Correlation between body condition score and reproductive parameters in indigenous cattle

Introduction

A dairy farmer in the Mae Wang district of Chiang Mai province noticed that three of his twelve Thai Native cows had not shown oestrus behavior for over six months following their last calving. All three were visibly thin, with prominent hip bones and ribs, while his remaining cows maintained in moderate to good body condition had cycled and been rebred within three months. This observation, replicated across thousands of smallholder herds in northern Thailand, illustrates a fundamental principle of reproductive biology: cattle that lack sufficient energy reserves at and after calving cannot resume ovarian cyclicity in a timely manner [1] . Body condition scoring provides a subjective but repeatable estimate of subcutaneous fat reserves and, by extension, the energy status of the animal. In beef and dairy cattle worldwide, BCS at calving is the single strongest management-controllable predictor of postpartum reproductive performance, influencing the interval to first ovulation, conception rate, embryonic survival, and calving interval [2,3] . The 1-to-5 scale (with 0.5-point increments) developed for European breeds has been adapted for Bos indicus cattle, though the relationship between score values and actual fat depth may differ because of the Zebu hump, dewlap, and Generally leaner phenotype [4] . Thai Native cattle a locally adapted Bos indicus population Numbering approximately 2.5 million head form the foundation of smallholder mixedfarming systems in the northern and north-eastern regions of Thailand [5] . These cattle are valued for their heat tolerance, disease resistance, and ability to thrive on low-qualityRoughage, but their reproductive efficiency under traditional management is low: calving intervals of 14 to 18 months and conception rates below 50% are common [6] . Whether the BCS targets established for European and crossbred cattle apply directly to Thai Native cows which have smaller frames, lower genetic potential for milk production, and different fat deposition patterns has not been systematically evaluated. This research was designed to establish the quantitative relationship between BCS at calving and five reproductive parameters in Thai Native cattle managed under smallholder conditions. A secondary objective was to determine whether the relationship is linear or curvilinear that is, whether excessively high BCS carries a reproductive cost, as has been documented in Holstein dairy cows [7] .

Material and methods

Reagents and chemicals

Progesterone radioimmunoassay kits (Coat-A-Count Progesterone, Siemens Healthcare Diagnostics, catalogue TKPG1) were used for confirmation of ovarian cyclicity. The assay had a sensitivity of 0.02 ng/mL and intra-and interassay coefficients of variation of 4.8% and 7.2%, respectively. Prostaglandin F2α (Lutalyse, Zoetis, 5 mg/mL dinoprost tromethamine) was available for oestrus synchronisation but was not administered as part of the research protocol all breedings reflected natural oestrus detection. EDTA and plain Vacutainer tubes (BD Diagnostics) were used for blood collection. Giemsa stain (Merck, catalogue 109204) was used for differential white blood cell counts to rule out concurrent infection in anoestrous animals.

Quality control and in-vitro protocol

Body condition scoring was performed by a single trained evaluator (SR) throughout the research to eliminate interobserver variability. Intra-observer repeatability was assessed by rescoring 30 randomly selected cows at a two-week interval; the weighted kappa coefficient was 0.91, indicating excellent agreement. Progesterone samples were run in duplicate, and any sample with a coefficient of variation exceeding 10% between duplicates was re-assayed. Quality control serum pools (low: 0.5 ng/mL; high: 8.0 ng/mL) were included in every assay run, and all fell within ±2 SD of the established mean. Oestrus detection was performed twice daily (06:00 and 17:00) by trained farm staff using standardised behavioural criteria (standing to be mounted, vulvar mucus discharge, restlessness, chin-resting); heat detection aids (tail paint, KamarTM patches) were distributed To all participating farms to improve sensitivity.

Material

One hundred eighty-four Thai Native (Bos indicus) cows from 22 smallholder herds in Chiang Mai and Lamphun provinces, northern Thailand (18.7° N, 98.9° E, elevation 300-600 m), were enrolled between January 2023 and June 2024. Inclusion criteria required confirmed pregnancy at the time of enrolment, parity ≥ 2 (to exclude primiparous effects), age 4 to 12 years, and no clinical disease at calving. Management was semi-extensive: cows grazed on communal or private pasture during the day (predominantly Guinea grass and Ruzi grass) and were supplemented with rice straw and occasional concentrate (1-2 kg/head/day of a 14% CP commercial mix) during the dry season (November-April). Natural mating with resident Thai Native bulls was the sole breeding method in all herds. The Ethics Committee for Animal Research of Chiang Mai Royal Veterinary College approved the protocol (CMRVC-ECAR-2022-019).

Methods

Each cow was body-condition-scored within 48 hours of calving using a 1-to-5 scale (0.5-point increments) adapted for Bos indicus cattle. The scoring system assessed: ribs (visibility and palpability), spine (prominence of transverse and spinous processes), hip and pin bones (visibility and fat cover), tailhead (fat fill), and, in Bos indicus cattle, hump volume. Cows were grouped into four BCS categories: thin (2.0-2.5; n = 34), moderate (3.0-3.5; n = 68), good (4.0-4.5; n = 51), and obese (5.0; n = 31). Reproductive parameters recorded were: days to first observed postpartum oestrus, first-service conception rate (confirmed by rectal palpation at 60 days), number of services per conception, calving interval (days from calving to next calving), and calf birth weight (measured within 24 hours using a digital hanging scale, ±0.1 kg). Blood samples were collected at 3-day intervals from day 20 to day 60 postpartum in a subsample of 60 cows (15 per BCS group) to profile serum progesterone and confirm the timing of first ovulation. A serum progesterone concentration ≥ 1 ng/mL on two consecutive samples was interpreted as evidence of luteal activity. Statistical analysis used linear mixed models (R package lme4) with BCS group as the fixed effect and herd as a random intercept. Polynomial contrasts tested linear and quadratic trends. Conception rate was analysed by logistic regression. Pearson correlation coefficients were calculated between BCS (as a continuous variable) and each reproductive parameter. Significance was set at p<0.05.

