Dairy breeders spend considerable time choosing the next round of bulls to use. That’s important because improvements in genetics has a significant influence on the generations that follow. Nevertheless, future performance will depend on how epigenetics regulates the DNA acquired through breeding. When epigenetics enters the picture, breeders will need to re-consider how they breed and manage their dairy cattle.
What’s Epigenetics?
Epigenetics underlies processes that affect health, fertility, longevity and many traits of dairy cattle. Epigenetic effects differ from direct genetic effects because the animal’s DNA sequence is not changed by epigenetic processes. Rather, epigenetic processes act by regulating whether genes within DNA sequences are “turned on” or “turned off” without any change in the DNA sequence. (Read more: FORGET GENOMICS – EPIGENOMICS & NUTRIGENOMICS ARE THE FUTURE)
Genetic and Epigenetic Differences
Traits such as milk yield, milk protein, conception rate, somatic cell count and udder conformation are heritable, meaning that differences among animals in these traits can be accounted for by family relationships among sires, dams and ancestors. Heritability ranges from around 3% to over 50% for various traits, therefore 3 to 50% of differences among animals are accounted for by differences in their DNA sequences.
The non-genetic variation in traits is included in what we refer to as environmental effects. Weather, feed, facilities, management practices and everything else that cattle are affected by in a herd fits into environmental effects. Many responses of cattle to environmental effects are regulated by epigenetic or closely-related processes at the cellular level in animals.
Epigenetic effects do not change an animal’s DNA sequence (genome). Instead, epigenetic effects alter how individual genes or groups of genes are controlled or as geneticists say “silenced or differentially regulated” throughout an animal’s life. Originally, epigenetic effects were thought to represent only alterations that could be passed to the next generation without changing in the animal’s genetic code. More recently it seems that epigenetic effects may impact various tissues and organs during certain periods in the animal’s life, without being passed to the next generation.
Epigenetic Triggers
Animal scientists are using the term “developmental programming” to define practices that may trigger epigenetic effects. Developmental programming may act through epigenetic or similar pathways to influence almost any trait of interest in dairy cattle. For dairy farmers, it matters little whether the action occurs through one mechanism or another, as long as responses are predictable and repeatable.
Repeatability means that there is a fairly predictable pattern of an action causing a specific or response separated by weeks, months, years or generations. That makes it challenging to determine cause and effect, without careful observations, good records and repeated verification.
Epigenetic effects may be triggered by conditions associated with natural biological process or by adverse conditions such as negative energy balance, heat stress, exposure to toxins or other disturbances. Epigenetic effects can be either positive or negative, so as we learn more it will be useful to incorporate management practices that stimulate positive effects and limit negative ones.
Epigenetic Effect #1 Calf Feeding and Future Performance
One epigenetic or epigenetic-like effect is the latent response to feeding higher levels of milk or replacer to heifer calves. Calves fed at higher levels produce more milk in first lactation about 2 years later, so the response occurs beginning about 700 days after the action. Preliminary data suggest that heifers fed more milk develop more mammary epithelial cells that become milk-secreting cells when first lactation begins. This is the kind of epigenetic effect that one would see for stem cells that are dividing rapidly when the milk is being fed. The exact regulatory mechanism for this effect is yet to be determined.
Epigenetic Effect #2 Milking Frequency Immediately After Calving
Similar to the situation in calves fed more milk, it has been demonstrated that cows milked 4X daily during the first 3 weeks of lactation and then 2X daily thereafter produce considerably more milk than cows milked 2X from freshening. The 4X milking early in lactation apparently stimulates development of more milk-secreting cells and these then remain throughout lactation, even when milking frequency drops to 2X.
Epigenetic Effect #3 Embryo Survival
It is highly probable that negative epigenetic effects occur when eggs (oocytes) are developing within the ovary when a cow is under stressful conditions. Such can be the case for the egg ovulated by an energy and/or health stressed cow that comes into heat 80 days post calving. The egg ovulated at day eighty actually started growing as an oocyte within her ovary about 3 weeks before calving.
Oocytes that develop under these stressful conditions have low survivability as embryos. Their fertilization rate is normal, but they degenerate and die at a higher rate in the first week after fertilization. This is a classical example of an adverse epigenetic effect. Our North Carolina State research team published the first report of this effect in 1992. It is referred to as the Britt Hypothesis and it has taken about 25 years for scientists to begin to understand this phenomenon at the DNA level.
Stay Tuned As We Learn More
There is a strong interest in understanding how epigenetics affect the developing fetus and how management of the pregnant cow influences the future long-term responses of the calf she is carrying. During fetal stages, tissues that will form muscles, mammary tissue, the immune system and all other systems undergo development. We will see a lot of new discoveries about epigenetics in these areas in the years ahead and this will give us tools to support development of better calves during pregnancy.
Husbandry practices trigger many of the epigenetic effects, both good and bad. Understanding how such effects are mediated will give us husbandry tools to improve both DNA-based genetics and ways to regulate the DNA in a beneficial manner.
The Bullvine Bottom Line
The Bullvine found that this information shared by Jack Britt assisted us in better understanding the topic of epigenetics. Yes, epigenetics is yet one more piece of the puzzle that progressive breeders are likely to use in the future to both breed and manage their dairy herds.
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