How does the cow's reproductive system work?

Answer 1

A cow's reproductive system is controlled by hormones that are issued from the ovaries and from the brain. It is a very complex system, much like that of the human female, and requires a lot of time and effort to explain, making it nearly impossible to generalize without generalizing it too much.

Proestrus is when the follicles attain final growth and secrete estrogen, which causes the cow to come into heat.

Estrus is the period where the cow is the most receptive to breeding with a bull.

Metestrus occurs 12 hours after the end of standing heat, and is when ovulation occurs. This is the time where the corpus luteum forms and produces progesterone.

Diestrus, the longest time of the Estrous cycle, is when the corpus luteum continues to produce progesterone in the hope that sperm will have fertilized the ovum during this time. If not, then the CL regresses at the end of diestrus (which is at the end of the 21 days after her initial estrus period), and the cycle begins again.

Once the ovum is fertilized (or the nucleus of the sperm head merges in with the nucleolus of the ovum), the ovum becomes a zygote and it starts dividing rapidly. The Zygote becomes a blastocyst when a little pocket starts to form inside the blastocyst beside the now-called embryo. The pocket bursts, sending the embryo out further into the cow's uterus. After a little more division, cells specific to becoming a placenta attach themselves to the wall of the uterus, and holds the embryo there for the rest of the gestation period. Typically, scientists have said that when the embryo attaches itself to the uterine wall, this is when gestation officially begins. The placenta is also responsible for sending hormones to the cow to signal the halt of any other ovules from producing in the corpus luteum of the ovaries, and ultimately preventing the cow going into estrus, and instead focus on her body reserves to help this embryo grow into a fetus, and ultimately into a baby calf. The placenta is responsible for the heightened progesterone levels in the cow, which indicates on-going pregnancy.

When the gestation period is up, or the calf is getting too big for the womb, the fetal calf starts sending stress signals through the placenta to the cow's brain, which signal the uterus to start labour contractions. Estrogen levels increase rapidly, sending more signals for the onset of labour. Muscle contractions through the uterus are partly involuntary, and partly voluntary, enabling the cow to judge how hard she should push and sometimes when. The calf assumes a "diver's" position when exiting the womb through the birth canal.

Once the calf is out on the ground, oxytocin levels increase to stimulate milk production, and even mothering-up to urge the calf to get up and nurse. Throughout the next 10 to 48 hours (or even up to 10 days), uterine contractions continue to expel the placenta, and the uterus also starts to shrink in size. Estrogen levels are not at their highest during this 45 to 80 day rest period from birth to the cow's first estrus after calving, and so the cow doesn't go back into her normal cycling until after this period of time.