Stephanie L. Hansen, PhD
Associate Professor - Beef Feedlot Nutrition
Department of Animal Science
Iowa State University, Ames, IA
The objective of the study was to compare the effectiveness of various TM repletion strategies on TM status and performance of steers fed diets containing the antagonists sulfur (S) and molybdenum (Mo). Seventy-two Red Angus steers were blocked by body weight (BW, 560 ± 14 lbs) and assigned to one of two corn-silage based depletion diets (DEP), which were fed for the entirety of the trial. Depletion diets were either supplemented with Cu, Mn, Se, and Zn at NRC (1996) recommendations (control, CON), or not supplemented with these TM and supplemented with 0.3% S (CaSO4) and 2 mg of Mo/kg DM to deplete TM status (antagonist, ANT). On d 88 and 89, respectively, the heavy and light blocks were assigned randomly within block to one of three trace mineral repletion (REP) strategies. Repletion treatments within depletion diets were as follows: 1) an injectable TM (Multimin90; Multimin USA, Fort Collins, CO) containing Cu, Mn, Zn, and Se, and dietary TM (Cu, Mn, Zn, and Se) supplemented at 100% of nationally recommended concentrations (NASEM, 2016) from strictly inorganic sources (ITM), 2) a sterilized saline injection and TM supplemented at 150% of NASEM (2016) recommendations from strictly inorganic sources (ING), 3) or a sterilized saline injection and TM supplemented at 150% of NASEM (2016) recommendations from a blend of 75% inorganic and 25% organic sources (BLEND). This 2 × 3 factorial resulted in 6 treatments for the 62 d repletion period (n = 12 steers per treatment). Injections were provided subcutaneously as Multimin90 for ITM, or sterilized saline solution for ING and BLEND at a rate of 1 mL per 150 lbs BW. Data were analyzed using the GLIMMIX procedures of SAS 9.4 with steer as the experimental unit in the depletion period (n = 36 per treatment) and repletion period (n = 12 per treatment).
At the end of the depletion period, regardless of initial status, liver Cu was decreased approximately 90% in ANT compared to CON. Similarly, liver Se was decreased approximately 40% and liver Mn concentrations were also decreased in ANT compared to CON at the end of depletion. The interaction of S and Mo can create thiomolybdates, which can bind Cu in the rumen and be excreted as an insoluble complex. Additionally, increased concentrations of S have been shown to decrease Se status in ruminants. Producers may not realize their cattle have been exposed to high S or Mo until symptoms of TM deficiency appear, and these data suggest 90 d of exposure to these antagonisms can greatly deplete TM status. Trace mineral supplementation can improve status, however inorganic forms are highly ruminally available and can be subjected to antagonisms by thiomolybdates, S, or diet. While organic TM are ruminally insoluble and thus potentially more available for absorption in the small intestine, injectable TM can improve TM status by avoiding ruminal interactions providing TM directly to the tissues.
In the repletion period, there were no interactions between DEP diet and REP strategy, suggesting the pattern of TM repletion within a REP strategy was similar across diets with or without antagonists. This is clearly noted in the effect of ITM on steers fed ANT and CON diets in the repletion period. Although ANT had lesser Cu status at the start of repletion than CON, both treatments receiving ITM increased liver Cu by approximately 44 mg Cu/kg DM when measured 14 d post injection. Liver Cu was improved by d 14 in ITM, while it took 28 d and 42 d for Cu status to improve in steers supplemented with 150% of national recommended concentrations from BLEND and ING treatments, respectively. Similarly, liver Se was improved by d 14 in ITM, on d 28 ITM and BLEND were greater than ING, and there were no differences due to repletion treatment on d 42.
There has been less research focused on how dietary antagonists may affect Mn and Zn status of cattle. In this study, S and Mo decreased liver Mn and plasma Zn at the end of the depletion period, while liver Zn was unaffected. Due to random treatment assignments, steers assigned to received ITM had lesser Mn concentrations at the start of the repletion period. Interestingly, while there was a decrease in liver Mn concentrations of BLEND and ING on d 14, steers receiving ITM maintained their status at this time, suggesting ITM had additional available Mn to utilize in response to the stressors.
Although ANT steers had lesser DMI during repletion, there were no differences in final BW due to treatment. Interestingly, ANT steers had more favorable G:F during the repletion period, which suggests some kind of compensational gain occurred as TM status of the steers improved. This intensive experiment was designed with the intent of examining the TM repletion response in steers fed a diet containing high S and Mo. Further work is needed to determine the optimal TM supplementation strategy to overcome dietary antagonists and create the greatest efficiency of production with the least environmental impact. These data suggest any of the TM supplementation strategies utilized would be sufficient depending on the urgency of TM repletion, with ITM having the most rapid improvement in Cu and Se status.