by South Dakota State University Associate Professor and Extension Meat Science Specialist Amanda Blair, Ph.D.
As beef producers, our management efforts tend to be heavily focused on events such as breeding and calving or on decisions such as nutritional inputs, pasture management, or genetic selection. But once cows are successfully bred, how often do we consider the development that’s occurring during gestation or the impact that our management decisions have on the developing fetus? Fetal programming is a relatively new area of study investigating that very concept. In fact, there is a growing body of evidence that suggests the gestational environment can cause permanent changes in the fetal genome resulting in lifelong impacts on the phenotype of the offspring.
Consider that the majority of livestock raised for food production is subjected to the gestational environment for a considerable amount of time relative to their entire lifespan. For example, a steer raised for beef and slaughtered at 16 months of age would spend approximately 37 percent of its entire lifespan, from conception to slaughter, in utero. However, gestation is often thought of as a fairly uneventful time of production. In many areas of the country, beef cattle producers implement low-cost feeding programs during all or portions of gestation. However, if feed is limited or forage conditions are poor this strategy can potentially cause a deficiency in protein and/or energy if cows are not supplemented. As a result, the fetus may receive inadequate levels of nutrients, potentially altering fetal development. Applying management strategies during gestation could enhance our goals of optimizing production efficiency.
It is important to understand that a substantial portion of development occurs during the embryonic and fetal periods. During these stages of life, cellular, tissue, organ, metabolic and hormonal systems are established, and if nutrients are restricted these developmental processes could be negatively impacted. Take muscle for example, in the beef fetus, the majority of muscle cells are developed beginning at about the third month of gestation until about seven or eight months of gestation. While these muscle cells will grow larger through the remainder of gestation and after birth, there are no additional muscle cells added after this time. In short, cattle are born with a set number of muscle fibers; these fibers can grow larger, but they can’t grow more. This is significant because any challenges or restrictions that occur during gestation could limit muscle development and result in reduced muscle mass throughout the lifetime of the animal. It is, also, important to note that muscle has a relatively low priority for nutrients during development. If nutrients are limited, the priority will be for brain, heart and other organs before they are partitioned to support muscle development. On the other hand, enhancing nutrient availability during gestation can increase muscle fiber development, thus increasing muscle fiber number and muscle mass of the offspring. Understanding these developmental concepts and learning how to program cows to manipulate the developmental processes of the offspring is the focus of many research efforts.
This concept of fetal or developmental programming is commonly referred to as the “Barker Hypothesis” or the “Fetal Origins Model.” Pioneering research conducted in human medicine by Dr. David Barker and his colleagues provided a connection, linking undernourished mothers with offspring that had low birth weights and an increase in adiposity and metabolic disorders. Barker went on to suggest that deficiencies in maternal nutritional status could alter metabolism, structure and physiology of the offspring. Additional research has suggested that maternal under-nutrition during pregnancy may cause offspring to develop a thrifty phenotype, meaning increased adiposity and reduced muscle mass, that is more prepared to deal with sparse nutrient availability. Animal and meat scientists have further investigated this concept and demonstrated that modifications to the gestational environment have the potential to influence the postnatal health, growth performance, reproductive performance, longevity, and composition of the offspring.
So how can the gestational environment influence the development of the fetus and subsequently the postnatal phenotype? It is generally accepted that alterations to the epigenome are responsible for the changes observed with fetal programming. With few exceptions, all cells in the body have the same genetic code, or DNA. Epigenetics is the term used to describe stable, heritable factors, other than the DNA sequence, that influence development. These factors can modify the expression of a gene (turn it on or off), but not the actual genetic code, thereby providing each cell with its unique identity. It is the expression of particular genes that allow for each cell to develop and function as intended by allowing a muscle cell to act as a muscle cell and a fat cell to work as a fat cell. These factors can be influenced by variation in the gestational environment such as nutrient availability and maternal stress, resulting in changes to the epigenome of the fetus. These changes can subsequently alter the development of fetal tissues and the subsequent growth and composition of the offspring. Given that the phenotype of any individual is a combination of its inherited genetic code and the environment in which it is raised, the ability of an offspring to fully express its genetic potential can be limited by insults to the epigenome. For example, if we selectively breed for heavy-muscled cattle in our herd, but restrict nutrients during development, the offspring may not be able to fully express their genetic potential for muscle growth.
In recent years, there has been an interest in understanding how to manage gestating females to either minimize the effects of challenging environmental conditions or to maximize productivity. Researchers across the country have focused on understanding the impacts of gestational management on various traits ranging from reproductive efficiency and longevity to offspring health and feeding performance. To better understand how prenatal management could be manipulated to produce carcasses with desired characteristics, our meat science research group at South Dakota State University has worked on several collaborative projects. The majority of our research has focused on altering maternal nutrition during mid-gestation because this is when the majority of muscle is developed and when fat cells begin to develop, and muscle and fat make up the majority of the carcass. We have learned that maternal energy restriction during mid-gestation influences fat deposition in intramuscular (marbling) and subcutaneous (backfat) fat deposits without impacting muscle mass. Additionally, when gestating females were subjected to either a control diet or a metabolizable protein restriction during mid- and late- gestation, the offspring from cows on the control diet showed upregulation of genes found in pathways associated with muscle tissue development, while calves born to protein-restricted cows showed upregulation of genes involved in fat tissue development.
It is necessary to recognize areas where we can improve our current management practices to maximize the genetic potential we have bred into our calves. The knowledge that the gestational environment can influence fetal development and subsequent, long-term production outcomes is another tool that has the potential to enhance these efforts. While research in this area is ongoing, producers should begin to recognize this phase of production as more than an uneventful span of time. Gestation is a time of major developmental milestones and while these events can set the stage for long-term productivity, the management of the gestating cow can, also, profoundly influence the outcomes.
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