Thermal Comfort and Its Effects on Reproductive Efficiency in Dairy Cows in the Hot Humid Climate of Nigeria

This study investigates the relationship between thermal comfort indicators and reproductive efficiency in dairy cows managed in the hot-humid climate of southern Nigeria, quantifying the extent and duration of heat stress and its consequences for reproduction. Heat stress is a major but undercharacterised constraint on dairy productivity in Nigeria's tropical zones, affecting hypothalamic-pituitary-gonadal axis function, oestrus expression, fertilisation rate, and embryo survival. This study conducts a year-round thermal monitoring study at five commercial dairy farms in Rivers, Edo, and Lagos States, continuously measuring temperature-humidity index, rectal temperature, respiration rate, and sweating rate in Holstein Friesian cows using data loggers and physiological sensors. Reproductive records including oestrus detection rate, conception rate, days to first service, and services per conception are extracted from farm records and correlated with temperature-humidity index data. Blood samples assess progesterone, oestrogen, cortisol, and heat shock protein 70 concentrations at low and high THI periods. Findings reveal that cows experience THI above the critical threshold of 72 for an average of 8.4 months per year across all farms. During high THI periods, conception rates decline from 58 to 32 percent and days to conception extend by 42 days. Serum HSP70 increases 2.8-fold during peak heat stress, confirming cellular stress responses. Cortisol elevation during heat stress negatively modulates LH pulsatility. The study recommends shade provision, cooling systems, and modified breeding timing as priority heat stress management interventions for Nigerian dairy farms.

Keywords: heat stress, reproductive efficiency, dairy cows, hot-humid climate, Nigeria.