Managing Reproductive Disorders in Dairy Cows
Early Embryonic Loss
Pregnancy loss contributes to reproductive inefficiency in lactating dairy cows because fertility assessed at any point during pregnancy is a function of both conception rate and pregnancy loss. Conception rates at 28 to 32 days post-AI in lactating dairy cows range from 40 to 47% (Pursley et al, 1997; Fricke et al., 1998), whereas conception rates in dairy heifers are nearly 75% (Pursley et al., 1997). Similarly, pregnancy loss in lactating dairy cows is greater than that in dairy heifers (20% vs. 5%; Smith and Stevenson, 1995). Although the specific factors responsible for early embryonic loss in dairy cows are not known, they may be similar to those factors responsible for reduced conception rates.
Timing of Early Embryonic Loss
Early embryonic loss in cattle is difficult to study because no sensitive test similar to that used for women and mares exists. The fertilization rate after AI in beef cows is 90%, whereas embryonic survival rate is 93% by Day 8 and only 56% by Day 12 post AI (Diskin and Sreenan, 1980). In dairy cattle, only 48% of embryos were classified as normal on Day 7 after AI (Weibold, 1988). Thus, substantial pregnancy loss probably occurs within two weeks post AI.
Rectal palpation from 40 to 60 days post AI is the most common method of pregnancy diagnosis in dairy cattle. Several studies have used pregnancy diagnosis based on rectal examination to establish a conception rate from which pregnancy loss can be determined as gestation ensues. Using this technique, pregnancy loss is about 10%, with greater losses in lactating cows compared with heifers (Thurmond et al., 1990; Markusfel-Nir, 1997). Furthermore, the risk of pregnancy loss was more than four times greater during the first compared with the second and third trimesters of gestation (Markusfel-Nir, 1997).
Recently, transrectal ultrasonography was used to determine the timing of pregnancy loss from 28 days post AI to calving in lactating dairy cows (Vasconcelos et al., 1997). Pregnancy diagnosis was conducted at 28, 42, 56, 70, and 98 days post AI for 1,600 dairy cows in three herds with a rolling herd average >23,000 pounds. The conception rate of cows at 28 days post AI was 32%, and overall pregnancy loss from day 28 to calving was nearly 25%, with most losses occurring during the first 60 days of gestation (Figure 3).
Factors Affecting Early Embryonic Loss
Because fertility assessed at any point during pregnancy is a function of both conception rate and pregnancy loss, factors associated with pregnancy loss may be similar to those responsible for low fertility. Nutrition can have a major impact on dairy cow fertility. A recent review (Ferguson, 1996) indicated that nutritional causes of low fertility are more like first due to energy management, second to excessive protein feeding, and third to trace element and vitamin deficiencies. In addition, greater body condition score losses from calving to breeding result in reduced fertility (Ferguson, 1996).
Specific physiologic mechanisms responsible for pregnancy loss in lactating dairy cows are unknown, but may include lactational stress associated with increased milk production (Oltenacu et al., 1980; Nebel and McGilliard, 1993), negative energy balance (Butler and Smith, 1989), toxic effects of urea and nitrogen (Butler et al., 1995) or reduced ability to respond to increased environmental temperature (Stevenson et al., 1984; Hansen et al., 1992). Beef cows losing weight have a higher incidence of early embryonic loss than those gaining weight (Dunn and Moss, 1992). This suggests that negative energy balance may be involved when a high incidence of early embryonic loss is observed in dairy cows. Recommendations for minimizing the severity of negative energy balance in high-producing dairy cows include maximizing dry matter intake in early lactation and feeding diets containing .78 Mcal NEl per pound and 5%-7% total fat (DM basis).
High circulating urea and ammonia from the feeding of diets high in degraded intake protein (DIP) may adversely affect early embryonic development (Butler, 1998). Further, the feeding of excess DIP may exacerbate negative energy balance and related reproductive problems. Elrod and Butler (1993) reported increased early embryonic loss for heifers fed an energy-restricted diet containing high levels of DIP. Ferguson (1996) indicated that cows fed high amounts of DIP showed more irregular intervals between first and second service. Dietary DIP should be restricted to 10%-12% (NRC, 1989; DM basis). Also, nitrate concentrations in water and forages should be evaluated when herds are experiencing a high incidence of abortions or early embryonic loss (Davison et al., 1965).
