Abstract
A method for the prediction of bull fertility has long been sought but the answer has often been inconclusive. Therefore, the relationship between sperm quality at laboratory tests and fertility was investigated in Bos indicus (n = 7) and Bos indicus × Bos taurus (n = 7) bulls. Ejaculates were collected, processed and frozen in mini-straws, and the straws were split into two groups. One group of straws was used for AI (Investigation 1) and the other group was used for laboratory tests (Investigation 2 and 3). In Investigation 1, bull fertility was determined from 6,101 first service inseminations. The mean ± SEM number of first service inseminations and conception rates (CR) in cows/heifer at 60 to 90 days post AI as determined by rectal palpation were 436 ± 6 and 50.7 ± 0.5%, respectively. Conception rates between bulls did not differ (P > 0.05). Practically, bulls or ejaculates would either be culled or kept. Bulls were therefore grouped as low fertility (47.7 ± 0.4 %, n = 2; 1 SD (1.9) below mean CR of all bulls) and adequate fertility (51.3 ± 0.5% n = 12) bulls. In Investigation 2, sperm concentration, motility, viability and subjective morphology were evaluated. Fourier Harmonic amplitudes (HA 0 - 5) of 100 live sperm nuclei of individual bulls were determined by image analysis. Percentages of viable spermatozoa in an insemination unit had a positive linear relationship with the bull fertility (P < 0.02). Sperm nuclear shape described by HA 0-5 differed between fertility groups (P < 0.05). Adequate fertility group bulls had longer sperm nuclei, indicated by a greater HA 2 (P < 0.01) than that of low fertility bulls. Multiple regression indicated a linear relationship between HA 0-5 of spermatozoa and bull fertility (P < 0.04, R² = 0.79). The HA 2 had the best correlation (r = 0.82, P < 0.001) to the bull fertility. Based on HA 0 - 5, low and adequate fertility bulls differed (MANOVA, P < 0.05) and a canonical correlation of 0.89 indicated a strong association between sperm nuclear shape and fertility groups. In Investigation 3, sperm binding to uterine and oviductal explants and to zona pellucida were enumerated. Zona binding was linearly related to bull fertility and was able to discriminate between low fertility vs. adequate fertility bulls (P < 0.05). Although sperm binding to tissues or sperm motility in the explants' effluent was not related to bull fertility, the percentages of acrosome reacted spermatozoa in the effluents of isthmus, and ampular explants were inversely related to bull fertility groups (P < 0.05). In conclusion, Fourier harmonic analysis and zona binding assay predicted bull fertility and could distinguish the low fertility bulls even though fertility differed only by 6%. The nuclear shape was related to bull fertility with the length of sperm nuclei, as measured by harmonic amplitude 2, being the best predictive of bull fertility.