FERTILITY COMPARISON BETWEEN BREEDING AT 24 HOURS OR AT 24 AND 48 HOURS AFTER COLLECTION WITH COOLED EQUINE SEMEN

It has become a common practice in the equine breeding industry to sen d 2 insemination doses for breeding: with transported cooled semen, on e to be used for the initial insemination upon arrival, and the other to be held a second insemination the next day. One fertile stallion an d 36 fertile mares were used to determine if breeding once with 1 dose of semen cooled for 24 h would improve fertility compared with breedi ng twice, 1 d apart, with half the dose of semen cooled for 24 h on th e first day of breeding and half cooled for 48 h on the second day of breeding. Mares were given two intramuscular injections of 10 mg PGF(2 )alpha 14 d apart. Following the second injection, mares were teased w ith a stallion and their ovaries were scanned by transrectal ultrasono graphy daily. When a dominant follicle (>35 mm diameter) was detected, 1500 units hCG were injected intravenously, and the mares were insemi nated. Semen was collected in advance of anticipated breeding, mixed i n nonfat dry milk solids-glucose extender to a concentration of 25 mil lion, sperm/mL, and placed in 2 commercial cooling containers for 24 o r 48 h of storage prior to breeding. Mares were randomly assigned to 1 of 2 insemination treatment groups: 1) Group T1 (n=18), in which mare s were inseminated on the day of hCG injection with 500 million sperma tozoa cooled for 24 h, or 2) Group T2 (n=18), in which mares were inse minated on the day of hCG injection with 250 million spermatozoa coole d for 24 h, and again on the following day with 250 million spermatozo a cooled for 48 h. Pregnancy status was confirmed by transrectal ultra sonographic examination at 14 and 16 d after ovulation. Pregnancy rate s were the same for both insemination treatment groups (12/18; 67%). T here was no advantage to holding half of the insemination dose for reb reeding on the following day. (C) 1998 by Elsevier Science Inc.