During a farm visit a cattle farmer asked me this question.
The reason for this was the umpteenth twin pregnancy on his dairy farm.
I will try to explain this complex matter in understandable language.
Twin pregnancies are timeless and therefore nothing special. However, we do see an upward trend in twin pregnancies.
In almost all cases, a twin pregnancy is caused by double ovulation.
There are a number of (animal-related) factors that significantly increase the chance of a twin pregnancy:
1. The genetic predisposition
Some “breeds” and / or “cow families” give birth to more twins than others.
2. The number of pregnancies of the individual cow
With a first pregnancy, the chance of a twin pregnancy is only 1%. In subsequent pregnancies, this percentage can rise to over 10%.
3. The milk production of the individual cow
More than 35% of the high yielding cows (more than 40 kg of daily milk production within 14 days before a natural ovulation) often have a double ovulation.
On the other hand, this percentage is around 10% for dairy cows that have a lower daily production in the same period.
4. A disrupted cycle
Around 38% of cows with a disrupted cycle have a double ovulation in the subsequent cycle.
In cows with a normal cycle, this percentage is around 16%.
Mainly stress-related matters, such as (no animal requirement) nutrition, overcrowding, rough handling, heat stress, have a negative influence on the cow's cycle.
5. The liver is partly responsible for a low progesterone level
With a higher feed intake in combination with a high milk production, the liver breaks down more progesterone. This results in a weak Corpus luteum (yellow body).
6. The influence of ovulation synchronization or Ovsynch
Some think hormone programs are the basis of many twin pregnancies.
In my opinion it is often precisely the "difficult to get pregnant" cows that are treated with a hormone program.
The cause of “difficult to get pregnant” is often a disturbed hormone balance, resulting in a weak yellow body or even NO yellow body. This combination increases the risk of a double ovulation.
The follicle's eggs are fertilized by applying a single ovulation synchronization. Because this follicle is formed when there is a shortage of progesterone, the chance of a double ovulation is greater.