The Making of a Calf
by Troy Smith for Angus Productions Inc.
NASHVILLE, Tenn. (Aug. 5, 2010) — Just about every cow-calf producer has had cause to wonder why some of his or her cows failed to become pregnant. According to University of Tennessee reproductive physiologist Lannett Edwards, there are plenty of possible reasons. Speaking during the Applied Reproductive Strategies in Beef Cattle (ARSBC) Conference in Nashville, Tenn., Edwards said management may contribute the fact that, in many herds, only 76%-80% of cows wean calves each year. Consequently, she added, a basic understanding of the process of making a calf is important to achieving a calf crop of 90% or better.
“Reproduction is a complex process dependent upon a multitude of factors,” Edwards stated, adding that producers need not feel overwhelmed by the complexity. “It’s important, though, to be aware of the many factors.”
During her presentation, Lannett Edwards highlighted the challenges related to the "pilgrimage" both egg and sperm make to the site of fertilization and the production of an embryo competent to develop into a healthy calf.
During her presentation, Edwards highlighted the challenges related to the “pilgrimage” both egg and sperm make to the site of fertilization and the production of an embryo competent to develop into a healthy calf. For example, a heifer has about a million oocytes at birth. But during a heifer's lifetime, a relative few oocytes will undergo the dynamic process of follicle growth and development to a size capable of ovulating. Why certain oocytes are destined to develop while others do not is unknown.
Edwards said the complexity of changes occurring during the maturation of a chosen one make the oocyte very susceptible to environmental stressors, especially elevated body temperature due to heat stress or illness. Consequently, severe stress can reduce female fertility due to direct effects on the oocyte.
Compared to the oocyte, Edwards explained, sperm cells make an epic journey from testes to epididymis, where they are held until released through ejaculation. During ejaculation, sperm are mixed with seminal fluid from accessory glands. But the greatest challenges are faced after deposition in a cow. Of the billions ejaculated, as few as 100 sperm actually reach the site of fertilization. Once a single sperm cell fertilizes the oocyte, a reaction occurs immediately to prevent other sperm from entering.
The fertilized oocyte, or zygote, then begins a series of cell divisions to become an embryo. During this time — up to 14 days after fertilization — the embryo is particularly fragile. This is believed to be the reason, Edwards said, why some females conceive but do not have successful pregnancies. Research suggests that while conception among a group of heifers may be 90%-95% successful, calving rate to a single breeding may be only 70%-75%. The majority of loss, said Edwards, occurs before Day 14. This may be due to genetic abnormalities or poor semen quality and other factors, but also because of environmental stress.
“At 15 to 17 days after fertilization, the embryo secretes hormones, which help prevent its destruction,” Edwards explained. “But during the first two weeks after fertilization, anything managers can do to minimize stress will help improve the outcome of reproduction.”
Editor's Note: This article was written under contract or by staff of Angus Productions Inc. (API), which claims copyright to this article. It may not be published or redistributed without the express permission of API, publisher of the Angus Journal, Angus Beef Bulletin, Angus e-List and Angus Beef Bulletin EXTRA. To request reprint permission and guidelines, contact Shauna Rose Hermel, editor, at 816-383-5270.