Introduction

Deterioration in cow reproductive traits has become a major problem in dairy cattle production. The negative association between production and reproduction has been reported in several dairy breeds of cattle (Pryce and Veerkamp 2001; Chagas et al. 2007). Age at first calving and calving interval are among measures which could be used to describe the reproduction efficiency of a cow. These parameters are influenced extensively by dairy management practices and impact the economic outcome of the dairy enterprise (Van Raden and Klaaskate 1993; Grohn and Rajala-Schultz 2000). There are several studies which examined the trends in reproductive performance including calving age and calving interval. Hare et al. (2006) reported that ages at first calving declined over time for 5 breeds of US dairy cattle. Decreasing ages at first calving for dairy population was reported by another study from Spain (Gonzalez-Recio et al. 2004).

De Vries and Risco (2005) determined the trends and seasonality of reproductive performance in Georgia and Florida dairy herds and concluded that average calving interval increased from 399 days in 1976 to 429 days in 2000. In another study, trends in reproductive performance in 1772 Ohio dairy herds from 1992 to 1998 revealed that herd average calving interval lengthened from 13.6 to 14.1 months (Rajala-Schultz and Frazer 2003). In contrast Refsdal (2007) indicated that average calving interval has varied between 12.4 and 12.6 months in Norwegian cattle from 1985 to 2005 without showing a specific trend.

In Iran during the last decades dairy industry has expanded progressively and the efforts have been to improve production and reproduction of dairy population. Fars province is one of the important dairy-production areas of the country. Most of the commercial dairy herds in the province are under registration of a dairy herd improvement program by Agricultural Jihad Organization (AJO), Ministry of Agriculture. The objective of this study was to determine the trends in calving ages and calving interval as important indicators of reproductive performance from 2000 to 2005 for registered dairy herds in the province.

Materials and methods

Study population and study design

This is a retrospective study which was conducted in Fars province, southern Iran. Target population consisted of all dairy cows which were under registration of the dairy herd improvement program, by AJO of the province. They are Holsteins and crossbred cows (Holsteins-Indigenous) with varying degree of purity, overall known as Iranian Holsteins and have non-seasonal calving programs. A convenient sample of 12 registered dairy herds (about 7% of all herds) was selected as study population. In the study herds, cows were artificially inseminated as a routine and artificial insemination (AI) was done by a trained herd owner or an AI technician. Cows were housed in open-shed barns and milked three times per day. Their rations were based primarily on corn silage, alfalfa hay and some concentrates with ground barley as the main energy source. They had veterinary consultants for disease management and nutrition.

All cows which calved between 21 March 2004 to 20 March 2005 were selected for the study. Overall 1437 cows from different parity groups were included and their birth dates and calving dates were retrieved retrospectively from database of the AJO. All data used in this study had been recorded in real-time by AJO staff. Calving interval was calculated as the difference between calving dates from succeeding parities. Calving interval was restricted to 260 to 750 days. Calving age was calculated as the difference between birth date and calving date. Records for a cow was excluded if calving age was <16 mo or >180 mo. Due to low number, sixth or higher calving records were removed from the analysis. Finally, records for 1134 cows were included in the statistical model.

Statistical analysis

Statistical analysis was conducted using SAS, 9.1 software (SAS institute Inc, Cary, NC, USA). Descriptive statistics were calculated as means and standard deviations. To determine the time trend in calving interval (3022 records) and calving age (4231 records), proc-mixed procedure was used. To account for clustering of data and for appropriate handling of missing and unbalanced data, herds were considered as repeated effect in the models. A variance component covariance structure was chosen to fit the model according to Akaike information criterion (Littell et al. 1998). The effect of year, season of calving, and parity on calving interval was evaluated with estimation of regression coefficients in the mixed model. Season of calving was dichotomized to warm (Apr-Sep) and cold (Oct-Mar) months. Year and parity treated as continuous variables. Interaction term for year by season and for year by parity was included in the model.

To demonstrate the time trend in calving ages at first, second, third, fourth and fifth calving, analysis was conducted for each calving separately and first-order autoregressive covariance structure was specified according to Akaike information criterion (Littell et al. 1998). The P-value less than 0.05 was considered as statistically significant in all analysis.

Results

Summary statistics for calving interval according to year, season of calving, and parity are shown in Table 1. The time trend for calving interval by parity is presented in Fig. 1. The results of regression analysis indicated that calving interval decreased significantly in the study period; season of calving had significant effect on calving interval whereas parity had no significant effect in the model (Table 2). Overall calving interval was 435 days in 2000 and declined to 389 days in 2004. Cows which calved during warm months (Apr-Sep) had significantly longer calving interval than cows with calving in cold months (Oct-Mar).

Fig. 1
figure 1

Trends in calving interval by parity and calving year for dairy cows in Fars province, southern Iran during 2000–2004. Parity 1 (♦), parity 2 (■), parity 3 (▲), parity 4 (x), parity 5 (*)

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Table 1 Summary statistics for calving interval by parity, season and calving year for dairy cows in Fars province, southern Iran during 2000–2004
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Table 2 Effect of year, parity and season of calving on calving interval for 3022 calving in Fars province, southern Iran during 2000–2004
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Summary statistics for calving age are shown in Table 3. Trends in ages at first and second calving are shown in Figs. 2 and 3, respectively. Summary results for regression analysis are shown in Table 4. Age at first calving was decreased significantly from 30 months in 2000 to 26 months in 2005. The corresponding measures for age at second calving were 44 and 41 months, respectively. The decline was observed for third calving too; however, ages at calving for higher parities showed opposite trends (Table 3). The increasing trend was significant for age at fifth calving; it was 81 months in 2000 and reached to 84 months in 2005.

