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Effect of Heat Stress on Early Embryonic Development in the Beef Cow¹

Oklahoma Agricultural Experiment Station, Stillwater 74078

Abstract

Hereford and Hereford x Angus cows (n = 31) were utilized to determine the effects of heat stress on early embryonic development and survival. After acclimation to handling, cows were cannulated via the jugular vein on d 7 and assigned to either a control (C) chamber environment of 22 C, 35% relative humidity (RH) or one of two heat stress treatments. Ambient temperature was maintained at 37 C for 12 h followed by a decrease to 33 C for the remainder of the day in both treatment groups. Relative humidity was maintained at 27% in treatment 1 (TRT 1) and 38% in treatment 2 (TRT 2). On d 8 to 16, daily measurements of respiration rate (RES), rectal temperature (REC) and water intake were taken along with samples of blood, which were analyzed for hematocrit (HEM) and plasma concentration of protein (PP), progesterone (P₄), estradiol-17β (E₂), thyroxine (T₄) and glucose (GLU). The uterus was recovered and flushed with saline on d 17 to recover the conceptus and uterine contents. Conceptus (if present) and corpus luteum (CL) wet weight were determined. Cows subjected to TRT 2 had increased RES and REC (P<.01), while HEM was decreased (P<.05) compared with C cows. Plasma T₄ concentration was decreased (P<.10) in TRT 2 compared with TRT 1 and cows, while P₄ concentration were not significantly different. Corpora lutea wet weights were reduced (P<.10) in heat-stressed cows vs C cows. Although heat stress did not significantly alter pregnancy rates (C, 82; TRT 1, 67; TRT 2, 55%), conceptus wet weights were reduced (P<.01) in heat-stressed cows. Compared with nonpregnant cows, pregnant cows had reduced concentrations (P<.05) of P₄ in plasma. Results indicate that heat stress during early pregnancy in cattle reduces conceptus weight and may possibly increase embryonic mortality.

¹ Journal article no. 5021 of the Agr. Exp. Sta., Oklahoma State Univ. We acknowledge the assistance of Rick Rasby, Jeff Phipps and Wesley Warren for their contributions to data collection. Gratitude also is expressed to Dr. Selwyn J. Barron for his aid in the use of the environmental chamber.

² Dept. of Anim. Sci.

³ Address reprint requests to: Dept. of Anim. Sci., Oklahoma State Univ., Stillwater, OK 74078.

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