Reproductive Management of Dairy AnimalsChallenges limiting reproductive efficiency of dairy animals include interrelationships among body condition

Reproductive Management of Dairy AnimalsChallenges limiting reproductive efficiency of dairy animals include interrelationships among body condition, dry matter intake, transition from the dry period to lactation, onset of normal estrous cycles, detection of estrus and embryonic survival. Reproductive inefficiency of dairy cattle causes great frustration for dairy producers (Call and Stevenson, 1985). Even under optimal conditions, the reproductive process is less than perfect because of the multiple factors involved in producing a live calf. To manage the complexities of the estrous cycle and the annual reproductive cycle, understanding of many interrelated physiological functions is critical. Furthermore, reproductive efficiency involves successful management of not only the cows but also the people who milk, feed, house, inseminate, and care for them (Forde et al., 2011)
Efficient reproduction in a dairy herd is the primary determinant of profitability (Call, 1978). To improve reproductive efficiency, the limiting factors must be identified. In general, detecting estrus is the major limitation to achieving a pregnancy (Barr, 1975). But once insemination occurs, two sources of pregnancy failure exist, which include, but are not limited to, fertilization failure and embryonic death. Fertilization failure follows procedures or practices that fail to facilitate union of a viable egg with a viable sperm. Once fertilization occurs, embryonic death occurs may be due to failure of normal embryonic development, recognition of pregnancy or normal maintenance of pregnancy (Crowe and Williams, 2012).

To maximize the chances of a renewed pregnancy for every heifer or cow that calves into the herd, a number of important time dependent components of the estrous cycle must be managed. It is critical to understand each component of the estrous cycle as well as the annual reproductive cycle (calving interval) and determine where limited time and resources might be concentrated best to reach AI-breeding goals (Sheehy et al., 2016). Maximal reproductive efficiency requires management of the calving interval. This consists of three major components: the elective waiting period, the active AI-breeding period, and gestation including the dry period (Diskin et al., 2012).

Elective waiting period
The first component of a calving interval is the traditional rest period or the elective waiting period (EWP). The duration of this period is partly a management decision. This period varies from 40 to 70 days. Part of its duration is based on the physiological need for the reproductive tract of the cow to undergo a healing process or involution. Some, but not all, studies indicate that longer elective “rest” periods improve conception rates (Britt, 1975), possibly because of improvements of various uterine traits. Research indicates that when cows calve without complication, this healing process requires no more than 40 days (Kiracofe, 1980; Marion et al., 1968). Involution includes macro and microscopic changes that prepare the reproductive tract, especially the uterus, for a renewed pregnancy (LeBlanc, 2010).

Periparturient Period
Parturition in the cow is a process that requires attention, care, and cleanliness. The risk for death is greater at calving than at any other period of the life cycle (Mather and Melancon, 1981). A multitude of calving related disorders predispose cows to ill health, loss in milk production and reduced reproductive efficiency (Stevenson and Call, 1989). Whatever can be done to reduce one or more of these disorders will result in the reduced incidences of other disorders because of their strong interrelationships. A number of important physiological adaptations must occur during the EWP (Crowe et al., 2014). The cow must adapt to the increased demand for nutrients by the mammary gland because of the onset of lactogenesis (Bauman and Currie, 1980). During late gestation, the feto-placental unit is a major nutrient consumer and orchestrates a homeorhetic priority of nutrient utilization (Bell, 1995). Once parturition occurs, the mammary gland becomes the major nutrient user (Short and Adams, 1989). As a result, an energy prioritization is manifested that places higher priorities on the use of nutrients for maintenance and growth of the cow and for milk secretion than for the onset of estrous cycles and the initiation of a new pregnancy (Pontes et al., 2015).
Onset of Estrous Cycles
The factors limiting the onset of estrous cycles in lactating cattle, a number of events that must occur before cows begin estrous cycles after calving (Pascottini et al., 2015). Ovarian follicular waves are reinitiated within the first week after parturition, and the first dominant follicle usually ovulates (Beam and Butler, 1998; Savio et al., 1990). Because follicular waves begin soon after calving, concentrations of blood FSH are sufficient, but a major limiting factor to ovulation is the reinitiation of adequate LH secretion in the form of circhoral LH pulses to support final follicular maturation and subsequent ovulation of a dominant follicle (Canfield and Butler, 1990, 1991; Roche et al., 2000).

The stimulation of appetite to ensure adequate DMI in normal, healthy cows is essential to provide nutrients for maximizing milk secretion, follicular growth, ovulation, uterine involution, and the initiation of pregnancy (Pursley et al., 2015). However, lactating dairy cows experience a postpartum negative energy balance that reaches its nadir during the first or second week after calving and recovers at a variable rate. The first ovulation occurs approximately 10 to 15 days after the nadir of energy balance and sometime before the peak in daily milk secretion (Butler and Smith, 1989; Zurek et al., 1995). During high milk production in early lactation when metabolic demands are enormous, major amounts of nutrients are required for mammary synthesis of lactose, protein, and triglycerides that cannot be met by dietary intake (Bell, 1995). Nevertheless, dairy cows with greater DMI, despite having a negative energy balance, produced more milk, lost less BW, and ovulated earlier post-partum than those with lower intakes (Staples et al., 1990; Zurek et al., 1995). Further, cows with greater intakes also reached the nadir of energy balance earlier and experienced a more severe, but shorter, period of negative energy balance, suggesting that when cows are more efficient in partitioning dietary and stored nutrients toward milk synthesis, they are also better able to recover ovarian cyclicity. Therefore, reestablishment of estrous cycles is correlated highly with the re-initiation of circhoral LH pulses, and the onset of these LH secretory patterns is related to the timing of the postpartum nadir of energy balance (Canfield and Butler, 1990).

Lactating dairy cows remain in negative energy balance until 6 to 10 wk after calving (Stevenson et al., 1997). Depending on when this negative energy balance is resolved, which can be manipulated cleverly by altering the postpartum duration and frequency of multiple daily milkings (Bar-Peled et al., 1995), the EWP and early active AI-breeding components of a calving interval are concurrent with insufficient nutrient intakes. Normally, peak DMI are achieved just after or coincident with zero energy balance (approximately 7 wk; Ingvartsen and Andersen, 2000) and after peak daily milk secretion. In the face of these nutrient deficits and negative energy balances, conception rates are <50% in lactating dairy cows inseminated during the first 100 DIM. In contrast to the majority of reports of first postpartum ovulations in milked cows occurring by the end of the first month after calving (Stevenson and Call, 1989; Stevenson et al., 1997), indicate that a variable proportion of lactating cows have not initiated estrous cycles before the end of the EWP. Studies indicate that 4 to 58% of cows have low concentrations of progesterone in blood serum and remain anestrus by 40 to 97 DIM (Stevenson, 2001).

