Around the world, dairy cows are finding it increasingly difficult to "get pregnant". Several hypotheses have been put forward to explain this drop in fertility, but a team from Laval University has just identified a factor that has passed under the radar of most researchers until now. A cow’s fertility may indirectly depend on factors linked to her "father’s" health, diet and rearing conditions, suggests the study published by these researchers in the scientific journal Epigenetics.
"Fertility is the ability to produce fertile oocytes. In cows, it is estimated from the number of conceptions per insemination. Over the past 20 years, the fertility rate has fallen from around 50% to 35% in Holstein cows", points out Marc-André Sirard, professor in the Department of Animal Sciences and researcher at the Centre de recherche en reproduction, développement et santé intergénérationnelle.
This phenomenon has been studied from many angles, but little attention had been paid to the epigenetics of male breeding stock. "Factors such as diet, disease and rearing conditions mean that certain regions of the genome can become caviated (by methyl groups), blocking the reading of genetic information and its translation into proteins. These epigenetic marks can be passed on to offspring," explains Marc-André Sirard.
To find out more, Sirard’s team studied 12 male progenitors chosen from a population of 100 bulls. Six of these sires had produced offspring distinguished by very high fertility; the other six had produced offspring of particularly low fertility. The researchers compared the epigenetic marks present on the DNA contained in the spermatozoa of the bulls in each group. These sperm came from semen samples collected several years earlier, during the same period as those used to conceive the offspring included in the study.
"Our analyses revealed that there was a distinct epigenetic profile between bulls with highly fertile offspring and those with less fertile offspring," summarizes Professor Sirard. Ten regions of the genome displayed particularly distinct epigenetics, and they were mainly located on the X sex chromosome. The consequence is that the epigenetic marks found on this chromosome of a progenitor are all passed on to his descendants."
The decline in dairy cow fertility has very real repercussions for milk producers, continues the researcher. For a cow to give milk, she must be pregnant," he points out. A drop in fertility means that more inseminations are needed to obtain a pregnancy, that more time elapses between two pregnancies, that cows have fewer calves during their lifetime, and that cows have to be replaced when they reach the age of 4 or 5, whereas they have a longevity of 25 years. To counter this drop in fertility, producers are resorting more and more to hormone treatments to trigger heat. This practice can have negative repercussions on the health of the cows, and it arouses mistrust among consumers."
"By modifying the diet or rearing conditions of bulls, we could eliminate the epigenetic marks that prevent the expression of certain fertility-related genes in their offspring."
-- Marc-André Sirard This study delivers two practical messages for mitigating infertility in dairy cows. "When choosing a sire’s semen, we should consider the epigenetics of the regions of the genome that we have associated with the fertility of their "daughters". What’s more, epigenetics is a reversible phenomenon. By modifying the bulls’ diet or rearing conditions, we could eliminate the epigenetic marks that prevent the expression of certain fertility-related genes in their offspring", concludes Professor Sirard.
The authors of the study, published in Epigenetics, are Ying Zhang, Clément Plessis, Julien Prunier, Hélène Martin and Marc-André Sirard of Université Laval, and Rémi Labrecque of SEMEX Boviteq.
