The effects of steroid implant and dietary soybean hulls on estrogenic activity of sera of steers grazing toxic endophyte-infected tall fescue pasture

Authors: Shappell, Nancy; Flythe, Michael; Aiken, Glen

Submitted to: Frontiers in Veterinary Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/21/2015
Publication Date: 8/21/2015
Citation: Shappell, N.W., Flythe, M.D., Aiken, G.E. 2015. The effects of steroid implant and dietary soybean hulls on estrogenic activity of sera of steers grazing toxic endophyte-infected tall fescue pasture. Frontiers in Veterinary Science. DOI: 10.3389/fvets.2015.00030.

Interpretive Summary: Cattle grazed on tall fescue pasture can consume ergot alkaloids that are produced by a fungal endophyte and cause a toxicosis. “Fescue toxicosis” decreases blood circulation and certain hormones to adversely affect growth and reproductive performances of cattle. Consequently, fescue toxicosis annually costs the U.S. beef industry approximately 1 billion dollars. Results of an earlier experiment reported that feeding soybean hulls to steers, with a daily consumption of 5 lb/steer, reduced the severity of toxicosis and increased daily weight gain, and further showed that daily weight gain was substantially increased with feeding soybean hulls combined with an estradiol-progesterone ear implant. It was concluded that the benefit of feeding soybean hulls was due to a dilution effect of the soybean hulls on the consumption of ergot alkaloids; however, estrogenic activity of soybean hulls could also have caused some positive adjustment of the circulation system. Blood serum from the steers used in the previous experiment was found to be elevated in steers fed soybean hulls and without implantation, and in those with ear implantation and without feeding soybean hulls. However, the highest estrogenic activity was determined in steers with implants and fed soybean hulls. The hormone prolactin, which is typically depressed in steers with fescue toxicosis, was also highest with the combination treatment. Estrogen activity in soybean hulls and particularly in combination with an estradiol-progesterone implant could have been a factor in the improved performance and reduced severity of toxicosis with these treatments. Feeding soybean hulls in combination with ear implants is a management option with potential to increase beef calf performance and well-being on endophyte-infected tall fescue.

Technical Abstract: Soybean hulls (SBHs), a co-product of soybean meal milling, have been fed to cattle pasturing on endophyte-infected tall fescue in attempts to increase rate of gain. Literature reports indicated some symptoms associated with fescue toxicosis were ameliorated by the use of steroidal implants containing estradiol (E2) and progesterone (IMP), feeding SBHs, or the combination of the two. While the mechanism for amelioration was unclear, the SBHs were postulated as acting as a diluent of the toxic factors of the fescue. We believe estradiol and phytoestrogens of SBHs might be exerting positive effects through relaxation of the persistent vasoconstriction found in animals consuming the ergot alkaloid associated with endophyte-infected fescue. If this were true, the estrogenic activity of serum of steers receiving SBHs, IMP, or a combination of the two should be elevated. Using the cellular proliferation assay of estrogenicity (E-Screen), estradiol equivalents (E2Eqs) were determined on both SBHs and the serum of steers from a previously reported study. The estrogenic activity of SBHs ranged from 5.0 to 8.5 ng Eqs g-1 DM (mean 6.5, n=4 different commercial sources of SBHs). At the feeding rate used, the theoretical calculated circulating blood E2Eq could be physiologically relevant (~ 80 pg mL-1, based on 2.3 kg SBHs d-1, 300 kg steer, 5.7% blood volume, and 10% absorption). Assay of serum samples indicated that steroidal implants or SBHs feeding did increase the estrogenicity of serum in grazing cattle. The E2Eq for IMP and SBH treatments were 56 and 151% greater than the control, with an apparent additive effect of combined treatments (SBH+IMP), increasing E2Eq by 211%. Similarly serum prolactin was greatest for the SBH+IMP group (188 ng mL-1), similar to values seen in steers grazing on endophyte-free fescue. Prolactin from the SBH treatment was higher than IMP or control groups (146 vs 76 and 60 ng mL-1, respectively). Research is needed to determine if additional E2Eqs from dietary phytoestrogens or exogenous sources of estradiol can further reduce symptoms of fescue toxicosis. The E-Screen assay was an effective tool in monitoring serum for estrogenic effects of dietary supplementation with SBHs or estrogenic implants.