MMN May 2024 – Risk Factors for Twinning

As dairy cattle have been selected over the years for higher milk production, a higher rate of twinning has also been observed. In fact, the rate has tripled over the last 30 years! There are many downsides to a cow having twins, with greater risk of calving difficulties and metabolic issues. Also, mixed sexed twins are more often than not freemartins (infertile) and the cow generally tends to produce less milk and is more likely to be culled than dams producing a single calf.

Twinning occurs either as a result of double ovulation (non-identical twins) or from an embryo splitting very early on in development, producing identical twins. Double ovulation is by far the most common scenario. The risk increases with age, with several studies reporting an incidence of less than 1% for first parity animals to over 5% in cows of highest parity. This increasing incidence may also be linked to higher milk yields with increasing parity.

As milk yield rises, so does dry matter intake. Cows with a higher dry matter intake have an increase in blood flow through the liver, resulting in lower progesterone levels. A reduction in blood progesterone increases the likelihood of double ovulation.

The level of milk production in the fortnight prior to oestrus is positively associated with the incidence of double ovulation in Holstein cows. With a milk yield of 40 litres, the incidence of double ovulation is around 25%. However, at 50 litres, the likelihood increases to around 50% (research by Paul Fricke, University of Wisconsin).

Therefore, nutrition has a key role to play, with cows on a rising plane of nutrition and in good or increasing body condition increasing the likelihood of double ovulation, through a higher milk yield and feed intake.

While reducing feed intake is not a recommended strategy for minimising the risk of twins, in herds that use synchronisation protocols for AI (and have a high incidence of twins), it may be worth considering the type of protocol used. For example, increasing the level of progesterone in the blood during the growth of the pre-ovulatory follicle may reduce the risk. This can be done by breeding cows to the first timed AI following a Double Ovsynch protocol.

Cows carrying twins have significantly higher energy requirements during pregnancy (50-70% more), and their dry matter intake in the pre-calving period is lower than those carrying just one calf. In addition, they have a shorter gestation length, meaning that they are less likely to have a full three weeks on a close-up ration. Therefore, it is good practice if cows are known to be carrying twins, to introduce them to the close-up diet earlier, ideally for the whole of the dry period. Keeping them in the close-up group will also reduce social stress, with no pen move from the far off to the close-up group.

Post-calving checks and keeping the cow in the fresh pen for longer to closely monitor her will help with early detection of any health issues and ensure she has a good appetite before being introduced to the main herd. This may help to mitigate some of the negative effects experienced in cows carrying twins.

Bulls have no effect on twinning so the genetics for twinning lies in the cows. Although it is lowly heritable, twins can run in certain cow families and a cow carrying twins is at greater risk of subsequent twinning.

lorna.macpherson@sac.co.uk; 07760 990901