The objective of this study was to estimate genetic parameters coupled with genetic and phonotypic trends for age at first calving (AFC) and first and second calving interval (CI1 and CI2) in Iranian Holstein cows. Records of reproduction from 1983 to 2007 for AFC, CI1 and CI2 were 261116, 163234 and 92661, respectively. Single and two trait animal model was used to estimate genetic parameters by restricted maximum likelihood procedures using Average Information algorithm in ASREML software. Estimates of heritability were 0.19±0.005 for AFC, 0.04±0.003 for CI1, and 0.04±0.004 for CI2. Genetic trends for AFC, CI1 and CI2 were -0.75±0.11, 0.004±0.02, -0.02±0.01day per year and phonotypic trends were -2.35±0.38, -1.13±0.39, -0.28±0.23 day per year, for AFC, CI1 and CI2, respectively. The genetic, phonotypic and environmental correlations were -0.049, 0, 0.002 for AFC and CL1, 0.014, -0.004, -0.004 for AFC and CL2, and 0.877, 0.081, 0.043 for CL1 and CL2, respectively.

Introduction: Age at first calving (AFC) has an important effect on profitability and reproductive management of dairy cattle. Every month increase in AFC beyond 24 months increases the cost of production. The time between birth and first calving represents a period in which replacement heifers are not generating income. Instead this rearing period requires considerable capital expenditures including feed, housing, and veterinary expenses. These expenses constitute 15% to 20% of the total expenses related to milk production. A basic approach to reduce this cost is to decrease the time between birth and her first freshening. Worldwide recommendations for one particular AFC might be an incorrect management goal for all of the cattle on all of the farms, since the recommendation might not represent the management goals and/or capabilities of a particular production system or farm. We realize that each dairy has its own set of unique management and environmental conditions, which makes a universal AFC and BW after first calving, a difficult goal to achieve. The AFC has a profound influence on the total cost of raising dairy replacements in which older calving heifers are more expensive to raise than younger ones.Materials and methods: A total of 19499 calving records belonged to 96 herd from 1996 to 2008 were used to estimate genetic components and genetic trend for age at first calving in Holstein dairy cows of Iran. Data were analyzed using a univariate model and Wombat software. Linear regression of estimated breeding values on calving year was used to estimate genetic trend.Results and Discussion: Estimated genetic trend was positive for some years and was negative for others and showed that reducing age at first calving has not been considered in the selection strategies; however, the phenotypic trend was decreased. The age at first calving for Yazd, Markazi, and southern Khorasan provinces were the highest and for Kermanshah, East Azarbayjan, and Ardebil provinces were the lowest compared to the other provinces. Most analysis shows that the financial benefit afforded to heifers that freshen at a low AFC seems to at the least offset any milk lost in the first lactation. The NRC (2001) suggests a post weaning BW equal to 82% of her mature body weight. This can be attained with a maximal pre-pubertal ADG of 2.0 lbs/d if a traditional pre-weaning program is employed or 1.8 lbs/d if an intensified pre-weaning program is employed. Due to the well-defined link between insufficient BW at calving and increased mortality and disease in first calf heifers, attaining this aim post calving BW is of critical importance. Ettema and Santos (2004) conducted an economic analysis of the AFC study that was discussed above. Rearing prices for the medium and high AFC groups were $40.34 and $107.89, respectively, more than that of the low AFC collection. Income for each AFC collection was adjusted for the cost of rearing, assessed feed to increase milk yield, stillbirths, diseases, open days, culling, mortality, labor cost, and the value of milk and calf produced as well as the value of a cow at the end of the 310 day studies. Adjusted income was $119.73 and $9.08 more for the medium and high AFC, respectively, than for the low AFC. These values were not significantly diverse implying no single AFC had an economic benefit over another. Nevertheless, these authors (Ettema and Santos, 2004) did not study the net present value of money in their analysis as St-Pierre (2002) did. If this had been considered, it would presumably shift the economic improvement to the low AFC heifers.Conclusion: Good climatic and weather conditions can be effective factors for reducing the age at first calving and cause to increase the fertility of heifers. However, management methods had a significant effect on this trait in some provinces. The primary benefits of reducing AFC include reducing rearing costs as well as reducing the amount of timein which the heifer is only a capital drain on farm resources. The primary disadvantage of reducing AFC is that it is frequently associated with a reduction in first lactation milk yield. Despite this reduction in first lactation milk yield, production per year of herd life is usually increased by reduced AFC. First lactation may be influenced by AFC, future lactations are definitely not. Furthermore, stay ability and health of cows is not influenced by reduced AFC as long as first calf heifers freshen at an adequate weight.

