Reproduction, Twinning and Genomics
by Texas A&M AgriLife Extension Livestock Specialist Joe Paschal, PhD.
Reproduction in cattle is generally thought to be lowly heritable (0.10-0.20 percent), and as a result selection response is low. Regardless of what reproductive trait is being selected, there is not a lot of variability in breeding value from which to select. These reproductive traits include: age at first estrus, or ovulation, pregnancy or calving; reproductive tract score, an indicator of fertility; calving interval, one of my favorites; number of calves born in a lifetime, a useful measure if the time is a reasonable one. The added fact that the generation in cattle is between four and five years, which is the average length of time it takes for animals to be produced and then be replaced by their offspring. It is wonderful to find old cows that have produced a calf every year of their productive life. Although I would prefer single births, some cows make up the difference with a set, or two, of twins as in the case of MISS LOF 144.
Calved in 2009, she apparently didn’t breed on time but still had twins in 2011, again in 2012, a bull calf in 2013, and twins in 2014 and again in 2015. An amazing feat. Twinning in Bos taurus beef cattle accounts for around 1 percent of all calvings. In Bos indicus the rate is about 0.5 percent; while in dairy it is much higher: 4 to 5 percent, one source reported 8 percent. Twins were, and sometimes still are, sought as a method to improve the level of efficiency of beef production even though they are lighter individually at birth and at weaning their combined weight nearly doubles the weaning weight of singles. The U.S. Meat Animal Research Center has studied twins for many years and even successfully selected to increase twinning rate. However, twins are not produced without costs. Even naturally-occurring ones can cause calving difficulties and reproductive tract damage that decreases return to estrus and rebreeding pregnancy percentages, delaying the next calf crop. At worst, calving difficulties can cause the loss of the cow and calf. Usually one twin is larger than the other, and sometimes a cow will deliver her first calf and then walk away from it when she delivers her second, abandoning the first.
Twins actually come in two genetic types, identical and fraternal. Identical twins occur when the fertilized egg divides completely into two, instead of just dividing to increase the number of cells in the growing embryo, and makes a second viable embryo. This embryo is genetically identical to the first and both calves will be the same sex. The calves can still have some phenotypic differences in growth and markings, but usually these are minor. The second type of twin is called a fraternal twin, because it is produced from a separate egg ovulated about the same time as the first. Cows usually ovulate one egg, but as they get older and in the fall months they may ovulate more than one egg at a time under natural conditions. Both eggs are viable and can be fertilized but, in reality, not many are. If both eggs are of the same gender, then the twins are normal when calved. However, if the twins were fertilized by sperm cells carrying different X and Y chromosomes, then one calf would be a heifer (XX, the egg only has an X chromosome) and the other, getting its Y from the sperm cell, is a bull (XY). At least three sets of MISS LOF 144’s twins were this type.
During development of these twins of different sex, the male fetus develops more rapidly and secretes a hormone that suppresses the sex organ development of the heifer fetus, creating an infertile female. This occurs about 90 percent of the time; in the other 10 percent, normal development occurs. Interestingly enough, there actually may be more freemartins than thought as often the twin is lost or absorbed early in the pregnancy. If the twin was a male fetus and the female survived to calving it could explain infertility in some heifers, perhaps those with infertile or small reproductive tracts. Females born to males can be tested to determine if they are freemartins with one of several commercial genomic tests, usually by looking for Y chromosomes in cells. Freemartins usually have very small and misshaped vulvas that have tufts of hair in addition to infertile reproductive tracts; but a genomic test is best to determine true freemartinism.
Researchers have been looking for genes that control fertility expression and timing. A least one gene has been identified by Australian researchers on chromosome 6 in the Booroola Merino sheep. One copy of the gene results in an additional lamb per gestation. Researchers have identified several markers in swine and at least six markers have been identified in cattle as being associated with fertility, particularly ovulation rate. A very promising one is located on chromosome 10, but none are commercially available yet for cattle.
Fertility is still a trait we have to manage by providing an optimal environment, selecting heifers that express estrus early and attain 60-65 percent of their body weight by around 14 months of age, and selecting cows that breed and rebreed early and within a 12-month period. Fertility can also be selected by culling infertile or subfertile heifers and bulls based on reproductive tract scores in heifers and breeding soundness examinations in bulls.