Body condition scores of cows at parturition seem to have little relationship with conception or with the number of AI services per conception (Domecq et al., 1997). However, loss of BCS between parturition and AI may negatively influence conception, because cows with BCS <3 at calving were less likely to be inseminated and loss of BCS between calving and 45 DIM was associated with more days open and delayed intervals to first service (Suriyasathaporn et al., 1998). Cows that lost 0.5 to 1.0 unit of body condition had greater conception at first service than those that lost >1.0 unit (Butler and Smith, 1989). Loss of BCS during the first 30 DIM contributed to the failure of multiparous cows to conceive at first service more than any other variable (Domecq et al., 1997). For example, pregnancy rates of dairy cows were less after the Ovsynch protocol when their BCS <2.5 before AI compared with those with BCS ?2.5 (Moreira et al., 2000). Furthermore, decreasing the percentage of the herd in lesser body condition before AI would increase net revenues (Moreira et al., 2000). Energy balance during the dry period and early lactation, as monitored by BCS, is a better indicator of conception at first service than are health disorders and other risk factors. In contrast, first test-day milk yield and the fat-to-protein ratio in milk were more reliable indicators of disease, fertility, and milk yield than was BCS or loss of body condition (Heuer et al., 1999). Therefore, maintaining adequate body condition and DMI and balancing diets for cows that are consuming >4% of their BW in daily DMI are challenges (Chase, 1993). Milk production and DMI of dairy cows are stimulated by increased dietary protein, but, unfortunately, decreased fertility often is associated with excess feeding of ruminally degradable or RUP as assessed by elevated blood or milk concentrations of urea (Butler, 1998). Concentrations of milk urea N exceeding 19 mg/dl are associated with altered uterine pH (Butler, 1998) and reduced fertility (Butler et al., 1996). The latter occurs when the negative energy balance is exacerbated by excess RDP intake (Butler, 1998) and excess loss in body condition after calving (Broster and Broster, 1998; Grummer, 1995).

Programmed Breeding
Programmed breeding is a method to schedule and control the insemination program of lactating cows in the herd. The advantages for programming estrous cycles include: 1) convenience of scheduling labor and tasks; 2) controlling the occurrence of estrus, ovulation, or both; and 3) knowing the stage of the estrous cycle and reproductive status of groups of cows in the herd. These reproductive statuses include: 1) open cows scheduled for first services; 2) open cows scheduled to be reinseminated; 3) open cows designated as culls; and 4) cows confirmed pregnant. Therefore, programmed breeding can be applied to at least two distinct groups of cows: 1) those that are scheduled for their first postpartum inseminations and 2)those that are open at pregnancy diagnosis but are reprogrammed to be inseminated in a new breeding cluster (Herlihy et al., 2012).

Breeding clusters: The breeding cluster is one method that can be used to organize groups of cows for programmed breeding (Folman et al., 1984). For example, if the EWP is 50 day before scheduled AI, then a breeding cluster of cows can be organized to fall within a certain range of DIM to fit the targeted first breeding date. These cows can be identified easily using DHIA software, computer records, or spreadsheet programs, or simply by keeping a chronological list of calving dates. In a herd of 200 cows, a cluster that calves during a 3 wk period can be organized so the freshest cow in the cluster meets the minimum acceptable EWP at the time of AI. When the EWP is 50 day, a cluster would consist of cows that are 50 to 70 DIM during the targeted breeding week. Therefore, the average interval to first insemination for that cluster would be 60 day. Cows failing to conceive should return to estrus during the breeding week of the next cluster of cows, which would be estrus synchronized for AI 3 wk after the first cluster. This clustering method allows first services and repeat inseminations to occur during the same week, thus concentrating most inseminations during 1 wk out of every 3 wk. This same sys-tem can be employed for AI of replacement heifers when they reach an acceptable age and weight to enter a breeding cluster (Aungier et al., 2012).

Targeted Breeding Program: This program has been promoted by one of the PGF2? manufacturers (Pharmacia & Upjohn) for synchronizing the AI breeding of lactating cows in a herd (Nebel and Jobst, 1998). Injections of PGF2? are administered 11 to 14 day apart. This interval is based simply on the fact that sufficient time must pass after the first injection so those females responding to the first injection (their corpus luteum regresses and they come into estrus) have a new CL that is mature enough to respond to a second injection (at least on day 6 of the estrous cycle). In addition, those females that were not in a stage of the estrous cycle with a CL that could regress after the first PGF2? injection should be responsive 11 to 14 day later (Beal, 1998; Odde, 1990). Targeted breeding requires that the first injection (so called set-up injection) be given 14 d before the EWP ends. No cows are inseminated after the first injection, although up to 50% may show estrus in response to it. The second injection (first breeding injection) then is given just before the end of the EWP, so first services can occur when cows are eligible for AI breeding. The Targeted Breeding program then recommends that when no estrus is detected after the second injection, a third injection (second breeding injection) be given 14 day after the second injection. If no standing estrus is detected after this third injection, then one fixed-time insemination can be given at 72 to 80 hr after this third injection of PGF2? (Sheldon et al., 2009).

Modified Targeted Breeding Program: The Modified Targeted Breeding program also is promoted by Pharmacia & Upjohn for synchronizing the AI breeding of lactating cows in a herd. This program was designed to force the majority of cows into the early luteal phase of the estrous cycle with a presynchronizing injection of PGF2? before the administration of GnRH. Then a 100-µg injection of GnRH is given 7 day before a second PGF2? injection. The GnRH injection alters follicular growth by inducing ovulation of the largest follicle (dominant follicle) in the ovaries to form a new or additional CL (Pursley et al., 1995). Based on random distribution of cows throughout the estrous cycle, about 64% will be at a stage of the cycle in which a dominant follicle will ovulate in response to the GnRH-induced LH release from the pituitary gland (Vasconcelos et al., 1999). Thus, estrus usually does not occur until after a PGF2? injection regresses the natural CL, the GnRH-induced CL (formed from the follicle induced to ovulate by the first GnRH injection), or both. Therefore, a new group of follicles emerges from the ovaries (as shown by transrectal ultrasonography) within 1 to 2 d after the first injection of GnRH (Pursley et al., 1995). From that new group of follicles, a newly developed dominant follicle emerges, matures, and ovulates after estrus is induced by PGF2?. Following the PGF2? injection, cows are inseminated based on detected estrus, or in the absence of estrus, one timed AI (TAI) can be administered at 72 to 80 hr after PGF2?.

