A cow’s gestation cycle can be compared to the seasons of any other environment. There are different processes taking place during each “season” that critically impact the fetus’ development in the time it is in-uterine. Each season presents a different concern for the development of the fetus as well as its future genetic potential being available post-birth.
Early gestation
Traditionally, early gestation tends to not be a main concern in production. The fetus is very small and has minimal nutrient requirements, so attention tends to go towards the cow’s calf on the ground already. However, this early gestation stage has key developmental stages for the fetus.
Once there is a maternal recognition of pregnancy following fertilization, a cow’s hormone levels adjust to produce progesterone, which is the hormone that maintains pregnancy. The uterus also begins to prepare an environment for a fetus and implants the placenta to the uterine wall and develops blood vessels for feeding.
A fetal heartbeat can develop as soon as 21 days, followed by additional organs. Survival organs—the heart and the brain—require an increased level of nutrients during this developmental phase. Other tissues and organs have a lower priority, so if there is a nutrient deficiency present, these other tissues and organs will suffer.
Around 45 to 60 days, sex organs begin development in the fetus. Follicles will then begin to develop on the ovaries in female fetuses. Beef cattle are born with all of the eggs they will have in their lifetime, so follicular development is important for reproductive productivity in the future. Typically, producers focus on reproductivity post-weaning, but it is a concern much sooner than that.
There is also some early muscle fiber and fat cell development in these beginning stages. Similar to follicles, cattle are also born with all of the muscle fibers they will ever have in their lifetime, so early gestation is crucial to set up the future muscling of a calf.
The placenta is the key to making sure requirements are met throughout gestation, Kincheloe said. The majority of placental growth takes place in the first half to two-thirds of gestation but will continue to grow throughout the pregnancy. Late gestation sees blood flow increase by four and a half times its previous level in uterine and placental blood flow, because that is when the majority of calf growth is occurring. For this increased blood flow to happen, circulation must be established early, or nutrient availability and oxygen carrying capacity will decrease and growth will suffer.
Mid-gestation
A fed steer harvested at 15 months of age spends 40 percent of its life in gestation. Mid-gestation is believed to be a key timeframe when it comes to outcomes in feeding cattle, said Dr. Amanda Blair, a professor and extension meat science specialist at South Dakota State University who discussed fetal programming at the 2019 NCBA Cattlemen’s College.
Secondary muscle fibers make up the majority of muscle mass in the calf after it’s born, and these peak during mid-gestation but development ends before birth. Fat cell development begins in mid-gestation but can continue to grow after birth. However, the number and distribution of fat cells in the body can be limited if stressors such as poor nutrition occur during mid-gestation.
Although muscle fiber and fat cell developments are fundamental for future carcass quality, the vital organs are going to be prioritized over muscle and fat development if necessary.
In a University of Wyoming study on fetal programming, all cows in the study were bred and started on native pasture with a crude protein percentage of about 5-6 percent. At mid-gestation, half of the cows stayed on the native range and the other half were moved to an improved pasture providing around 11 percent crude protein. In late-gestation, all cows were moved back to the native range but were provided an alfalfa supplement.
The research determined no differences in birth weight or 205-day weaning weight. However, there was shown to be a decrease in average daily gain, final weight and hot carcass weight in the protein-restricted calves. But these weight differences didn’t translate to a difference in rib eye area yield grade or marbling. There was a decrease in backfat thickness and tenderness, however.
Late gestation
Dr. Janna Kincheloe, a livestock systems extension specialist at North Dakota State University who also spoke about fetal programming at the 2019 NCBA Cattlemen’s College, noted that cows tend to receive the most attention during late gestation. Roughly 75 percent of calf growth occurs during this period, with the fetus weighing around 20 lbs. at the beginning of late gestation and then growing to its final birth weight.
In another study conducted by the University of Wyoming, cows were separated into two groups: one group received 100 percent of their energy requirements, and the other received 65 to 70 percent of energy requirements.
Results showed a decrease in calf birthweight by around 4 lbs. for the energy-restricted cows. There was also a 7 percent increase in calving death loss. In addition, weaning weight was reduced by 26 lbs.
The University of Nebraska, Lincoln also conducted a study during late gestation supplying one group of cows a 42 percent crude protein supplement vs. a control group. Calves from each group did not show a difference in birthweight, but supplemented dams displayed an increase in calf weaning percentage. Calves also had a higher weaning weight, a higher percentage for heifers calving in the first 21 days, and overall higher pregnancy rates.
When it comes down to it, different stages of the gestation cycle impact a fetus’ development in different ways due to distinctive processes taking place in each stage. How the gestational environment is managed in each stage can critically impact the calf’s development and help it reach its full genetic potential. — Anna Miller, WLJ correspondent
