Final answer:
The hormone responsible for inducing labor in late-term cows is oxytocin, which stimulates uterine contractions and is secreted by the posterior pituitary gland. Insulin, involved with blood sugar regulation, is not associated with parturition.
Step-by-step explanation:
The hormone primarily responsible for inducing parturition, or labor, in late-term cows is not insulin, but rather oxytocin. During the final stages of pregnancy, the posterior pituitary gland secretes oxytocin, which plays a crucial role in stimulating the uterine contractions necessary for childbirth. The uterine smooth muscles become increasingly sensitive to oxytocin towards the end of pregnancy due to the increased number of oxytocin receptors.
Oxytocin's release is triggered by the stretching of tissues in the uterus and cervix during childbirth, and this, in turn, intensifies the contractions through a positive feedback loop until the calf is born. Oxytocin also promotes the milk ejection or "let-down" reflex postpartum by stimulating the contraction of myoepithelial cells surrounding the mammary glands, which aids in breastfeeding.
In summary, while insulin is associated with the regulation of blood sugar levels, it is oxytocin that is critical for labor and delivery in cows as well as in other mammals, including humans.