Animal Science Department


First Advisor

Ty B. Schmidt

Date of this Version



A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science Major: Animal Science Under the Supervision of Dr. Ty B. Schmidt Lincoln, Nebraska August 2018.

Copyright (c) 2018 Lauren E. Kett


Forty-nine crossbred feeder lambs (wethers, n = 49; 53.3 ± 3.7 kg BW) were utilized to evaluate the interaction of b - adrenergic agonist (bAA) supplementation and heat stress on growth performance and carcass composition. Utilizing and 3 x 2 factorial design, lambs were randomly assigned to one of three bAA supplementation: 1) Control, CON, 2) Ractopamine Hydrochloride at 40 mg/hd/d, RHCL, and Zilpaterol Hydrochloride at 2.5 mg/hd/d, ZHCL for a period of 20 d and one of two environmental conditions (Thermal Neutral: TN and Heat Stress: HS). The TN environment had a constant thermal heat index (THI) of 16.6°C. Within the HS environment, a cyclic design was utilized to achieve a THI of 29.5ºC from 10:00 to 20:00 h and a THI of 24.5ºC from 22:00 to 08:00 h. Starting at 08:01 and continuing to 09:59 h, temperature and RH were gradually increased to achieve a THI of 29.5ºC at 10:00 h and reduction of temperature and RH from 20:01 to 21:59 h to achieve a THI of 24.5ºC at 22:00 h. Regardless of bAA supplementation (P = ≥ 0.09), lambs exposed to the HS environment had reduced DMI (P < 0.001), ADG (P = 0.002), and final BW (P = 0.03). In addition, exposure to the HS environment (regardless of bAA supplementation; P = ≥ 0.07) decreased HCW (P < 0.001), percent change in LM area (P = 0.004) and percent change in LM depth (P = 0.005). There was a bAA x environment interaction associated with RHCL supplementation and heat stress (P = 0.003). Lambs supplemented RHCL in the HS environment had reduced (P = 0.003) respiration rates, when compared to CON and ZHCL supplemented lambs. Supplementation of ZHCL decreased adipose tissue (P = 0.05) and increased percent fat free lean (P = 0.01), when compared to RHCL and CON lambs. Within the current study, both heat stress and bAA supplementation had an impact on growth performance and carcass composition. However, the data does not indicate that there was any significant interaction between bAA supplementation within a heat stress environment on growth performance or carcass composition in feeder lambs.

Advisor: Ty B. Schmidt