2.5. Milk yield

Milk production is the primary factor affecting the incidence of double ovulation in lactating dairy cows [21].

High milk production near the time of ovulation can increase the incidence of double ovulation, which would therefore subsequently result in an increase in twinning. Current dairy management strategies are aimed at maximizing the milk production of the cow. This intensive feeding strategy would in turn increase the incidence of double ovulation in the dairy cattle population in general. It is therefore not unexpected that given the annual increases in milk production that there should be an overall increase in twinning rates as well.

#### 2.6. Hormonal influences

It was found [31] that the rate of twinning increased with increases in milk production, incidence of cystic ovarian disease, and the use of common pharmaceuticals, including gonadotropin-releasing hormone (GnRH), prostaglandin (PGF2α), and antibiotics. For farmers and the milk industry, only a functional cow with regular and healthy offspring is profitable. Any kind of disorder in relation to the reproductive system will cause losses, by increasing the number of required inseminations and elongating the calving interval. To avoid or at least to reduce these issues, a hormonal treatment could help to reach a successful pregnancy [32, 33]. For the "OV-Synch method," the cows get two intramuscular injections of gonadotropinreleasing hormone (GnRH) as well as one prostaglandin (PGF2α) injection in a specific order to synchronize the ovulation of a herd [21]. Another benefit beside the synchronization is the use of this injection combination to treat cysts of the ovaries [34]. Due to the effect of hormones on the cows' body, 14.1% of the cows have shown a double ovulation, and with that, in 5.2% of the cases, a twin pregnancy was detected [21].
