EVALUATION OF DIFFERENT COOLING RATES, EQUILIBRATION PERIODS AND DILUENTS FOR EFFECTS ON DEEP-FREEZING, ENZYME LEAKAGE AND FERTILITY OF TAURINE BULL SPERMATOZOA

We studied the effects of 2 diluents (Tris and milk), 4 cooling rates (10 degrees C/30 degrees C to 5 degrees C for 1 or 2 h), 2 equilibrati on periods (0 and 2 h) and their interactions on the freezability, glu tamic oxaloacetic transaminase (GOT) leakage and fertility of frozen-t hawed semen in 18 ejaculates from 3 Friesian bulls. The means of pre- and post-freezing motility, GOT leakage and fertility rates (52.81% ba sed on follow up of 267 inseminated cows) were significantly (P<0.01) influenced by the bulls, cooling rates and equilibration periods, but not by diluents or the interactions studied. The mean prefreeze motili ty of spermatozoa following 1 h of cooling from 10 degrees C to 5 degr ees C was significantly lower (60.38%) and that after 2 h of cooling f rom 30 degrees C to 5 degrees C was higher (72.38%) than 2 h of coolin g from 10 degrees C to 5 degrees C (66.57%) or 1 h of cooling from 30 degrees C to 5 degrees C (67.96%). The mean post-thaw motility observe d following 2 h of prefreeze cooling was, however significantly greate r (45%) than after 1 h of cooling (35%) for both the initial temperatu res. Leakage of GOT pre- and post-freezing was significantly less foll owing 2 h of cooling from 30 degrees C to 5 degrees C (17.26 and 27.36 mu mole/L) than after 1 h of cooling from either 10 degrees C (19.71 and 30.13 mu mole/L) or 30 degrees C (18.95 and 29.58 mu mole/L) and c ooling from 10 degrees C to 5 degrees C(21.43 and 34.48 mu mole/L). Th e conception rates for semen frozen at the above cooling rates (66.13, 48.65, 56.67 and 42.25%, respectively) were inverse to GOT leakage. A n equilibration period of 2 h over that of 0 h at 5 degrees C adversel y affected the prefreeze motility and GOT leakage, but it significantl y improved postthaw motility (44.03 vs 35.49%) and fertility rates (57 .86 vs 47.24%). These findings suggested that both Tris- and milk-base d diluents were equally efficacious for cryopreservation of bovine sem en, and that slow cooling of semen straws over a period of 2 h from 30 degrees C to 5 degrees C as compared with faster cooling rates or a l ower initial temperature (10 degrees C), plus at least 2 h of equilibr ation time at 5 degrees C were essential for optimal freezability, low er enzyme leakage and higher fertility rates within the tropics.