The logistic regression analysis revealed statistically significant effects of all three explanatory variables. As shown in (Table 3), sex had a significant association with fitness classification (Wald χ
2 = 5.1782; P = 0.0229), suggesting that female goats had higher odds of being ranked in the high-fitness group compared to males. Season of birth also demonstrated a significant association (p = 0.0012), indicating that environmental and management conditions during the early developmental phase influenced long-term fitness outcomes. The year of birth exhibited the most robust statistical response (Wald χ
2 = 70.2874; p<0.0001).
Logistic regression estimates for predictive variables
A detailed analysis of the logistic regression estimates is presented in (Table 4 and Fig 1). Among the categorical contrasts, the female group exhibited 1.493-fold higher odds of being classified as high fitness compared to males (p = 0.0229). Seasonal differences indicated that animals born in the rainy season (R vs. Au) were significantly less likely to fall in the high fitness group (OR = 0.354; p<0.0001), potentially due to postnatal stressors or suboptimal nutrition during that time. Animals born in summer season showed higher odds of survival than all other seasons.
The year-wise contrasts yielded critical insights. Goats born in 2019 and 2021 had notably reduced odds of achieving high-fitness status compared to those born in 2023 (OR = 0.042 and 0.043, respectively; both p<0.001). Conversely, goats born in 2022 showed significantly higher odds (OR = 0.366; p = 0.0007), possibly reflecting improved management or breeding practices in that year.
These findings corroborate earlier reports that shifts in climate, disease prevalence and feed availability can markedly influence phenotypic outcomes
(Feng et al., 2020 and
Do et al., 2021).
The ROC curve value 0.73 shows the good performance of logistic regression model in binary classification by plotting true positive rate (sensitivity) vs false positive rate (1-specificity) across various probability thresholds (Fig 2).
Effect of season of birth on fitness classification
The analysis revealed that the season of birth significantly influenced the fitness of goats. As depicted in Fig 7, goats born during the autumn and winter seasons had a higher probability of high fitness compared to those born in summer. The Wald Chi-square test for the season of birth yielded a value of 10.5526 with a p-value of 0.0012, indicating a strong statistical association.
Parameter estimation using bayesian analysis
Bayesian estimation was performed using the ‘GIBBSF90+’ and ‘PostGibbsF90’ modules from the ‘BLUPF90’ software suite
(Misztal et al., 2014). The model was run for 25,000 Gibbs samples with a burn-in of 5,000 and a thinning interval of 20, retaining 1,000 posterior samples for inference. The aim was to partition the total phenotypic variance of the fitness score into genetic and residual components and estimate the heritability.
The posterior mean for the additive genetic variance (Animal) was estimated at 0.03675±0.00216, with a 95% Highest Posterior Density (HPD) interval ranging from 0.0084 to 0.0734, suggesting a modest yet credible genetic contribution. The residual variance was much larger (0.1665 ±0.00135), indicating a predominant non-genetic contribution to the phenotype. Total phenotypic variance was estimated at 0.20325±0.00062, yielding a heritability estimate of 0.1791±0.01031 (HPD interval: 0.049 to 0.349) as depicted in (Table 5).
Interpretation of bayesian posterior parameters and heritability
The Bayesian analysis provided posterior distributions for four key parameters: additive genetic variance (Animal), residual variance (Residue), phenotypic variance and heritability. These were inferred from the Gibbs sampling process, which after 5,000 burn-in cycles and thinning, yielded 1,000 retained samples shown in (Fig 3, 4 and 5). The Bayesian estimates of effects of sex, season of birth, year wise trend on fitness is shown in (Fig 6, 7 and 8 respectively) and Breeding value estimated of fitness of selected Beetal goats is shown in (Fig 9).
Additive genetic variance (Animal effect)
The mean posterior estimate of the additive genetic variance was 0.0367±0.0022, suggesting a relatively modest influence of heritable factors on the immune response phenotype under investigation. The 95% HPD interval (0.0084-0.0734) indicates that while only some variation is due to additive genetic effects, it is never zero. A Geweke diagnostic value of 0.04, combined with low autocorrelation across lags, confirmed chain convergence and parameter stability.
Residual variance
Residual variance was estimated at 0.1665±0.0014, with a narrow HPD interval (0.1368-0.1992), reflecting the dominance of unexplained variability in the model. This is typical of complex threshold traits like fitness where environmental, epigenetic and non-additive genetic factors play substantial roles
(Naeem et al., 2012 and
Lawless et al., 2014).
Phenotypic variance and heritability
Total phenotypic variance was calculated as 0.2033± 0.0006, encompassing both genetic and residual components. The heritability (h
2) estimates of 0.1791± 0.0103, with a posterior range of 0.049-0.349, suggests that the trait is moderately heritable. This degree of heritability is consistent with findings in goat immune trait studies using similar stimulation models
(Khanduri et al., 2018 and
Wu et al., 2019), indicating that while genetic selection may improve immune responsiveness, environmental management remains crucial.
