INTRODUCTION

The return rate, defined as the proportion of sows that return-to-estrus (RTE) after artificial insemination (AI) without establishing pregnancy, is a critical reproductive performance indicator in commercial swine production systems. An elevated return rate reflects unsuccessful conception or early embryonic loss and contributes to prolonged non-productive days, thereby increasing production costs [1]. In practice, the return rate varies substantially with herd management practices, sow parity, environmental conditions, and underlying physiological factors [2-5].

Marked differences in return rate have been reported among countries and production systems. In high-performing herds, return rates as low as 6.6% have been documented, whereas in ordinary herds they may exceed 11.7%–18.5% [6, 7]. In the United States, reported return rates were 15.6% in gilts, 13.9% in parity 1–2 sows, and 11.6% in higher-parity sows [8]. In Brazil, an overall return rate of 10.7% was observed, with particularly high values associated with short lactation length (LL) or large litter size at weaning (LSW) [9]. Similarly, a high return rate of 19.2% across gilts and sows has been reported in Bhutan [10].

Seasonal variation in return rate has been consistently demonstrated. Increased return rates are generally observed during summer and early autumn. For example, return rates were reported to peak in July and August in the Netherlands [11], while higher rates during May–August compared with September–November were observed in the USA [12]. In another study from the United States, Koketsu [7] reported a progressive seasonal increase in return rate, reaching a peak of 17.4% during October–December in ordinary herds.

Parity also plays an important role, with primiparous sows consistently showing higher return rates than multiparous sows [8, 13]. Gilt management, particularly age at first mating, has been identified as a key determinant of return rate. Increasing age at first insemination has been associated with reduced return rate in gilts [14], whereas mating at excessively early or late ages is linked to suboptimal reproductive outcomes [15–17]. In addition, management-related factors such as the weaning-to-service interval (WSI) and LL significantly influence return rate. A WSI exceeding 6 days has been associated with increased return rate [11, 18, 19], and short lactation periods have also been shown to elevate return rate [9, 20].

Despite extensive investigations into RTE in commercial swine herds, important knowledge gaps remain. Most previous studies have evaluated mixed-parity populations, with limited emphasis on primiparous sows, a physiologically distinct group that faces the combined demands of continued growth, first lactation, and post-weaning reproductive recovery. Moreover, key reproductive timing variables specific to first parity, such as age at first AI (AFAI), age at first farrowing (AFF), age at first AI after weaning (ASAI) and month of first post-weaning insemination (MSAI), have rarely been evaluated simultaneously with classical productivity traits, including number of piglets born alive (NBA), litter birth weight (LBW), LSW, litter weight at weaning (LWW), and LL. In addition, evidence derived largely from temperate production systems may not be fully applicable to tropical environments, where chronic heat stress, seasonal fluctuations, and management constraints can substantially alter reproductive responses. The relative contributions of WSI, seasonal insemination timing, and first-parity productivity traits to RTE under tropical conditions therefore remain insufficiently characterized.

The present study aimed to identify and quantify reproductive, management, and seasonal risk factors associated with RTE following the first post-weaning AI in primiparous sows raised under tropical conditions. Specifically, the study evaluated the associations between RTE and AFAI, AFF, NBA, LBW, LSW, LWW, LL, WSI, ASAI, and MSAI using a large-scale retrospective dataset. By focusing exclusively on primiparous sows and integrating variables spanning gilt development, first lactation performance, and post-weaning reproductive management, this study sought to provide evidence-based insights to optimize breeding strategies and reduce RTE in tropical swine production systems.

MATERIALS AND METHODS

Ethical approval

This study was conducted exclusively using pre-existing farm management and reproductive records obtained from a commercial Landrace × Yorkshire sow herd located in central Vietnam. No experimental procedures, invasive interventions, or direct animal handling were performed solely for research purposes. All records were anonymized prior to analysis to ensure confidentiality and protection of farm identity.

Study period, location, and environmental conditions

This retrospective study was conducted from March to July 2025 on a commercial swine farm housing approximately 5,000 Landrace × Yorkshire breeding sows in the central region of Vietnam. The farm is located in a tropical climate zone characterized by persistently high humidity and year-round warm to hot temperatures. The primiparous sows included in the study were born between October 2022 and September 2024, and all post-weaning AI events occurred between November 2023 and May 2025. Monthly mean minimum and maximum ambient temperatures (°C) were as follows: January (15.8–20.4), February (17.1–23.6), March (19.2–25.4), April (25.0–32.9), May (25.1–31.5), June (27.3–34.5), July (27.1–34.0), August (26.8–34.6), September (25.1–32.5), October (22.7–31.4), November (20.8–29.4), and December (15.5–22.1). Monthly relative humidity data were unavailable; however, the annual mean humidity was estimated at 85–86%.

