Genetic and Phenotypic Trends of Productive and Reproductive Traits in Iranian Holstein Dairy Cattle of Isfahan Province

Document Type: Research Articles

Authors

1 Department of Animal Science, Faculty of Agriculture, Shahid Bahonar University of Kerman, Kerman, Iran

2 Department of Animal Science, Faculty of Agriculture, University of Jiroft, Jiroft, Iran

Abstract

Productive and reproductive traits have great importance in international dairy cattle breeding objectives. Production, reproduction (days open and calving interval) and pedigree data of Iranian Holstein dairy cattles were collected between 2002 and 2012 from 47 herds in Isfahan province. The data set consisted of 30179 records from 18837 registered IranianHolstein dairy cattles. Single trait repeatability animal model was used to estimate genetic parameters by restricted maximum likelihood procedures. Estimates of heritability of milk, fat and protein yields were 0.30 ± 0.014, 0.065 ± 0.0089 and 0.083 ± 0.0098 respectively and also these value for calving interval (CI) and days open (DO) were 0.074 ± 0.0094 and 0.023 ± 0.0057 respectively. Low heritability for reproductive traits indicates the importance of using available relatives’ information for selection of these traits. Average increase in breeding value for milk, fat and protein over the 11 years period were 130, 0.22 and 0.09 kg/year, respectively. The means of breeding value of DO increased by 0.08 d/year from -0.17 d in 2002 to 0.58 d in 2012 and for CI decreased irregularly by -0.01 d/year from 0.05 d in 2002 to -0.03 d in 2012. In general, all traits showed negative phenotypic trends for the studied period.

Keywords


INTRODUCTION

In dairy farms in Iran, producer revenue and costs depend on the production level and number of animals raised to marketing. Reproduction traits are a source for animals breeding and high production, thus cow productivity has more contribution in the revenue and costs of dairy farms. Therefore, cow productivity has been identified as a key factor affecting the efficiency and economic viability of the dairy cattle industry. Reproductive failure causes economic losses due to reduced production as a result of prolonged calving intervals (Olori et al. 2002). Breeding programs for dairy cattle in Iran have been based primarily on increasing both production traits and reproduction performance. Reproductive efficiency of a cow is measured by factors such as age at first calving, calving interval, days open and number of services per conception (Nilfrorooshan et al. 2004). Calving intervals (CI) reflect the periods that a cow reproduces again (Hare et al. 2006). Days open (DO) is calving interval to pregnancy (time from calving to conception). Reproductive traits have been neglected in the past in most dairy cattle genetic improvement programs worldwide, mainly because these traits are known to exhibit a low heritability (Kadarmideen, 2004). Most breeding programs give more weight to yield traits than the reproductive performance in selection indices. Use of these programs has caused to genetic improvement in yield and depression in reproductive traits (Faraji-Arough et al. 2011). Breeding program for increasing production in dairy cattle has negative side effects on fertility traits (Pryce et al. 1997; Roxström et al. 2001). Selection for higher production under one management system may lead to more health risks compared to other management systems. Therefore, management and genetics have to be integrated to develop an effective program for improvement of health and fertility in dairy cattle (Windig et al. 2006). Reproduction problems make economic losses in two ways: first, the production loss as a result of prolonged CI; second, increase of replacement costs because of fewer calves per cow (Boichard et al. 1998; Olori et al. 2002). Genetically, most studies of the association between milk yield and reproductive measures in dairy cattle showed a negative relationship among them (Jones et al. 1994; Shook, 1989; Van Dorp et al. 1998). Higher milk yield per lactation has been associated with more DO (Berger et al. 1981; Hansen et al. 1983) and longer CI. Estimating genetic and phenotypic trends in a population provide the animal breeders with essential information to develop more successful breeding programs and also to assess the effectiveness of their selection procedure. Despite remarkable changes in selection indices in different countries (Interbull, 2000) in the last fifteen years, the main emphasis in selecting bulls and cows in Iran is based on estimated breeding value of milk yield (Sadeghi-Sefidmazgi et al. 2009). Although several investigations have been carried out in Iranian Holstein cows on the genetic trend of 305-day milk yield (Razmkabir, 2005; Sahebhonar, 2007; Khorshidie et al. 2012) and one research on the genetic trend of persistency of milk yield (Khorshidie et al. 2012), but the genetic trends for CI and DO have not yet been appropriately evaluated. The primary objective of this study was to estimate the genetic parameters of productive (milk, fat and protein) and reproductive (CI and DO) traits. The secondary objective was to calculate breeding values and to demonstrate the genetic and phenotypic trend of these traits.

