Muscle, fat cell development during mid-gestation can be impacted by nutrient restriction
“If we think about a feedlot steer, what percent of their life – from conception to harvest – is spent in gestation, assuming they are harvested at 15 months of age?” asks South Dakota State University Extension Meat Science Specialist Amanda Blair.
With nine months in gestation and 15 months spent growing, about 40 percent of a steer’s life is spent in gestation, she says.
“When we think about the growth of a feedlot steer, gestation is a pretty significant portion of their life,” Blair says. “Mid-gestation is considered to be a critical time point in regards to outcomes from feeding cattle.”
Feedlot performance, growth performance, yield grade and quality grade all have links to mid-gestation, which is defined as three to six months of gestation.
Blair explains, “Mid-gestation is a key developmental time point for both muscle and fat.”
Muscle and fat
Early in gestation, primary muscle fibers are established. Those fibers act as a scaffolding for secondary muscle fibers to develop on and around.
“These secondary muscle fibers make up the bulk of the muscle mass of an individual, and production of these fibers peaks in mid-gestation,” Blair says. “Then, it tails off sometime in late gestation, ending before birth.”
Cattle, humans and other livestock species are born with the number of muscle fibers they will have throughout their lives.
“Any insult that would happen during mid-gestation could influence muscle production for the rest of an animal’s life,” she continues. “We can make muscle fibers bigger, but we can’t ever make more.”
Fat cells are established during early gestation, as well, but they continue to develop postnatally.
“However, similarly to muscle, any insult that might occur can alter the establishment of cells early and could also influence the distribution of those cells in the body of the animal postnatally,” Blair adds.
While muscle and fat are often thought of first in animal production, Blair says, “On the whole, however, they rank fairly low in importance when it comes to the developing fetus.”
She adds, “If resources are limited, growth priority is definitely going to be given to vital organs, such as the brain, heart and lungs, and muscle and fat development could be compromised.”
Researchers at South Dakota State University fed cows at mid-gestation such that they maintained a body condition score of five or lost a body condition score.
“The loss was due to primarily energy restriction,” Blair explains. “At the end of mid-gestation, cows were placed back on a 100 percent energy level diet.”
She continues, “This meant cows were at either positive or negative energy level during mid-gestation, depending on the treatment they received.”
Results showed no difference in birthweight, which initially led researchers to believe there was no impact of nutritional deficit in mid-gestation.
“However, if we look at some of the subsequent results, we did find calves exposed to negative energy status during gestation had lighter receiving weights when entering the feedlot,” Blair says. “They were also lighter at days 28 and 57 in the feedlot. This was mitigated by the end of the trial.”
At slaughter, there were no differences in body weight, hot carcass weight or ribeye area.
No influences in meat color or tenderness were seen in the study, either.
“We did, however, see a difference in the ratio of marbling to back fat, indicating this restriction may have shifted the distribution of fat in the carcass,” Blair explains. “In this case, we saw more marbling and less backfat in the cattle from restricted dams.”
A decreased tendency for backfat translated to a tendency for improved yield grade in nutrient restricted calves.
Graduate students involved in the project also noted previous literature showed impacts in health from nutrient restriction during mid-gestation, so the study also looked at health.
“To determine if there were impacts on the immune system, we vaccinated them with a novel antigen, which would cause an immune reaction but it would not make them sick,” Blair says. “We did not want to compromise the rest of the study.”
The results showed calves from dams with negative energy status during mid-gestation had a reduced immune response when challenged.
Blair comments, “I think this result warrants further research in this area.”
“While the research shows benefits to carcass in the form of marbling and yield grades, the effects on receiving weight and potential health in the feedlot is a concern,” Blair summarizes. “We have seen some positive things, but we also know there are negative impacts from energy restriction in mid-gestation.”
Another study at University of Wyoming restricted protein in mid-gestation.
In the study, cattle grazed on native range, receiving five to six percent crude protein. At mid-gestation, half of the cows were moved to improved pastures that provided about 11 percent crude protein. At the end of mid-gestation, all the cows were moved to the same pasture and were provided an alfalfa supplement.
“Again, in this study, there were no differences in birthweight or 205-day adjusted weaning weight,” Blair says. “However, the calves exposed to this protein restriction at mid-gestation had average daily gains and decreased final body weight, which translated to a decrease in hot carcass weight.”
There were no differences seen in ribeye area, yield grade or marbling scores, but a decrease in backfat thickness and tenderness was seen.
South Dakota State University furthered the work to look at the differences seen when cows were fed either 100 percent or 80 percent of their metabolizable protein requirements.
“We found a similar result,” Blair says. “Those restricted in mid-gestation had a reduction in tenderness.”
Research done in sheep showed increase in connective tissue cells as a result of nutrient restriction.
“Connective tissue impacts tenderness and equals toughness,” she comments. “This has not been validated in beef work.”
Amanda Blair presented during the 26th Annual Cattlemen’s College, held in late January 2019 in conjunction with the National Cattlemen’s Beef Association Cattle Industry Convention and Trade Show.
Saige Albert is managing editor of the Wyoming Livestock Roundup. Send comments on this article to firstname.lastname@example.org.