CONCEPT OF HEAT CYCLE IN DAIRY CATTLE & IMPORTANCE OF AI
With only a one letter difference, the words “estrous” and “estrus” look and sound similar, but there’s an important difference. Estrus is a noun and refers to the short period in which a cow is sexually receptive and will stand to be bred. Estrus is commonly called “heat.” Estrous is an adjective used to describe things related to estrus, such as the behaviors associated with estrus (estrous behaviors) or the period from one estrus to the next (estrous cycle).
Normal Estrous Cycle in Cattle
A good understanding of the normal estrous cycle in cattle can help producers address reproductive challenges in both heifers (young female dairy or beef animals that have not yet had their first calf) and cows (female dairy or beef animals that have had at least one calf). This understanding is also critical when using a drug regimen to control and synchronize estrous cycles in cattle.
A heifer has her first estrus, or heat, at puberty. The age at puberty is influenced by genetics, nutrition, and body weight. Heifers fed an appropriate diet will generally reach puberty between 9 and 15 months of age. Dairy heifers tend to reach puberty earlier, at 9 to 12 months, whereas beef heifers tend to reach it a bit later, at 13 to 15 months. The age at puberty for some cattle breeds, such as Brahman, can be as late as 24 months.
After puberty, a heifer continues to have regular estrous cycles every 21 days (the normal range is every 18 to 24 days). The estrous cycle in cattle is complex and regulated by several hormones and organs (see Figure 1).
Figure 1 —Phases of the bovine estrous cycle. Note: E2 = estrogen and P4 = progesterone. (Figure used with permission from Current Conceptions, Inc. from Pathways to Pregnancy and Parturition, 3rd ed.1 Image cannot be reprinted in any other publication in hard copy or electronic form without written permission from Current Conceptions, Inc.)
The Follicular Phase: Waves of Ovarian Follicles
The follicular phase of the estrous cycle refers to the narrow period of time right before estrus (heat) and ovulation (release of the egg for possible fertilization). During this phase, there is rapid growth of a dominant ovarian follicle and increased estrogen production.
A follicle is a structure containing an egg, also called an ovum, and other cells that can produce estrogen. A heifer or cow will generally have two or three groups of ovarian follicles—called waves—develop during a single estrous cycle (the range is one to four waves). One follicle in each wave will become the dominant follicle. Early in the estrous cycle, when the progesterone level is high, the dominant follicle will not ovulate. Instead, it will regress and allow another wave of follicles to emerge. The last wave occurs later in the estrous cycle when the progesterone level is low. The follicle that emerges as dominant during this wave will not regress. Instead, it will grow larger and produce increasingly more estrogen.
Estrus (Heat) and Ovulation
The high estrogen level produced by the dominant ovarian follicle causes the heifer or cow to show signs of estrus. This means she’s sexually receptive—she’s said to be “in heat”—and will stand to be bred or mounted by other cows, commonly referred to as “standing heat.” The heifer or cow may show other signs of estrus, such as having a clear mucous vaginal discharge and an increased activity level. She may also vocalize more and try to mount other cows. Estrus is considered the beginning, or “Day 0,” of the estrous cycle.
Besides causing the heifer or cow to show signs of estrus, the high level of estrogen produced by the dominant ovarian follicle also triggers the hypothalamus—a section of the brain—to release a surge of gonadotropin releasing hormone (GnRH) right before ovulation. GnRH causes the pituitary gland in the brain to release two other hormones—follicle stimulating hormone (FSH) and luteinizing hormone (LH)—which travel in the blood to the ovary to control what happens to the follicles.
FSH is secreted during and shortly after estrus and causes a new wave of ovarian follicles to develop. A surge of LH causes the dominant ovarian follicle to rupture approximately 24 to 32 hours later, thus releasing the egg. This is ovulation and marks the transition from the follicular phase to the luteal phase.
The Luteal Phase: The Developing Corpus Luteum
During the luteal phase of the estrous cycle, the remnants of the newly ruptured ovarian follicle develop into the corpus luteum and begin to produce progesterone, a steroid hormone needed to support and maintain a potential pregnancy if the egg is fertilized. Over the first 10 days of the estrous cycle, the corpus luteum matures and increases in size. The corpus luteum reaches its maximum size and produces the most progesterone at mid-cycle (around Days 9 and 10). Under the influence of high progesterone, waves of follicles continue to emerge and regress without a dominant ovarian follicle rupturing. A dominant follicle won’t rupture again until the progesterone level falls during the next follicular phase.
No Pregnancy
If the egg isn’t fertilized or the early embryo fails to develop, the uterus releases the hormone prostaglandin F2-alpha around Days 16 to 20 of the estrous cycle. Prostaglandin F2-alpha causes the corpus luteum to regress. This is called luteolysis and leads to a drop in progesterone.
The low progesterone environment allows the dominant ovarian follicle to emerge from the last wave of follicles and mature instead of regress. The dominant follicle produces enough estrogen to cause estrus, thus starting the next estrous cycle.
Pregnancy
If the heifer or cow becomes pregnant, the embryo prevents the uterus from releasing prostaglandin F2-alpha and the corpus luteum continues to release progesterone. The high progesterone level stops the heifer or cow from cycling—she won’t go into heat or ovulate. In a normal, healthy pregnancy, the embryo develops into a fetus. Gestation (the period between when the animal becomes pregnant and when she calves) lasts about 283 days. It’s desirable for a heifer to have her first calf when she’s 2-years-old. For that to happen, the heifer must reach puberty and become pregnant by 14 to 15 months of age.