 

MATERIALS AND METHODS

Data

In this study, data were provided by the Vahdat corporation in the Isfahan province. The data were comprised of production, reproduction and pedigree databases of Iranian Holstein cattle calved in the Isfahan province between 2002 and 2012 in 47 herds. The production traits were consisted of milk, fat and protein yields and reproduction traits were CI and DO. The initial data set included 37075 lactation records. Records were edited to include only those cattle with a valid identification, valid sire and dam registration number. Records with unknown birth and calving dates were deleted. Age at calving with in each lactation was restricted by removing outlier data, using the following ranges, as used by Mostert et al. (2006): 20-42 months for lactation 1, 30-54 months for lactation 2, 40-67 months for lactation 3, 50-75 months for lactation 4 and 60-90 months for lactation 5. Records with CI less than 300 days or greater than 600 days and days open less than 30 days or greater than 300 days were also discarded. The final data set comprised 30179 lactation records of 18837 animals from 47 herds, 1224 sires and 13957 dams. The descriptive statistics for the traits described are in Table 1.

 

Statistical model

Statistical analyses of all traits considered in this study were performed using single trait repeatability animal models in ASREML (Gilmour et al. 2006). Variance and covariance estimates obtained from these analyses were used to estimate heritability. The following statistical models were applied to obtain variance components:

Model 1 (for milk, fat and protein yields):

 1

Where:

Yijkmns and Yijklmns: observation of 305-d milk, fat, protein yields (kg) or DO and CI respectively.

µ: overall population mean.

Li: fixed effect of lactation yield (i=1, . . . ,5).

HYSj: fixed effect of herd-year-season of calving.

HYSk: fixed effect of herd-year-season of insemination.

an: age at calving fixed effect coefficient.

bn : DO fixed effect coefficient.

cn: days in milk fixed effect coefficient.

αm: additive genetic random effect.

βn: permanent environmental random effect.

eijkmns and eijklmns: residual random effect.

 

RESULTS AND DISCUSSION

Means

Selection for milk yield in dairy cattle is mostly made on the basis of 305-d milk yield (Bilal et al. 2008; Seyedsharifi et al. 2008; Bilal and Khan, 2009).In this study, means ofmilk, fat and protein yields were 11779.65, 303.9 and 286 kg/yr respectively. Mean of milk yield for Holsteins dairy cattle of Khorasan Razavi province in Iran for first, second and third parity were 7126.27, 7985.86, 8246.53 kg, respectively and for fat yield 305-d were 230.61, 252.46 and 259.86 kg, respectively (Teimurian, 2009). Samore et al. (2008) reported mean of protein 269, 296 and 300 kg for first, second and third parity, respectively. Means of CI and DO were 386.8 and 108.3 days, respectively. Estrada-Leon et al. (2008) in Mexico, Hammoud et al. (2010) in Egypt, Shirmoradi et al. (2010) in Iran, Ghiasi et al. (2011) in Iran and Gunawan et al. (2011) in Indonesia, reported mean of CI 453.9, 393.8 and 360.9 days in Brown swiss, Iranian Holsteins and Bali dairy cattles, respectively. Mean of DO in this research was in agreement with results of Iranian Holstein cattle (Ghiasi et al. 2011; Deljoo Isaloo et al. 2012) but less than the mean of DO in Friesian cattle (130.7) (Hammoud et al. 2010). Mean of DO for US Holstein cattle was 113 days (Oseni et al. 2004). Hultgren and Svensson (2010), Makgahlela et al. (2008), Farhangfar and Naeemipour Younesi (2007), Chookani et al. (2010) and Hare et al. (2006) reported mean of first calving interval in Jerseys (390 day), Ayrshire (398 d), Holsteins (404 d), Guernsey (406 d) and Brown swiss (407 d). Ansari-Lari and Kafi (2010) using 8204 calving records in five herds, reported that mean of DO and CI in Holstein cows in Fars province were 134 and 403 d, respectively.

 

Variance component

Estimates of the variance components are shown in Table 2. Estimates of additive genetic variance for all traits were less than residual variance. In reproductive traits the residual variance effects comprised of a large proportion of the total variation, therefore heritability estimates for these traits were low. Most previous research concluded that additive genetic variation for reproductive traits is very low in proportion to phenotypic variation, which lead to heritabilities for those to be close to zero and selection for improving of these traits would not be worthwhile (Berger et al. 1981; Hansen et al. 1983).

