Automated systems based on wearable sensors for livestock monitoring are becoming increasingly popular. Specifically, wireless in-body sensors could yield relevant data, such as ruminal temperature. The collection of such data requires an accurate characterization of the in-to-out body wireless channel between the in-body sensor and the gateway. The aim of this study is to experimentally characterize the in-to-out-body propagation loss for cows and horses at 433, 868, and 1400 MHz. Measurements were conducted in vivo on five different fistulated cows and five horse cadavers using specialized robust in-body capsule antennas inside the animals' abdomen. Next, the in-body antenna gain was deembedded from the wireless channel, and the in-to-out body propagation loss was obtained as the difference between measured unobstructed line-of-sight path losses and in-to-out-body path losses. The measurements showed a body propagation loss of (mean +/- standard deviation) 30.8 +/- 4.1 dB, 44.5 +/- 4.8 dB, and 54.2 +/- 4.7 dB for cows at 433, 868, and 1400 MHz, respectively. For horses, the body propagation losses were 23.2 +/- 3.8 dB, 31.0 +/- 4.7 dB, and 44.4 +/- 3.2 dB at 433, 868, and 1400 MHz, respectively. These results are important to determine the wireless range of WBANs to optimize the network topology and estimate the associated network cost for large-scale monitoring systems.