The reality of twins in beef cattle
The concept of improving the rate of reproduction in beef cattle by selection for an increased rate of twinning has been debated with both optimism and pessimism since the early days of animal breeding.
With the success of selection for multiple births in sheep, there was hope that the technique would also work in cattle.
In general, however, beef cattle farmers have been opposed to twin births due to the number of problems associated with twinning.
Successful reproduction in a cow-calf production system is the cornerstone of biological and economic efficiency.
Cattle evaluations have focused mainly on production traits, and genetic improvement in reproductive traits through the use of selection tools has been limited.
It is globally accepted, however, thatfromaneconomicperspective, reproductive traits are twice as important as production traits in a cow-calf production system.
Reproduction of cattle is dependent on several factors, including breed type (for example Bos taurus or B. indicus), animal class and location.
The interaction between environment, health, genetic factors and management also has a major influence on reproductive performance.
Twinning is rare
Cattle are uniparous: female animals are likely to produce only one offspring per gestation. In most beef cattle breeds, twinning is rare, with multiple births most likely being due to multiple ovulations with a frequency of not more than 1%. In dairy breeds, by contrast, the frequency is between 3% and 5%. The regulation of the twinning rate has been achieved via embryo transfer, genetic selection, hormonal treatments or the immunologic suppression of hormones.
In cattle, twinning has advantages and disadvantages, depending on whether one is running a dairy or beef operation, and what production system is being used.
Genetic selection
The heritability of twinning, and indeed of most reproductive traits, is very low. Heritability (h2) for the twinning rate has been estimated to be between 0.01 and 0.09. However, it has been suggested that with genetic selection, twinning in cattle may be increased to a level of economic importance. This could be achieved through the use of selection criteria primarily focusing on multiple observations of ovulation rate when applied to replacement heifers and sires.
The genetic correlation between ovulation rate and the twinning rate is 0,75 or higher.
In cattle, the rate of twinning is regarded as a quantitative trait,
which means that it is dependent on the combination of several genes modified by environmental factors.
With the development of new genetic technologies such as quantitative trait loci (a locus is a specific, fixed position on a chromosome where a particular gene or genetic marker is located) and marker-assisted selection, it may be possible to select more effectively for twinning in beef cattle.
If loci-affecting traits related to reproductive performance, including twinning, can be identified, DNA markers will be useful for selecting genetically superior animals for several reproductive traits, thus improving response to selection.
Disadvantages of twinning
Some disadvantages associated
with twinning include lower calf survival rate, dystocia, stillbirths, abortions, calf abandonment, retained placentas, longer intervals between conceptions, increased culling rate (lower cow reproductive performance) and freemartin (infertile) heifers. Thelowerproductiveperformance in cows is caused mainly by increased difficulties during gestation and a prolonged intercalving period due to difficult births. Moreover, with the increase in twin births in a herd, the risk of unsuccessful reconceptions increases, and thereby the profitability of the herd decreases drastically.
It has been reported that cows calving twins have longer calving intervals and increased culling rates than those carrying only one calf.
In addition, twinning places the
cow at greater risk for developing metabolic disorders such as ketosis and displaced abomasum.
Advantages of twinning
In theory, beef cattle productivity in a herd can be improved through an increase in prolificacy or an increase in the frequency of twins. When raising twins, a beef cow will most likely wean more total calf weight than a cow with only one calf.
Twinning, therefore, offers the potential for an increase in beef production efficiency in a herd. Researchers have illustrated the potential for a decrease in production costs of about 20% to 30% per unit of beef returns when an increase in the total weight of calves is observed at weaning using twinning.