Consideration for an optimal calving interval
Detection of estrus in dairy cattle
Reproduction in high-producing dairy cows
Interpreting indexes of reproductive efficiency
The maintenance of an optimal calving interval is essential for optimizing milk production from the cows within the herd. The following includes discussions on:
CONSIDERATIONS FOR AN OPTIMUM CALVING INTERVAL
A blanket recommendation for timing of postpartum inseminations does not exist. For years, dairy farmers believed that the ideal calving interval should be 12 months, and that cows should be bred at the first estrus after 40 days postpartum. This management decision would result in the shortest interval from calving to pregnancy and minimize calving interval. Now as production averages are steadily increasing they may consider a
13-month calving interval ideal. Cows tend to be open 2 days longer for every 450 kg increase in milk yield. Statistically speaking the calving interval for a cow producing 13,600 kg/year was 30 days longer than for a cow producing 6,800 kg. Decisions for timing of first insemination may be based on producer preference for length of calving interval, for season of the majority of calving to maximize returns for milk produced, or to ensure adequate milk production during months when meeting quota is essential.
Because low and medium producing cows do not maintain peak lactation as long as high producing cows, farmers may do better by freshening them as quickly as possible.

Early lactation is more profitable than later lactation because milk production is higher. High producing cows are profitable because they produce more milk for a longer time. Rapid freshening increases production of low producers because they reach high production levels more often. With high producers, frequent freshening may be less of a problem because they maintain high levels of production for a greater part of their lactation.
Two factors seem to play a role in cow's calving interval, but whatever the cause, management seems to be an overriding factor. First, high producers tend to have silent heats more often than low producers. Because the silent heats are easy to miss, cows do not get bred back as early as possible. Second, dairy farmers may give high producers a longer time before being bred back. They may decide to skip a heat and breed her back during the next one.
DETECTION OF ESTRUS IN DAIRY CATTLE
Producers should regard estrous detection as a specific management task. It is often advantageous to assign a particular employee or family member to be in charge of estrous detection, so that one person is responsible for heat checking and recording all heat dates. An understanding of the estrous cycle, knowledge of the signs of estrus, regular visual observation, and accurate record keeping are central to improving efficiency in this area.

The Importance of Heat Detection. In breeding programs which rely on artificial insemination, efficient and successful heat detection protects against extended periods of nonproductivity. Inadequate estrous detection leads to missed breeding opportunities or low conception rates for heifers and cows, which in turn leads to costly extended calving intervals.
Improved estrous detection practices allow for heifers and cows to be artificially bred to genetically superior sires and for cows to achieve optimal calving intervals.
It has been estimated that the dairy producer loses $2-4 per day in net income when calving intervals exceed 12.5 months.
To maintain the ideal 12.5-month calving interval, cows must be pregnant by 100 days after calving. As a guideline, 90 percent of cows should have been observed in heat by 40 to 50 days after calving. This then allows for approximately three breeding opportunities before the 100 days have elapsed after calving.
A side benefit of the close visual observation required in efficient heat detection is that it is a further means of monitoring the overall health and condition of the cows.

Signs of Estrus Estrus is the period of sexual receptivity that normally occurs every 18 to 24 days in nonpregnant heifers and cows. A figure illustrating the various stages of esrus is shown later. The signs of estrus can be divided into three main categories:
Primary, - Stands immobile when mounted
Secondary - behavioral - bellowing, increased activity, walking the fenceline, charging, butting, licking, sniffing, Flehmen posture (inverted nostrils), circling, chin-resting, and mounting, - and physical - manure on flanks, roughened tailhead, loss of hair on tailhead, abrasions on tailhead, red vulva, swollen vulva, clear mucus discharge from vulva, and
Miscellaneous - depressed appetite, depressed milk production, frequent urination, metestrous bleeding

Primary Signs of Estrus The most dependable sign of estrus is when a heifer or cow stands immobile when mounted by herdmates or a bull. The duration of standing heat varies greatly among individual cows and heifers, ranging between 6 and 24 hours, with an average duration of 16 hours. Also, the length of heat for any one cow can be very different from one estrous cycle to the next. Because approximately 5 percent of all pregnant cows and heifers will stand immobile when mounted by herdmates, caution must be practiced even when this sign is observed so as to avoid inseminating pregnant heifers and cows. Good records will indicate if the animal has been bred and/or diagnosed pregnant.