Husbandry, feeding, and general management

Replacement gilts were group-housed until sexual maturity and subsequently transferred to individual metal crates with fully slatted floors. Both gestation and farrowing units were equipped with ventilation fans and evaporative cooling pads. Automated feeding systems were used throughout all production stages. During the gilt phase, feed allowance was approximately 2.5–2.6 kg/day. During gestation, gilts received 2.2–2.4 kg/day of diets containing 14% crude protein, 0.7% lysine, 8%–9% crude fiber, 7%–8% fat, and 15 MJ/kg metabolizable energy. Lactating sows were fed ad libitum diets with increased protein and lysine contents (16% crude protein, 1.2% lysine). After weaning and until the subsequent AI, sows received 2.5–3.0 kg/day of lactation feed. Water was available ad libitum via nipple drinkers.

Gilts were first bred at a minimum body weight of 135 kg, typically during the second or third estrus. Natural farrowing was allowed, with induction performed on day 116 of gestation using cloprostenol (Hanprost, Hanvet, Hanoi, Vietnam) if spontaneous farrowing did not occur. Dystocia was managed using oxytocin (Dona oxytocin, DonaVet, Dongnai, Vietnam) or manual assistance. All hormonal treatments were administered perivulvarly. Cross-fostering was conducted after the first 24 h postpartum, whereas split suckling was not practiced. Preweaning mortality averaged approximately 7%. Piglets were weaned at a mean age of 25 days, with 11.7% weaned before 21 days of age.

Reproductive management and estrus detection after weaning

Following weaning, sows received an intramuscular injection of Vitamin AD3E (DonaVet, Dongnai, Vietnam) and were exposed daily to intact Meishan boars for at least 6 h at a ratio of 1 boar per 12 sows. Estrus detection was performed once daily in the early morning using the back pressure test in the presence of a boar. Detection duration ranged from a few seconds to approximately 20 seconds, depending on estrus intensity.

Sows that failed to exhibit estrus within 7 days post-weaning received a second injection of ADE. If estrus was not detected by day 14, alternate-day feed restriction was initiated to stimulate follicular growth. For sows not exhibiting estrus by days 18–21 post-weaning, hormonal treatment with equine chorionic gonadotropin (eCG) and human chorionic gonadotropin (hCG) (Heat 5x, Dong Bang Co. Ltd., Seoul, Korea) was administered intramuscularly in the neck. Boar exposure and photoperiod were subsequently increased to ≥12 h and ≥16 h/day, respectively, to promote estrus expression.

AI

Sows exhibiting estrus were inseminated using extended semen with a total motility of ≥75% and containing at least 3 × 10⁹ sperm cells per dose. Semen collected from Duroc boars was evaluated microscopically, extended using MR-A® 3 (Kubus, Barcelona, Spain), stored at 15°C, and used for AI within 3 days of collection. Insemination was performed three times in gilts at 8–12 h intervals and twice in sows at 20–24 h intervals.

RTE determination

Following AI, sows were monitored for RTE from days 18 to 45 post-insemination. Estrus detection was conducted once daily in the morning by trained technicians using a Meishan boar to provide visual and olfactory stimulation. Sows showing behavioral signs of estrus were further examined by stimulation of the vulva, mammary glands, and inguinal area, followed by confirmation of the standing reflex using back pressure.

Data collection and inclusion and exclusion criteria

A substantial portion of this dataset had been previously used to investigate factors associated with prolonged WSI in primiparous sows [21]. Records from 8,830 animals were initially retrieved. Gilts without AI records, females inseminated but not farrowed, sows that farrowed once but were not inseminated after weaning, sows with fewer than 45 days since AI, and those with incomplete, biologically implausible, or logically inconsistent records were excluded. Ultimately, data from 5,111 primiparous sows were retained for analysis.