Ovsynch program is similar to the previous program, except it requires no detection of estrus. In fact, it is described more accurately as an ovulation synchronization program; hence the name, Ovsynch. A 100-µg injection of GnRH is given 7 day before a PGF2? injection, then a second 100-µg injection of GnRH is administered 48 hr after PGF2?, with one fixed-time insemination given 0 to 24 hr later. Following the first GnRH injection, a dominant follicle is induced to ovulate as described previously. Following the second GnRH injection, in the absence of elevated concentrations of progesterone after CL regression is induced by PGF2?, the preovulatory LH surge is induced so the preovulatory follicle ovulates between 24 and 34 hr later (Pursley et al., 1995). Cows will show estrus in this program, with about 8 to 16% around the time of the PGF2? injection and up to 30% by or shortly after the second GnRH injection (Stevenson et al., 1999). If cows are detected in estrus at any time, they should be inseminated according to the AM-PM rule (Trimberger, 1948) to maximize conception, and the injections of PGF2?, GnRH, or both should be eliminated.

Active AI-breeding period
The second component of the calving interval is the period between the end of the EWP and when the first or subsequent estrus is detected, followed by AI and eventual conception. The duration of this period is a function of the estrus detection rate and the level of individual cow fertility. Whether or not hormones are used to induce estrus before first and subsequent services, the percentage of cows detected in estrus depends on the efficiency of detecting estrus in all cows (Stevenson, 2000). The level of cow fertility depends upon a number of factors, including the fertility of the service sire, correct thawing and handling of semen, AI-breeding technique, and timing of insemination (Stevenson et al., 1983). Level of fertility and estrus detection rates (EDR) are rate limiting to the establishment of pregnancy in a timely fashion.

Detection of Estrus
The greatest limiting factor to successful fertilization is detection of estrus. Two important challenges exist for detecting estrus: accurately recognizing signs of estrus and identifying all possible periods of estrus in breeding heifers and cows. One might be quite accurate in detecting cows in estrus but still have a major estrus detection problem because too many estrous periods go unobserved. Problems are caused by a lack of diagnostic accuracy (errors of commission) and a lack of efficient detection of all periods of estrus (errors of omission). Based on elevated progesterone on the day of AI, detection errors of commission averaged 5.1 (Reimers et al., 1985) and 13% (Anon, 1992) and ranged from 2 to 60% in herds.

Signs of Estrus
Mounting activity is stimulated strongly by estrogen and inhibited by progesterone (Allrich, 1994). Thus, mounting frequency is considerably greater for cows in proestrus or estrus than for cows that are out of estrus or in midcycle with a functional CL (Helmer and Britt, 1985). Once four or more sexually active animals are in estrus in the same pen, standing and mounting activity normally will be maximized (Hurnik et al., 1975) and should increase efficiency of detected estrus. A number of environmental conditions either stimulate or restrict interactions among cows and influence whether they show estrus (Stevenson, 2000). Cows that are eating or are crowded in holding pens or alleys do less mounting. Cows also show less mounting activity when housed on slippery alleys, frozen ground, or any surface that makes footing tenuous (Britt et al., 1986). Cows in estrus are more likely to mount one another if the other cows are loose rather than tied. Perhaps this indicates that freedom to interact before mounting is important for maximum expression of mounting activity. Cows that have foot problems show less mounting activity regardless of whether the problem is structural, subclinical, or clinical (Leonard et al., 1994). Many of the foot problems that affect mounting activity might be alleviated by proper foot care (e.g., foot baths or keeping dry cows on dirt) and regular hoof trimming.

Estrus Detection Aids
Senger (1994) described the ideal estrus detection system as having the following characteristics: 1) continuous surveillance of the cow; 2) accurate and automatic identification of the cow in estrus; 3) operation for the productive lifetime of the cow; 4) minimal labor requirements; and 5) high accuracy and efficiency (95%) for identifying the appropriate physiological events that correlate with estrus, ovulation, or both. Several estrus detection aids are available on the market to assist dairy producers in identifying estrus (Stevenson, 2000). These include inexpensive tail paints, tail chalks, heat mount detectors such as the Kamar or Bovine Beacon and more expensive electronic gadgetry including pedometers, which measure increased physical activity (walking) associated with estrus or pressure-sensitive, rump-mounted devices such as the HeatWatch system, which detects and records standing activities by cows in estrus. Further, biochemical prototype sensors that are able to measure progesterone inline during the milking procedure have been tested (Claycomb et al., 1996; Koelsch et al., 1994).

Pregnancy detection
Direct methods of pregnancy detection: Various methods are available to determine pregnancy status, these include return to oestrus (Senger, 1994), rectal palpation of the reproductive tract (Cowle et al., 1948) and ultrasound scanning to observe the reproductive tract (Frickle et al., 2002). In practice return to oestrus is fraught by the difficulties associated with oestrous observation, so currently most pregnancy detection in cows is carried out by ultrasound scanning of the reproductive tract to detect the presence or absence of the early embryo and foetal fluid. Using this method pregnancy status is generally determined from day 28 onward of pregnancy. This method while routinely used, is too late to allow rebreeding at the optimal time (i.e., 18 to 24 days post initial AI) for non-pregnant cows as the normal oestrous cycle is 18 to 24 days.

Indirect methods for pregnancy detection: Indirect methods for early pregnancy diagnosis use qualitative or quantitative measures of hormones or conceptus specific substances in maternal body fluids as indirect indicators of the presence of a viable pregnancy (Cordoba et al., 2001). Commercially available indirect methods for pregnancy diagnosis in dairy cows include milk progesterone tests and tests for pregnancy-associated glycoproteins (PAGs) in blood or milk (Lopez-Gatiuos et al., 2005). Progesterone assays are more useful as a non-pregnancy test on day 21. However, it is inaccurate as a test for pregnancy as reversion to low P4 in non-pregnant cows is highly variable due to early embryonic losses. PAG measurement is a viable method of determination of pregnancy status in dairy cows (Cordoba et al., 2001). However, accuracy of PAG detection is only good after day 35 to 40. Interference may also occur from PAG carry over from previous pregnancy for 40–50 days giving rise to a risk of false positives. It also may give false positive results after embryo loss.

Gestation and Dry Period
The third component of a calving interval is gestation, including the dry period. The duration of gestation is fairly constant and cannot be shortened significantly without adversely affecting the health or viability of the newborn (Bazer and First, 1980).

Body Condition and Body Weight
The performance of dairy cows in their next lactation is usually dependent on getting them into the appropriate body condition before the dry period of the current lactation begins. The BCS of cows at the end of lactation should be 3.25 to 3.75 (Wildman et al., 1982; Studer, 1998). Management of body condition is critical in the last 100 day of lactation, because when cows are too thin, it is more energy efficient for them to regain body condition and weight during lactation than during the dry period (Moe, 1981). When BCS are less than optimal, energy density of the diet should be increased during mid to late lactation. Compared with cows losing condition, cows gaining condition during the dry period yielded more milk in the first 120 d of lactation and had an accelerated rate of increased milk yield.

