342. Effects of Supplementation of an Active Dry Yeast Probiotic Saccharomyces Cerevisiae CNCM I-1077, on Holstein Steer Feedlot Performance When Fed a High-fiber Finishing Ration.

Abstract The objectives of this study were to evaluate the effects of supplementation of an active dry yeast probiotic, Saccharomyces cerevisiae CNCM I-1077, during periods of heat stress on feedlot performance and final carcass characteristics of Holstein steers fed a high-fiber ration within a commercial finishing feedyard. Measured parameters included initial body weight (IBW), final body weight (FBW), average daily gain (ADG), total weight gain (TWG), dry matter intake (DMI), days on feed (DOF), gain to feed (G:F), and final carcass characteristics during the summer of 2024. Hourly ambient air temperature and humidity were recorded to calculate the temperature humidity index (THI). Holstein steers (n = 1,075; IBW of 540 ± 15.0 kg) were randomly allocated to treatments consisting of 1) SC-1077 (Saccharomyces cerevisiae CNCM I -1077 at 2 x 1010 CFU/hd/d; n = 524 steers) delivered via micro-machine, or 2) CON (no SC-1077; n = 551 steers). Steers were allocated to treatment by lot with a total of 4 lots per treatment. Response variables were analyzed using linear mixed models in R version 4.5.1, with treatment as a fixed effect and arrival date included as a random effect. Treatment means are reported with associated SEM and significance declared at P ≤ 0.10. Steers experienced heat stress conditions (THI > 75) for 72% of the 135-day period. Within that period, steers experienced a total of 50 days of no relief (THI > 75) for 24 consecutive hours, and 16 days within the dangerous and emergency heat stress levels (THI > 79) for 24 consecutive hours. Amylase NDF organic matter (aNDFom) of the finishing ration averaged 18.2% (Table 1). Neither IBW, FBW, or TWG differed (P ≥ 0.24) between dietary treatments (Table 2). Fewer DOF were required for SC-1077 steers (P = 0.07) to reach target end point in comparison to CON. Neither DMI nor ADG differed between treatments groups (P ≥ 0.37), but SC-1077 steers DMI was numerically lower than CON (P = 0.37) while SC-1077 steers ADG was numerically greater than CON (P = 0.51). Because of these numerical differences, SC-1077 steers were more fficient (G:F; P = 0.04) compared to CON. There were no statistical differences in carcass characteristics (P ≥ 0.19) between the two treatments. In summary, feeding SC-1077 to Holstein steers exposed to periods of extreme heat stress during the final 100 days on feed improved feed efficiency and decreased DOF compared to the CON, without impacting carcass characteristics. The improvements in feed efficiency, G:F and DOF, resulted in savings of $36.07/hd with a return on investment of 4.6:1. For image description, please refer to the figure legend and surrounding text. For image description, please refer to the figure legend and surrounding text.