Variable definitions

AFAI and AFF were calculated by subtracting the date of birth from the date of first AI and first farrowing, respectively. NBA was defined as the number of live-born piglets. LBW was calculated as the total weight of live-born piglets with individual birth weights >0.85 kg. LL was defined as the interval from farrowing to weaning. LSW and LWW were defined as the number and total weight of weaned piglets per litter, respectively. WSI was calculated as the interval between weaning and the first post-weaning AI. ASAI was defined as sow’s AFAI after weaning. MSAI corresponded to the calendar month of the first post-weaning AI.

Classification of independent variables

All continuous variables were categorized prior to analysis to account for nonlinear associations with RTE. AFAI, AFF, NBA, LBW, LSW, LWW, LL, ASAI, WSI, and MSAI were categorized according to predefined biologically relevant intervals without modification.

Statistical analysis

Statistical analyses were performed using IBM SPSS Statistics for Windows, version 22.0 (IBM Corp., Armonk, NY, USA). A two-step analytical approach was applied. Initially, univariable logistic regression analyses were conducted to evaluate associations between each independent variable and RTE in primiparous sows. Subsequently, all independent variables, regardless of their statistical significance in univariable analyses, were included in a forward multivariable logistic regression model to identify independent predictors of RTE while adjusting for potential confounding effects.

Spearman’s rho correlation coefficients were used to assess collinearity among independent variables, and variables with r ≥ 0.7 were excluded from the same model to avoid multicollinearity. Model fit was evaluated using the Hosmer–Lemeshow goodness-of-fit test. Model performance and parsimony were compared using the Akaike information criterion and the Bayesian information criterion. Multicollinearity among retained variables was further assessed using variance inflation factors. The discriminatory ability of the final model was evaluated using the receiver operating characteristic (ROC) curve, and model accuracy was quantified as the area under the ROC curve (AUC).

RESULTS

Descriptive statistics

The overall RTE of the investigated primiparous Landrace × Yorkshire sows was 8.6% (440/5,111) and varied across categories of physiological and management-related factors.

With respect to AFAI, the lowest RTE was observed in the 240–250-day group at 6.5% (38/583), whereas the highest RTE was observed when AFAI exceeded 285 days, at 11.8% (107/909). Sows inseminated before 230 days of age also exhibited a relatively high RTE of 10.7% (56/523), indicating a quadratic relationship between AFAI and reproductive outcome.

A similar pattern was observed for AFF. The lowest RTE (6.5%; 61/941) occurred in sows with AFF of 360–375 days. In contrast, sows that farrowed after 390 days showed higher RTEs (9.7–11.6%), followed by those with AFF below 345 days (10.6%; 49/462), further supporting a quadratic association between AFF and the likelihood of post-insemination RTE.

RTE declined with increasing NBA, from 10.0% (134/1334) in sows with ≤10 piglets to 6.9% (83/1199) in those with >14 piglets. A comparable downward trend was noted for LBW, with RTE decreasing from 11.1% (62/559) in litters <12 kg to 8.3% (152/1,828) in litters >18 kg. For LSW, the lowest RTE was 7.4% (143/1929) in the 13–14 piglet group, whereas the highest RTE was 10.7% (23/214) in litters with >14 piglets. RTEs were broadly comparable across LWW (7.6%–9.3%) and LL categories (7.1%–9.3%).

Marked seasonal variation was observed for MSAI. The lowest RTE occurred in April–May (4.9%; 37/755), followed by December–January (6.0%; 68/1,126). In contrast, higher RTEs were recorded during June–July (13.5%; 18/133), August–September (16.7%; 47/281), and October–November (12.0%; 116/965). Regarding ASAI, the lowest RTE was observed at 380–400 days (5.5%; 52/954), whereas higher RTEs were recorded in sows with ASAI <380 days (10.6%–13.0%) and >430 days (10.8%–11.8%).

The lowest RTE by WSI category was observed in sows inseminated 2–6 days after weaning (7.3%; 234/3210), followed by those with WSI >20 days (7.7%; 64/835). In contrast, higher RTEs were recorded in sows with very short (0–1 days: 13.4%; 9/67) or prolonged WSI (7–8 days: 12.9%; 77/597; 9–10 days: 17.6%; 25/142; 11–20 days: 11.9%; 31/260), indicating a nonlinear relationship between WSI and RTE.

Univariate analysis

Univariate logistic regression analysis identified significant associations between post-weaning RTE and AFAI, AFF, NBA, LBW, MSAI, ASAI, and WSI. No significant associations were observed for LSW, LWW, or LL (Table 1).

