by Sally Colby
There’s more to feeding pregnant beef cows than adjusting the quantity and nutrients of a ration. Dr. Katie Wood, assistant professor of ruminant nutrition and physiology at University of Guelph, said feeding pregnant cows has a significant impact on both the offspring and the future performance of that offspring.
Wood reviewed the contributions to the phenotype (physical traits) of an animal: herd genetics (DNA) and how the animal interacts in the environment (feed, health, stress).
“In the past 20 to 25 years, we’ve learned more about the interaction between the gene itself and the environment,” said Wood. “We’re starting to learn that the environment can have permanent, lasting changes on the genetic code.” Wood added that such changes may be permanent, and potentially heritable.
This relatively new approach to optimizing production includes several subfields: nutrigenetics, fetal programming and epigenetics.
Nutrigenetics is the “you are what you eat” aspect, or how the genetic composition of an animal influences response to a given diet. Fetal, or developmental, programming is “you are what your mother ate” and takes into account pregnancy, the cow’s diet during gestation and stress, and how those factors influence the offspring.
The third, epigenetics, is a fairly new concept. Wood explained that some genetic changes are thought to be heritable and possibly permanent. “‘Epi’ comes from the Greek and means ‘above and beyond,’ so the study of epigenetics is above and beyond that animal’s genetics,” she said. “You are what your grandmother ate.”
Beef producers can use these concepts to improve their bottom line. For example, the ideal feedlot animal is efficient, grows quickly and puts on marbling without excess external fat. However, as the animal goes through life, random stresses may redirect the outcome of the phenotype. “Most of these events are random – things we don’t have a lot of control over,” said Wood. “But we may actually be able to influence the direction. If we learn more about what happens in the fetal and neonatal period, we may be able to direct the animal as it matures toward a more ideal phenotype.”
The challenge becomes determining how to manage or remove disruptions in the animal’s life cycle. For example, many important traits are established during fetal development. “In early life, there is development of the uterus and placenta,” said Wood. “Toward the end of the first trimester, we see developments of the organs in the fetus, including the testes and ovaries. That’s important in reproductive health.”
Later in gestation, primordial follicle development in female calves determines the number of eggs the animal will have throughout her lifetime. “That occurs late in the first trimester and the beginning of the second trimester,” said Wood. “In the late first trimester and early second trimester, muscle cells develop, starting with primary myocytes then secondary myocytes. These determine the number of muscle fibers the animal will have as an adult.” During the third trimester, fat cells begin to develop, but disruptions in fetal growth during the early part of this period can potentially impact marbling.
As body systems undergo changes during gestation, stress events can impact the development of these systems. Stressors can be environmental, such as heat or cold stress, or health issues in the cow. While it’s impossible to control some stressors, producers can manage nutritional impacts such as protein, fat and minerals.
Wood reviewed a study that demonstrates the effects of supplementation in late gestation. A three-year Nebraska study involved 170 late gestation Hereford cross cows grazing on 6% protein rangeland. Half were supplemented with one pound of a cottonseed-based protein supplement three times a week, and half received no supplement. Following calving, cows were on either meadowgrass pasture or received cool season grass hay in a dry lot. The heifer calves from these cows were managed as a group, and exposed to a bull for 45 days as yearlings.
Despite receiving more protein during late gestation, there was no impact on the birth weight of calves from supplemented cows. However, the daughters from supplemented cows were heavier at weaning, pre-breeding and at pregnancy check.
Pregnancy rates of offspring from protein-supplemented dams were also influenced. The daughters of supplemented cows had a 93% pregnancy rate versus an 80% pregnancy rate among daughters of non-supplemented dams. Calving distribution was also influenced, with 77% of heifers from supplemented dams calving in the first 21 days versus about 50% of heifers from non-supplemented dams calving in the first 21 days.
Research conducted in Wyoming and South Dakota examined the effects on male offspring whose dams were fed protein supplements. Cattle were managed as a group on winter range (6.5% protein) until day 120 of gestation, then half the cows were switched to improved pasture (wild rye and wheatgrass – about 11% protein). Cows remained on the improved pasture for 60 days. Steer calves from these cows were managed as a group and fed out for slaughter.
Wood said there was no difference in birth weight between calves born on range versus improved pasture, but offspring from dams that spent 60 days on improved pasture were heavier at weaning. They had higher ADG, which translates to heavier weight at slaughter, and more fat over the 12th rib. Steers also had higher marbling scores.
Researchers looked further at marbling traits and found interesting results. “Regardless of whether the animals were on improved pasture or native range, the size of the fat cells didn’t change,” said Wood. “But steers from cows on improved pasture had more fat cells. This shows they have potential for a higher level of marbling.”
It’s well known that in late gestation (during the last 180 days of pregnancy) fetal growth is significant. With that comes high nutrient demand: protein to support growth, energy for the cow and minerals for bone development. If the cow is not receiving adequate nutrition, her metabolism might change to shuttle nutrients toward the developing calf, and in some cases, the cow’s own muscle reserve is used to support dramatic late gestation growth.
Woods’s hope is to develop generate multigenerational standard operating procedures for feeding cattle, targeting key outcomes such as the ideal feedlot animal or replacement heifers as well as how to best deal with environmental interferences such as drought, heat stress or cold stress.
“It’s a work in progress,” said Wood. “We know a lot about the timing of some stresses, and we know that when those nutritional insults occur can really matter and change outcomes. What you feed pregnant cows matters much more than you think. It’s likely to have long-term impact on the herd, whether it’s in the feedlot or in replacement females.”