Conception rates after artificial insemination or embryo transfer in lactating dairy cows during summer in Florida

The objective was to compare conception rates to embryo transfer relative t o AI, during summer heat stress, in lactating dairy cows. Holstein cows (n= 180; 50 to 120 d postpartum) were allocated randomly to 1 of 3 groups: arti ficial insemination (AI, n=84), embryo transfer using either embryos collec ted from superovulated donors (ET-DON, n=48), or embryos produced in vitro (ET-IVF, n=48). Embryos from superovulated donors were frozen in 10 % glyce rol and were rehydrated in a 3-step procedure, in decreasing concentrations of glycerol in a sucrose medium before transfer. Embryos produced in vitro were frozen in 1.5 M ethylene glycol, thawed and transferred without rehyd ration. Blood samples were collected from AI and ET recipients on Days 0, 7 and 22 for measurement of progesterone in plasma. Conception rare was esti mated for the three groups at Day 22 (progesterone>1 ng/mL) and confirmed a t Day 42 by palpation per rectum. Conception rate estimates at Day 22 did n ot differ among groups (AI, 60.7%; ET-DON, 60.4%; ET-IVF, 54.2%), but conce ption rates at Day 42 differed (AI, 21.4%; ET-DON, 35.4%; ET-IVF, 18.8%; AI versus ET: P>0.10 and ET-DON versus ET-IVF: P<0.05). In cows considered pr egnant at 22 d but diagnosed open at 42 d, the interestrous intervals were 28.8+/-2.2, 35.2+/-3.5 and 31.6+/-2.9 d, respectively, for AI, ET-DON and E T-IVF groups. Transfer of embryos collected from nonheat-stressed superovul ated donors significantly increased conception rates in heat stressed dairy cattle. However, transfer of IVF-derived embryos had no advantage over AT. Where appropriate mechanisms are in place to attenuate the effects of heat stress, embryo transfer using frozen-thawed donor embryos increases concep tion rates. (C) 1999 by Elsevier Science Inc.