OR = Odds ratio, 95% CI = 95% confidence interval.

Multivariate analysis

Multivariate logistic regression analysis showed that MSAI, WSI, ASAI, and NBA were the most important predictors of RTE in primiparous sows (Table 2). Because of strong correlations with ASAI, AFAI and AFF were excluded from the final model (Spearman’s rho >0.7). The model explained a modest proportion of the variance (Nagelkerke R² = 5.2%) and demonstrated good fit, as indicated by a non-significant Hosmer–Lemeshow test (p = 0.915). Variance inflation factor values ranged from 1.008 to 1.158, indicating no multicollinearity. Model discrimination was fair, with an AUC of 0.655.

Compared with sows with NBA of 1–10 piglets, those with 11–12 and 13–14 piglets showed no significant difference in RTE (p > 0.05). However, sows with >14 piglets exhibited a significantly lower RTE (Odds ratio [OR] = 0.66, 95% confidence interval [CI] = 0.49–0.88, p = 0.005).

MSAI exerted a pronounced effect on RTE. Relative to sows inseminated in August–September, those inseminated in December–January (OR = 0.47, 95% CI = 0.30–0.74, p = 0.001), February–March (OR = 0.57, 95% CI = 0.38–0.84, p = 0.005), and April–May (OR = 0.30, 95% CI = 0.19–0.48, p < 0.001) had significantly lower odds of RTE. No significant differences were observed for June–July or October–November (p > 0.05).

Using ASAI of 380–400 days as the reference, sows with ASAI of 332–370 days (OR = 1.84, 95% CI = 1.15–2.94, p = 0.012) and 370–380 days (OR = 1.76, 95% CI = 1.08–2.87, p = 0.023) showed moderately increased odds of RTE. Substantially higher odds were observed in sows with ASAI of 430–450 days (OR = 2.12, 95% CI = 1.42–3.14, p < 0.001) and >450 days (OR = 2.15, 95% CI = 1.37–3.38, p = 0.001).

Relative to sows inseminated 2–6 days post-weaning, those inseminated at 7–8 days (OR = 1.77, 95% CI = 1.33–2.34, p < 0.001) and 11–20 days (OR = 1.63, 95% CI = 1.08–2.46, p = 0.020) had moderately higher odds of RTE, whereas sows inseminated at 9–10 days showed a marked increase (OR = 2.25, 95% CI = 1.41–3.60, p < 0.001). No significant differences were observed for sows inseminated at 0–1 days or >20 days after weaning (p > 0.05), which may be partly attributable to the small number of sows with very short WSI.

OR = Odds ratio, 95% CI = 95% confidence interval.

DISCUSSION

Comparison with previous studies

In the present study, the RTE of primiparous sows (8.6%) was slightly lower than values reported in several previous investigations, many of which included mixed-parity populations. Peltoniemi et al. [22] reported an RTE of 10.4% in Finnish herds, whereas Koketsu et al. [7] documented an RTE of 11.6% in high-performing herds in the United States. Other studies from the United States reported RTE values of 13.9% for parity 1–2 sows [8] and 10.1% for primiparous sows [20]. Similarly, Vargas et al. [9] observed an RTE of 10.7% in Brazilian herds. In Spanish, Portuguese, and Italian herds, the RTE in primiparous sows was reported to be 12.3% [13]. Overall, the RTE observed in this study is broadly consistent with previously published data, indicating that reproductive performance of primiparous sows in the present herd was within expected ranges.

Effects of WSI on RTE

The association between WSI and RTE observed in this study is consistent with earlier findings. Higher farrowing rates have been reported in sows with a WSI of 0–6 days compared with those with 7–20 days [19, 23]. Likewise, improved farrowing rates were observed in sows with WSI of 3–5 days compared with 0–2 and 9–12 days [24], and in sows with WSI of 4–6 days compared with 7–20 days [25]. The increased RTE associated with WSI of 7–20 days may be explained by impaired follicular development and altered metabolic status. Longer WSI has been linked to smaller follicle size at weaning and reduced litter size, indicating compromised follicular quality and ovulation capacity [26]. Such sows are also more likely to exhibit shortened estrus and estrus-to-ovulation intervals, increasing the risk of suboptimal insemination timing [27]. In addition, greater protein and body weight losses during lactation are associated with poorer follicular quality [28] and prolonged WSI [29]. These metabolic deficits may delay post-weaning ovarian recovery and impair embryonic development and survival, particularly in sows losing more than 5% of body weight [30, 31]. Consequently, a WSI of 7–20 days likely reflects inadequate physiological readiness for breeding and contributes to increased RTE. In contrast, when WSI exceeds 20 days, reproductive function may have sufficient time to recover [29], resulting in RTE levels comparable to those observed during the early post-weaning period. Conversely, the high RTE observed in sows with WSI of 0–1 days may be related to suboptimal follicular development [25] and an increased risk of ovarian cyst formation [32].