Secondary Signs of Estrus Secondary signs are not as definitive as standing heat but are valuable in determining when animals should be watched closely for estrus.
Behavioral When an animal is approaching or is in standing heat, she may be observed bellowing with greater frequency than would normally be encountered. Increased activity by animals near or in heat has been a consistent sign observed but is difficult to detect in casually observed animals. Heifers and cows in heat will often be seen walking the fenceline as if searching for other animals.
Various types of interactions occur between animals which are in or near heat. These include charging behaviour, butting and licking. These behaviours occur at all stages of the estrous cycle but appear to increase in frequency at estrus. Cows in heat will also exhibit increased sniffing activity. It is thought that this behaviour is in response to a pheromone produced by the reproductive tract at this time. After sniffing the vulva or urine of other cows, animals may infrequently display the Flehmen posture (inversion of the nostrils), similar to that exhibited by the bull.

Estrous animals will often interact together by circling each other and then chin-resting, wherein the chin is rested on the topline, usually near the tailhead, of a receiving animal. This may or may not lead to mounting activity (one animal attempting to mount other animals). The mounting animal is most likely approaching, in, or just past standing heat. The mounted animal will stand immobile if she is in heat or will attempt to escape the mount if she is not in heat.
When there are two or more animals close to estrus or in estrus, they form what is termed a "sexually active group"
(SAG). Members of a SAG can be observed interacting with each other at any time of the day or night. Producers should note which cows are active in this group, since they may be in heat.
Physical Physical signs which indicate that a particular heifer or cow has been mounted and is thus probably in heat include: mud and/or manure on flanks, roughened tailhead and/or loss of hair, and presence of abrasions about the tailhead. Other physical signs include a red and/or swollen vulva and a clear, mucus discharge from the vagina which looks like and has the consistency of egg white.

Miscellaneous Signs of Estrus In addition to the primary and secondary signs' of estrus, there are several miscellaneous signs that can be observed. These signs are the least dependable and occur infrequently. Among these are included depressed appetite, depressed milk production, and frequent urination. Careful interpretation of these nonspecific estrous signs must be made because they can also be present in diseased states. Another miscellaneous sign is that of metestrous bleeding. This is a bloody discharge from the reproductive tract seen on the vulva. This usually occurs 1 to 3 days after standing heat, regardless of whether or not the animal has been bred and conceived. When metestrous bleeding occurs, it is too late to breed the animal, but the producer should note its occurrence, particularly if standing heat was not observed, so as to expect estrus in 17 to 19 days.
The behavioral and physical characteristics of estrus described above apply to heifers and cows kept in free-stall or pasture environments. Animals in tie-stalls will exhibit less noticeable behaviour because of their confinement.

However, if animals in tie-stalls are turned out daily into a common holding lot, then the estrous behaviour observed will be as described above.
Factors Influencing the Detection of Estrus
There are many factors that can influence the dairy producer's ability to detect estrus in cows and heifers. Improvement of heat detection can only be achieved when producers recognize these factors and understand how they affect the success of estrous detection.
Identification Accurate and complete record keeping is essential to increasing the efficiency of heat detection. Proper identification of cows in estrus can help ensure the accuracy of records. Legible ear tags, freeze brands, or other clear and precise methods of cow identification are important in visual observation of heat detection. Also, proper outdoor lighting should be provided, because this will help in identifying cows in heat during the dark hours of the day.
Time of Visual Observation Since the mounting activity of cows does not occur at a constant frequency, cows should be observed for estrus two to three times daily for 30-minute periods. Primary observations for estrus should not occur at