Dry Period
The dry period is a critical component to subsequent performance of dairy cows rather than an insignificant rest period between lactations. Nutrients required during this period include the maintenance and growth of the cow plus that required by the developing feto-placental unit (Bell, 1995). The diet for the far-off dry period should be balanced for high fiber and less energy density, whereas that for the close-up dry period should contain higher energy density with less fiber (Van Saun, 1991). During the last 10 day before parturition, DMI decrease significantly (Bertics et al., 1992) and restrict nutrient intake during a time of increasing demand for protein, energy, and other nutrients by the feto-placental unit and the maternal tissues (McNamara and Hilliers, 1986). Preventing this prepartum decline in DMI by placing feed refusals into ruminal can-nulae of late-gestating cows tended to increase their subse-quent milk yield during the first 4 wk of lactation (Bertics et al., 1992).

Factors affecting fertility
Timing of Ovulation and Insemination
Ovulation occurs 24 to 32 hr after the onset of estrus (Walker et al., 1996) and is triggered by the same hormonal mechanism that causes the cow to display estrus (Allrich, 1994). Once the egg has ovulated, its estimated viable life is <12 hr, unless it becomes fertilized (McLaren, 1974). Secondary signs of estrus may be visible for up 40 hr before and up to 20 hr after the onset of estrus (Stevenson, 2000). When GnRH is administered at random stages of the estrus cycle, up to 64% of the cows will have a dominant follicle that can ovulate in response to the GnRH induced LH release (Vasconcelos et al., 1999), and ovulation after GnRH will occur between 24 and 34 hr (Pursley et al., 1995).

If frozen thawed semen from most bulls is handled properly, it is estimated to have a viable life span of <48 h in the female reproductive tract (McLaren, 1974). Sperm are not capable of fertilizing the egg immediately upon thawing and deposition into the uterine body of the female because they must traverse the uterine horns to the uterotubal junction, enter the oviduct, and complete a maturation process known as capacitation (Hawk, 1983). In general, normal, motile sperm need about 6 to 10 hr to reach the lower portion of the oviduct, during which the process of capacitation is completed (Hawk, 1987). The subsequent 12 to 16 hr represent the period of maximal fertile life of the sperm, followed by rapidly declining motility and fertility.

The key to proper timing of insemination and maximizing fertilization rates is to inseminate cows at a time to allow ovulation to occur when adequate numbers of motile sperm are present in the oviduct. Based on a twice daily estrus detection program, cows submitted for insemination should be inseminated about 12 hr after first detection in estrus (Trimberger, 1948). The exact time when estrus begins is unknown, but on the average, the female detected in estrus at either daily observation period has been in estrus for about 6 hr. So when inseminated 12 hr after first detection, the female actually is bred about 18 hr after the onset of estrus or approximately 6 to 12 hr before ovulation. This breeding scheme allows ample time for transport and capacitation of sperm and a synchronized over-lap of the fertile lives of both the egg and the sperm, even if the timing is off by as much as 6 h. Conception rates were greater for cows that were in estrus during the morning and inseminated the same afternoon (52%) than for cows observed in the afternoon and serviced the next morning (47%; Reimers et al., 1985). In contrast, other studies, which were based on nonreturn rates, have shown that conception rates after once daily AI were not different from those for cows inseminated based on the AM-PM rule (Foote, 1979; Nebel et al., 1994). Results were best when inseminations were based on standing estrus and when AI occurred between 8 and 11 a.m. (Nebel et al., 1994). Optimal timing of AI should occur between 4 and 12 h after the first standing event detected by the HeatWatch system (Dransfield et al., 1998) or between 6 and 17 hr after increased pedometer readings (Maatje et al., 1997).

Based on elevated progesterone on the day of AI, 5.1 (Reimers et al., 1985), 13 (Anon, 1992), or 19% (Sturman et al., 2000) of cows are inseminated when not in estrus. Inseminating pregnant cows can lead to induced embryonic death or abortion. Approximately 10% of pregnant dairy cows express estrus (Erb and Morrison, 1957). To avoid aborting a previous pregnancy in these cows, semen should be deposited mid cervically (Macmillan et al., 1977) thereby reducing the potential for aborting a pregnancy but still giving the cow a chance to conceive if she is open and really in estrus. More careful submission and rejection of questionable previously inseminated cows can reduce unnecessary use of semen, reduce abortions, and minimize long calving intervals (Sturman et al., 2000).

Semen Handling Techniques
Another cause of fertilization failure is improper handling of semen. Not following recommended procedures for retrieving, thawing, and protecting straws until safely inside the cow results in damaged sperm membranes, cold and heat-shocked sperm, or impaired sperm motility (Foote and Parks, 1993). The final result is an overall reduction in the fertile life of the frozen-thawed sperm in the straw. To maintain maximum fertilization rates, recommended semen handling techniques must be followed (Curry, 2000; Woelders, 1997): 1) when removing straws for thawing, prevent exposure of other straws by keeping them below the frost line of the tank; 2) thaw straws in water at 37°C (95°F) for at least 40 second; and 3) once thawed, prevent “cold shock” of sperm cells by thermally protecting the breeding unit in the French gun by keeping it at near body temperature until the semen is deposited in the female (Brown et al., 1982). Although prevention of “cold shock” is very important to maximal sperm survival, thawing semen at the proper temperature is even more critical (Stevenson, 1997).

AI Techniques
Actual insemination technique may or may not be a major factor contributing to failure of fertilization. Improper placement of the semen in the reproductive tract can be a limiting factor when the technician is unsure where the tip of the breeding gun is placed upon deposition of semen. Fewer numbers of motile sperm gain access to the oviduct when semen is placed in the cervix than in the uterus (Hawk, 1983). The target for insemination is the uterine body. When in doubt, deposition of the semen slightly inside one or both uterine horns is less likely to compromise fertility than placement only in the cervix. Inseminations into the uterine body or both uterine horns produced similar nonreturn rates (McKenna et al., 1990). Because most of the inseminate is expelled from the female by retrograde flow (Hawk, 1983), it is critical that all of the semen be placed in the uterus. Errors in semen placement are common among professional technicians. Below-average technicians placed the inseminate in the target site (body of uterus) only about one-third of the time compared with 85.7% accuracy by above average technicians (Graham, 1966). Nearly 25% of the time, semen was not even placed in the uterus by below-average technicians. Few cows will conceive when the semen is placed in the vagina.

Embryonic Losses
Once the eggs are fertilized, the next obstacle is loss of the early embryo that occurs during the cleavage stage of pregnancy. By 3 day after fertilization, the fertilized egg will undergo at least three cell divisions to produce an eight-cell egg. During this time before it enters the uterus at day 3 or 4, the embryo is free floating in the lower oviduct. Cell divisions continue normally as the morula forms (32 or more cells), and eventually the blastocyst develops by day 7 or 8 (McLaren, 1974). It is at this stage (late morula or early blastocyst) that embryos are recovered from donor cows for transfer to recipi-ent females (Hasler, 1992). This is a critical period of development when early losses can occur. Sometime around day 5 to 7, just after the embryo enters the uterus and when the morula is developing into the blastocyst, is when early embryo death often occurs (Ayalon, 1978). These losses, in addition to fertilization failures, are significantly greater in repeat-breeding heifers and cows. Losses of embryos at this early stage of pregnancy generally are not detected, because the cow returns to estrus at a regular interval of about 3 wk.

