Simple SummaryResidues of herbicidal active substances such as glyphosate in ruminant feed lead to oral exposure in animals. Consequently, the possible toxic influences of glyphosate on the health of ruminants are of concern. While some studies have analyzed the effects of glyphosate residues on dairy cows, studies with fattening bulls are scarce. Therefore, the present feeding study on German Holstein bulls was conducted within a real-life in vivo scenario that could occur in Germany before restrictions on glyphosate usage are enacted and might still be realistic in other countries. As well as feeding diets with or without glyphosate residues for several weeks, different concentrate proportions were employed to analyze the putative influences of varying energy and nutrient supplies and consequently different ruminal milieus on potential glyphosate effects. Glyphosate exposure did not adversely affect animals’ performances or other health-related characteristics upon the tested conditions. The observed putative effects of glyphosate on selected blood parameters were rather weak and not consistent. In contrast, concentrate feed and time markedly influenced most of the experimental parameters. In summary, upon the formerly realistic exposure conditions in Germany, all animals remained clinically inconspicuous throughout the trial.AbstractGlyphosate (GLY), the active substance in non-selective herbicides, is often found in ruminant feed. The present feeding study aimed to investigate the effects of GLY-contaminated rations and different concentrate feed proportions (CFP) on the health of fattening German Holstein bulls. Bulls were grouped by low (LC) or high (HC) CFP with (GLYLC, GLYHC) or without GLY-contaminations (CONLC, CONHC) in their rations. Intakes (dry matter, water) and body weight were documented continuously lasting over an average range from 392.2 ± 60.4 kg to 541.2 ± 67.4 kg (mean ± SD). Blood samples collected at the trial’s beginning, and after 7 and 15 weeks, were analyzed for hematological and clinical-chemical traits, functional properties of leukocytes, redox parameters and DNA damage. The average GLY exposures of 128.6 (GLYHC), 213.7 (GLYLC), 1.3 (CONHC) and 2.0 µg/kg body weight/d (CONLC) did not lead to GLY effects for most of the assessed parameters relating to animal health and performance. CFP and time displayed marked influences on most of the experimental parameters such as higher dry matter intake and average daily gain in HC compared with the LC groups. GLY effects were rather weak. However, the observed interactive effects between GLY and CFP and/or time occurring in an inconsistent manner are likely not reproducible. Finally, all animals remained clinically inconspicuous, which brings into question the physiological relevance of putative GLY effects.