 

Heritability and repeatability

The rate of genetic gain that could be made by selection is depends on the heritability of the trait, therefore high heritability of traits is an important index for response to selection. Estimation of heritability and repeatability of the traits is an essential genetic parameter which is required for animal breeding programs. Lower heritability of fat yield indicates that this trait greatly influenced by environmental conditions. In this research heritability estimated for milk, fat and protein yields were 0.305 ± 0.014, 0.065 ± 0.009 and 0.083 ± 0.098, respectively and for CI and DO were 0.074 ± 0.009 and 0.023 ± 0.006, respectively. Heritability estimated in these research for milk yield was in agreement with the results obtained Ojango and Pollott (2001) in Holstein Friesian cattle (0.30±0.04) and Gebreyohannes et al. (2013). Also, these estimates were in agreement with the results obtained by Mostert et al. (2010) for CI and Haile et al. (2009) for DO. These estimates for heritability of CI and DO are higher than the results obtained by Estrada-Leon et al. (2008) and Gunawan et al. (2011). Misztal et al. (1992) obtained much higher heritability estimates than those in the present study of 0.26, 0.149 and 0.238, for yields of milk, fat, and protein, respectively. Visscher and Thompson (1992), also reported higher heritability estimates for yields of milk and fat of 0.39 and 0.36, in British cattle respectively. The heritability estimate obtained for CI was slightly larger than that obtained by Ojango and Pollott (2001) for first and first three parities (0.06±0.02 and 0.05±0.02, respectively). Heritability estimates by Haile et al. (2009) for DO in Ethiopian Boran cattle and their crosses with Holstein Friesian in central Ethiopia were 0.047 and 0.1, respectively. Estimates of heritability for CI were lower than those value obtained by Makgahlela et al. (2008) for South African cattle, Gressler et al. (2005) for Nellore in Brazil, Farhangfar and Naeemipour Younesi (2007) and Chookani et al. (2010) for Iranian Holstein, but higher than Wasike et al. (2009) for Boran cattle in Kenya. Vergara et al. (2009), Toghiani Pozveh and Shadparvar (2009) and Ghiasi et al. (2011) showed that heritability for CI was between 0.11 and 0.18. Different estimated values for heritability of production and reproduction traits in this research compared to above mentioned authors could be due to several factors such as: animal breed, management system, environmental factors, size and structure of data, model of analyses, and statistical methods employed. Low estimates of heritability for CI and DO indicated that these traits might be greatly influenced by environmental conditions. Therefore, improvements in nutrition and reproductive management would likely have a larger impact on reducing CI and DO than the genetic selection (Vergara et al. 2009). Also, due to the low heritability of reproduction traits, selection for improving these traits in dairy cattle would not worthwhile (Kadarmideen, 2004; Makgahlela et al. 2008). Therefore there is a genetically improvement potential for these traits through selection, and this could be achieved by increasing the amount of information used in genetic evaluation (e.g., using information offspring). In this research productive traits had higher repeatability than reproductive traits. Repeatability estimates were 0.592 ± 0.006, 0.329 ± 0.009 and 0.328 ± 0.009 for milk, fat and protein, respectively.

 

Table 1 Summary statistics for productive and reproductive traits

2

CI: calving interval and DO: days open.

SD: standard deviation.

 

Table 2 Variance components estimates for production and reproduction traits

3

CI: calving interval and DO: days open.

VP: phenotypic variance;  VA: additive genetic variance; VEP: animal permanent environmental variance and Ve: error variance.

 

For reproductive traits, repeatability was 0.215 ± 0.009 and 0.052 ± 0.009 for CI and DO traits, respectively.

 