The next critical stage is around day 15 or 16, when the embryo must be developed sufficiently to override the spontaneous uterine secretion of PGF2?, which normally causes the CL to regress and initiates another period of estrus and a new estrous cycle (Northey and French, 1980). This process is facilitated by trophoblastic production of large quantities of interferon tau that are integrated into a multifactorial antiluteolytic function to conserve the established pregnancy (Thatcher et al., 1995). Additional embryonic losses can occur during the period of 25 to 40 day after insemination. These so-called late embryonic deaths probably occur partly as a result of some failure in the attachment of the developing placenta to the uterine wall. The fragile connections of developing placental cotyledons to the uterine caruncles of the endometrium transfer gases, nutrients, and waste products between the uterus and the developing calf (Perry, 1981). The first should as early as possible to identify non-pregnant cows and the second sometime after day 70, because only 3.4% of pregnancies in lactating dairy cows are lost after day 70 (Vasconcelos et al., 1997).

Clinical Mastitis and Abortions
Evidence is mounting that cows with mammary infections are predisposed to early pregnancy losses because of disruption of normal luteal maintenance. Cows that had clinical mastitis during the first 45 day of gestation were at 2.7 times greater risk for abortion during the next 90 day than cows without mastitis (Risco et al., 1999). Several reproductive traits (days to first service, days open, and services per conception) were impacted negatively in cows with clinical mastitis compared with healthy controls (Barker et al., 1998; Schrick et al., 1999). This phenomenon was manifested when both gram-negative and gram-positive pathogens were isolated from milk secretions of cows with clinical mastitis. The mechanism by which mastitis interferes with pregnancy and other reproductive traits seems related to the secretion of PGF2?. This evidence includes: 1) quarters of cows experimentally induced with coliform mastitis have greater concentrations of PGF2? in their milk than control quarters, and 2) intravenous endotoxin infusion increased concentrations of blood plasma prostaglandins, thromboxane B2, and cortisol, whereas concentrations of progesterone were decreased (Cullor, 1990).

Male Related Factors and Sire Usage
If a herd bull is used during summer, it is susceptible to heat stress. Even short periods of heat stress cause a marked reduction in semen quality that may last for more than 4 to 5 wk after the end of the heat-stress period. Long-term heat stress will reduce motility, sperm quality, and sex drive (mounting activity) of the bull (Foote, 1978). Even when sires at bull studs are maintained in air conditioned facilities or well-ventilated barns during summer, the quality of semen often is compromised to some degree. Choice of AI sires for the breeding herd should consider the fertility status of the breeding females. Beyond selecting for the primary trait of milk yield, choice of AI sires also should emphasize longevity (Weigel et al., 1995), which usually is accomplished by emphasizing selection of the overall classification score, dairy character, and udder depth (Funk, 1993).

Reproductive technologies
Advantage of using embryo transfer of either fresh or frozen-thawed embryos is the bypassing of the early embryo losses that occur before day of transfer (i.e., day 7). This may be an important advantage given the steady decline in conception rates and the ability to exploit further the genetic superiority of a few cows in the herd. Recent advances in the synchronization of follicular waves and the efficiency of embryo production have resulted from controlling follicular dominance (Guilbault et al., 1998). Increase in transfers of traditionally produced embryos because of advances in the in vitro production of embryos as the result of transvaginal ultrasound-guided aspiration of oocytes and the discovery of the effects of follicular status on oocyte quality and competence for embryonic development. Cloning of embryos has increased in recent years. Clones also have been derived from fetal and adult somatic cells (Wilmut et al., 1997). About 15 to 20% survive to the blastocyst stage and pregnancy rates generally are high by day 35. However, substantial losses occur between day 60 and 90, yielding pregnancy rates of about 25%. Few losses occur during the middle trimester, but substantial losses during the last trimester result in about 10 to 20% of the calves surviving to term (Spell and Robl, 2000).

Refinement of molecular biology tools related to increased availability of higher quality embryos has favored the emergence of screening of potential transgenic produced cattle and embryo sexing (Guilbault et al., 1998). Other technologies including cell sorting have allowed successful separation of X-and Y-bearing sperm (Johnson, 2000). The potential applications of commercially available sexed semen for cattle production are many (Hohenboken, 1999). Insemination of sexed, frozen-thawed sperm into both uterine horns or into the uterine body produced similar pregnancy rates, which were about 90% of the rate achieved with unsexed, frozen-thawed controls that had 7 to 20 times more sperm/insemination dose (Seidel et al., 1999). The accuracy of sexing approached 90% males or females.

Summary and Conclusions
Components and parts of the calving interval is the key management steps in maintaining reproductive efficiency in the dairy herd. Successful reproduction begins in the previous pregnancy with assessment of body condition about 5 month before parturition or 3 month before the dry period. Changes should be made in the nutrition program to allow cows to reach the appropriate BCS by end of lactation. Maximal DMI ensure that milk yield, onset of estrous cycles, and initiation of pregnancy can occur in a timely manner, if the programmed breeding protocols and good detection of estrus are in place. Use of the Ovsynch program is likely to be the most efficient and least costly way to pre-pare clusters of cows for their best chance to conceive at first AI service. Using tail chalk or other sophisticated electronic detection aids to identify normal returns to estrus at 18 to 24 day after TAI will ensure greater return rates of cows not pregnant to the first TAI. For those open cows not detected in estrus, weekly pregnancy diagnosis by ultrasonography or palpation is critical to resynchronize open cows by applying a TAI pro-gram such as the Ovsynch protocol. Once diagnosed pregnant twice, these cows should be safely pregnant with little further fetal losses after 98 day.

Factors governing reduced reproductive performance in dairy cattle are numerous and often difficult to diagnose. In general, those factors resulting from inadequate detection of estrus or fertilization failure (e.g., semen handling and AI techniques) are resolved more easily than those related to embryonic death. Although detection of estrus and timing of insemination require more labor and common sense, they are more manageable than many causes of embryonic death. Although diagnosing those causes may be difficult, they usually are related to some source of stress experienced by the lactating cows. A continuous effort should be focused on reducing various stressors that lower reproductive efficiency.

References
Allrich, R. D. 1994. Endocrine and neural control of estrus in dairy cows. J. Dairy Sci. 77:2738-2744.

Anon. 1992. Progesterone tests show dairy farmers are breeding cows not in heat. Agribus. Dairyman 7:16-18.

Aungier SPM, Roche JF, Sheehy M, Crowe MA. 2012. Effects of management and health on the use of activity monitoring for estrus detection in dairy cows. J Dairy Sci. 95:2452–66.

