Simple SummaryBrazilian feedlot operators subjectively decide how long animals must stay in the feedlot based on a visual evaluation of weight and fatness. Typically, remuneration for commodity meat producers is based on carcass weight, and the lack of a metric to define the ideal slaughter endpoint often leads to erroneous decision-making, mainly due to not knowing the actual carcass gain of the animals. The use of “days on feed” (DOF) is a metric that allows the producer to precisely determine when the animal’s gain drops and, consequently, the feed efficiency in the feedlot. Thus, this study aimed to evaluate the effect of DOF on the performance, intake, feed efficiency, and carcass characteristics of Nellore heifers that received high supplementation during the growing phase. This study generated information that can help to determine the biological slaughter endpoint for young heifers finished in the feedlot. Our results suggest a relationship between the proportion of ether extract of the shrunk body weight, backfat thickness, and body weight can be used as a reference point to make decisions about the biological endpoint of slaughter in the feedlot based on days on feed.AbstractTwo studies evaluated the productive characteristics of young Nellore heifers receiving different days on feed (DOF) to determine the biological slaughter endpoint. In Experiment 1 (Exp. 1), fifty-one Nellore heifers [324 ± 19.3 kg of body weight (BW); 16 ± 1 months] were split into three DOF lengths (45, 75, or 105 days), while in Experiment 2 (Exp. 2), thirty-six Nellore heifers (362 ± 25.5 kg of BW; 18 ± 1 months of age) were split into three DOF lengths (45, 90, or 135 days). In both studies, all animals were distributed in complete randomized blocks according to initial BW and stratified via carcass ultrasound. The diet was supplied ad libitum, allowing 3% of refusals. The point at which the animals achieved 25% of ether extract of shrunk body weight (EESBW) was defined as the biological endpoint. Thus, relationships were made between some characteristics obtained in both studies. Positive linear relationships were found between backfat thickness (BFT) vs. EESBW(p< 0.001,r= 0.84) and BFT vs. body weight (p< 0.001,r= 0.77). Our results suggest that the biological slaughter endpoint for young Nellore heifers is 6.97 mm of backfat thickness or 402 kg shrunk body weight, corresponding to around 75 DOF.