It’s The Calf’s Call

Labor, it is nothing new to beef cattle producers. Be it spring or fall, calving season brings with it a bucket load of challenges and needs from the cowherd. While the bulls are enjoying their vacation, the females are hard at work.

Understanding the stages of labor and their duration is critical for providing proper assistance. In addition to the stages of labor, it is important to understand what is taking place physiologically within the female during labor.

Labor is initiated by the fetus. Stress signals trigger the release of fetal cortisol (stress hormone), which initiates parturition (labor) through a cascade of physiological and hormone-based events (of all the hormones associated with labor, cortisol is the only one coming from the fetus). In normal cases, as the fetus reaches full term and full size, it is possible that the limitation of space in the uterus is a stressor, thus the fetus will release fetal cortisol to initiate the labor process. This cortisol is then absorbed by the maternal unit causing initiation of stage one of labor.

The rise in fetal cortisol results in the conversion of progesterone to estrogens through placental enzymes. In addition, cortisol will cause an increase in uterine production of prostaglandin, this will cause relaxin secretion and regression of the corpus luteum (CL), aiding in the removal of the progesterone block, allowing contractions to start (See Table 1 for hormone definitions).

Table 1: The Hormones of Labor.

Estrogens cause an increase in uterine contractions. The increase in estrogen comes from the conversion of progesterone to estrogen. Fetal cortisol causes the increased synthesis of the enzyme, which causes the conversion of progesterone to estrogen.

Oxytocin's role in labor is to facilitate contractions that are initiated by estrogen and prostaglandin. As the fetus moves through the birth canal, pressure on the cervix stimulates sensory neurons and causes oxytocin to be secreted. This oxytocin further stimulates contractions. As contractions continue, more pressure is put on the cervix, causing increased secretions until peak pressure is reached. This is an example of classic positive feedback. As more oxytocin is secreted from increased pressure on the cervix, which stimulates contracts, and more pressure is put on the cervix.

Fetal cortisol also causes the secretion of prostaglandin from the uterus. Thus prostaglandin causes relaxin secretion. Relaxin’s primary function during parturition is to relax the pelvic ligaments allowing the fetus to pass through the birth canal. Following birth, contractions will continue to expel the fetal membranes (placenta).