Impact on milk production
Milk production decreases when THI exceeds 70. This already happens at a temperature from 22 degrees. In times when the THI exceeds 80, milk production can drop to 38%. There is a clear 48-hour delay between the rise in THI and the drop in milk production. The dairy cows in the middle of their lactation (80 – 120 days on the milk) are most sensitive to heat stress. The old milking cows have a lower milk production and are therefore less sensitive to heat stress.
Dairy cows in early lactation are less dependent on feed intake. As a result, they produce less heat per kilogram of milk produced. Second calf cows also seem to be most sensitive to heat stress. The third and older calf cows are least affected by heat stress. It takes until November before milk production starts to rise again. In October, milk production is therefore on average 2 kilograms lower than in January. So the effects of heat stress last longer than just during the periods when it’s really hot.
Effects on udder health
The tank cell count has been examined for udder health. The tank cell count starts to rise from March and continues to rise until August. Then the tank cell count decreases again. A strong correlation has been found between the tank cell count and the THI.
The companies with a high tank cell count (> 250) rise on average a lot higher than the companies with a low tank cell count (<100). The companies with a higher tank cell count probably have more animals with a cell count above 250. These animals in particular are causing the large increase during the summer months.
Impact on reproduction
Various index numbers have been investigated for reproduction. The main results are that the insemination rate and conception rate are negatively affected. For example, decreases of 2 – 63% were found for the insemination rate, and the conception rate decreased by 1 – 60%. The insemination rate decreases because the length and intensity of the heat decreases. As a result, the animals don’t show the heat that well.
The conception rate decreases because less animals become pregnant. This has to do with the quality of the eggs, which is negatively influenced by heat stress. As a result, cows that calve in the spring and summer have more difficulty getting pregnant than cows that calve in the fall or winter. It takes an average of two cycles to recover from heat stress, so the negative effects of heat stress can last until autumn.