Ayalon, N. 1978. A review of embryonic mortality in cattle. J. Reprod. Fertil. 54:483-493.

Barker, A. R., F. N. Schrick, M. J. Lewis, H. H. Dowlen, and S. P. Oliver. 1998. Influence of clinical mastitis during early lactation on reproductive performance of Jersey cows. J. Dairy. Sci. 81:1285-1290.

Bar-Peled, U., E. Maltz, I. Bruckental, Y. Folman, Y. Kali, H. Gacitua, A. R. Lehrer, C. H. Knight, B. Robinzon, H. Voet, and H. Tagari. 1995. Relation-ship between frequent milking or suckling in early lactation and milk production of high producing dairy cows. J. Dairy Sci. 78:2726-2736.

Barr, H. L. 1975. Influence of estrus detection on days open in dairy herds. J. Dairy Sci. 58:246-247.

Bauman, D. E., and W. B. Currie. 1980. Partitioning of nutrients during preg-nancy and lactation: A review of mechanisms involving homeostasis and homeorhesis. J. Dairy Sci. 63: 1514-1529.

Bazer, F. W., and N. L. First. 1983. Pregnancy and parturition. J. Anim. Sci. 57:425-460.

Beal, W. E. 1998. Current estrus synchronization and artificial insemination programs for cattle. J. Anim. Sci. 76(3): 30-38.

Beam, S. W., and W. R. Butler. 1998. Energy balance, metabolic hormones, and early postpartum follicular development in dairy cows fed prilled lipid. J. Dairy Sci. 81:121-131.

Bell, A. W. 1995. Regulation of organic nutrient metabolism during transition from late pregnancy to early lactation. J. Anim. Sci. 73: 2804-2819.

Bertics, S. J., R. R. Grummer, C. Cadorniga-Valino, and E. E. Stoddard. 1992. Effect of prepartum dry matter intake on liver triglyceride concentration and early lactation. J. Dairy Sci. 75:1914-1922.

Britt, J. H. 1975. Early postpartum breeding in dairy cows. A review. J. Dairy Sci. 58:266-271.

Britt, J. H., R. G. Scott, J. D. Armstrong, and M. D. Whitacre. 1986. Determi-nants of estrous behavior in lactating Holstein cows. J. Dairy. Sci. 69: 2195-2202.

Broster, W. H., and V. J. Broster. 1998. Body score of dairy cows. J. Dairy Res. 65:155-173.

Butler, W. R. 1998. Review: Effect of protein nutrition on ovarian and uterine physiology in dairy cattle. J. Dairy Sci. 81:2533-2539.

Butler, W. R., and R. D. Smith. 1989. Interrelationships between energy bal-ance and postpartum reproductive function in dairy cattle. J. Dairy Sci. 72:767-783.

Butler, W. R., J. J. Calaman, and S. W. Beam. 1996. Plasma and milk urea nitrogen in relation to pregnancy rate in lactating dairy cattle. J. Anim Sci. 74:858-865.

Call, E. P. 1978. Economics associated with calving intervals. Pages 190-201 in Large Dairy Herd Management. C. J. Wilcox and H. H. Van Horn, ed. Univ. Presses Florida, Gainesville.

Call, E. P., and J. S. Stevenson. 1985. Current challenges in reproductive management. J. Dairy Sci. 68:2799-2805.

Canfield, R. W., and W. R. Butler. 1990. Energy balance and pulsatile luteiniz-ing hormone secretion in early postpartum dairy cows. Domest. Anim. Endocrinol. 7:323-330.

Canfield, R. W., and W. R. Butler. 1991. Energy balance, first ovulation and the effects of naloxone on LH secretions in early postpartum dairy cows. J. Anim. Sci. 69:740-746.

Chase, L. E. 1993. Developing nutrition programs for high producing dairy herds. J. Dairy Sci. 76:3287-3293.

Claycomb, R. W., M. J. Delwich, C. J. Munro, and R. H. BonDurant. 1996. Enzyme immunoassay for on-line sensing of milk progesterone. Trans. Am. Soc. Agric. Eng. 39:729-734.

Cordoba MC, Sartori R, Fricke PM. 2001. Assessment of a commercially available early conception factor (ECF) test for determining pregnancy status of dairy cattle. J Dairy Sci. 84:1884–9.

Cowie TA. 1948. Pregnancy diagnosis tests: a review. Commonwealth agricultural bureaux joint publication No. 13, Oxford, UK. p. 11–7. 15.

Crowe MA, Diskin MG, Williams EJ. 2014. Parturition to resumption of ovarian cyclicity: comparative aspects of beef and dairy cows. Animal. 8:1–14.

Crowe MA, Williams EJ. 2012. Triennial lactation symposium: effects of stress on postpartum reproduction in dairy cows. J Anim Sci. 90:1722–7.

Cullor, J. S. 1990. Mastitis and its influence upon reproductive performance in dairy cattle. Pages 176-180 in Proc. Int. Symp. Bovine Mastitis, Indianapo-lis, IN, Mastitis Council, Inc., and Am. Assoc. Bovine Practitioners, Ar-lington, VA.

Curry, M. R. 2000. Cryopreservation of semen from domestic livestock. Rev. Reprod. 5:46-52.

Diskin MG, Parr MH, Morris DG. 2012. Embryo death in cattle: an update. Reprod Fert Develop. 24:244–51.

Domecq, J. J., A. L. Skidmore, J. W. Lloyd, and J. B. Kaneene. 1997. Rela-tionship between body condition scores and conception at first artificial in-semination in a large dairy herd of high yielding Holstein cows. J. Dairy Sci. 80:113-120.

Dransfield, M. G. B., R. L. Nebel, R. E. Pearson, and L. D. Warnick. 1998. Timing of insemination for dairy cows identified in estrus by a radio-telemetric estrus detection system. J. Dairy Sci. 81:1874-1882.

Erb, R. E., and R. A. Morrison. 1957. Estrus after conception in a herd of Holstein-Friesian cattle. J. Anim. Sci. 16:267-270.

Folman, Y., M. Kaim, Z. Herz, and M. Rosenberg. 1984. Reproductive man-agement of dairy cattle based upon synchronization of estrous cycles. J. Dairy Sci. 67:153-160.

Foote, R. H. 1978. Factors influencing the quantity and quality of semen harvested from bulls, rams, boars and stallions. J. Anim. Sci. 47(2):1-11.

Foote, R. H., and J. E. Parks. 1993. Factors affecting preservation and fertility of bull sperm: A brief review. Reprod. Fertil. Dev. 5:665-673.

Forde N, Beltman ME, Lonergan P, Diskin M, Roche JF, Crowe MA. 201. Oestrous cycles in Bos Taurus cattle. Anim Reprod Sci. 124:163–9.

