The percentage of dystocia decreased significantly (P < 0.05) with increasing live body weight of cows, which ranged from 5.3% for cows that weighed 600–650 kg to 8.3% for cows that weighed 350–400 kg (Table 1). These results are in accordance with those obtained by Anderson (¹⁹⁹²) who found high negative correlation between dam weight and the incidence of dystocia.

The percentage of incidence of dystocia in Friesian cows decreased significantly (P < 0.05) with age progression, which decreased from 7.4% at 3–5 years of age to 4.6% at 11–13 years as shown in Table 1. These results agreed with those obtained by Roughsedge and Dwyer (²⁰⁰⁶) who found that first-calf heifers account for the majority of calving difficulties and associated calf losses. High rates of dystocia among first-calf heifers and young cows are mostly due to their smaller size at first parturition than at subsequent calvings.

Results in Table 1 revealed that the effect of parity on the percentage of dystocia in Friesian cows showed similar trend to age of cows, which decreased significantly (P < 0.05) from 7.7–4.6% with advancing parity from 1st to 8th parity. These results are in accordance with those obtained by Eriksson et al. (²⁰⁰⁴) who found that the frequencies of difficult calvings and stillbirths were approximately 6% at first parity and 1–2% at later parities for Charolais and Hereford cattle.

Results in Table 2 indicated that the percentage of incidence of dystocia in Friesian cows was significantly (P < 0.05) higher with twinning than single calving (15.5% vs. 6.5%, respectively). These results agreed with those obtained by Kirkpatrick (²⁰⁰²) who indicated that the inconsistency could be a reflection of two competing dynamics which increases the incidence of dystocia attributable to malpresentation. Echternkamp et al. (²⁰⁰⁷) found increased incidence of dystocia for twin or triplet births compared with single births.

The percentages of incidence of dystocia in Friesian cows tended to be higher with born male than female calves (7.8% vs. 6.5%) as shown in Table 2. These results are in agreement with those obtained by Johanson and Berger (²⁰⁰³) and Ettema and Santos (²⁰⁰⁴) who found that calf sex affected the incidence of dystocia, and it was higher in males than in females.

The percentage of incidence of dystocia increased significantly (P < 0.05) with increasing birth weight of born calves (Table 2). Calf birth weight is the trait most highly correlated with incidence of dystocia (Anderson ¹⁹⁹²).

The percentage of incidence of dystocia was not significantly (P > 0.05) different among the different years of study and ranged from 6.2–7.4%, with an average of 6.9%, as shown in Table 3. These values are higher than the reported dystocia rates in dairy cattle internationally, which are generally <5% (Mee ²⁰⁰⁸). Year of calving affected the incidence of dystocia (Zaborski et al. ²⁰⁰⁹).

The highest percentage (P < 0.05) of incidence of dystocia was detected in winter season (8.5%), while the least percentage (P < 0.05) was noticed in summer season (4.8%) as shown in Table 3. These results agreed with those obtained by season of calving internationally, which are generally <5% (Mee ²⁰⁰⁸). Colburn et al. (¹⁹⁹⁷) suggested that greater calf birth weight and calving difficulty may be expected in winter following severe spring temperatures.

Results in Table 3 revealed that the percentage of incidence of dystocia in Friesian cows was significantly (P < 0.05) higher with feeding winter ration compared with summer ration (8.2% vs. 5.1%). These results are in accordance with those obtained by Zaborski et al. (²⁰⁰⁹) who found that nutrition had an effect on dystocia. Manipulating feeding level during pregnancy offers an alternative method for manipulating the birth weight of calves. Little is known about the effects of nutrition in early gestation on placental development or birth weight of calves (Hickson et al. ²⁰⁰⁶).

The incidence of dystocia had an adverse effect on the reproductive performance of dairy Friesian cows as shown in Table 4. The first estrus, first service, service period, days open and calving interval were significantly longer (P < 0.05) in cows that exhibited dystocia compared to normal cows (58.1, 92.5, 150.5, and 428.5 vs. 49.2, 74.0, 123.2 and 400.6 day, respectively). However, gestation period was nearly similar for normal and cows that exhibited dystocia. These results indicated that dystocia led to increasing the service interval, service period, days open and calving interval by 8.9, 18.5, 27.3, and 28.0 day, respectively. These results are in agreement with those obtained by Lopez de Maturana et al. (²⁰⁰⁷) who reported that the effect of dystocia was statistically significant, implying a delay in the pregnancy periods of the cow.

The incidence of dystocia resulted in a significant (P < 0.05) reduction in conception rate, where the conception rate at 90, 120, and 150 days, and the entire lactation for cows that exhibited dystocia reduced by 10.7%, 11.5%, 12.02%, and 12.5% compared with normal cows, respectively. These results are in accordance with those obtained by Lopez de Maturana et al. (²⁰⁰⁷) who showed that dystocia resulted in impaired fertility because it decreased the incidence of success at first insemination by 12%.

The number of service per conception for cows exhibited that dystocia was significantly higher (P < 0.05) compared to normal cows (3.4 vs. 2.7). These results might be due to the lower conception rate as well as the longer service period for cows that exhibited dystocia. These results agreed with those obtained by Lopez de Maturana et al. (²⁰⁰⁷) who found that the effect of dystocia was statistically significant. This fact indicates that 0.5 more inseminations are needed to impregnate the cow in the next reproductive cycle after a difficult calving.

Data in Table 4 showed that lactation period was significantly longer (P < 0.05), while average daily milk yield was significantly lower (P < 0.05) for cows with incidence of dystocia compared to normal cows. However, total milk yield tended to lower for cows with incidence of dystocia compared to normal cows. These might be due to the longer days for cows with incidence of dystocia. These results are in accordance with those obtained by Berry et al. (²⁰⁰⁷) who found that milk yield was less in cows that experienced dystocia at calving compared with those that did not.