By Vet Annie Kerr
Twins in dairy cows can result in significant losses to a dairy herd. This is because they significantly increase the risk of assisted calvings, retained foetal membranes, uterine infections, metabolic disease (e.g. ketosis and Left Displaced Abomasums- LDA), culling and also extend the calving to conception period. These issues impact the health of the cow and the profitability of the herd.
The potential problems with twins begin very early on with fusing of the placental blood vessels (resulting in freemartins). As the pregnancy develops, total foetal mass becomes the critical issue as a cow with twins is attempting to support 60% more foetal mass. When foetuses, placenta, and fluids are considered, cows carrying twins lose more condition before and after calving compared to cows carrying a single pregnancy.
As a result of their poor dry matter intakes (relative to their single carrying cohorts) through pregnancy, cows carrying twins have significantly lower insulin concentrations through the dry period. This results in fat mobilization and higher NEFA (nonesterified fatty acids) and ketone (betahydroxybutyrate, BHB) concentrations during the last month of pregnancy - both of which significantly increase the risk of type II ketosis and subsequent metabolic disease.
The problems continue around calving with cows carrying twins typically calving 10-14 days earlier than their single carrying cohorts, which can pose problems in dry cow selection therapy.
Roughly 18% of twins require assistance at calving and following assistance, survival rates in twins were only 73% compared to the 95% in assisted single deliveries. When all of the above is taken into account the net cost of a typical set of twins is believed to be approximately £130.
The typical UK incidence of twins is believed to be roughly 2-5% in cows compared to 1% in heifers, however there is a large variation between herds with incidences of 14-15% in some herds.
The average heritability of twinning among Holstein sires has increased over time. Individual bulls have a Predicted Transmitting Ability (PTA) which can range from 1.6% to 8% with a higher incidence of twinning being reported for certain cow families.
The incidence of twins has increased in recent years, in particular in holsteins. This is due to increased milk production and increased feed intake which leads to a higher incidence of double ovulation which is the primary cause of twinning.
As part of the normal oestrus cycle the cow’s ovaries go through waves of follicular development (typically three in maiden heifers and two in adult cows), however one of the follicles is selected to become ‘dominant’ secreting oestrogen and suppressing the development of other follicles (this follicle then goes on to ovulate) – it is this increasing oestrogen level which is in part responsible for the expression of oestrus.
However, during periods of heat stress follicular quality is reduced, compromising oestrogen production (which helps explain the reduction in oestrus expression described above) and as the heat wains, multiple (poor quality) follicles are allowed to develop and ovulate – increasing the incidence of twins. This is further exacerbated by the increase in dry matter intakes which occurs as the heat recedes resulting in an ovarian ‘flush’.
Twins are detectable at pregnancy diagnosis using ultrasound around 40 days (however they can be picked up less reliably from 30 days) which can allow you to consider the following management strategies:
Monitor body condition scoring more closely.