Fricke PM. 2002. Scanning the future – ultrasonography as a reproductive management tool for dairy cattle. J Dairy Sci. 85:1918–26.

Funk, D. A. 1993. Optimal genetic improvement for the high producing herd. J. Dairy Sci. 76:3278-3286.

Graham, E. F. 1966. The use of a dye indicator testing, training, and evaluat-ing technicians in artificial insemination. Pages 57-60 in Proc. 1st Tech. Conf. Artif. Insem. Reprod. Natl. Assoc. Anim. Breed.

Grummer, R. R. 1995. Impact of changes in organic nutrient metabolism on feeding the transition dairy cow. J. Anim. Sci. 73:2820-2833.

Guilbault, L. A., F. Pothier, H. Twagiramungu, and M. A. Sirard. 1998. New technologies to improve the reproductive efficiency of dairy cattle. Can. J. Anim. Sci. 78 (Suppl.):113-129.

Hasler, J. F. 1992. Current status and potential of embryo transfer and repro-ductive technology in dairy cattle. J. Dairy Sci. 75:2857-2879.

Hawk, H. W. 1983. Sperm survival and transport in the female reproductive tract. J. Dairy Sci. 66:2645-2660.

Hawk, H. W. 1987. Transport and fate of spermatozoa after insemination of cattle. J. Dairy Sci. 70:1487-1503.

Helmer, S. D., and J. H. Britt. 1985. Mounting behavior as affected by stage of estrous cycle in Holstein heifers. J. Dairy Sci. 68:1290-1296.

Herlihy MM, Giordano JO, Souza AH, Ayres H, Ferreira RM, Keskin A. Nascimento AB, Guenther JN, Gaska JM, Kacuba SJ, Crowe MA, Butler ST and Wiltbank MC. 2012 Presynchronization with double-Ovsynch improves fertility at first postpartum artificial insemination in lactating dairy cows. J Dairy Sci 95: 7003-7014.

Heuer, C., Y. H. Schukken, and P. Dobbelaar. 1999. Postpartum body condi-tion score and results from the first test day milk as predictors of disease, fertility, yield, and culling in commercial dairy herds. J. Dairy Sci. 82:295-304.

Hohenboken, W. D. 1999. Applications of sexed semen in cattle population. Theriogenology 52:1421-1433.

Hurnik, J. F., G. J. King, and H. A. Robertson. 1975. Estrous and related behaviour in postpartum Holstein cows. Appl. Anim. Ethol. 2:55-68.

Ingvartsen, K. L., and J. B. Andersen, 2000. Integration of metabolism and intake regulation: A review focusing on periparturient animals. J. Dairy Sci. 83:1573-1597.

Johnson, L. A. 2000. Sexing mammalian sperm for production of offspring: the state-of-the-art. Anim. Reprod. Sci. 60(6):93-107.

Khoury MJ, Ioannidis JP. 2014. Medicine. Big data meets public health. Science. 346:1054–5.

Kiracofe, G. H. 1980. Uterine involution: Its role in regulating postpartum intervals. J. Anim. Sci. 51(Supp. 2.):16-28.

Koelsch, R. K., D. J. Aneshansley, and W. R. Butler. 1994. Milk progesterone sensor for application in dairy cattle. J. Agric. Eng. Res. 58:115-120.

LeBlanc S. 2010. Monitoring metabolic health of dairy cattle in the transition period. J Reprod Dev. 56:S29–35.

Leonard, F. C., J. O’Connell, and K. O’Farrell. 1994. Effect of different hous-ing conditions on behaviour and foot lesions in Friesian heifers. Vet. Rec. 134:490-494.

López-Gatius F, Santolaria P, Mundet I, Yániz JL. 2005. Walking activity at estrus and subsequent fertility in dairy cows. Theriogenology. 63:1419–29.

Maatje, K., S. H. Loeffler, and B. Engel. 1997. Predicting optimal time of insemination in cows that show visual signs of estrus by estimating onset of estrus with pedometers. J. Dairy Sci. 80:1098-1105.

Macmillan, K. L., E. D. Fielden, and R. J. Curnow. 1977. Factors influencing A. B. conception rates. VIII. Effects of non-oestrous inseminations and re-turn patterns after second inseminations. N.Z. J. Exp. Agric. 5:123-127.

Marion, G. B., J. S. Norwood, and H. T. Gier. 1968. Uterus of the cow after parturition: Factors affecting regression. Am. J. Vet. Res. 29:71-75.

Mather, E., and Melancon, J. 1981. The periparturient cow—A pivotal entity in dairy production. J. Dairy Sci. 64:1422-1430.

McKenna, T., R. W. Lenz, S. E. Fenton, and R. L. Ax . 1990. Nonreturn rates of dairy cattle following uterine body or cornual insemination. J. Dairy Sci. 73:1779-1783.

McLaren, A. 1974. Fertilization, cleavage and implantation. Pages 143-165 in Reproduction in Farm Animals. E. S. E. Hafez, ed. 3rd ed. Lea & Febiger, Philadelphia, PA.

McNamara, J. P., and J. K. Hilliers. 1986. Adaptions in lipid metabolism of bovine adipose tissue in lactogenesis and lactation. J. Lipid Res 27:150-157.

Moreira, F., C. A. Risco, M. F. A. Pires, J. D. Abrose, M. Drost, M. DeLor-enzo, and W. W. Thatcher. 2000. Effect of body condition on reproductive efficiency of lactating dairy cows receiving a timed insemination. Theriogenology 53:1305-1319.

Nebel, R. L., and S. M. Jobst. 1998. Evaluation of systematic breeding programs for lactating dairy cows: a review. J. Dairy Sci. 81:1169-1174.

Nebel, R. L., W. L. Walker, M. L. McGilliard, C. H. Allen, and G. S. Heck-man. 1994. Timing of artificial insemination of dairy cows: fixed time once daily versus morning and afternoon. J. Dairy Sci. 77:3185-3191.

Northey, D. L., and L. R. French. 1980. Effect of embryo removal and intrau-terine infusion of embryonic homogenates on the lifespan of the bovine corpus luteum. J. Anim. Sci. 50:298-302.

Odde, K. G. 1990. A review of synchronization of estrus in postpartum cattle. J. Anim. Sci. 68:817-830.
Pascottini OB, Dini P, Hostens M, Ducatelle R, Opsomer G.2015. A novel cytological sampling technique to diagnose subclinical endometritis and comparison of staining methods for endometrial cytology samples in dairy cows. Theriogenology. 84:1438–46.

Perry, J. S. 1981. The mammalian fetal membranes. J. Reprod. Fertil. 62:321-335.
Pontes GCS, Monteiro PLJ, Prata AB, Guardieiro MM, Pinto DAM, Fernandes GO. 2015. Effect of injectable vitamin E on incidence of retained fetal membranes and reproductive performance of dairy cows. J Dairy Sci. 98:2437–49.

