University of Florida
What is heat stress?
There are several sources of heat that affect cattle.
- The first and most important is heat from the sun.
- This is amplified when the humidity is high.
- Metabolic heat from digesting feed. This heat is higher for forage compared to grain.
In response to extreme heat, cows will:
- Regulate internal heat production (eat less, reduce growth, lactation, and activity)
- Regulate heat exchange (increase blood flow to the skin, increase sweating & panting)
We would prefer that cattle increase their heat exchange and keep their production high.
Heat stress is when the cow's internal temperature increases above normal levels.
We can also expect more areas of the country to be affected by heat and humidity. So, how do we select cattle that can cope with heat stress?
In swine, poulty and dairy production we use climate control to manage heat stress. This is not feasible in beef cow-calf production where cattle are not confined and we can't control their environment.
There is lot of genetic variation in thermal tolerance in beef cattle. This means that we can make progress through genetic selection!
About 20% of the variation in thermal tolerance is due to genetic variation.
The hard part will be identifying which animals are genetically superior for core body temperature regulation. This requires exposing animals to heat stress and seeing which ones perform the best. Need phenotypes and tools to make selection decisions.
Dr. Mateescu research is focusing on using Angus x Brahman cross cattle to collect heat tolerance traits.
They put a temperature sensor in a blank CIDR (no hormones, just the CIDR). They left these in heifers for 5 days.
Under high heat stress, only 100% Brahman were able to regulate their body temperature and stay neare 39 degree Celcius.
Under lower heat stress, Angus and 3/4 Angus could not effectively regulate their temperature, whereas Brahman, 1/4 Angus, 1/2 Angus and Brangus could regulate their temperature.
Even when looking within 725 Brangus, we see a lot of variation in internal body temperature under heat stress. Animals with smoother, finer hair had lower internal body temperatures (were cooler).
We have to select for internal body temperature using genomics because collection of the phenotype is very expensive. The cow of the future will have both high productivity and resistance to heat stress.