Angus sire field fertility and in vitro sperm characteristics following use of different sperm insemination doses in Brazilian beef cattle.

The primary objective was to determine if Angus bull fertility varied by number of sperm inseminated. A secondary objective was to characterize the potential impact of random variation on fertility using two identical sperm per dose treatments, which differed only in straw color. Computer-assisted sperm analysis (CASA) and flow cytometry (FC) were used to identify post-thaw sperm characteristics associated with field fertility differences between bulls. Ejaculates from five Angus bulls were collected, extended, and cryopreserved at 10, 20, 20 or 40 × 10 sperm per dose in color-coded 0.5-mL French straws. Multiparous cows (n = 4866) from ten Brazilian farms were synchronized for first-service timed artificial insemination (TAI). Bull identification and straw color were recorded at TAI. Pregnancy per TAI (P/TAI) did not differ between sperm doses (43.8, 45.3, 43.8 and 47.1% for 10, 20, 20 or 40 × 10 sperm respectively; P = 0.31) nor was there an interaction between bull and dose (P = 0.53). The P/TAI differed between bulls and ranged from 40.7 to 48.1% (P < 0.01). The overall P/TAI between the two control groups were not different (45.3 vs 43.8%); however, the numerical variation within bull ranged from 0.5 to 4.9 percentage points. Numerous CASA and FC post-thaw sperm characteristics differed among bulls (P < 0.05), but these characteristics did not explain the fertility difference between bulls. Principal component analysis provided a multivariate description of the CASA and FC data, where three principal components (Prin1, Prin2, and Prin3) accounted for a combined total of 88.7% of the data variability. The primary components of each PCA axis were flow cytometric measures of sperm viability and DNA fragmentation (Prin1), and CASA-derived sperm movement patterns (Prin2) and motility (Prin3); however, the relative influence of these characteristics varied by bull. Although fertility differences between bulls were detected, neither sperm per dose nor post-thaw in vitro sperm analyses (CASA and FC) were able to explain the observed differences in field fertility between bulls, further illustrating the difficulties in predicting bull fertility.