Pursley JR, Mee MO, Wiltbank MC. 2015. Synchronization of ovulation in dairy cows using PGF2? and GnRH. Theriogenology. 44:915–23.

Pursley, J. R., M. O. Mee, and M. C. Wiltbank. 1995. Synchronization of ovulation in dairy cows using PGF2a and GnRH. Theriogenology 44:915-923.

Reimers, T. J., R. D. Smith, and S. K. Newman. 1985. Management factors affecting reproductive performance of dairy cows in the northeastern United States. J. Dairy Sci. 68:963-972.

Risco, C. A., G. A. Donovan, and J. Hernandez. 1999. Clinical mastitis associ-ated with abortion in dairy cows. J. Dairy Sci. 82:1684-1689.

Roche, J. F., D. Mackey, and M. D. Diskin. 2000. Reproductive management of postpartum cows. Anim. Reprod. Sci. 60-61:703-712.

Savio, J. D., M. P. Boland, N. Hynes, and J. F. Roche. 1990. Resumption of follicular activity in the early postpartum period of dairy cows. J. Reprod. Fertil. 88:569-579.

Schrick, F. N., A. M. Saxton, M. J. Lewis, H. H. Dowlen, and S. P. Oliver. 1999. Effects of clinical and subclinical mastitis during early lactation on reproductive performance of Jersey cows. Pages 189-190 in Proc. Natl. Mastitis Council Ann. Mtg., Natl. Mastitis Counc., Madison, WI.

Seidel, G. E., Jr., J. L. Schenk, L. A. Herickhoff, S. P. Doyle, Z. Brink, R. D. Green, and D. G. Cran. 1999. Insemination of heifers with sexed sperm. Theriogenology 52:1407-1420.

Senger PL. 1994. The estrus detection problem: new concepts, technologies, and possibilities. J Dairy Sci. 77:2745–53.

Senger, P. L. 1994. The estrus detection problem: New concepts, technologies, and possibilities. J. Dairy Sci. 77:2745-2753.

Sheehy MR, Fahey A, Aungier SPM, Carter F, Crowe MA, Mulligan FJ. 013. A comparison of serum metabolic and production profiles of dairy cows that maintained or lost body condition 15 days before calving. J Dairy Sci. 100:1–12.

Sheldon IM, Cronin J, Goetze L, Donofrio G, Schuberth HJ. 2009. Defining postpartum uterine disease and the mechanisms of infection and immunity in the female reproductive tract in cattle. Biol Reprod. 81:1025–32.

Short, R. E., and D. C. Adams. 1989. Nutritional and hormonal interrelation-ships in beef cattle reproduction. Can. J. Anim Sci. 68:29-39.

Spell, A., and J. M. Robl. 2000. Somatic cell cloning in the beef industry. Pages 95-100 in Proc. 49th Beef Cattle Short Course, Univ. of Florida, Gainesville.

Staples, C. R., W. W. Thatcher, and J. H. Clark. 1990. Relationship between ovarian activity and energy status during the early postpartum period of high producing dairy cows. J. Dairy Sci. 73:938-347.

Stevenson, J. S. 1997. Help prevent sperm cold shock. Hoard’s Dairyman 142:29.
Stevenson, J. S., and E. P. Call. 1989. Reproductive disorders in the peripartu-rient dairy cow. J. Dairy. Sci. 71:2572-2583.

Stevenson, J. S., J. F. Smith, and D. E. Hawkins. 2000. Reproductive out-comes of dairy heifers treated with combinations of prostaglandin F2?, nor-gestomet, and gonadotropin-releasing hormone. J. Dairy Sci. 83:1-8.

Stevenson, J. S., M. K. Schmidt, and E. P. Call. 1983. Factors affecting repro-ductive performance of dairy cows first inseminated after five weeks post-partum. J. Dairy Sci. 66:1148-1154.

Stevenson, J. S., Y. Kobayashi, and K. E. Thompson. 1999. Reproductive performance of dairy cows in various programmed breeding systems in-cluding Ovsynch and combinations of gonadotropin-releasing hormone and prostaglandin F2?. J. Dairy Sci. 82:506-515.

Studer, E. 1998. A veterinary perspective of on-farm evaluation of nutrition and reproduction. J. Dairy Sci. 81:872-876.

Sturman, H., E. A. B. Oltenacu, and R. H. Foote. 2000. Importance of insemi-nating only cows in estrus. Theriogenology 53:1657-1667.

Suriyasathaporn, W., M. Nielsen, S. J. Dieleman, A. Brand, E. N. Norrdhuiz-en-Stassen, and Y. H. Schukken. 1998. A Cox proportional-hazards model with time-dependent covariates to evaluate the relationship between body-condition score and the risks of first insemination and pregnancy in a high-producing dairy herd. Pre. Vet. Med. 37:159-172.

Thatcher, W. W., M. D. Meyer, and G. Danet-Desnoyers. 1995. Maternal rec-ognition of pregnancy. J Reprod. Fertil. (49):15-28.

Trimberger, G. W. 1948. Breeding efficiency in dairy cattle from artificial insemination at various intervals before and after ovulation. Nebraska Ag-ric. Exp. Stn. Res. Bull. 153:1-26.

Van Saun, S. J. 1991. Dry cow nutrition. The key to improving fresh cow performance. Vet. Clin. North Am. Food Anim. Pract. 7:599-620.

Vasconcelos, J. L. M., R. W. Silcox, G. J. Rosa, J. R. Pursley, and M. C. Wilt-bank. 1999. Synchronization rate, size of the ovulatory follicle, and preg-nancy rate after synchronization of ovulation beginning on different days of the estrous cycle in lactating dairy cows. Theriogenology 52:1067-1078.

Vasconcelos, J. L. M., R. W. Silcox, J. A. Lacerda, J. R. Pursley, and M. C. Wiltbank. 1997. Pregnancy rate, pregnancy loss and response to heat stress after AI a two different times from ovulation in dairy cows. Biol. Reprod. 56(1):140.

Weigel, D. J., B. G. Cassell, I. Hoeschele, and R. E. Pearson. 1995. Multiple-trait prediction of transmitting abilities for herd life and estimation of eco-nomic weights using relative net income adjusted for opportunity cost. J. Dairy Sci. 78:639-647.

Wildman, E. E., G. M. Jones, P. E. Wagner, R. L. Boman, H. F. Troutt, and T. N. Lesch. 1982. A dairy cow body condition scoring system and its rela-tionship to selected production characteristics. J. Dairy Sci. 65:495-501.

Wilmut, I., A. E. Schnieke, J. McWhir, A. J. Kind, and K. H. S. Campbell. 1997. Viable offspring derived from fetal and adult mammalian cells. Na-ture 385:810-813.

Woelders, H. 1997. Fundamentals and recent development in cryopreservation of bull and boar semen. Vet. Q. 19:135-138.