This study was carried out to investigate the effect of age at first calving (AFC) on calving interval (CI) and dry period length (DP) of dairy cows in semiarid climate of Iran. Records of 57870 first lactation dairy cows were collected from 601 herds during 1995 to 2009 by the Animal Breeding Center of Iran. The analysis was performed using mixed models procedure. Variance components were estimated via restricted maximum likelihood method in a univariate animal model using DMU software. The data were divided in two groups as AFC£26 mo (group 1) and AFC>26 mo (group 2). The average AFC for all data, group 1 and group 2 were 26 (±2.5), 24 (±1) and 28 (±2.5) mo, respectively. The average CI and DP were 375 (±47), 67 (±20) day for group 1 and 380 (±50), 69.5 (±22) day for group 2, respectively. The estimates of heritability of CI and DP were 0.037 (±0.008), 0.022 (±0.006) for group 1 and 0.026 (±0.009), 0.00 for group 2, respectively. Phenotypic trend of CI and DP was negative and significant for two groups (P<0.05). The difference between average estimated breeding values of CI and DP for group 1 were lower than group 2 and significant (P<0.01). The results of this study showed that cows calving at a young age in semiarid climate of Iran had lower CI and DP during their first lactation.

The objective of this study was to estimates the genetic and environmental components for the lactation curve parameters, milk yield, age at first calving (AFC), calving interval (CI) and somatic cell count (SCC) in Iranian Holstein cows. The dataset consisted of 210625 test day records from 25883 cows with milk yield in the first parity recorded from July 2002 to September 2007 in a total of 97 herds in Iran. The lactation curve and the selected lactation parameters were the scaling factor to represent yield at the beginning of lactation (a), the factor associated with the inclining (b) and declining (c) slopes of the lactation curves and the first 100-day milk yield, second 100-day milk yield, third 100-day milk yield, peak yield (Ymax), days in milk at peak yield (b/c), persistency (s), lactation length (LL) and the 305-day milk yield. The incomplete gamma function (Wood function) was used to estimate lactation curve and lactation parameters from daily milk records. Among the 100-day milk yield periods, the second 100-day milk yield had the highest herita-bility (0.29±0.024) and the highest genetic correlation with the 305-day milk yield (0.996±0.00). Lactation curve parameters had low h2 (0.017±0.007 to 0.051±0.011). The b / c had a relatively high genetic correla-tion with the 305-day milk yield (0.52±0.08), a moderate genetic correlation with CI (0.32±0.14) and nega-tive genetic correlations with measures of somatic cell count. This suggested that b / c could be used as a criterion to improve 305-day milk yield and resistance to subclinical mastitis.

The objective of this study was to estimate genetic parameters coupled with genetic and phonotypic trends for age at first calving (AFC), open days (OD) and milk yield (MY) of first lactation in Iranian Holstein cows. Records of reproduction and production for genetic evaluation from 1984 to 2005 for AFC, OD and MY were 71736, 20126 and 20126, respectively. Single and two-variable animal models were used to estimate genetic parameters by restricted maximum likelihood procedures and WOMBAT software. heritability of AFC, OD and MY was estimated 0.20±0.02, 0.04±0.01, 0.47±0.9 respectively. Genetic and phonotypic trends of AFC were negative so that phenotype and breeding value average had decreased from 1984 to 2005. Genetic and phonotypic trends of OD and MY were positive. The genetic and phonotypic correlations between OD and MY were +0.23 and +0.07 respectively.