Trends in estimated breeding values and phenotypic

Comparison of genetic and phenotypic trends helps to assess genetic improvement which due to superior performance. Over the 11-years period, the average increase in mean breeding value for milk, fat and protein were 130, 0.22 and 0.09 kg/yr, respectively. Genetic trends for CI and DO have irregular trends as some years are positive while the other years are negative. The trends of these traits showed that selection for decreasing CI has not been performed in Iranian Holstein cows. This could be due to the fact that selection was only focused on production traits, and there were no selection on reproduction traits in breeding program of Iranian Holstein cows. In reproductive traits, an increase observed in the mean breeding value for DO and a decrease in CI at the rate of 0.08 and -0.01 d/yr, respectively. CI and DO decreased from 398.17 and 115.42 d in 2002 to 341.85 and 58.25 d in 2012 (Figures 4 and 5). The reason for declining phenotypic trend of CI and DO in current study might be due to increasing calf price relative to milk price in recent years that encourages farmers to reduce waiting period and open days after calving. Mean breeding values and phenotypic trends in productive and reproductive traits by year of birth were plotted against time (Figures 1-5). In general, all traits showed negative phenotypic trends for the studied period. Negative trend for milk production in whole period was due to a negative trend for last trend (2011 to 2012). Deljoo Isaloo et al. (2012) analyzed over 10891 records collected from 23 years (1988 to 2010) from animal breeding center of Iran.

 

Figure 1 Genetic and phenotypic trend of milk yield (kg)

 3

 

Figure 2 Genetic and phenotypic trend of fat yield (kg)

3 

 

Figure 3 Genetic and phenotypic trend of protein yield (kg)

4 

 

Figure 4 Genetic and phenotypic trend of calving interval (day)

4 

 

Figure 5 Genetic and phenotypic trend of days open (day)

5

 

Genetic trends for DO and CI for Iranian Holstein cattle were -0.002 and - 0.04, and phonotypic trends were 0.006 and -0.04, respectively. Ansari-Lari et al. (2009) reported a decrease in DO from 435 days in 2000 to 389 days in 2005 in Iranian Holstein cattle in Fars province. Also, Deljoo Isaloo et al. (2012) reported a negative trend for these traits in Iranian Holstein cattle.

 

CONCLUSION

In general, results of this investigation show that genetic improvement for production traits was at an acceptable level in Holstein cattle of Iran. However breeding programs to increase genetic merit of milk volume were more effective than milk components. Also indicated that phenotypic trends of reproduction traits are improving in Iranian Holstein dairy cattle. Specific causes of such improve in reproductive performance are not clear, but could be due to improvements in nutrition and reproductive management. Low heritability and repeatability in some productive (fat and protein) and reproductive traits have been deprived of improve in the phenotypic trends during some periods which may be partly attributed to responses to direct and indirect genetic selection. Also, genetic selection may not always yield substantial additive gains. However, because of economic importance of productive and reproductive traits an attempt should always be made to keep these traits at their optimum.

 

ACKNOWLEDGEMENT

This paper is adapted from MS Thesis of Z. Aghajari  (supervisors; Dr. Ahmad Ayatollahi Mehrgardi). Authors would like to thanks the Animal Science Department of Shahid Bahonar University of Kerman for the support during doing this research.

Ansari-Lari M., Rezagholi M. and Reiszadeh M. (2009). Trends in calving age and calving intervals for Iranian Holstein in Fars province, southern Iran. Trop. Anim. Health Prod. 41, 1283-1288.

Ansari-Lari M., Kafi M., Sokhtanlo M. and Nategh Ahmadi M.H. 2010. Reproductive performance of Holstein dairy cows in Iran. Trop. Anim. Health Prod. 42, 1277-1283.

Berger P.J., Shanks R.D., Freeman A.E. and Laben R.C. (1981). Genetic aspects of milk yield and reproductive performance. J. Dairy Sci. 64, 114-122.

Bilal G., Khan M.S., Bajwa I.R. and Shafiq M. (2008). Genetic control of test-day milk yield in Sahiwal cattle. Pakistan Vet. J. 28(1), 21-24.

Bilal G. and Khan M.S. (2009). Use of test-day milk yield for genetic evaluation in dairy cattle: a review. Pakistan Vet. J. 29(1), 35-41.

Boichard D., Barbart A. and Briend M. (1998). Genetic evaluation for female fertility in French dairy cattle. Interbull. Bull. 18, 99-101.

Chookani A., Dadpasand M., Mirzaei H.R., Rokouii M. and Sayad Nezhad M.B. (2010). An estimation of genetic parameters for some reproductive traits and their relationships to milk yield in Iranian Holstein cattle. Iranian J. Anim. Sci. 40(4), 53-61.

Deljoo-Isaloo H.A. and Pasha-Eskandari-Nasab M. (2012). The estimation genetic and environmental parameters and genetic and phenotype and genetic trend for reproduction traits of Holstein cattle was Khoramdare culture technology. J. Anim. Sci. 92, 52-58.

Estrada-Leon R.J., Magana J.G. and Segura Correa J.C. (2008). Genetic parameter for reproductive trait of Brown swiss cow in the tropic of Mexico. J. Anim. Vet. Adv. 7, 124-129.