Statistical diagnostics and model strength
• Effective sample sizes were above 60 for most Para-meters, with phenotype at 364.4, ensuring precision.
• Low lag autocorrelations (
e.g., <0.1 for lag-1) and positive convergence diagnostics further validated the quality of MCMC estimation.
• The Geweke diagnostics were <0.05 for all parameters, suggesting that the Markov chains had converged adequately. Autocorrelation values for lags 1, 10 and 50 were all low, supporting the independence of retained samples. Effective sample sizes were above the recommended threshold in most cases (
e.g., 364.4 for phenotype), reinforcing the robustness of posterior estimates as shown in (Table 6).
Collectively, these results offer compelling evidence that fitness in indigenous goats, though partially heritable, is significantly shaped by environmental exposures and non-additive factors. Such insight informs selective breeding strategies aiming to enhance disease resilience without compromising adaptability.
In this study, data on Beetal goats spanning from 2015 to 2023 were extracted from records at the Directorate of Livestock Farms, GADVASU. A total of 530 animal records belonging to 230 dam families sired by 52 males were used, out of which 230 death records were reported and 466 animals survived upto one year of age. Using logistic regression, non-genetic factors such as sex, season of birth and year of birth were evaluated for their influence on survivability upto one year of age. The findings indicated significant effects of these factors, with females and animals born in summer seasons showing a higher likelihood of being categorized into the high fitness group.
The Breeding values for fitness showed a range of 1.117 with maximum value of 0.303 and minimum value of -0.814 with average value of -0.068. The heritability estimate of 0.1791±0.01031, derived from Bayesian analysis with Gibbs sampling, suggested a moderate genetic influence on the trait under consideration. It suggests that family selection should be practiced for breed improvement programme. These outcomes underscored the complex interplay between environmental exposures and host genetic architecture in shaping immune fitness in goats.
A logistic regression model was employed to assess the influence of sex, season of birth (SoB) and year of birth (YoB) on the classification of goats into high-fitness and low-fitness categories. These fitness categories were foundational for downstream miRNA profiling and differential expression analyses. The observations form logistic regression analysis are in line with previous studies that have identified year-to-year environmental variability and perinatal conditions as critical determinants of phenotypic resilience and immune development in ruminants
(Brogaard et al., 2016; Bilbao-Arribas et al., 2019;
Liang et al., 2016). Such stratification provides a biologically relevant framework to investigate the endogenous miRNA profiles associated with high and low fitness groups.
The trend of effect of season of birth may be attributed to favorable climatic and nutritional conditions prevailing during the cooler months, which could positively impact neonatal immunity and survivability. Goats born during extreme summer are often exposed to higher heat stress and lower feed availability, potentially impairing their early growth and immune programming. These findings underscore the importance of birth season in shaping lifetime health and productivity traits in livestock. The trend of year of birth showed mixed trend but in later years, it showed a positive trend depicting improved management or breeding practices in these years.
The Bayesian estimates and MCE from post-gibbs sample estimates suggest that approximately 17.9% of the variation in the fitness could be attributed to additive genetic effects as heritability, while the remainder arises from environmental and non-additive genetic factors. Such low to moderate heritability is consistent with complex immune traits as documented in livestock studies by
Brogaard et al., (2018) and
Qi et al., (2018), where host resistance traits were found to be moderately heritable.
The results of the study indicate that the 95% highest posterior density regions (HPDs) for the major gene parameter, particularly for the major gene variance, indicating the statistical significance of the major gene component by
Inan et al., (2025).
Practical implications for breeding programs
Moderate heritability (0.1791±0.01031) for fitness suggests family selection using top dam families with high breeding values. Prioritize females and summer-born kids, which show higher survivability per logistic regression. Optimize birth seasons to avoid summer heat stress.
Study limitations
This study was confined to a single farm (DLF, GADVASU, Ludhiana), restricting generalizability to broader Beetal populations or varying management practices across Punjab; the dataset covered a relatively short 9-year period (2015-2023), potentially overlooking long-term environmental trends or rare events; additionally, dependence on farm records risks unmeasured recording inaccuracies and the binary survival metric (up to 12 months) neglects post-yearling fitness or multi-trait dynamics.
Suggestions for future studies
To address the single-farm limitation and enhance generali- zability, multi-farm or multi-regional studies across diverse Beetal populations in Punjab and beyond should incorporate longer-term data (
e.g., 15+ years) to capture climatic variability and generational trends; expanding the dataset to include post-12-month survival, health metrics and multi-trait analyses (
e.g., growth, reproduction alongside fitness) would provide a holistic breeding value framework, while integrating genomic data (
e.g., miRNA expression or SNP markers) with these phenotypic records via advanced Bayesian models could validate BVF rankings, identify causal genetic factors and support practical selection programs for improved goat fitness.