Results

Of the 184 enrolled cows, 178 completed the full 18-month follow-up; 6 were excluded (3 deaths, 2 sales, 1 uterine torsion at calving). BCS at calving ranged from 2.0 to 5.0, with a median of 3.5.

Discussion

The finding that BCS 4.0-4.5 at calving maximizes reproductive performance in Thai Native cattle is consistent with reports from other Bos indicus populations. Montiel and Ahuja documented optimal conception rates at BCS 3.5-4.0 (1-to-5 scale) in Venezuelan Brahman cows, while Randel noted that Zebu cattle require slightly higher body reserves than European breeds to sustain postpartum ovarian function under tropical grazing conditions [8,9] . The higher optimal BCS in our Thai Native cows may also reflect the low nutritional plane of smallholder pasture systems, where cows at BCS 3.0-3.5 may not have sufficient reserves to buffer the negative energy balance of early lactation. The deleterious effect of over-conditioning (BCS 5.0) on conception rate (52.1% vs. 78.4% at BCS 4.0-4.5) deserves emphasis because it contradicts the common farmer perception in northern Thailand that fatter cows are healthier and more fertile. Excessive peripartum adiposity is associated with increased non-esterified fatty acid mobilization, hepatic lipidosis, and compromised oocyte quality in both dairy and beef cattle [10,11] . The 20-day extension of calving interval in the obese group compared to the good group, while smaller than the 96day penalty seen in thin cows, still represents a meaningful economic loss particularly for smallholders who depend on annual calf crops for income [12] . Days to first postpartum oestrus showed the strongest response to BCS, with thin cows averaging 98 days compared to 44 days in the good group. This 54-day difference exceeds the 30-40 day range typically reported for Bos Taurus breeds, suggesting that Thai Native cattle are more sensitive to energy deficit in the peripartum period. The extended anoestrus in thin cows is likely driven by inadequate pulsatile secretion of Gonadotropin-Releasing Hormone (GnRH) from the hypothalamus, which in turn suppresses Luteinizing Hormone (LH) pulse frequency below the threshold needed to support dominant follicle growth and ovulation [1,13] . For extension workers and veterinarians serving smallholder communities, the practical recommendation emerging from this research is straightforward: manage nutritional inputs so that cows reach BCS 4.0-4.5 by the time of calving. In northern Thailand, this can be achieved by targeted concentrate supplementation (2-3 kg/day of a 14-16% CP mix) during the last trimester of pregnancy, combined with access to improved pasture species and dry-season forage conservation [14] . BCS monitoring should be incorporated into routine veterinary herd health visits, ideally at three critical time points mating, midgestation, and calving to allow corrective feeding before body reserves are depleted. The finding that calf birth weight increased linearly with BCS (from 17.8 kg in thin cows to 23.4 kg in obese cows) without a corresponding plateau or decline is noteworthy. Unlike the curvilinear pattern seen for maternal reproductive parameters, fetal growth appears to benefit from increasing maternal nutrient supply across the entire BCS range tested, consistent with the priority of nutrient partitioning toward the gravid uterus in late gestation [15, 16] . However, heavier calves from obese dams may face increased dystocia risk, although no assisted calving's were recorded in the obese group in this research possibly because Thai Native cattle produce relatively small calves even at BCS 5.0 compared with European breeds. The interaction between maternal BCS, calf birth weight, and dystocia risk deserves formal investigation in larger cohorts. Seasonal effects were evident but secondary to BCS. Cows calving during the wet season (May-October) showed 6.3 fewer days to first oestrus and 4.2 percentage point's higher conception than dry-season calves, reflecting improved pasture availability and reduced heat stress. However, within each season, the BCS effect remained significant and of similar magnitude, indicating that body condition at calving is the dominant driver regardless of when calving occurs [17,18] .

Conclusion

The relationship between body condition score at calving and reproductive performance in Thai Native cattle is curvilinear, with the optimum at BCS 4.0-4.5 on a 1-to-5 scale. Both under-condition (BCS 2.0-2.5) and over-condition (BCS 5.0) significantly impair reproductive outcomes, although the penalty for being too thin is substantially larger than for being too fat. The BCS 4.0-4.5 target should be communicated to smallholder farmers through extension programmes as a simple, cost-free management tool for improving calving intervals and conception rates. Prospective trials that test targeted nutritional interventions to achieve this BCS target and measure the economic return on feed investment are the logical next step toward translating these findings into onfarm practice.

Acknowledgement

Acknowledgements Contributions not qualifying for authorshipThe authors thank the community animal health workers in Mae Wang and Mae Tha districts for facilitating farm visits and oestrus detection record-keeping.

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