Faraji-Arough H., Aslaminjad A.A. and Farhangfar H. (2011). Estimation of genetic parameter and trends for age at first calving and calving interval in Iranian Holstein cattle. J. Agric. Sci. 7, 79-87.

Farhangfar H. and Naeemipour Younesi H. (2007). Estimation of genetic and phenotypic parameters for production and reproduction traits in Iranian Holsteins. J. Sci. Technol. Agric. Nat. Res. Water Soil. Sci. 11(1), 431-441.

Ghiasi H., Pakdel A., Nejati Javaremi A., Mehrabani Yeganeh H., Honarvar M., Gonzalez-Recio O., Jesus Carabano M. and Alenda R. (2011). Genetic variance components for female fertility in Iranian Holstein cattle. Livest. Sci. 139(3), 277-280.

Gilmour A.R., Gogel B.J., Cullis B.R. and Thompson R. (2006). ASReml User Guide Release 3.0. VSN International Ltd, Hemel Hempstead, HP1 1ES, UK.

Gressler M.G.M., Pereira J.C.C., Bergmann J.A.G., Andrade V.J., Paulino M.F. and Gressler S.L. (2005). Genetic aspects of weaning weight and some reproductive traits in Nellore cattle. Arq. Bras. Med. Vet. Zootec. 57, 533-538.

Gunawan A., Sari R., Parwoto Y. and Uddin M.J. (2011). Non genetic factors effect on reproductive performance and pre weaning mortality from artificially and naturally breed of Bali cattle. J. Indonesian Trop. Anim. Agric. 36, 1-8.

Haile A., Joshi B.K., Ayalew W., Tegegne A. and Singh A. (2009). Genetic evaluation of Ethiopian Boran cattle and their crosses with Holstein Friesian in central Ethiopia: reproductive traits. J. Agric. Sci. 147, 81-89.

Hammoud M.A. and EI-Zarkouny S.Z. (2010). Effect of sire, age at first calving, season and year of calving and parity on reproductive performance of Friesian cattle under semiarid conditions in Egypt. Arch. Zootech. 13(1), 60-82.

Hansen L.B., Freeman A.E. and Berger P.J. (1983). Association of heifer fertility with cow fertility and yield in dairy cattle. J. Dairy Sci. 66, 306-314.

Hare E. and Norman H.D. (2006). Trends in calving ages and calving intervals for dairy cattle breeds in the United States. J. Dairy Sci. 89, 365-370.

Hultgren J. and Svensson C. (2010). Calving interval in dairy cows in relation to Heifer rearing conditions in southwest Sweden. Reprod. Domest. Anim. 45, 136-141.

Interbull. (2000). National Genetic Evaluation Programmers for Dairy Production Traits Practiced in Interbull Member Countries 1999-2000. Department of Animal Breeding and Genetics, Uppsala, Sweden, Bulletin 24.

Jones W.P. and Hansen L.B. (1994). Response of health care to selection for milk yield of dairy cattle. J. Dairy Sci. 77, 3137-3152.

Kadarmideen H.N. (2004). Genetic correlations among body condition score, somatic cell score, milk production, fertility and conformation traits in dairy cattle. Anim. Sci. 79, 191-201.

Khorshidie R., Shadparvar A.A., Ghavi Hossein-Zadeh N. and Joezy Shakalgurabi S. (2012). Genetic trends for 305-day milk yield and persistency in Iranian Holsteins. Livest. Sci. 144, 211-217.

Makgahlela M.L., Banga C.B., Norris D., Dzama K. and Ngambi J.W. (2008). Genetic analysis of age at first calving and calving interval in south African Holstein cattle. Asian J. Anim. Vet. Adv. 3(4), 197-205.

MisztalI., Lawlor T.J., Short T.H. and VanRaden P.M. (1992). Multiple-trait estimation of variance components of yield and type traits using an animal model. J. Dairy Sci. 75, 544-551.

Mostert B.E., Theron H.E. and Kanfer F.H.J. (2006). Test-day model for south African for participation in the international evaluations. South African J. Anim. Sci. 36, 58-70.

Mostert R.B., Van der Westhuizen R. and Theron R. (2010). Calving interval genetic parameters and trends for dairy breeds in south Africa. J. Anim. Sci. 40, 156-162.

