Identification of factors associated with pregnancy loss in lactating crossbred beef cows.

This study aimed to identify factors associated with pregnancy loss in lactating crossbred beef cows managed under extensive conditions. The research was conducted in three management sectors of a commercial farm located in Boquerón, Paraguay. A total of 1554 reproductive records from Brangus and F1 Zebu crossbred cows, diagnosed as pregnant at approximately 30 days of gestation, were analyzed. Cows were bred by fixed-time artificial insemination (FTAI) followed by exposure to cleanup bulls. Pregnancy status was evaluated by ultrasonography at 30-day intervals from 30 days of gestation until day 210. Cows remained in the same management groups until calving. Survival analyses were performed using Kaplan-Meier estimators (PROC LIFETEST, SAS 9.4). Factors evaluated in the survival analyses included management sector, season of conception, breed, age, origin of pregnancy, estrus expression at FTAI, body condition score (BCS) at FTAI, body weight (BW) at FTAI, and BCS dynamics. Cumulative pregnancy loss from 30 days of gestation to calving was 9.7%, with 51.6% of the losses occurring before 90 days. No association of management sector, breed, age, origin of pregnancy, and BCS at FTAI were observed (P > 0.05). However, pregnancy losses were associated with the season of conception (P = 0.04), with differences in the survival curves between the dry season and the rainy season. Estrus expression at FTAI was also associated with pregnancy loss (P = 0.02), as cows failing to express estrus showed different survival curve when compared to cows that exhibited estrus. Body weight at insemination also had an association (P = 0.009), as heavier cows presented a different survival curve when compared with lighter cows. Even though BCS at FTAI was not independently associated with pregnancy loss, cows that gained BCS between 30 and 90 days of gestation experienced lower losses than those that maintained or lost condition (P = 0.005). These findings suggest that season of conception, BCS dynamics (as a proxy of nutritional status), and estrus expression at FTAI may contribute to pregnancy maintenance in beef cows. Strategies aimed at improving body condition, estrus expression, and, the timing of the breeding season may help reduce pregnancy loss and enhance reproductive efficiency in tropical cow-calf systems.