Managing Reproductive Disorders in Dairy Cows

 

Twinning

Twinning is an unavoidable outcome of reproduction in dairy cattle and is undesirable in a dairy operation because it reduces overall profitability and reproductive efficiency (Eddy et al., 1991; Beerepoot et al., 1992). One study estimated that every twin birth incurs a $108 economic loss compared with a singleton birth (Beerepoot et al., 1992). Twinning also reduces reproductive performance by increasing average days open and services per conception of the dam during the subsequent lactation (Nielen et al., 1989). In addition, the etiologies of many periparturient diseases in cattle are associated with twin births. Cows calving twins have higher risks for stillbirth, retained placenta, metritis, displaced abomasum, ketosis, and acidurea (Pfau et al., 1948; Markusfeld, 1987; Nielen et al., 1989). Incidences of abortion (29.3% vs. 12.0%), neonatal calf mortality (15.7% vs. 3.2%), reduced birth weight (43.5 vs. 30.6 kg), and retained placenta (34% vs. 7%) also are greater among twins compared with singleton calves, probably due to the reduced gestation length and increased incidence of dystocia among cows calving twins (Pfau et al., 1948; Erb and Morrison, 1959; Neilen et al., 1989; Day et al., 1995). Culling rates are also greater for cows calving twins (Eddy et al., 1991). One impact of twinning is a reported reduction in the number of fertile heifers available for use as replacements in the dairy herd (Table 3). This decrease arises from increased neonatal calf mortality of twins and a skewed gender ratio resulting in more homozygous male pairs.

 

Table 3. Effect of twinning on the number of replacement heifers per pregnancy.

 

Replacement Heifers per Pregnancy

Reference

Single Births

Twin Births

Erb & Morrison, 1959

0.43

0.34

Nielen et al., 1989

0.48

0.42

Day et al., 1995

0.42

0.29

 

Mechanisms of Twinning

Cattle are a monotocous species meaning that, under most circumstances, a successful pregnancy results in the birth of a single calf. The physiologic mechanism responsible for regulating the number of follicles that become dominant within each follicular wave usually results in selection of a single dominant follicle capable of ovulation (see Figure 2). Initiation of an induced or naturally occurring ovulatory stimulus causes release of a single oocyte from the dominant follicle at ovulation. If the subsequent events from fertilization to parturition occur normally, the pregnancy will result in birth of one calf. On occasion, however, two follicles are selected to continue growth from among the group of growing follicles in a follicular wave resulting in a phenomenon termed codominance. If the appropriate stimulus for ovulation occurs naturally or is induced when codominant follicles are present, two oocytes, one from each follicle, will be released. If the subsequent events from fertilization to parturition occur normally for both oocytes, twins will result. The single cell that is formed after an oocyte is fertilized by a sperm is called a zygote. Thus, twins that occur as a result of ovulation and fertilization of two oocytes are called dizygous twins.

Most twins in cattle are of the dizygous type (Erb and Morrison, 1959; Johansson et al., 1974; Ryan and Boland, 1991). Dizygous twins can be the same or opposite in sex and are no more alike phenotypically or genetically than siblings with the same parents born during different gestations. Ovulation of a single dominant follicle also can, on rare occasions, result in twins. Twins resulting from ovulation and fertilization of a single oocyte are called monozygous twins. Monozygous twins are genetically and phenotypically identical and, therefore, are always of the same sex. The mechanism by which monozygous twinning occurs is not clearly understood, but monozygous twinning can be considered a natural cloning of the original zygote in vivo. The rate of monozygous twinning in cattle is low, with estimates ranging from 7.4% (Erb and Morrison, 1959) to 13.6% (Ryan and Boland, 1991) of all twin births or less than 0.3% of all births.

 

Twinning Rates in Dairy Cattle

Risk factors for twinning in cattle include effects of breed type, parity. (Nielen et al, 1989; Ryan and Boland, 1991). The percentage of twins born also varies among seasons of the year, with a trend toward more twin births during the summer months. This seasonal effect on twinning has been attributed to an increased plane of nutrition during the fall when cows calving during the summer would have conceived, a decreasing light period, and a decrease in the viability of early stage embryos conceived during summer months compared with those conceived during cooler fall months (Cady and Van Vleck, 1978; Nielen et al., 1989). High cumulative milk production and previous twinning are additional factors that increase the risk of twinning (Nielen et al., 1989; Kinsel et al., 1998). In general, the twinning rate for most beef breeds of cattle is less than 1% (Rutledge, 1975). The reported incidence of twinning in dairy cattle ranges from 2.5 to 5.8% and is dramatically affected by parity, ranging from 1% for first parity to nearly 10% during later parities (Table 4).

 

Table 4. Effect of parity on twinning rate (%) or relative risk (R)a of twinning in dairy cattle.

 

No. of

Parity

All

Refb

Calvings

1

2

3

4

5

6

Parities

1

937

0.7

5.0

4.2

5.0

7.0

 6.7c

4.2

2

7,387

1.3

4.4

5.3

4.6

5.8

6.0

4.6

3

11,951

0.8

2.7

4.1

4.5

4.9

 4.8c

3.2

4

19,755

0.9

2.1

3.5

3.4

3.7

3.2

2.5

5

24,843

1.0

7.0

7.5

7.9

 9.1c

-

4.2

6

19,497

1.3

6.0

9.4c

-

-

-

-

7

52,362

1.0

2.9

3.2

3.9

3.3

4.1c

2.4

8

8,521

R=0.2

R=1.5

R=1.6

R=1.6

R=2.0c

-

5.8

aRelative risk (R) of twinning for each parity group is calculated against all others pooled together.

bReference: 1=Pfau et al., 1948; 2=Erb & Morrison, 1959; 3=Nielen et al., 1989; 4=Eddy et al., 1991; 5=Ryan & Boland, 1991; 6=Berry et al., 1994; 7=Kinsel et al., 1998; 8=Markusfeld, 1987.

cIncludes all cows ³ the parity listed.

