The Affect of Heat Stress on Bulls

Posted in Category(ies):  Other NewsTipsDairyBeef

Heat stress is a real problem, particularly for stock bulls at the moment.

A report from Kentucky University states that: "Bulls Natural service is still utilized on many dairy farms. Therefore, the importance of maintaining fertility in bulls is important. Bulls can be heat stressed when exposed to temperatures of 80°F (26.7°C) for merely 6 hours a day and the effects can last 9 weeks or more after the heat stress.

 

Studies have shown sperm motility is reduced by 10%, abnormal sperm count is increased by 20% and total sperm concentration is reduced by 61%. The overall result is fewer opportunities for normal, healthy sperm to reach the ovum and produce a pregnancy."

View the full report here.

 

Johnson et Al. 1963

The sperm development cycle in bulls is approximately 60 days long. Depending on the nature of the problem affecting the testes, such as heat, freezing temperatures, stress or infectious diseases, and the severity and length of the problem, semen quality will vary. 

This means severe weather from last March may still be affecting some stock bulls fertility today.

Managing Heat Stress in Dairy Cows (full article NADIS here)

"The comfort zone for a dairy cow is wide varying from around -15°C to +25°C. Minus 15 °C is called the Lower Critical Temperature (LCT) and 25 °C the Upper Critical Temperature (UCT). At temperatures below the LCT the cow will increase her dry matter intake to keep warm or convert feed to heat rather than produce milk.

The comfort zone or thermoneutral zone for a dairy cow is wide varying from around -15°C to +25°C. Minus 15 °C is called the Lower Critical Temperature (LCT) and 25 °C the Upper Critical Temperature (UCT). At temperatures below the LCT the cow will increase her dry matter intake to keep warm or convert feed to heat rather than produce milk. At temperatures above the UCT, cows have two main control strategies to maintain their thermal balance:

1. Increasing heat dispersion - in particular evaporation, by increasing subcutaneous blood flow, panting, drooling etc. These activities increase the maintenance energy needs of the animal by an estimated 20% at 35°C. This means that part of the cow's production energy will be re-directed to thermal regulation.

2. Limiting heat production - by reducing all activity and changing the feeding pattern. As the majority of heat production in dairy cows is essentially due to rumen fermentations the cow will reduce her DM intake by 10-30%. She will also be selective in what she will eat - namely less roughages.  The latter increase rumen activity and therefore heat production.  Also, rumination, which produces heat, decreases dramatically.

If an animal fails to control her thermal balance, she becomes heat stressed, her feed intake will decline and so will her milk yield. As the ambient temperature increases above the UCT, milk yields can fall by as much as 20%. There will also be a reduction in fertility, including an increase in embryonic loss. There is also evidence of an increase in the risk of clinical mastitis.

High-yielding cows generate more heat than dry cows irrespective of ambient temperatures. A cow yielding 18 litres per day will generate 28% more body heat than a dry cow. A cow yielding 31 litres a day produces 48% more body heat than a dry cow. In broad terms, each cow produces the same heat output as a 1.4kW electric heater. There is evidence that heat stress is most marked when it comes in short bursts with no time for the cow to adapt to the rising temperatures."

A number of our AI Technicians are now back AI'ing in dairy herds that had stopped AI'ing due to the current weather conditions, bull breakdown and concerns around bull fertility. 

Posted on Thursday, 28 June 2018  |  By Progressive Genetics
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