Reproductive Management of Early Embryonic Loss
At present, there is no practical way to reduce early embryonic loss in lactating dairy cows. However, recognizing the occurrence and magnitude of early embryonic loss may actually present management opportunities by taking advantage of new reproductive technologies that increase AI service rate in a dairy herd. One such technology is the use of transrectal ultrasonography for early pregnancy diagnosis. If used routinely, transrectal ultrasonography has the potential to improve reproductive efficiency within a herd by reducing the period from AI to pregnancy diagnosis to 26 to 28 days with a high degree of diagnostic accuracy (Pierson and Ginther, 1984). Furthermore, use of ultrasound could minimize embryonic loss that may occur after manipulation of the reproductive tract and conceptus during pregnancy diagnosis using rectal palpation (Paisley et al., 1978; Vaillancourt et al., 1979).
There are several reasons that transrectal ultrasound is not widely used among bovine practitioners for pregnancy diagnosis at present. First, ultrasound machines are relatively expensive, costing between $15,000 to $20,000. Second, most ultrasound machines are large and require an external power source thereby making them cumbersome to use under field conditions. Because of these factors, use of ultrasound has been restricted research or specialized procedures such as fetal sexing, transvaginal oocyte recovery, or embryo transfer. Fortunately, several companies are currently marketing newer generations of ultrasound machines that are cheaper, smaller, and battery operated. Continuation of this trend will foster future use of this technology by bovine practitioners.
There are two main caveats to using ultrasound for routine early pregnancy diagnosis in a dairy herd. First, when using ultrasound for early pregnancy diagnosis, emphasis must be given to identifying nonpregnant rather than pregnant cows. Of cows diagnosed pregnant at 28 days post AI, 14 to 16% experience early embryonic loss by 56 days post AI (Vasconcelos et al., 1997; Fricke et al., 1998). Therefore, cows diagnosed pregnant at 28 days post AI using ultrasound should be scheduled for reexamination around 56 days post AI, when the rate of embryonic loss per day begins to decline (Vasconcelos et al., 1997; Figure 3). Second, a management strategy must be developed to return the nonpregnant cows to service as quickly as possible after pregnancy diagnosis. Such strategies include administration of PGF2a to cows with a responsive CL, use of estrus detection aids, or a combination of both methods. Unfortunately, the service rate was only 58% when using a system combining PGF2a and Kamar heat mount detectors (Britt and Gaska, 1998), probably due to the inherent inefficiencies of estrus expression and detection in lactating dairy cows.
An attractive strategy for managing reproduction in a dairy herd would combine use of synchronization of ovulation and timed AI (Ovsynch), an estrus detection aid, and early pregnancy diagnosis using ultrasound. Every two weeks, groups of cows past the voluntary waiting period would receive their first postpartum insemination after synchronization of ovulation using Ovsynch. This would dramatically reduce median days to first AI by eliminating estrus detection for the first postpartum breeding. At the time of AI, an estrus detection aid such as a Kamar device or estrus detection tail paint would be applied to the cow. This would aid in detection of cows that return to estrus between 18 to 28 days post AI due to failure of conception or early embryonic loss. Cows detected in estrus during this period could then be inseminated based on the detected estrus. At 28 days post AI, a veterinarian using ultrasound would identify any nonpregnant cows, which would be scheduled for resynchronization using Ovsynch along with the next group of cows. This would eliminate reliance on estrus detection for the next breeding, thereby reducing the interval from pregnancy diagnosis to rebreeding. All cows diagnosed pregnant at 28 days post AI would be scheduled for a second ultrasound examination at 56 days post AI to determine if pregnancy loss had occurred. This is an aggressive reproductive management system that would improve reproductive efficiency by maximizing AI service rate in the herd. This is accomplished through use of early pregnancy diagnosis using ultrasound. Although estrus detection would not be completely eliminated using this system, it would be minimized through the use of Ovsynch and timed AI.