Comparison of semen quality in young and mature Holstein bulls measured by light microscopy and flow cytometry.

Random samples of cryopreserved, milk-extended semen, collected from 20 Holstein bulls at about 14 mo of age (young) and again at about 4 yr of age (mature), were evaluated at thawing and during 3-h incubation to compare semen quality of young versus mature bulls. Evaluation by differential interference contrast microscopy showed greater proportions of cytoplasmic droplets in semen from young versus mature bulls. Mature bulls exhibited greater proportions of intact acrosomes in freshly thawed semen than did young bulls. Evaluation of sperm chromatin structure by flow cytometry after staining with acridine orange showed lower values for mature versus young bulls, indicating resistance of DNA in nuclear chromatin to acid denaturation increased with age. Correlations between ages for most sperm morphology, acrosome integrity, and flow cytometry variables were high and positive. Nonreturn rate for young bulls was positively related to morphologically normal sperm and acrosomal integrity and negatively related to flow cytometry traits. Results suggest semen quality of young bulls was related to subsequent quality as mature bulls. With flow cytometry, differences were detected between semen samples that were not evident with light microscopy.