الفهرس 
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Abstract
The current study aimed to evaluate the role of LDL and Cysteine on semen cryopreservation of Holstein bull semen during different stages of semen freezing. Semen was collected twice weekly from 4 Holstein bulls, only ejaculates possessed 75% motility or more were pooled. This study was divided into three experiments, experiment 1 pooled semen was spilt into 5 parts, one part was diluted with Tris-egg yolk (control) and the other four testes media, whole egg yolk was replaced by 4, 6, 8 and 10% low density lipoproteins (LDL). Experiment 2, pooled semen was divided into 5 equal fractions, one fraction was diluted with tris-egg yolk(control) while the others were diluted with tris-egg yolk extender supplemented with cysteine at levels of 5, 10, 15 and 20mM. Experiment 3, pooled semen was divided into 4 parts, one part diluted with tris-egg yolk, second part diluted with 6% LDL and other diluted with 6% LDL+5 or 10mM cysteine, respectively. Extended semen was equilibrated at 5 ºC for 4 h, filled in 0.25 ml French straws, and frozen in liquid nitrogen (-196 ºC). Sperm motility, live sperm, sperm abnormality, plasma membrane integrity and acrosome integrity were assessed at post dilution, post equilibration and post thawing. In conclusion, extenders containing 6% LDL and/ or supplemented with 10mM Cysteine improved the freezability and fertility of Holstein bull spermatozoa.
Key words: Holstein bull semen, LDL, Cysteine, extender, freezing, Conception rate.