Nilfrorooshan M.A. and Edriss M.A. (2004). Effect of age at first calving on some productive and longevity traits in Iranian of the Isfahan province. J. Dairy Sci. 87, 2130-2135.

Ojango J.M.K. and Pollot G.E. (2001). Genetics of milk yield and fertility traits in Holstein-Friesian cattle on large scale Kenyan farms. J. Dairy Sci. 79, 1742-1750.

Olori V.E., Meuwissen T.H.E. and Veerkamp R.F. (2002). Calving interval and survival breeding values as a measure of cow fertility in a pasture-based production system with seasonal calving. J. Dairy Sci. 85, 689-696.

Oseni S., Tsuruta S., Misztal I. and Rekaya R. (2004). Genetic parameters for days open and pregnancy rates in US Holsteins using different editing criteria. J. Dairy Sci. 87, 4327-4333.

Pryce J.E., Veerkamp R.F., Thompson R., Hill W.G. and Simm G. (1997). Genetic aspects of common health disorders and measures of fertility in Holstein Frisian dairy cattle. J. Anim. Sci. 65, 353-360.

Razmkabir M. (2005). Estimation of genetic trends for production traits in Iranian Holstein cattle. MS Thesis. TehranUniv., Tehran, Iran.

Roxström A., Strandberg E., Berglund B., EmanuelsonU. and Philipsson J. (2001). Genetic and environmental correlations among female fertility traits and milk production in different parities of Swedish red and white dairy cattle. Acta Agric. Scandinavica. 51, 7-14.

Sadeghi Sefidmazgi A., Moradi-Shahrbabak M., Nejati-Javaremi, A. and Shadparvar A. (2009). Estimation of economic values in three breeding perspectives for longevity and milk production traits in Holstein dairy cattle in Iran. Italian J. Anim. Sci. 8, 359-375.

Sahebhonar M. (2007). Estimation of genetic trends for production traits and specification of some effective factors on these trends in Iranian Holsteins. MS Thesis. TehranUniv., Tehran, Iran.

Samore A.B., Groen A.F., Boettcher P.J., Jamrozik J., Canavesi F. and Banganato A. (2008). Genetic correlation patterns between somatic cell score and protein yield in the Italian Holstein-Friesian population. J. Dairy Sci. 91, 4013-4021.

Seyedsharifi R., Nasab M.P. and Sobhani E.A. (2008). Estimation of genetic parameters and breeding values for test-day and 305-days milk yields in some Iranian Holstein herd. J. Anim. Vet. Adv. 7(11), 1422-1425.

Shirmoradi Z., Salehi A.R., Pahlavan R. and Mollasalehi M.R. (2010). Genetic parameters and trend of production and reproduction traits in Iranian Holstein cattle. J. Anim. Prod. 12, 21-28.

Shook G.E. (1989). Selection for disease resistance. J. Dairy Sci. 72, 1349-1362.

Teimurian M. (2009). Estimation of genetic parameters and genetic trend for production traits of Holstein dairy cattle of Khorasan Razavi. MS Thesis. FerdowsiUniv., Mashhad, Iran.

Toghiani Pozveh S. and Shadparvar A.A. (2009). Genetic analysis of reproduction traits and their relationship with conformation traits in Holstein cows. Livest. Sci. 25(1), 84-87.

Van Dorp T.E., Dekkers J.C.M., Martin S.W. and Noordhuizen J.P.T.M. (1998). Genetic parameters of health disorders and relationships with 305-day milk yield and conformation traits of registered Holstein cattle. J. Dairy Sci. 81, 2264-2270.

Vergara O.D., Elzo M.A. and Ceron Munoz M.F. (2009). Genetic parameters and genetic trends for age at first calving and calving interval in an Angus-Blanco Orejinegro-Zebu multibreed cattle population in Colombia. Livest. Sci. 126, 318-322.

Visscher P.M. and Thompson R. (1992). Univariate and multivariate parameter estimates for milk production traits using an animal model. I. Description and results of REML analyses. Genet. Select. Evol. 24, 415-430.

Wasike C.B., Indetie D., Ojango J.M.K. and Kahi A.K. (2009). Direct and maternal (co)variance components and genetic parameters for growth and reproductive traits in the Boran cattle in Kenya. Trop. Anim. Health Prod. 41, 741-748.

Windig J.J., Calus M.P.L., Beerda B. and Veerkamp R.F. (2006) Genetic correlations between milk production and health and fertility depending on herd environment. J. Dairy. Sci. 89, 1765-1775.