Abstract

The first service conception rate of cows after using two protocols was investigated regarding various factors including Days in Milk (DIM), seasons of AI and insemination based on the observed Heat or Timed Artificial Insemination (HAI or TAI). Lactating Holstein cows with corpus luteum (n=503) were assigned to receive the G6G or short-term Modified-Cosynch protocols. Cows received G6G (n=217) that consisted of PGF2α, 2-day, GnRH, 6-day, GnRH, 7-day, PGF2α, 56h, GnRH. Short-term Modified-Cosynch group (n=286) received PGF2α, 48h, Estradiol Benzoate (EB), and GnRH was administered at the time of AI. In all groups, cows were inseminated upon detection of estrus (HAI) or underwent Timed Artificial Insemination (TAI) 16h after the second GnRH dose or 32h after EB injection. Pregnancy was diagnosed by means of transrectal ultrasonography from 31 to 35 days after insemination. The conception rate of conceiving after short-term Modified-Cosynch was higher than the G6G protocol (34.6 vs 24.9%; Odds Ratio (OR)=1.6; P=0.02). Inseminated cows after 60 DIM had significantly higher conception rate and odds of conceiving (35.5%; OR=1.8) than the cows were inseminated ≤60DIM (23.7%; P=0.004). The contestant rate of cows was inseminated underwent timed artificial insemination was higher in winter compared with other seasons (39.3 vs 23.2%; P=0.007). In conclusion, short-term Modified-Cosynch protocol can be used to obtain satisfactory conception rate of cows with corpus luteum. Days in milk after calving, season and type of insemination should be considered when selecting the appropriate program for postpartum inseminated cows.

Keywords:Postpartum; Dairy cows; Heat; Prostaglandin; GnRH; Days in milk; Season