2. Factors impacting twinning

It has been well documented that [13–15] the natural frequency of twin calvings is influenced by a wide variety of genetic and environmental factors ranging from 0.5 to 4% depending on these factors.

Mainly, environmental and genetic factors influence the number of twin Holstein Friesian calves born every year. The factors having most influence over multiple births are age, parity, seasons and climate, feeding, milk yield, hormonal influence, and genetic predisposition.

#### 2.1. Genetics

It has been shown that there is a far higher incidence of twins in dairy cattle as opposed to beef cattle [16]. The frequency of twin births also shows variation by breed [17], with incidences of multiple ovulations much more frequently observed in Holstein Friesian cows than in other dairy cattle breeds [15].

The results drawn from a model calculation estimating the genetically determined number of calves per calving (or litter size), the real incidence of twinning, and the percentile contribution of triplet and quadruplet pregnancies are shown in Table 1.

#### 2.2. Age

Age is also a contributing factor when dealing with the rate of occurrence of twin and multiple births. Cows that have had several lactations have a higher number of twin and multiple births compared to that of heifers [19].

Twin Calving and Its Connection to Other Economically Important Traits in Dairy Cattle http://dx.doi.org/10.5772/intechopen.72905 63


Table 1. Average litter size and percentage of triplet and quadruplet births predicted from percentage of multiple birth [18].

Twinning not only increased from first to second parity, but also from second to third parity, to then more or less plateau for the remainder of the reproductive life of the cow [9].

Older cows have a tendency to carry twins more frequently due to the cows' maturity and the increased occurrence of polyovulation [20]. The increase in incidence of double ovulations is approximately linear with increasing calving numbers [21]. One explanation that has been proposed for this is that in twin pregnancies the increase in embryonic mortality in younger cows is due to the uterus having insufficient functional capacity. The total area of the uterus in heifers and in young cows is usually not sufficient to support several embryos [22].

#### 2.3. Seasonality

Superficially from an economical viewpoint, the idea of an excess number of calves per calving can seem advantageous. Nevertheless, when the outcomes are analyzed, twin calving has several negative consequences. Increased rates of abortion [5], dystocia [6], higher mortality of the calves around or during parturition [7], and increased rates of placenta retention [8] are seen far more frequently in twin-calving cows. Twins born tend to be less developed than single-born herd mates of the same age, they have higher mortality rate [9], and rebreeding the dam can be delayed [10], leading to a longer calving interval and decreased profits. Use of sexed semen has been shown to result in a lower frequency of twin pregnancies [11]. However, this lower twinning rate is due to damages caused to spermatozoa by the physical forces during the sexing process, which in itself has other negative effects that are well known such

Another important aspect to consider is the qualities of the dams with a predisposition to twin calving, as the cows with higher lactation performance and better fertility tend to be more predisposed to multiparous pregnancies [6]. And hence some of the most advantageous and desirable traits in a dairy cow can lead to twinning, which is overall considered to be a

It has been well documented that [13–15] the natural frequency of twin calvings is influenced by a wide variety of genetic and environmental factors ranging from 0.5 to 4% depending on

Mainly, environmental and genetic factors influence the number of twin Holstein Friesian calves born every year. The factors having most influence over multiple births are age, parity, seasons and climate, feeding, milk yield, hormonal influence, and genetic predisposition.

It has been shown that there is a far higher incidence of twins in dairy cattle as opposed to beef cattle [16]. The frequency of twin births also shows variation by breed [17], with incidences of multiple ovulations much more frequently observed in Holstein Friesian cows than in other

The results drawn from a model calculation estimating the genetically determined number of calves per calving (or litter size), the real incidence of twinning, and the percentile contribution

Age is also a contributing factor when dealing with the rate of occurrence of twin and multiple births. Cows that have had several lactations have a higher number of twin and multiple births

of triplet and quadruplet pregnancies are shown in Table 1.

as decreased conception rates [12] and is not dependent on the dam.

negative trait.

these factors.

2.1. Genetics

2.2. Age

dairy cattle breeds [15].

compared to that of heifers [19].