Effects of MSAI on RTE

The seasonal pattern identified in this study aligns with previous reports demonstrating reduced fertility from summer to late autumn. Higher RTE or lower farrowing rates during warmer months have been documented in herds from the United States, Spain, Finland, and Japan [7, 12, 13, 22, 33]. In contrast, studies conducted in Thailand and Nigeria reported comparable or even higher farrowing rates during hot seasons [34, 35]. The elevated RTE observed between June and November compared with December and May may be attributed to increased ambient temperatures. Maximum temperatures occurring 2–3 weeks before AI exert the strongest negative effect on farrowing rate [36, 37]. High ambient temperatures reduce feed intake in lactating sows [38, 39], resulting in increased body weight loss, which has been linked to impaired follicular development [40, 41]. Moreover, heat stress induces hormonal disturbances during early pregnancy [42–44]. Collectively, these factors may explain the higher RTE in sows inseminated from June to November compared with December to May. The higher RTE observed in October–November compared with April–May remains unclear, as ambient temperatures are typically higher in April–May. One possible explanation is the cumulative negative effect of prolonged heat exposure during the preceding summer and autumn, whereas sows inseminated in April–May may have benefited from recovery during cooler months from December to March.

Effects of AFAI, AFF, and ASAI on RTE

This study identified optimal timing windows for key reproductive indicators, with AFAI of 240–250 days, AFF of 360–375 days, and ASAI of 380–400 days being associated with the lowest RTE. Bertoldo et al. [15] reported increased risks of pregnancy loss when AFAI was either <200 or >400 days. Similarly, Iida et al. [36] observed lower farrowing rates in gilts and sows with AFAI between 269 and 400 days compared with those inseminated before 269 days, and Koketsu et al. [17] demonstrated reduced gilt farrowing rates with increasing AFAI. In contrast, Tummaruk et al. [14] reported a decrease in RTE with increasing AFAI, while Holm et al. [16] found higher RTE in primiparous sows with lower AFAI. These discrepancies may reflect differences in the AFAI ranges evaluated, as Tummaruk et al. [14] included gilts with AFAI ≥150 days, whereas Holm et al. [16] applied a threshold of ≥120 days. Collectively, these findings provide practical guidance for optimizing insemination timing to reduce reproductive failure.

Effect of NBA on RTE

The inverse association between NBA and RTE observed in this study contrasts with the findings of Tummaruk et al. [14], who reported no significant effect of NBA on subsequent RTE. One possible explanation is that NBA reflects overall reproductive efficiency, encompassing ovulation rate, fertilization success, embryo quality, and implantation capacity. Higher NBA may therefore identify sows with superior reproductive potential, which are more likely to sustain pregnancy following subsequent AI, resulting in lower RTE.

Lack of effects of LBW, LSW, LWW, and LL on RTE

After adjustment for other variables, LBW, LSW, LWW, and LL were not significantly associated with RTE. This is consistent with previous reports. Hidalgo et al. [45] found no difference in farrowing rate between sows with LLs of 3 and 5 weeks. Similarly, Arend et al. [46] reported no difference in pregnancy or farrowing rates between primiparous sows nursing 12 piglets and those nursing 15–16 piglets, supporting the present finding for LSW. In contrast, Vargas et al. [9] reported higher RTE in sows weaning 12 piglets compared with those weaning 7–11 piglets and observed the highest RTE in sows with LLs of 15–19 days. Koketsu et al. [33] also reported reduced farrowing rates in sows with shorter LLs (8–19 days) compared with longer durations (20–28 days). Although LL, LSW, and LWW may contribute to depletion of sow body reserves during lactation, their effects on RTE appear to be indirect and mediated through WSI [21]. Likewise, LBW does not directly reflect ovarian function to the same extent as NBA. Because WSI and NBA were retained in the multivariate model, the effects of LL, LSW, LWW, and LBW were no longer statistically significant, indicating that their influence on RTE is weaker and mediated through more proximal determinants.