For the current research the records of 62 farms in Isfahan province were available. To estimate genetic and phonotypic trends of reproductive traits such as age at first calving, calving interval, days open and number of insemination to conception rate the records of 63, 866 cows including 167793 reproductive records were employed. Farms were covered by Vahdat industrial Agriculturists and Dairymen Cooperative of Isfahan during 1986 to 2012. Genetic parameters estimated by linear animal model using Restricted Maximum Likelihood Method (REML). Genetic and phonotype trends were estimated by using regression of average breeding values and phonotype values on calving year. Heritability of age at first calving (0.19±0.007), calving interval (0.06±0.005), days open (0.041±0.004) and number of insemination to conception (0.071±0.004) were estimated. Phonotypic trends of age at first calving, calving interval, days open, number of insemination to conception in first parity and number of insemination to conception in second parity onward were estimated to be -3.48±1.08, -0.41±0.4, -0.42±0.2, 0.013±0.0025 and 0.031±0.01 respectively. Estimated genetic trends for age at first calving, calving interval, days open, for number of insemination to conception in first parity and number of insemination to conception in second parity onward were estimated to be 0.07±0.06, -0.56±0.5, -0.02±0.06, 0.0015±0.001 and 0.0005±0.005, respectively.

Retrospective data from Sebele Dairy Farm were analysed for reproductive performance of Friesian/Holstein cows. Three hundred records from 1989 to 1995 were used. The herd was zero grazed and the data was analysed for age at first calving (AFC), calf birth weights (BW), calving interval (CI), days open (DO), calving rate (CR), reproductive wastage (RW) and calving difficulty (CD); dystocia, after-birth retention and misrepresentation of foetus) and calving weights (CW). The effects of year of calving (YOC) and year of birth (YOB) were tested. The year of calving affected age at first calving (P<0.05) whereby in 1989/90 and 1990/91 cows were younger than the rest of the years. Calving weight at first calving was significantly affected (P<0.01) by year of calving. YOC had an effect (P<0.05) on both calving interval and days open. CW and CR were significantly affected (P<0.001 and more) by the year of calving. Reproductive wastage tended (P=0.065) to be affected by YOC while no effect (P>0.05) was observed for BW and CD. The year of birth of cows (YOB) had an effect (P<0.05) on age at first calving and it also significantly affected (P<0.01) CW of first calving heifers. Heifers born in the 1980’s had lower live weight at calving than those born in the 1990’s. Age categories (1=20–39 months; 2=40–69 months and 3=>70 months) tended (P=0.062) to influence the birth weights of calves. As cows’ age increased, birth weights of their calves decreased. However, age category did not affect (P>0.05) calving interval or days open. Male calves were heavier (P<0.05) at birth than females (38.7±1.3 kg vs.36.8±0.7 kg). CW was lower for heifers than for cows and represented 84% of the cows’ weights. Heifers had higher (P<0.01) CR than cows (100 vs.64.9%), however RW was higher (P<0.01) for heifers than cows (25.0 vs.11.0%). RW was significantly correlated with birth difficulty (r=0.50; P<0.001). It seems that a low temperature-humidity index in the month of conception resulted in higher CR nine months later. It is recommended that when cows are zero grazed, feed quality should be assured by nutrient balancing and should not affect performance between years.

In this study 700 records from 501 Holstein heifers (born during 2006-2012) were used to investigate of reproductive performance. The linear model was employed for Analysis of Age at First Services (AFS), Age at First Calving (AFC) and Interval from First to Last services (IFL) and the Generalized Linear Model for Conception Rate (CR). Results showed that the herd on CR was significant but the difference of other traits were not significant among herds. Conception rate least square means were not significant among different seasons and service numbers. Season of birth had significant effect on AFC so that heifers born during winter had lower AFC than summer born heifers. Comparison of lest square means for reproductive parameters in the studied herds with optimum levels showed that the AFS, AFC and IFL in most herds were higher than optimum (p<0.05). Conception rate was suitable in all herds and even in one herd was higher than optimum that shows using high quality sperms and ideal estrus detection.