Aim: This study was undertaken to compare the reduction in color of two dyes methylene blue (MBRT) and resazurin dye (RRT) with other tests of bull semen quality and to examine their relationship to fertility. Methods: One hundred sixty-four ejaculates from 59 bulls were examined, processed, and used for 30 016 inseminations. Results: Bulls used in artificial insemination have been selected for high semen quality and fertility, and semen from these bulls averaged 80.6% unstained sperm, only 11 % had abnormalities, and fertility ranged from 64% to 76%. The MBRT and RRT were run with standardized sperm numbers to prevent sperm concentration from influencing the dye reduction time. A short RRT was correlated with higher fertility (r=-0.26) but MBRT was not correlated (r=-0.05, P>0.05). The correlations were low, but are typical and reflect the statistical effect of the large binomial variance associated with pregnancy or nonpregnancy being coded as 1 or 0. Conclusion: The fact that the RRT was significantly correlated with fertility when sperm numbers were standardized, and published reports that resazurin is useful for monitoring other semen characteristics, indicate that this dye may provide valuable information not given by other simple laboratory tests.

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