Limitations

Several limitations of this study should be acknowledged. First, all data were derived from a single commercial herd managed under tropical conditions; therefore, the findings may not be directly generalizable to herds operating in different climatic regions, management systems, or production settings. Second, the study population comprised a single genetic line, which restricts extrapolation of the results to herds using other genotypes.

Third, several key physiological and management-related variables known to influence reproductive performance were not available, including body condition score at weaning, lactation feed intake, postpartum health status, and sow body weight loss. The absence of these potential confounders may have influenced the observed associations, particularly in the context of tropical heat stress. Fourth, the lack of regional humidity data precluded calculation of the temperature–humidity index, thereby limiting a more precise evaluation of heat stress effects.

Fifth, RTE could not be categorized into regular, irregular, or late return patterns because such information was not recorded by the farm, potentially obscuring biologically relevant differences in underlying reproductive mechanisms. In addition, the exclusive focus on primiparous sows limits the applicability of these findings to multiparous animals. Finally, as this was a retrospective observational study based on routinely collected farm records, causal inferences cannot be drawn from the observed associations.

CONCLUSION

This large-scale retrospective study demonstrated that RTE in primiparous Landrace × Yorkshire sows under tropical conditions was 8.6%, a value comparable to or slightly lower than those reported in previous studies. Multivariate analysis identified MSAI, WSI, ASAI, and NBA as the most influential predictors of RTE. Specifically, lower RTE was observed in sows inseminated during December–May, with an optimal WSI of 2–6 days, an ASAI of 380–400 days, and an NBA >14 piglets. In contrast, reproductive traits such as LBW, LSW, LWW, and LL were not independently associated with RTE after adjustment for confounding factors, indicating that post-weaning reproductive timing and physiological readiness play a more critical role than litter performance traits alone.

These findings provide clear, evidence-based guidance for improving reproductive efficiency in primiparous sows under tropical conditions. From a management perspective, maintaining a WSI of 2–6 days, avoiding excessively early or delayed ASAI, and strategically scheduling AI during cooler months may substantially reduce RTE. In addition, the association between higher NBA and reduced RTE suggests that first-parity performance can serve as a useful indicator for identifying sows with superior subsequent reproductive potential. Collectively, these strategies may help reduce non-productive days, improve herd fertility, and enhance overall productivity in tropical swine production systems.

A major strength of this study is the large sample size derived from a well-documented commercial herd, allowing robust statistical analysis focused exclusively on primiparous sows. The integration of variables spanning gilt development, first-parity performance, seasonal effects, and post-weaning reproductive management provides a comprehensive evaluation of RTE determinants in a physiologically distinct group often under-represented in previous research.

Future studies should incorporate additional physiological and management variables, including body condition score at weaning, lactation feed intake, sow weight loss, postpartum health disorders, and temperature–humidity index, to further elucidate mechanisms underlying RTE. Expanding analyses to multiple herds, diverse genotypes, and multiparous sows would improve generalizability. Prospective or intervention-based studies evaluating targeted management adjustments may also help establish causal relationships.

Overall, this study highlights the central role of post-weaning reproductive timing, seasonal conditions, and first-parity performance in determining RTE in primiparous sows. Optimizing AI timing and mitigating environmental and metabolic stressors appear to be more effective strategies for reducing RTE than relying solely on traditional litter performance indicators. These findings contribute valuable insights for refining reproductive management practices in tropical swine production systems.

DATA AVAILABILITY

The supplementary data can be made available from the corresponding author upon request.

AUTHORS’ CONTRIBUTIONS

NHN, DTKL, NVT, BVD, BTAD, and PS: Conceived and designed the study. NHN: Collected data. NHN and PS: Analyzed data, interpreted the results, and drafted and revised the manuscript. All the authors participated in scientific discussion, read, reviewed, and approved the final manuscript.

COMPETING INTERESTS

The authors declare that they have no competing interests.

PUBLISHER’S NOTE

Veterinary World remains neutral with regard to jurisdictional claims in the published institutional affiliations.

ACKNOWLEDGMENTS

We are grateful to the farm owners for their consent to data collection and to the veterinarians for their assistance in data collection. The authors did not receive any funds for this study.

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