Effect on fertility of freezing large numbers of straws of bovine spermatozoa in a mechanical freezer.

Abstract

Fertility of semen frozen in a mechanical forced-vapor freezer was compared with that frozen in static nitrogen (N) vapor. Semen from 10 Holstein bulls was extended in milk-10% glycerol at 30 x 10(6) progressively motile spermatozoa/ml and packaged in .5-ml French straws. Straws were divided into three equal groups . bull-1 . collection day-1 and frozen in a mechanical freezer at full or half loads (7,250 +/- 250 or 3,500 +/- 250 straws in 5-straw goblets on metal canes) or in static N vapor (330 +/- 30 straws held singly on horizontal racks). For field distribution, straws frozen in static N vapor also were placed in 5-straw goblets on canes after being frozen. Each of the two goblets on a cane held a different treatment. Based on 75-d nonreturn rates for 23,137 first service inseminations, fertility was 1.6 percentage points higher (P less than .05) for semen frozen in static vapor (66.3%) than that frozen in a fully loaded mechanical freezer (64.7%). The difference of 1.3 percentage points in fertility favoring static vapor over freezing a half load in a mechanical freezer (65.0%) approached significance (P = .06); half and full loads did not vary (P greater than .05). In conclusion, the small increase in fertility favoring static over forced-vapor freezing supports use of the static vapor method unless the savings in time and labor of freezing large numbers of straws at one time in a mechanical freezer are considered to offset the small difference in fertility.