 

The effect of parity on twinning rate is not clearly understood but may be explained by an increased ability of older cows to support twins throughout gestation, an increase in the rate of double ovulation, or an interaction of both of these factors. Increased uterine capacity of cows calving twins has been reported (Ryan and Boland, 1991). Furthermore, the incidence of double ovulation in lactating dairy cows is around 14% (Kidder et al., 1952; Fricke et al., 1998), and, as with the incidence of twinning, increases with parity (Labhsetwar et al., 1963). Kinsel et al. (1998) reported an increase rate of twinning over a 10-year period. The single largest contributor to this increase was the increase in peak milk production that occurred over that period. They also suggested that feeding higher energy diets to high producing cows may be increasing the incidence of double ovulations, and hence the rate of twinning. This nutritional effect is similar to the practice of "flushing" in ewes (Dunn and Moss, 1992), but further research is required in dairy cattle.

Increasing levels of dietary bypass protein can increase ovulation rate and incidence of twinning in ewes (Nottle et al., 1988). Therefore, the high levels of bypass protein fed to lactating cows may partially account for the increased twinning rate. The overall twinning rates reported for dairy cows in recent studies are greater than those reported in many earlier reports (Day et al., 1995), indicating that twinning rate may be increasing over time in the dairy cattle population as a whole. If twinning is related to nutrition and/or milk production (Nielen et al., 1989), this increase in twinning would not be unexpected considering recent trends in feeding practices and yearly increases in milk production per cow.

 

Freemartinism

Freemartinism in heifers results from twinning when embryonic membranes of a male and female conceptus fuse during gestation resulting in exchange of blood between the male and female fetuses. Endocrine factors or cells from the male calf cause abnormal development of the reproductive organs of the female calf resulting in infertility. Freemartinism occurs in about 92% of heifers born as a result of heterosexual twin pregnancies (Buoen et al., 1992). Thus, about 8% of heifers from heterosexual twin pregnancies will be fertile, presumably because the fetal membranes fail to fuse or because membrane fusion occurs after the critical period of reproductive organ differentiation (Buoen et al., 1992).

The earliest developmental abnormalities of the female reproductive tract resulting in freemartinism occur between 49 to 52 days post fertilization (Jost et al., 1972). Interestingly, freemartinism has been documented in singleton female calves, which probably results due to loss of a male twin after fusion of the embryonic membranes but before parturition (Wijeratne et al., 1977). In addition, many bulls born twin to a heifer exhibit various degrees of impaired reproductive function including inability to produce semen, reduced sperm production, or increased incidence of abnormal spermatozoa (Dunn et al., 1979). From a practical perspective, bulls born twin to a heifer can be used for breeding purposes after passing a breeding soundness and semen quality examination (Long, 1979).

 

Management of Twins

Management of cows carrying twins depends on accurate identification of the presence of twins early during gestation. Cows carrying twin pregnancies can be accurately identified at 40 to 55 days post AI using transrectal ultrasonography (Echternkamp and Gregory, 1991; Davis and Haibel, 1993; Dobson et al., 1993). Palpation per rectum between 50 to 70 days post AI also results in an acceptable degree of accuracy among experienced bovine practitioners (Day et al., 1995).

Several management scenarios could be considered upon diagnosis of a twin pregnancy in a dairy cow. Continued management of the cow carrying twins could be avoided either by culling the cow or by aborting the twin pregnancy, usually through administration of an ecbolic agent such as PGF2a. Several factors would argue against aborting a twin pregnancy with the intent of rebreeding the cow. First, the estimated average lactation length of cows subjected to induced abortion and rebreeding would approach 500 days (~18.5 month calving interval) based on average reproductive performance and management indices for lactating cows (Table 5). Second, the risk for a twin pregnancy during the subsequent gestation is increased because cows calving twins are at greater risk for subsequent twinning (Nielen et al., 1989). Based on these considerations and depending on the value of the dam and calf, culling may be a better alternative to aborting the pregnancy.

 

Table 5. Estimated intervals and cumulative days in milk associated with events after a management decision to terminate the pregnancy of a cow carrying twins.

 

Interval

Cumulative

Mean interval from:

(Days)

Days in Milk

Calving to twin pregnancy diagnosis and induction of abortion

184a

184

Induction of abortion to second conception

  84b

268

Second conception to dry off

232c

500

aAverage days open (144 days; voluntary waiting period = 60 days, conception rate = 40%, and service rate = 40%)+ day of gestation at diagnosis of twins (40 days).

bMedian days to second conception using AI breeding (84 days; conception rate = 40%, and service rate = 40%).

cAverage gestation length (282 days) - average dry period (50 days).

 

If a cow carrying twins is to be maintained in the herd until parturition, several management practices should be considered. First, based on research in beef cows (Wheeler et al., 1979; Koong et al., 1982), dairy cows carrying twins should be fed a higher plane of nutrition, especially during the last trimester of gestation (Nielen et al., 1989; Day et al., 1995). Second, because the gestation length of cows calving twins is reduced by 7 to 10 days (Pfau et al., 1948; Nielen et al., 1989; Ryan and Boland, 1991), most cows calving twins would miss the transition diet feeding period beginning about two to three weeks before the estimated calving date. Earlier dry off and feeding of a transition diet may reduce the incidence of postpartum metabolic problems associated with cows calving twins (Ryan and Boland, 1991). Finally, assistance at calving for cows carrying twins may reduce complications associated with dystocia and may reduce economic losses by reducing the rate of neonatal calf mortality.

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