Cows are more sensitive to heat stress then humans, with production losses starting in the mid 70 degrees in moderate humidity and in the low 80’s in low humidly. The economic costs of heat stress will vary depending on geographic location and facilities. For California cows with access to shade but not fans or sprinklers, losses are estimated to average $110/cow/year. Losses are higher in cows without access to shade.
Providing adequate shade may be the most cost-effective heat-stress mitigation available. Addition of sprinklers/soakers or misters/foggers can leverage heat stress reduction provided by shades. Optimal practices will differ from farm to farm, but the majority of western dairies should consider cooling the holding pen and exit lane sprinklers.
For severe heat stress events, the easiest way to tell if cows require emergency cooling is by counting respirations. If more than 5 out of 10 cows (50%) have breathing rates exceeding 100 breaths per minute, immediate action should be taken to reduce heat stress.
Economic Costs of Heat Stress in Dairy Cattle
Heat stress will of course vary depending on a farm’s geographic location and facilities. One study estimates that California dairies with minimal heat stress abatement (access to shade, but no fans or sprinklers) lose $110 per cow per year. Losses are higher in cows without access to shade.
Decreased milk production results in large part (but not exclusively) from reduction in Dry Matter Intake (DMI). California dairies with minimal heat stress abatement are estimated to experience average DMI reductions of 320 lb/cow/year, and a concurrent decrease in milk production of about 650 lb/cow/year. Embryos are also susceptible to increased body temperature resulting from heat stress.
Reproduction can suffer even more than milk production with Florida data estimating up to a 53% reduction in conception rates during the summer and into the fall. California operations with minimal heat abatement are estimated to experience in average increase of 12 days open and almost a 1% increase in reproductive culling. Production losses are not limited just to lactating cows.
Heat-stressed dairy cows have been observed to produce 1,000 to 2,000 pounds less milk during the next lactation.
When Does Heat Stress Start in Dairy Cows?
Heat stress results from a combination of factors including ambient heat & humidity, local air movement and absorption of radiant (sunlight) energy. Cattle are actually more sensitive to heat and humidity then humans. The best way to anticipate whether environmental conditions will have a negative effect on production is to examine a dairy-specific Temperature Humidity Index (THI) chart.