SPRING SUMMER BONUS MAGAZINE 2026

Inmany South African dairies, heat stress is often seen as primarily a lactation challenge. When temperatures rise above 30°C, and the humidity remains high, milk yield drops, feed intake declines and conception rates suffer. But if we genuinely want to beat the heat, wemust look earlier than the bulk tank. The actual cost of summer often begins quietly in the dry period, weeks before calving, where the heat load shapes colostrumquality, calf immunity, and the productivity of the next generation. Beat the heat before calving: protecting colostrum, calves, and the future herd Stephan van der Westhuizen - Technical Specialist: Dairy When the De Heus Heat Stress Index exceeds 68, cows begin to experience measurable heat stress. This index combines ambient temperature and relative humidity into a single value that reflects how effectively a cow can dissipate heat under real farm conditions, making it far more informative than temperature alone. In practice, producers can monitor these conditions using tools such as the De Heus CoolCare app, which translates local weather data into real-time HSI values and easy-to-interpret risk categories. Once the index rises above 72, the impact becomes more pronounced. As shown in Graph 1, average summer conditions in South Africa regularly exceed these thresholds, placing dry cows under sustained heat load during late gestation, precisely when mammary development and immune preparation for colostrum formation are most critical. Graph 1: Average Heat Stress Index for SA (Source South African Weather Service) Spring/Summer 2026 BONUS www.agribonus.co.za 50