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Influence of Ambient Temperature on Prolactin Concentrations in Serum of Holstein and Brahman x Hereford Heifers^1,2,

Oklahoma Agricultural Experiment Station, Stillwater 74078 , Michigan State University, East Lansing 48824 and and USDA, SEA—AR, Southwestern Livestock and Forage Research Station, El Reno, OK 73036

Abstract

Four Holstein and four Brahman x Hereford heifers about 8 mo of age were used in a study to determine whether breed influences the effects of ambient temperature on concentrations of prolactin in serum. Two heifers of each breed were stanchioned in each of two environmental chambers at 21 C for 7 d, after which chamber temperatures were changed to 7 or 31 C during 6 h. After 5 d at 7, 21 or 31 C, heifers were injected with 60 µg thyrotropin-releasing hormone (TRH). A switch-back design was used and each heifer was exposed to all treatments. Concentrations of prolactin in serum of heifers during exposure to 7, 21 or 31 C for 5 d were related to ambient temperature (9.0, 20.9 and 29.5 ng/ml, respectively; P>.001), but the response was not influenced by breed. Heifers of both breeds responded similarly to treatment with TRH, and prolactin in serum increased (P>.001) within 5 min from 7.0 ± 3.2 to 45.7 ± 8.2 ng/ml in heifers at 7 C, from 13.1 ± 1.6 to 97.2 ± 9.6 ng/ml in heifers at 21 C and from 18.2 ± 3.5 to 96.2 ± 11.3 ng/ml in heifers at 31 C. We conclude that concentrations of prolactin in serum of heifers are positively associated with ambient temperature and that the effects of temperature on basal and TRH-stimulated concentrations of prolactin do not differ significantly between Holstein and Brahman x Hereford heifers. Thus, differences in tolerance to heat were not related to differences in prolactin secretion.

¹ Journal Article No. 3870 of the Agr. Exp. Sta., Oklahoma State Univ., Stillwater; Michigan Agr. Exp. Sta. Journal Article No. 9937. This research was conducted in cooperation with the USDA, SEA-AR and supported in part by USPHS grant Am-15899.

² We thank Dr. R. Rippel (Abbott Laboratories, North Chicago, IL) for the TRH (pyro-glutamyl-histidyl-proline amide) and the National Institutes of Health for the bovine prolactin.

³ Anim. Sci. Dept., Oklahoma State Univ., Stillwater 74078.

⁴ Anim. Reprod. Lab., Dept. of Dairy Sci., Michigan State Univ., East Lansing 48824.

⁵ Present address: Dept. of Dairy Sci., The Ohio State Univ., Columbus 43210.

⁶ Southwestern Forage and Livestock Research Station, El Reno, OK 73036.