Effect of one or three timed artificial inseminations before natural service on reproductive performance of lactating dairy cows not observed for detection of estrus

The objectives were to determine the effects of one or three timed artificial insemination (AI) before natural service (NS) in lactating dairy cows not observed for detection of estrus on hazard of pregnancy, days nonpregnant, and 21-days cycle pregnancy rate. A total of 1050 lactating Holstein cows were subjected to a double Ovsynch program for their first postpartum AI. On the day of first AI (78 +/- 3 days in milk), cows were blocked by parity and randomly assigned to receive either one timed AI (1TAI, n = 533) or three timed AI (3TAI, n = 517) before being exposed to NS. Cows assigned to 1TAI were exposed to bulls 7 days after the first AI. Nonpregnant cows in 3TAI were resynchronized with the Ovsynch protocol supplemented with progesterone twice, with intervals between AI of 42 days, before being exposed to NS 7 days after the third AI. Cows were evaluated for pregnancy 32 days after each timed AI, or every 28 days after being exposed to NS. Pregnant cows were re-examined for pregnancy 28 days later (i.e., 60-day gestation). Exposure to heat stress was categorized based on the first AI being performed during the hot or cool season, according to the temperature-humidity index. Body condition was scored at first AI. All cows were allowed a period of 231 days of breeding, after which nonpregnant cows were censored. Pregnancy to the first AI did not differ between 1TAI and 3TAI on Day 60 after insemination (30.8 vs. 33.5%). Cows receiving 3TAI had a 15% greater hazard of pregnancy and a 17% greater 21-days cycle pregnancy rate than 1TAI and these benefits originated from the first 84 days of breeding. These changes in rate of pregnancy reduced the median and mean days nonpregnant by 9 and 10 d, respectively. Despite the long inter-AI interval in cows subjected to 3TAI, reproductive performance was improved compared with a single timed AI and subsequent exposure to NS. In dairy herds that use a combination of AI and NS, allowing cows additional opportunities to AI before onset of breeding with bulls is expected to improve reproductive performance. (C) 2012 Elsevier Inc. All rights reserved.