Fig. 2
figure 2

Trends in age at first calving for dairy cows in Fars province, southern Iran during 2000–2005

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Fig. 3
figure 3

Trends in age at second calving for dairy cows in Fars province, southern Iran during 2000–2005

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Table 3 Summary statistics for calving age (months) by parity and calving year for dairy cows in Fars province, southern Iran during 2000–2005
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Table 4 Effect of year of calving on calving age for 4231 calving in Fars province, southern Iran during 2000–2005
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Discussion

Increases in calving intervals over time for dairy cows have been reported in many countries (Gonzalez-Recio et al. 2004; Wall et al. 2003; Lucy 2001; Hare et al. 2006). However, in Florida dairy herds, no detectable trends occurred during a 52 years period (1923–1974) in calving interval and overall mean calving interval for this period was 401 days (Silva et al. 1992). Also, Refsdal (2007) reported that calving interval has been fairly constant between 12.4 to 12.6 months (372 to 378 days) during 1985–2005 in Norwegian cattle.

In the present study, there was a significant decrease in calving interval during the study period (Fig. 1) and the increasing trends which generally reported from other countries was not observed. In many studies, days open were evaluated in addition to calving intervals. We did not have data about days open in the study population. However, days open and calving interval would be expected to be highly correlated, as they only differ by the gestation period which is very similar for most cows. Therefore, it could be reasonable to think that days open probably had a similar decreasing trend in our population.

Several explanations could be provided for the observation of decreasing trend in calving interval. Foremost it should be noted that the baseline calving interval in the present study (430 days) is much higher than the corresponding features in above mentioned studies (Silva et al. 1992; Rajala-Schultz and Frazer 2003; De Vries and Risco 2005; Hare et al. 2006; Refsdal 2007). With decreasing trend for calving interval, study herds reached near baseline measure for other countries with some delay.

In other countries, increasing calving interval as a measure of the deterioration of reproductive performance has been attributed to large genetic trends for milk yield. The genetic trend for milk in Fars province during 2001–2005 is not large and shows that average breeding value has increased from ~ -350 kg in 2001 to ~ -20 kg in 2004 (Ministry of Agriculture, http://www.abc.org.ir/). On the other hand, efforts of AJO for improving management practices and better nutrition by education of producers intensified during the recent decades. Taken together, calving interval may have been improved in response to the efforts of AJO; and it is expected that the decline in fertility which was observed in other countries (due to negative genetic relationship between milk and fertility) would appear with some delay in Iranian Holsteins in study herds, too.

Another factor that might contribute to reduced reproductive performance is inbreeding. In contrast, crossbreeding has been shown to increase herd life, perhaps due, in part, to improvement in reproductive performance (Lucy 2007). Gradual increase in the average inbreeding coefficient was reported for Holsteins breed in the United States (http://www.aipl.arsusda.gov/). As noted above, study herds in the present study are known as Iranian Holsteins, they are crossbreed of Holstein breed with indigenous cows. The gradual increase in the average inbreeding coefficient has not been experienced by Iranian Holsteins, yet. Future studies are required to examine these possibilities.

Finally, we selected a convenience sample of 12 registered dairy herds, included all cows that had calved during 2004–2005 and followed them retrospectively for their previous calving interval. The possibility of selection bias in this limited number of herds could not be excluded and generalization of the observed trend to the target population should be done with caution. There is also the possibility that some of the poor performance cows may have calved again after the data cutoff for the study. If that happened, the mean we found might be biased down.

In the present study there was significant effect of season of calving on subsequent reproduction as measured by calving interval. Cows that calved in the spring and summer exhibited the longest calving interval (420 days) compared with those that calved in cold months (411 days). Significant seasonality in reproductive performance has been reported from southeastern United States (Al-Katanani et al. 1999) and Spain (Lopez-Gatius, 2003) and some other studies (De Vries and Risco 2005; Washburn et al. 2002; Silva et al. 1992; Ray et al. 1992). In Spain, Lopez-Gatius found significant decrease in conception rate during warmer summer months compared with cooler winter months between 1991 and 2000 in 4 herds (Lopez-Gatius 2003). De Vries and Risco (2005) found that spring-calving cows had the largest average days to conception whereas fall-calving cows had the fewest. These are consistent with the results of the present study. Reduction of fertility may be largely attributed to heat stress during warmer months. Considering these findings, delayed breeding to have most calving in fall and winter may be economically advantageous (Grohn and Rajala-Schultz 2000).

The mean age for first calving (28 months) is generally higher than what was reported by Hare et al. (2006) for US Holsteins breed (26.9 months). However, the trend is relatively consistent with results obtained by Hare et al. (2006) who reported calving at progressively younger ages with less decline for higher parities in US dairy cattle during 1980–2004. Similar result for reduction of age at first calving was found in Spain (Gonzalez-Recio et al. 2004). As suggested by Hare et al. (2006), the decrease in age at first calving might represent earlier maturity from better calf-raising practices.

In conclusion, reproductive performance as measured by calving interval and age at calving revealed relatively positive trends in the Iranian Holsteins in Fars province during 2000–2005. This could be attributed, at least partly to better management and nutrition practices during this period and also to the fact that large genetic trend for milk which has been observed in countries with decreasing reproductive performance has not occurred in these herds yet.