Transuterine embryo migration, distribution of sexes within uterine horns, and fetometry in Nellore (Bos indicus) cattle

There is no consensus about the occurrence of transuterine embryo migration under natural breeding circumstances, neither data related to this phenomenon for zebu cattle. In this study, 5431 reproductive tracts of Nellore cows and heifers were evaluated in an abattoir in the state of Mato Grosso do Sul. A total of 1030 animals (19.0%) were pregnant at the time of slaughter (including 2 twin pregnancies). Regarding singleton pregnancies (n = 1028), 39.5% of them, the fetus was located in the left uterine horn and the remaining 60.5% in the right. These frequencies differed statistically from parity. In all 1028 (100.0%) cases, the presence of a single CL was perceived in the ovary ipsilateral (whether right or left) to the pregnant uterine horn, indicating the absence of transuterine embryo migration of the conceptus. The overall sex ratio found was 51.5%, considering only pregnancies with sex identified, not differing significantly between the number of males and the number of females. The sex frequencies obtained from the total number of singleton pregnancies were 46.5% males, 43.9% females, and the remaining 9.6% corresponding to unknown sex fetuses. The supposed sex predilection for uterine horns was not observed because the difference between the numbers of males and females for the same uterine horn was not statistically significant. The crown-rump (CR) and crown-nose (CN) measures, for the conceptus with CR length 2.00 cm to 15.00 cm, showed a high linear correlation coefficient (r = 0.990865), being CN = (0.3027 × CR) + 0.4491, r2 = 0.9818, the equation that describes the behavior between the variables for this length interval. In conclusion, in Nellore cattle, the transuterine migration of conceptus may not exist, or consists of a rare event. In addition, the sex ratio and predilection are not distinguished from the symmetrical distribution, and regarding fetometry, an equation that involves CR and CN, could be useful for the veterinary field routine, especially in reproductive evaluation.