Cyclicity
When a heifer or cow has an estrous cycle that’s normal in length and she displays normal estrous behaviors during heat, she’s said to be “cycling.” When a heifer or cow isn’t pregnant but she’s not ovulating or showing signs of heat, she’s not cycling. Instead, she’s “anestrus,” which is sometimes described as being “acyclic.” Right after a heifer or cow calves, it’s normal for her to be anestrus for a short period of time. Sometimes a medical problem, such as ovarian cysts or an infection of the reproductive tract, can cause a cow to stop cycling. Another reproductive challenge is silent heat—when a heifer or cow appears to be anestrus but, in fact, she is cycling normally and just not showing signs of heat.
Oestrus cycle
Proestrum 2 days) Period of building up growth of graffian Follicle which helps for the
nourishment of ovum fluid contains hormone called „oestrogen‟. It causes changes in uterus, blood supply.
Oestrum: ( 1 day) During which the female is ready to receive male.
Metoestrum 4 days) Implantation of the embryo takes place C.L. takes place. Prevents the growth of graffian follicle thereby arrests oestrus cycle.
Diestrum : ( 14 days)Further development of uterus takes place. If the animal has not
conceived involution of uterus take place.
Symptoms of Heat:
1.Off feed 2. Drop in milk yield 3. restless and excited 4. Bellowing 5. Oedema / swelling of
genitalia 6. frequent utination 7. Transparent mucous discharge 8. cow/buffalo which are in heat will mount on other animals and allows mounting of other animals.
OPTIMUM TIME FOR CROSSING
Egg/ova from ovary – released about 12-18 hours after the onset of symptoms of heat. Ovum will survive upto 16 hours after the release. Sperm live for 12-14 hours.
Morning signs of heat are exhibited – AI done in the evening : 12 hours delay.
Proestrum : This marks the animal – coming in heat. GF – Ovary – growing – increased -secretion of follicular fluid – Estradiol – increase No.of Cilia – increased Vascularity of uterus
– increase in thickness ofEpithlial wall of vagina .
The vaginal wall adjustemnt is well filled to prevent possible damage to the wall when coitus occurs.
Oestrum : This is the period of desire. „Graffian Follicle‟ – Ripe or very turgid
This period is brought to an end by the rupture of the follicle (or) ovulation. Vulva becomes
swollen. Vulva and Vagina – congested.
Met Oestrum : This is the period when the organ returns to normal non congested condition.During this period the cavity of the GF from which ovum has been expelled becomes recognized and forms a new structure known as C.L.
1. It prevents the maturation of further graffian follicle
2. It is essential for the implantation of the fertilized egg.
3. It is intimately concerned which the development of mammary gland.
Diestrum :Longest part of estrus cycle
Implantation – uterine milk –for the nourishment of the embryo prior to implantation. Absenceof pregnancy. – returns to normal and thus the cycle continues.
ARTIFICIAL INSEMINATION.
Artificial insemination is the deposition of male reproductive cells
(sperm) in the female tract by mechanical means rather than
“Natural Service”
ADVANTAGES :
1.Increases usefulness of superior sires to extra ordinary degree.
2.Services of Superior Sires are greatly extended.
If the sires are used for Natural Service the animal can serve only 50-60
animals/year but under Artificial Insemination the amount of
semen secreted by the animal can be used to satisfy the
requirements of 1000 animals per year
DILUTION OF SEMEN
Average of Sperm/mL : 1000 million
Total volume of semen/2 ejaculate :6 mL
No. of motile Sperms : 90 %
Total number of motile sperms : 5400 million.
Expected wastage during processing : 10 %
(i.e.) filling and sealing
Net no.of sperms available : 5400-540= 4860
Minimum No. of sperms required / dose : 30 Million.
@ this rate no. of doses that could be prepared : 4860 /30 = 160 doses
So, total no. of doses that can be : 160 x 52 weeks = 8320 doses.
prepared / week
Frozen semen required / dose : 1 mL
3.No need to maintain Breeding Bull.The frozen semen can be stored in the Liquid Nitrogen1960C.
4.Semen can be quickly and easily transported by air to different continents.
5.Spreading of diseases is absolutely- NIL.
6.Overcomes the difficulty of size and weight between Dam and Sire.
7.Increase the rate of Conception because in the artificial insemination the semen is being
deposited in the mid cervix .
8.Outstanding animals located apart can be mated.
9.Helps in better record keeping.
10.Old and heavy sizes bulls ,injured / disabled sires can be used.
MERITS:
1.Semen can be stored in the frozen state , so progeny can be obtained even after the transfer, WHY even after death of bull-15-20 years.( atomic, radioactive, X-ray unit)
2.Semen is expanded and no. of animal can be crossed.
3.Frozen semen can be transported to destination once in a month from the semen bank.
DISADVANTAGES.
1.Some bulls semen may not freeze well.
2.If inferior bull semen is frozen and used –Extensive damage is caused.
3.Maintenance of frozen semen bank is not economical for a small area of operation.
4.Requires well trained technical personnel‟s and special equipments and hygienic measures are to adapted in preparation.
5.Improper cleaning of instruments and unsanitary condition may lead to lower fertility and may be nucleus for the spreading of diseases.
