Analysis of breed effects and genetic parameters of semen quality traits for frozen thawed semen in stallions Despite improvements in recent years in the techniques of freezing semen for reproduction purposes a considerable amount of stallions’ semen is still not suitable for providing frozen thawed semen that fulfils the minimum quality requirements. Semen quality traits vary considerably between horse breeds month and age of the stallions under similar environmental conditions. Therefore the objectives of our study were to analyse the systematic effects on the quality traits of semen (fresh and frozen thawed) collected in the breeding and non breeding season. 5.1 Breed and stallion effects on frozen thawed semen in Warmblood Light and Quarter horses The objectives of the present study were to analyze systematic effects on semen quality traits from fresh and frozen thawed semen collected in and outside season. A total of 4 681 reports on semen traits of 121 stallions representing Arabian Thoroughbred Quarter Horse and four warmblood breeds used for artificial insemination at the Lower Saxon National stud Celle were edited for analysis of gel free volume sperm concentration total number of sperm and total number of motile sperm in fresh semen and in frozen thawed semen progressive motility DNA fragmentation index and non viable sperm. Month year age breed and stallions’ effects were analyzed with a linear mixed model procedure. Breed differences were significant for sperm concentration total number of sperm total motile sperm number and DNA fragmentation index. Hanoverian stallions showed significant higher least squares means for sperm concentration total number of sperm and total motile sperm number in fresh semen whereas Thoroughbred had significant higher least squares means for DNA fragmentation index. Stallions with an age of 2 6 years had significant lower least squares means in sperm concentration total number of sperm and total motile sperm number and progressive motile sperm post thawing than the other age groups. Month was significant for all semen traits but progressive motile sperm post thawing. Month by age class interaction showed a significant influence. Inter stallion variance accounted for 27 71% and the two way stallion by month interaction for 2 7% of the total variance of semen quality traits. The largest proportion of inter individual variance among stallions was explained by the DNA fragmentation index. 5.2 Genetic parameters and estimated breeding values for traits of raw and frozen thawed semen in German Warmblood stallions Objectives of the present study were to estimate genetic parameters for frozen thawed semen traits of 271 fertile German Warmblood stallions and genetic correlations with raw semen quality traits. Semen samples were collected from stallions utilized for semen collection and artificial insemination (AI) on the Lower Saxon National Stud Celle and the North Rhine Westphalian National Stud Warendorf. Semen quality variables were analyzed in 63 972 raw (gel free volume concentration progressive motility number of sperm) and 3 681 frozen thawed samples (motility DNA fragmentation index (DFI) non viable sperm). A multivariate linear animal model was used to estimate additive genetic and permanent environmental variances among stallions as well as estimated breeding values (EBVs) for all semen traits. Heritability estimates were greatest for DFI (h2 = 0.45) and least for non viable sperm counts (h2 = 0.11). Additive genetic correlations between progressive sperm motility in raw semen and DFI (rg = 0.79) as well as non viable sperm (rg = 0.45) were negative. The EBVs for frozen thawed semen traits ranged from 49 to 181 with mean reliabilities of 0.28 to 0.43. The EBVs for progressively motile sperm post thawing and DFI were the most highly correlated traits with EBVs for stallion fertility (r = 0.38 and r = 0.17). Stallions with relatively greater EBVs for progressive motility in raw semen may be most suitable when freezing semen for storage and subsequently thawing it for AI. Using EBVs for semen traits in selection of stallions to AI mares appears as an option for genetic improvement to enhance fertility after AI.