7th World Congress on Genetics Applied to Livestock Production, August 19-23, 2002, Montpellier, France INBREEDING AND ITS EFFECTS ON MILK AND FAT YIELDS OF IRAN HOLSTEINS R. Tohidi 1 , R. Vaez Torshizi 1 , M. Moradi Shahrebabak 2 and M. B. Sayyad Nejad 3 1 Department of Animal Science, Tarbiat Modarres University, Tehran, Iran 2 Department of Animal Science, University of Tehran, Karaj, Iran 3 Animal Breeding Center of Iran, Karaj, Iran INTRODUCTION In recent years the widespread use of few genetically superior and outstanding proven sires has caused concern the industry with inbreeding problems. Inbreeding is the mating of animals that are related. It makes pairs of genes alike and thus may reduces genetic diversity of each animal and has harmful effects on production and reproduction traits (Falconer, 1989). The harmful impact of inbreeding on milk production traits has been reported in many studies. Miglior (1994), using records of Canadian Holstein cows, reported that on average the inbreeding depression per each 1% increase in inbreeding is 24.8 kg and 0.9 kg, respectively, for milk and fat yields. For U.S. Holstein cows, estimates were much larger, and were –29.6 kg and –1.1 kg (Wiggans, et al., 1995) and –37.0 kg and –1.0 kg (Cassell, 1999), respectively, for milk and fat yield for each 1% increase in inbreeding. For Iran Holsteins, there is no investigations dealing with inbreeding. The objective of present study was to calculate level of inbreeding and estimate the inbreeding depression on milk and fat yields for Iran Holstein population. MATERIALS AND METHODS Pedigree information. A pedigree information of 382831 registered Holstein cows and bulls, collected by Animal Breeding Center of Iran (ABCI) from 1979 to 2000, was used in this study. Inbreeding coefficient (F) of each animal was calculated using a program written by Sargolzaei (2000) based on Meuwissen and Luo (1992) algorithm. Ninety-four percent of animals in pedigree file were female and 6% were male. Also, 64%, 65% and 58% of animals had known sire, known dam and known sire and dam, respectively. The base population year was considered to be 1968 with inbreeding coefficient of zero, and therefore the trends of the inbreeding were calculated since then. Inbreeding Depression. A total of 60589 cows records for the first lactation, collected during 1993 to 2000 by Animal Breeding Center of Iran, were used to estimate the effect of inbreeding on milk and fat yield. The following single-trait animal model was fitted to the data: e Za Xb y + + = where y is the vector of first lactation milk or fat yield, is the vector of fixed effects, including herd-year-season, age at calving and inbreeding coefficients, a is the vector of additive genetic effects, and are design matrices for b and a , respectively, and is the vector of random residuals. The age at calving and inbreeding coefficients were fitted as b X Z e Session 01. Breeding ruminants for milk production Communication N° 01-55