feeding or when cows are moved to the milking parlour because chances for interactions are decreased at those times. The time of day that heat detection should occur varies with each farm, but detection usually should be done during the quiet times of the day. Around sunrise and sunset are good times to heat check. In winter, most heat checking should occur during the warmer parts of the day, while in summer, most heat checking should occur during the cooler parts of the day.
Detection Aids Several heat detection aids have proven effective in supplementing visual observations for estrus. A mount detector is a patch (Kmar patches) containing a red-ink-filled capsule which is glued on the tailhead. When the cow is mounted by a herdmate, the capsule within the patch is broken and the inner area of the patch turns red. Applying marking chalk or paint on the tailhead is also an effective heat detection aid. When a cow is mounted, some of the chalk or paint is rubbed off. Therefore, the producer can determine if the cow has been mounted and approximately how many times this has occurred. Testosterone-treated cows and surgically altered bulls (gomer bulls) or androgenized females wearing chinball markers can also help improve the efficiency of heat detection. Other detection means used have included pedometers

which record increased activity of individual cows, sniffer dogs which are trained to recognize pheromones produced by cows in heat, and vaginal probes which measure changes in electrical conductivity of vaginal mucus. It must be remembered, however, that heat detection aids should only be used to supplement, and not take the place of, visual observations.
Housing Conditions The conditions in which dairy cows and heifers are housed can greatly influence the detection of estrus. Providing shaded areas in the summer months and protection from winds in the winter months can improve the efficiency of heat detection on most farms. This protection is important because exposure to extreme hot or cold weather will decrease the estrous activity of most cows and heifers.
Providing good footing surfaces can also improve heat detection. The duration of estrus and the total number of standing mounts displayed are increased in cows on dirt floors as compared to when the same cows are housed in a concretesurfaced lot. In areas of animal traffic where concrete is used, it should be grooved to decrease the chance of cows slipping

and causing injury to themselves when they are active. In addition, footing surfaces that are wet or ice-covered should be eliminated. Once a cow has slipped and fallen, she will be less likely to interact with other animals, and this will therefore decrease the producer's ability to detect heat.
Another important factor affecting the success of heat detection is the number of animals in heat at the same time. An increase in the number of cows simultaneously in heat will increase the total number of interactions between cows and thus will improve the chance of detecting each individual cow in heat. By using prostaglandins, the dairy producer can synchronize estrous cycles and. have several cows in heat at the same time. However, if cows and heifers are housed in crowded conditions, the success of heat detection will be decreased because the animals will not have an opportunity to freely interact.
Miscellaneous Factors Although research has failed to identify any specific nutrients that can enhance estrous behaviour, all diets should meet the nutrient requirements of the cow so optimum production and health can be achieved.

Cows that develop ovarian cysts may exhibit frequent estrous behaviour that can be mistaken for normal estrus. Regular herd health checks performed by a veterinarian can detect potential disease states that may interfere with the expression of estrus. For example, pyometra will cause the CL to be retained on the ovary for extended periods of time, and therefore the cow will not come into estrus until this condition is treated.
Finally, many postpartum cows will have a silent estrus at the first and second ovulations after calving. Silent estrus is a condition wherein the ovary functions normally (ovulation occurs) but estrus is not exhibited.
When to Inseminate For many years, dairy producers followed the a.m.-p.m. rule when inseminating. This rule dictates that cows and heifers first observed in heat in the morning should be bred late in the afternoon. Likewise, cows and heifers first observed in heat in the afternoon should be bred the following morning. This is still a good principle, but many producers have now successfully gone to once-a-day insemination. This rule dictates that cows and heifers first observed in estrus in

the afternoon and the following morning should be bred late that morning. The once-a-day rule results in conception rates comparable to the a.m.-p.m. rule and also allows all breedings to occur at one time during the day.
REPRODUCTION IN HIGH-PRODUCING DAIRY COWS
Nature, as modified by genetic selection for increased milk production, has placed milk production as the greatest priority for high-producing dairy cows, therefore, reproduction is secondary to milk production. This ranking has created a strain in achieving acceptable reproductive efficiency while maintaining high levels of milk production.
The goal of dairy producers should be to have 100 percent of cows cycling by 40 days postpartum with the highest possible fertility. It is, therefore, critical that postpartum cows be fed and management properly. High-producing dairy cows are notorious for having a delayed return to estrus, an inadequate standing heat, and reduced rates of conception or failure to conceive. While low and average producing cows ovulate between 14 and 21 days after calving, high producers take about
20 days longer to ovulate.

