Background: Reproductive efficiency is one of the most important characteristics for beef cattle. Bos indicus cattle have
longer periods of postpartum anestrus, which contributes to poorer reproductive efficiency of zebu cows when compared with
that of Bos taurus, this facts are particular relevant in primiparous cows. These challenges are more important in primiparous
cows in pasture management systems because the intake of nutrients during the postpartum period is not sufficient to meet
requirements for growth as well as lactation. The objectives of this study were to evaluate the days postpartum at conception
and the pregnancy rate at the end of a 90 days breeding season in primiparous and multiparous cows.
Materials, Methods & Results: The study was conducted at the Experimental Farm of the Capim Branco, Federal University
of Uberlandia, in the breeding season of 2009/2010. It was used 14 primiparous and 84 multiparous Nelore cows. The females
were treated with the following protocol. On Day 0, all cows received an intravaginal device containing 1.9 g progesterone
(CIDR®) and 1.0 mg, i.m. of estradiol cypionate (ECP ®). On Day 7 the cows received 12.5 mg, i.m. dinoprost tromethamine
(Lutalyse®). On Day 9, the intravaginal devices were removed, and calves were temporally weaned. On Day 11 (48 h after
intravaginal device withdrawal), all cows were inseminated and calves reunited with their dams immediately after insemination.
All cows that returned to estrus before pregnancy diagnosis were re-inseminated 12 h after estrous detection. Pregnancy
diagnosis was performed by transrectal ultrasonography 30 days after TAI, confirmed 60 days after the end of breeding season.
The pregnancy rate at end of the breeding season was calculated by dividing the number of pregnant cows heifers by the
number of cows used in the breeding season. The pregnancy rate at end of the breeding season were analyzed by logistic
regression, and the number of days postpartum at conception were analyzed by general linear model, using a model that
includes the effect of animal category. Differences were considered significant when P < 0.05. The animal category did not
affect (P > 0.05) the number of days postpartum at conception. However, the animal category had effects (P < 0.001) on
pregnancy rate at end of breeding season. During the breeding season 42.90% (6/14) of the primiparous cows became
pregnant, compared to 88.1% (74/84) of the multiparous cows.
Discussion: The pregnancy rate at the end of a 90 days breeding season was affected by animal category, which may indicate
that heifers need more nutritional and management attention, to not have their reproductive efficiency compromised. The low
pregnancy rate at the end of a 90 days breeding season detected in primiparous cows was probably because of the intake of
nutrients during the postpartum period that was not sufficient to meet requirements for growth, lactation and reproduction in
this animal category. The days postpartum at conception were not affected by the animal category, probably due to the
utilization of ovulation synchronization protocols that induces cyclicity and allowed anestrous cows to be inseminated.
