CASE STUDY: Influence of Seminal, Physical, and Mating Behavior Traits of Bulls on Number of Calves Sired per Bull in a Multisire Herd

Professional Animal Scientist, Apr 2008 by Whitworth, W A, Forrest, D W, Sprott, L R, Holloway, J W, Warrington, B G

Semen was collected via electroejaculation (Electrojac III, Ideal Instruments, Chicago, IL). Spermatozoal motility, gross appearance, and rate of forward movement were evaluated immediately upon collection. A semen sample was mounted on a slide and stained with a commercially available eosin-nigrosin stain kit (Semen Analysis Kit, A.J.P. Scientific, Inc., Clifton, NJ) in order to determine morphological characteristics. The slides were observed under light microscopy (400 × magnification) and 200 cells per slide were counted to ascertain primary and secondary sperm abnormalities according to the standards set by the Society of Theriogenology (Chenoweth et al., 1992). Primary abnormalities categorized included pyriform heads, tapered heads, and microcephalic or macrocephalic sperm. secondary abnormalities assessed were detached heads, distal midpiece reflex, the Dag defect, pseudodroplets, bowed midpieces, bent principal pieces, coiled principal pieces, tail stump defect, abaxial tails, proximal droplets, and distal droplets (Barth and Oko, 1989). An aliquot of each ejaculate was analyzed via Western blot to determine the FAA classification of each bull (Bellin et al., 1994). Bulls were designated A (FAAprotein on the sperm head only), B (FAA protein on the sperm head and in seminal fluid), C (FAA protein in the seminal fluid only) or D (no detectable FAA protein). For the purposes of analysis, bulls exhibiting the FAA protein on sperm were classified as FAA-positive (classifications A and B) and those which did not exhibit the FAA protein on sperm were classified FAA-negative (classification C and D).

Bulls (n = 12) were mated with multiparous, crossbred (Angus and Angus ? Bos indicus breedtypes) cows (n = 305, 4 to 8 yr of age) for 90 d. Body condition score, sperm motility and morphology, SC, and SDR for each of the Bonsmara and Braunvieh bulls are reported in Table 1. Ten cows were eliminated from the study because they either were culled, became ill, or died. Pregnancy rate was determined via palpation per rectum at 60 d after the conclusion of the breeding period. After the ensuing calving season was completed, paternity was verified by DNA typing of calves (MMI Genomics, Davis, CA). A needle prick of the ear of each bull, cow, and calf was used to obtain a blood sample that was blotted onto a FTA Micro Card (Whatman Inc., Florham Park, NJ) to stabilize released nucleic acids. A punch from each sample was washed to remove associated heme proteins from the card. Commercial genotyping kits (Stockmarks II) obtained from Applied Biosystems (Foster City, CA) were used in the analysis. Genetic profiles for each cow and calf pair were examined to determine the sire of the calf.

Backward stepwise regression (SAS Inst. Inc., Cary, NC) was used to determine which of the traits accounted for the highest percentage of the variability in number of calves sired. Chisquare analyses were used to determine the effects of breed of bull on number of calves sired and on the number of calves born early (first 40 d of calving) or late (≥ 41 d of calving) by sperm morphology classification.