After calving and during the period of greatest milk production and energy deficiency, the uterus, ovary, hypothalamus, and pituitary glands of cows undergo a process of recovery and rebuilding for establishment of subsequent pregnancy. This period is characterized by hormonal maturation of the pituitary and hypothalamus, morphological and histological changes in the uterus, and establishment of new follicular populations on the ovary leading to first ovulation.
The magnitude of energy deficiency (negative energy balance) seems to affect the processes of follicular growth and development leading to first ovulation. Research has shown a strong negative correlation between loss of body weight
(negative energy balance) and increasing days to ovulation. Cows having a high rate of body weight loss during early lactation also show increased days to first heat, increased days open, and reduced conception to first, second and third services.
Inadequate standing heats in high-producing dairy cows may be demonstrated by a shorter standing heat period and a lack of intensity of estrus, both of which place a strain on poor heat detection procedures. Postpartum anestrus reduces

reproductive efficiency because cows do not exhibit estrus and ovulate fertilizable eggs. Cows having the most standing heats before 60 days are most fertile, and those cows not showing heat within 30 days postpartum are least fertile, require more service per conception and are most likely to become problem breeders.
Failure of high-producing cows to conceive is a multifaceted phenomenon not yet completely understood. A combination of events associated with negative energy balance such as changes in circulating concentrations of hormones and metabolites, as well as interactions of higher brain centres with the hypothalamus and pituitary may cause anestrus and infertility. Successful fertilization appears to be related to follicular estrogen levels. Estradiol is the single most important steroidal secretion of preovulatory follicles is responsible for behavioral signs of estrus. In addition, estradiol initiates the LH surge which causes ovulation of the oocyte and luteinization of the cells of the follicles which forms the corpus luteum.
Difficulties encountered in estrus expression and reproductive management in early lactation may be explained by the

reduced ability of the preovulatory follicle to secrete estradiol. A reduction in energy balance or a general nutrient deficiency or imbalance may also have deleterious effects on the uterine environment, thereby affecting embryo survival.
Ovarian follicles grow and develop for several weeks prior to first ovulation, and growth of follicles on the ovary is directly related to energy balance. After parturition, the ovary begins to develop many follicles. Smaller follicles (3 to 10 mm in diameter) cannot ovulate but represent the pool of follicles on the ovary from which preovulatory follicles are selected. Cows in the most negative energy balance have the most small follicles and the fewest large follicles (> 10 mm in diameter) and, therefore, have the fewest follicles that potentially can ovulate. Postpartum cows have their first ovulation about 10 days after they have reached their most negative energy balance. The secretion of Luteinizing Hormone (LH) is reduced during energy deficiency, and because LH is essential for follicle growth and maturation, fewer follicles are large enough to ovulate.
Ovarian follicles start growth and differentiation 40 to 40 days before they reach mature size and ovulate. An oocyte or follicle adversely affected by a negative energy balance, may show a delay of up to 60 days before the detrimental effects

on ovulation, fertilization, embryonic development, or establishment of pregnancy are dissipated. Most early postpartum cows ovulate a follicle which has developed during negative energy balance, therefore carry-over effects may partially explain infertility in early lactation. The corpus luteum is derived from the ovarian follicle as influenced by LH, is responsible for producing progesterone which establishes and maintains pregnancy. A corpus luteum adversely affected by negative energy balance appears to produce less progesterone than a normal corpus luteum, thereby negatively fertility.
Cows consuming the most feed in early lactation are more likely to ovulate before 22 days in lactation and more likely to produce more milk. Therefore, feeding the highest quality rations to maximize feed intake will, not only, maximize milk production, but also stimulate ovarian function and ovulation. The inclusion of fat (primarily bypass fats) may improve reproductive performance of early lactation cows depending on the effects of the dietary fat on energy balance. Cows showing improvement in milk yield but not in energy balance may not show a positive response in reproductive performance.

Fat feeding appears to have a positive effect on the corpus luteum, but not until the second or third postpartum estrous cycle.