. Although repeated-measures ANOVA was appropriate for the balanced longitudinal design of the present study, future investigations with larger sample sizes may benefit from the use of linear mixed-effects models to further account for individual variability and enhance statistical robustness. Further studies incorporating larger populations, steroid hormone profiling, assessment of local vasoactive mediators (e.g., nitric oxide), seasonal influences, and longitudinal fertility outcomes will strengthen mechanistic understanding and enhance the translational applicability of uterine Doppler ultrasonography in herd-level reproductive management.

Conclusion

This study demonstrates that Tharparkar cows display clear and repeatable uterine perfusion patterns across the estrous cycle, corresponding with changes in luteal function and vascular regulation. The Doppler hemodynamic values established here provide useful breed-specific reference standards and support the incorporation of transrectal spectral Doppler ultrasonography into reproductive evaluation and herd fertility management in Bos indicus cattle. Compared with Bos taurus breeds, the observed perfusion dynamics suggest distinctive vascular regulation that should be considered when applying reproductive monitoring and synchronization protocols to indigenous cattle. Future work should focus on characterizing these vascular signatures in cows with reproductive disorders, evaluating seasonal and herd-level variability, and integrating Doppler indices with endocrine profiling to advance Doppler-guided diagnostic and decision-making tools for indigenous breeds.

Data availability statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Ethics statement

The animal study was approved by Institute Animal Ethics Committee, ICAR-IVRI, Izatnagar (Approval no. 26-3/2020-21/JD(R)/IAEC). The study was conducted in accordance with the local legislation and institutional requirements.

Author contributions

MS: Data curation, Investigation, Writing – original draft, Writing – review & editing. CW: Investigation, Methodology, Writing – original draft, Writing – review & editing. NK: Data curation, Formal analysis, Software, Writing – original draft, Writing – review & editing. AR: Investigation, Methodology, Writing – original draft, Writing – review & editing. VY: Writing – review & editing, Data curation, Visualization. VC: Investigation, Methodology, Project administration, Writing – original draft, Writing – review & editing. LS: Methodology, Validation, Writing – original draft, Writing – review & editing. MK: Formal analysis, Investigation, Writing – original draft, Writing – review & editing. PR: Validation, Visualization, Writing – original draft, Writing – review & editing. AK: Conceptualization, Data curation, Formal analysis, Validation, Visualization, Writing – original draft, Writing – review & editing. KK: Software, Validation, Visualization, Writing – original draft, Writing – review & editing. GS: Methodology, Validation, Writing – original draft, Writing – review & editing. SS: Conceptualization, Investigation, Project administration, Resources, Supervision, Writing – original draft, Writing – review & editing.

Funding

The author(s) declared that financial support was received for this work and/or its publication. The authors acknowledge institutional support and access to facilities provided by ICAR-IVRI.

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The author(s) declared that generative AI was not used in the creation of this manuscript.

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RI, Resistance index; PI, Pulsatility index; BFV, Blood flow volume; TMAX, Timed-averaged maximum velocity; MUA, Middle uterine artery; UBF, Uterine blood flow; P4, Progesterone; CL, Corpus luteum.

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