Trace minerals are important components of many biological functions, including growth and development, and the immune response. Trace minerals are often supplemented to cattle, but interactions within the rumen and poor bioavailability can lead to decreased trace mineral status. Additionally, stressors such as transit and disease challenges, and perhaps growth-promoting technologies, may alter trace mineral needs, although this relationship has not been investigated. This research was designed to: 1) examine the effect of trace mineral source and concentration on ruminal dry matter digestibility and mineral solubility, 2) determine the effect of trace mineral status on the response to a trace mineral injection in feedlot steers, 3) determine the effect of a trace mineral injection prior to transit, on beef cattle response to transit and growth and carcass characteristics, and 4) investigate the interaction between the β- agonist ractopamine hydrochloride and a supplemental Zn-amino acid complex in growth performance of feedlot cattle. It was hypothesized that improvements to trace mineral status through supplementation would improve cattle health and growth performance. After completion of our first objective, it was determined that inorganic (sulfates) trace mineral supplementation decreased ruminal dry matter digestibility, but hydroxylated trace mineral supplementation had no impact on dry matter digestibility, relative to no trace mineral supplementation. Additionally, Mn and Cu from hydroxy sources were less soluble in the rumen, and Cu from hydroxy sources was equally soluble in the abomasum when compared with the sulfate source. Hydroxy trace mineral sources are less soluble in the rumen, which can prevent negative trace mineral and dietary component interactions within the rumen. After completion of our second objective, we determined that steers with mildly deficient trace mineral status experienced greater body weight loss in response to transit. Additionally, trace mineral injection can increase liver Cu and Se for up to 30 d post-injection, and can improve marbling score and ribeye area regardless of previous trace mineral supplementation. After completion of our third objective, it was determined that a trace mineral injection 28 d prior to transit, or feed and water restriction did not change the physiological response to transit or feed and water restriction, but did cause a decrease in growth performance for the 14 d period post-transit. However, there was no overall effect on growth performance or carcass characteristics when the entire feeding period was evaluated. Trace mineral injection is an effective way to rapidly increase trace mineral status, but may be most effective in improving growth performance and immune function in cattle with less than adequate trace mineral status. Finally, after completion of our fourth objective, it was determined that increasing supplementation of a Zn amino acid complex (30, 60, and 90 mg Zn/kg diet DM) to a diet that already contained 88 mg Zn/kg diet DM led to a linear improvement in average daily gain, feed efficiency, final body weight, and tended to increase hot carcass weight in steers that were also supplemented with ractopamine hydrochloride. The Zn requirements of cattle supplemented with a β-agonist may be greater than previously estimated, as demonstrated by the improved growth response to Zn supplementation in ractopamine hydrochloride supplemented cattle.

Overall, this research suggests that trace mineral supplementation to feedlot cattle above documented requirements may be most beneficial in cattle with inadequate trace mineral status, or cattle experiencing rapid growth in response to a growth-promoting technology