Submitted to: Beef International Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 5/17/2016
Publication Date: N/A
Interpretive Summary: While significant strides have been made with regard to reducing the detrimental effects of heat stress on feedlot cattle, morbidity and mortality associated with heat stress continues to be a significant issue for beef cattle feedlots in the U.S. In an effort to provide additional insight into this issue, and to potentially identify preventative management strategies, scientists within the ARS’ Livestock Issues Research Unit in Lubbock, Texas have been evaluating feed additives that may mitigate some of the detrimental effects of heat stress in beef cattle. In the present study, we report that supplementing yeast products during a period of of controlled-heat stress reduced body temperature, increased water intake, and decreased respiration rates in feedlot heifers. The reduced body temperature and respiration rates during the heat stress event for the yeast supplemented heifers clearly indicates that these heifers tolerated the heat stress event better than the control heifers. Interestingly, both water intake and the number of drinks per hour during the heat stress event was also increased in the yeast supplemented heifers. Whether or not the increased water intake was driving the reduced vaginal temperature and respiration rates observed in the present study remains unclear at this time. However, the majority of the data would indicate that supplementation with the live yeast and yeast cell wall product to feedlot heifers may mitigate some of the negative effects associated with a naturally-occurring heat stress event in feedlot cattle. This data will be of interest to feedlot producers, veterinarians, feedlot consultants, and scientists working in the field of beef cattle stress and management.
Technical Abstract: The objective of this study was to determine if supplementing the diet of near-finished beef cattle with a yeast product would mitigate the negative impact of a controlled HS on the physiological and endocrine responses. Crossbred beef heifers (n=111; BW=281.07 kg) were divided into 2 pens in a commercial feedlot. One pen was fed a standard finishing ration (CON), and the other was fed the same ration plus supplemented via top dress with a combination of a live yeast (1.5g/hd/d) and yeast cell wall product (2.5 g/hd/d; YEAST). After 50 d of supplementation, a subset of 32 heifers (n=16 hd/trt) were selected based on BW, phenotype, and temperament and transported to the USDA-ARS Bovine Immunology Research and Development complex in Lubbock, TX. The following day, all heifers were weighed, fitted with an indwelling jugular catheter and an indwelling vaginal temperature (VT) probe, and then moved into individual stanchions in an environmentally-controlled facility. For the next two days, the facility was maintained at thermoneutral (TN) conditions while the heifers acclimated to the bleeding stalls. On day 3, blood samples were collected at 1-hour intervals for serum during two 4-hour periods to assess differences between treatment groups in TN conditions. At 1400 and 2200 hours each day, a whole blood sample was collected from each heifer for complete blood cell counts. Additionally, respiration rates were collected at 1600 and 2400 hours. Beginning on day 4 and continuing through day 7, the heifers were exposed to a 4-hour period of maximum HS conditions on each day that was achieved by increasing the THI starting at 0700. At 0700 the THI was gradually increased to reach a targeted THI of 80 by 1400 hours. The THI was maintained at 80 from 1400 until 1800 hours. After 1800 hours the THI was gradually decreased in a manner to reach a targeted THI of 76 by 2200 which was maintained until 0700 the next day. There was no difference in BW loss (P = 0.14) or ADG (P = 0.53) between the treatments following HS. However, for the YEAST heifers, VT was reduced during the HS event (P < 0.01). The yeast supplemented heifers consumed more water/h (P < 0.01) and had increased drinking bouts (P < 0.01) during the HS event. YEAST heifers tended (P = 0.09) to have decreased respiration rates compared to CON heifers during the HS event. Overall, there were variable effects on hematological parameters between the two treatment groups. Hematocrit (P < 0.03), lymphocyte counts (P < 0.007) and eosinophil counts (P < 0.0001) were greater for the YEAST heifers in comparison to CON heifers, while neutrophil counts (P = 0.002) were greater in CON heifers. The reduced VT and tendency for reduced respiration rates during the HS event for the yeast supplemented heifers clearly indicates that these heifers tolerated the HS event better than the control heifers. Collectively, the data would indicate that supplementation with the live yeast and yeast cell wall product to feedlot heifers may mitigate some of the negative effects associated with a naturally-occurring HS event in feedlot cattle.