379 Effect of trace mineral source and dietary starch level on copper, manganese, and zinc concentrations in ruminal digesta of Nellore feedlot steers.

Abstract Ionization of metals from trace minerals supplemented in feedlot cattle diets can strongly bind to dietary fiber in the rumen which can ultimately impact intestinal absorption of these elements. Furthermore, high concentrations of free copper (Cu) and zinc (Zn) in ruminal fluid can be toxic to bacteria. Thus, this study aimed to evaluate the Cu, Mn, and Zn concentrations in the solid fraction of the ruminal contents collected from Nellore feedlot steers receiving different trace mineral sources and dietary starch levels. Briefly, 96 Nellore steers with an initial BW of 368 ± 3.6 kg were blocked by BW and allocated to four pens equipped with automatic waterers and electronically monitored feeding systems (Intergado, Betim, Brazil) to obtain individual animal feed intake. Pens were then randomly assigned to treatments. A 2 x 2 factorial arrangement of treatments was used. Factors included: 1) two levels of dietary starch (15 or 35% - Starch15 or Starch35) and 2) two trace minerals sources [Sulfate (STM) or Hydroxychloride (HTM)] used to supply 12, 40, and 60 mg/kg DM of supplemental Cu, Mn, and Zn, respectively. All finishing diets were formulated to meet or exceed 1.5 kg of ADG. At the end of 118 d feeding period, steers were slaughtered at a commercial abattoir. Immediately post slaughter, ruminal contents (approximately 500g wet weight) were collected from all steers and transported back to the laboratory. Ruminal samples were then divided into solid and liquid fractions via centrifugation at 12,000 x g. The supernatant was discarded and the solid fraction was dried in a forced air drying oven at 55°C for 72 h. Samples were then digested with nitroperchloric acid, filtered through ash-free paper, brought to a total volume of 10 mL with ultrapure water, and analyzed for Cu, Mn, and Zn concentrations using an ICP-AES instrument. Data were analyzed using a mixed effects model (PROC MIXED, SAS Inst. Inc., Cary, NC) for a completely randomized block design. Individual animal was considered the experimental unit. Significance was determined at P ≤ 0.05. There was no starch level by trace mineral interaction (P ≥ 0.05) for Cu, Mn, or Zn concentrations within the solid fraction of ruminal contents. Regardless of the starch level in the diet, Cu, Mn, and Zn concentrations were greater (P < 0.05) in the solid fraction of the rumen content from steers that received STM compared to HTM sources of Cu, Mn, and Zn (Figures 1, 2 and 3). These data indicate that Cu, Mn, and Zn from STM sources are found in greater concentrations in the ruminal digesta than HTM sources of Cu, Mn, and Zn. Future research examining the post ruminal availability of Cu, Mn, and Zn bound to ruminal digest is warranted.