Seasonal effects on ruminal fermentation and greenhouse gas emission patterns in non-lactating crossbred Saanen goats under tropical conditions: Evidence from respiratory chamber measurements.

Background and Aim: High ambient temperature (HTa) associated with climate change poses challenges to livestock production and exacerbates greenhouse gas (GHG) emissions in a self-perpetuating cycle. While HTa influences ruminal fermentation and enteric methane (CH₄) emissions in dairy cows, data on long-term seasonal HTa effects in heat-adapted tropical dairy goats under natural conditions remain limited. This study aimed to evaluate seasonal HTa effects on ruminal fermentation patterns, physiological responses, and daily CO₂ and CH₄ emissions in non-lactating crossbred Saanen goats using respiratory chamber measurements. The research gap addressed includes the scarcity of studies decoupling long-term acclimatization from short-term diurnal HTa using paired ambient and cooled chamber conditions in small ruminants under true tropical environments. Materials and Methods: Twelve nulliparous crossbred Saanen × Bach Thao goats were used in a completely randomized design with two seasonal periods (winter: December-February; summer: March-May) and six replicates. Goats were housed individually and fed a total mixed ration (80% corn silage, 20% concentrate). Ambient conditions, dry matter intake (DMI), water intake (WI), rectal temperature (Tr), respiratory rate (RR), plasma cortisol, and ruminal volatile fatty acids (VFAs) were measured. Daily CO₂ and CH₄ emissions were quantified over 24 h in respiratory chambers under ambient and 25°C air-conditioned conditions at mid-experiment points. Data were analyzed using linear mixed-effects models with season, chamber, and their interaction as fixed effects. Results: Summer daytime temperature-humidity index exceeded 80, with a seasonal ΔTa of 3.0°C. Long-term HTa increased total ruminal VFA concentration (53.2 ± 0.7 vs. 57.8 ± 1.0 mM, p < 0.05) and acetate:propionate ratio (2.3 ± 0.2 vs. 3.3 ± 0.4, p < 0.05) while decreasing NH₃-N (30.1 ± 1.0 vs. 26.1 ± 0.1 mg/dL, p < 0.05), without affecting DMI or WI. Daily CO₂ (378 ± 40 vs. 407 ± 32 g/day) and CH₄ (7.2 ± 1.7 vs. 9.2 ± 1.0 g/day) emissions did not differ between seasons (p > 0.05). However, attenuating daytime HTa to 25°C reduced nighttime CO₂ emissions (188 ± 12 vs. 160 ± 5 g/day, p < 0.05), linked to altered drinking behavior. Conclusion: Seasonal HTa under tropical conditions altered ruminal fermentation without changing daily enteric CO₂ or CH₄ emissions in crossbred dairy goats, highlighting their superior adaptability. Short-term HTa attenuation revealed subtle effects on nighttime CO₂ via drinking behavior. These findings provide baseline data for species-specific GHG mitigation and challenge cattle-centric models in warming regions.