Abstract
The effects of the time after calving at flush of the first treatment (TIME) and the age at the last calving (AGE) on embryo production following repeated superovulation treatment were investigated for the determination of appropriate timing for starting embryo production in Holstein donors. TIME on the number of normal embryos recovered showed a linear decrease from 131-190 d to ?? 461 d, and there were more normal embryos in the classes of donors treated during 70-130 d, 131-190 d, and 191-250 d than in the class of donors treated during ?? 461 d (P<0.05). AGE described curves with the peak at 5 yr, and a significantly greater numbers of normal embryos were recovered from donors calved at 4 yr and 5 yr than from donors calved at ?? 7 yr (P<0.05). However, there was no statistically significant difference among donors calved at 3 to 6 yr. It is desirable that donor selection be made as early as possible for genetic improvement, and these results suggest that it is possible to select donor by records of the first lactation and to start embryo production about 70 d after the second calving.