2. Factors impacting twinning

62 Ruminants - The Husbandry, Economic and Health Aspects

Generally, it is found that more twin and multiple births take place in summer, spring, and autumn than in the winter months. The percentage of pregnancies that were twins was found to be 2.41% in summer, 2.04% in spring, and falling to 1.79% in autumn [17, 23, 24]. A study in the USA (Minnesota) [9] showed that the percentage of twin births in April–June was 8.3%, July–September was 9.7%, October–December was 5.2%, and January–March was 2.3% in the northern hemisphere.

The postpartum conception rate was higher in autumn than in spring due to the higher feed quality in the autumn months and the lower temperature (so that heat stress is not a problem). The opposite was found to be true [25] when dealing with beef herds, in which twin births were seen to be 50% higher in autumn than in spring [19].

The University of Wisconsin performed an investigation on the North American sires and found that the highest rate in twinning was in the lead up to the summer solstice and the lowest was at the winter solstice [26]. One cause of most frequent summer twinning (p = 0.002) could be that during the autumn months, the weather is cooler, and the hot summer months that would subject the cow to heat stress, which could damage the embryo, are over [27]. The survival rate of the embryo or the fetus and the ovulation rates determine if there will be a single calf, twin calves, or multiple calves born [28]. During the study period, 1281 (3.94%) twin births occurred. The ratio of twin deliveries on the farms varied between 3.43 and 4.35%.

on the cows' body, 14.1% of the cows have shown a double ovulation, and with that, in 5.2% of

Twin Calving and Its Connection to Other Economically Important Traits in Dairy Cattle

http://dx.doi.org/10.5772/intechopen.72905

65

Data from 393,002 parturitions were recorded along with the cows' performance in the following production cycle [30]. Data were collected from 145,457 Polish Holstein Friesian cows. The animals initially calved between 2000 and 2012 and were either in use or culled by the end of 2013. Among the factors analyzed, the cow's age had the strongest effect on the rate of occurrence of multiple pregnancies, showing multiple pregnancies to be much more frequent in cows than in heifers. Each consecutive lactation raises the chance of multiple ovulations, and occurrence of twin births increases linearly with the frequency of double ovulation [21, 35]. The incidence of twin pregnancies goes from 1% in maiden heifers to about 10% in following lactations. The rate of twin pregnancies increases the most between the first and second lactation. After the second lactation, the incidence increases to a lesser extent [23]. It should also be noted that heifers have an increased mortality of one of the fetuses in multiple pregnancies, due to the high nutrient requirement of the growing body of the heifer coupled with

In an analysis [37] of 4000 cows calving between 2000 and 2010 in a herd in northern Hungary (Table 2), ages were compared at first breeding, at first conception, and at first calving of twinning and nontwinning cattle. The average age at first breeding at the farm was 17.9 months (544 days), and no significant difference was found between future twinning and nontwinning heifers. The average age of first conception was 18.3 months (557 days). There was no significant difference between nontwinning and twinning heifers. On average, heifers became

Reproductive traits Cows with single calving/n Cows with twin calving/n p-Value Age at first breeding, month 17.88 3569 17.92 392 0.753 Age at first conception, month 18.32 3410 18.34 376 0.868 Age at first calving, month 27.45 3632 27.47 395 0.830 Calving to first service interval BF, day 73.90 6208 73.92 722 1.000 Calving to first service interval AF, day 73.59 5207 78.09 275 0.043 Open days BF 109.3 5318 104.6 642 0.047 Open days AF 109.3 5207 123.1 275 0.001 Gestation period BF, day 278.7 9940 274.0 724 <0.001 Gestation period AF, day 278.8 5207 279.0 275 0.511 Calving interval BF, day 404.2 5698 392.2 680 <0.001 Calving interval AF, day 397.5 5207 410.3 275 <0.001 Total life span, month 60.12 3581 75.97 386 <0.001

Table 2. Reproductive traits of cows with single and twin birth before (BF) and after calving (AF) [37, 38].

3. Reproductive traits in twin- and single-calving cows before calving

the cases, a twin pregnancy was detected [21].

the demands of the twin fetuses [36].
