The tell-tale signs are there: fewer cows showing signs of estrus, inconsistent heats, more cows open after insemination. Heat stress is starting to take a toll on your herd’s reproductive performance. Heat stress can have long-term effects on conception and pregnancy rates, lower milk production and increase health problems, resulting in lowered profitability.
An Uphill Battle
For parts of the country commonly associated with mild weather, summer months can mean fighting the same battles herds in the southern portion of the country struggle with year ’round. The temperature humidity index (THI) takes into account the combined effects of air temperature and humidity. Cattle will show signs of heat stress at a THI of 72; with summer THI readings creeping to these levels and beyond, heat stress is a common battle, regardless of location.
Alongside things typically associated with heat stress, like less milk and component production and dry matter intake, heat stress can negatively influence:
- Estrus detection. Duration and intensity of estrus are negatively effected by heat stress. Cows attempting to reduce body heat production limit their physical activity, translating to fewer physical signs of estrus, which makes heat detection more difficult.
- Fertility. Pregnancy rate is reduced by decreased fertility resulting from impeded development of mature follicles. Premature follicles emerge around 40 days prior to ovulation, making oocytes susceptible to heat-related damage, which can compromise quality almost immediately.
- Follicle maturity. Heat stress during the estrous cycle causes earlier emergence of the dominant follicle in the second wave, more estrous cycles with three rather than two waves, and cycle extension in addition to negative changes in ovarian and follicular function.
- Embryonic development. Embryonic development, especially through day 40, is vulnerable to heat stress. Elevating the cow’s internal temperature results in a reduction in the number of embryos that continue to develop1.As the cow tries to thermo-regulate, her system will redistribute blood flow away from her core. This aids in dissipating heat, but reduces blood flow supporting fetal development and supplying the uterine body.
How to fight the heat
Producers can mitigate many negative effects of heat with proper cow comfort, nutrition and reproductive management. Combating the symptoms of heat stress requires focus on improving detection of estrus and managing heat-stress-related embryonic mortality2. Here are recommendations for improving summertime performance: