If a cow is to calve consistently, she must deliver her first calf early. Puberty is a function of breed, age, and weight. Beef heifers that are bred at 13–15 mo and calve at 22–24 mo have two advantages: they get closer attention from herdspeople by calving before the main herd starts to calve, and subsequently they have the extra time needed to rebreed with the mature cow herd. For heifers to breed at 14 mo, they should have attained at least 65%–75% of their projected mature weight; therefore, adequate nutrition is of major importance. The breeding season for virgin beef heifers should start 3 wk before that of the main cow herd. The above considerations do not apply to dairy cattle, which calve throughout the year; however, scheduling heifer calvings at the start of a seasonal calving dairy herd such as in New Zealand or Ireland represents an opportunity to "reset" the calving pattern. Lifetime profit of dairy replacement heifers is maximized when heifers calve at 23–25 mo of age. Thus, to maintain genetic progress and maximize profitability, heifer breeding strategies on dairy operations should include artificial insemination (AI) that results in attainment of pregnancy to allow calving at ~24 mo of age.
To compensate for the greater attrition rate usually seen with virgin heifers, a greater number should be bred than is needed to maintain or increase herd numbers, eg, 150%.
Irregularities of Estrus and Anestrus
Breeding will not occur if the cow is anestrous, or if estrus is undetected. Genuine anestrus is not common in dairy cows but is more common in postpartum beef cows in below target body condition. Anestrus, subestrus, or irregular estrous cycles in cows may result from a number of factors, including poor management or nutrition, disease, injury, or disturbances in endocrine functions. One of the most important management factors in artificially bred herds is failure to detect or observe estrus. The average duration of estrus is 18 hr, but in many cows it is appreciably shorter. A systematic program for detection of estrus is important if cows are to be bred at the right time. A producer must be familiar with signs of estrus. Aids in estrus detection that are valuable adjuncts to the heat-detection program include chalk marks on the tailhead, chemically or electronically activated devices attached to the tailhead of the cow that reveal when other cows have mounted, and a vaginal probe that measures the electrical conductivity of the vaginal mucus. Accidental access of bulls to cows and failure to keep proper breeding records often result in apparent anestrus because of pregnancy without a service history.
Although genuine anestrus is generally not common in dairy cows, "subestrus" with poor estrous expression or irregular estrous cycles in cows may result from a number of factors, including poor management or nutrition, disease, injury, or disturbances in endocrine functions. In many dairy herds, detection of estrus for AI is inefficient, because not all cows are identified in estrus because of human error, attenuated expression of estrus in high-producing cows, and adverse responses to heat stress. Systematic breeding programs for AI at a predetermined time (ie, timed AI [TAI]), without the need for estrus detection, coupled with early rebreeding of nonpregnant cows are successful options for reproductive management of lactating dairy cows. These systems optimize pregnancy rate by synchronizing follicle development, regression of the corpus luteum (CL), and precise induction of ovulation to provide a fixed TAI. Incorporation of TAI in dairy herd reproductive management programs reduces labor requirements for detection of estrus while improving overall reproductive performance and maximizing profit.
Silent heat refers to normal follicular development and ovulation without evident signs of estrus. Its frequency decreases as lactation progresses, so that incidence is low by 4 mo postpartum. Cows with true silent heats may be detected only through rectal palpation or ultrasound of the ovaries, via accelerometers/activity meters, or the use of progesterone assay in milk or plasma.
Cyclic changes occur in the ovary over 18–24 days. These changes generally can be recognized and the time of the cycle estimated, particularly in the 3–4 days before ovulation, at the time of ovulation, or 3–4 days after ovulation. The CL regresses 3–4 days before the onset of estrus; it becomes smaller in size and changes from a diestrous, liver-like consistency to one more fibrous. Estrus is evidenced by the presence of a palpable follicle, an absent or regressed CL, and firm uterine tone. The vaginal mucosa is edematous, the cervix is relaxed and hyperemic, and a variable amount of clear serous mucus is frequently seen at the vulva, which is puffy and swollen. The immediate postovulatory period is characterized by blood in the mucous discharge and an ovary with a corpus hemorrhagicum, which on palpation is recognized as a soft area (5–15 mm in diameter) in the ovary. The CL is detectable by day 4–5 as a small and somewhat softer structure than the mature CL, which reaches maximal size by day 7.
The examiner can sometimes predict the next estrus from previous observations around estrus, and the cow can be watched closely at the next anticipated estrus. The primary behavioral sign of estrus—standing to be mounted—is the accurate predictor of ovulation. In cows that are approaching ovulation, the appropriate time can be estimated, and the cow bred on secondary signs such as mounting other cows; however, breeding on secondary signs may be associated with wide variation in time to ovulation. Aged gametes (either semen or oocytes) are associated with reduced embryo viability.
Regimens have therefore been developed for administration of prostaglandins (PGs) and their analogues alongside other products to synchronize estrus and to reduce dependence on estrus detection. Prostaglandins are effective only if a cow has a functional CL. A range of synchronization options are in common use, including double prostaglandin regimens, GnRH and prostaglandin protocols, and regimens that use combinations of GnRH and prostaglandin alongside intravaginal progesterone-releasing devices. For estrus synchronization, the prostaglandin or its analogue is administered to all cows. In those in days 6–18 of the cycle, the CL will regress and estrus will occur in 2–7 days. The others may either have been in estrus recently or will be in a few days. Eleven days later, all cows will be between days 6 and 18 of their cycle, and prostaglandin is administered a second time. Most cows will be in estrus in 3–4 days and will ovulate in 4–5 days. Breeding is done either on signs of estrus or fixed time. Cystic ovary disease ( see Cystic Ovary Disease Cystic Ovary Disease ) may be responsible for irregularities of the estrous cycle, eg, follicular cysts (anestrus, nymphomania, and shortened cycles) and luteal cysts (anestrus).
Under certain circumstances, ovaries are nonfunctional. They can be recognized as smooth, small, bean-shaped structures on a single examination, or reveal no activity or change after several examinations over a period of 3 wk. The most common causes are low total energy intake during late winter or droughty summer pastures, and excessive loss of body weight in lactating dairy cows postpartum.
The stress of chronic or severe disease, injury, or ovarian tumors may interrupt ovarian activity and result in anestrus. Congenital defects, such as freemartinism and ovarian hypoplasia, result in estrual failure. Inactive ovaries are treated by correcting the basic cause; they usually do not respond to gonadotropin or steroid hormone treatment.