This study investigated the effects of feeding clinoptilolite (CLN; 2.5% of diet dry matter) with a particle size of either 30- or 400-mu m on ruminal fermentation characteristics, measures of nitrogen (N) utilization, and manure ammonia-N (NH3) emissions in feedlot cattle. The impact of directly applying 30- or 400-mu m CLN to the pen surface (2,250 kg/ha) on manure NH3-N emissions was also evaluated. Six beef heifers were used in a replicated 3 x 3 Latin square design with 21-d periods. Dietary treatments were 1) finishing ration with no supplement (CON), 2) CON + 30-mu m CLN (CLN-30), and 3) CON + 400-mu m CL (CLN-400). Intake was measured daily. To evaluate fermentation characteristics, ruminal fluid was collected on day 19. Indwelling pH loggers were used to measure ruminal pH from days 15 to 21. Blood was collected 3-h post-feeding on day 21 for metabolite analysis. Fecal grab and urine spot samples were also collected from days 19 to 21 to measure nutrient digestibility, route of N excretion, and in vitro NH3 emissions. There was no diet effect (P >= 0.12) on nutrient intake and apparent total tract digestibility, and ruminal short-chain fatty acid profile and pH. Ruminal NH3 concentration, which was lower (P = 0.04) for CLN-30 than CON heifers, did not differ between CON and CLN-400 heifers. Although there was no diet effect (P = 0.50) on plasma urea-N (PUN) concentration, proportion of urea-N excreted in urine was lower (P = 0.01) for CLN-30 than CON and CLN-400 heifers. Urinary NH3-N excretion, which was greater (P <= 0.04) for CLN-400 than CON heifers, did not differ between CLN-30 and CLN-400 heifers. Feeding CLN also increased (P <= 0.02) fecal excretion of potassium (K) and iron (Fe) and reduced (P = 0.01) urinary excretion of calcium (Ca). There was a treatment x time interaction (P = 0.01) for NH3 emission rate, which was greatest within the first 36 h of incubation and was lower for manure from CLN-400 compared to CON and CLN-30 heifers and pen surface application treatments. Cumulative NH3 emissions were lower (P < 0.01) for manure from CLN-400 compared to CON and CLN-30 heifers and the pen surface application treatments. Although surface application was ineffective, feeding 400-<mu>m CLN to finishing cattle could result in a beneficial decrease in manure NH3 emissions. However, changes in fecal and urine excretion of minerals like K and Ca, which suggest a decrease in bioavailability, need to be considered when feeding CLN in finishing cattle diets. Lay Summary There is interest in developing strategies that limit ammonia emissions from confined feeding operations, as it leads to both air and water pollution. Feeding zeolites like clinoptilolite (CLN) or directly applying them to pen surfaces to bind ammonia represents possible mitigation strategies. Although this could impact its effectiveness, there still is limited information on whether the particle size of CLN influences the binding of ammonia when fed or surface applied. Therefore, this study evaluated the effects of feeding supplemental CLN with a small (30-mu m) or large particle size (400-mu m) to finishing cattle on ruminal fermentation parameters, route of nitrogen excretion, and manure ammonia emissions. We also evaluated the effects of applying 30- and 400-mu m CLN directly to the pen surface on manure ammonia emissions. Only CLN with the small particle size (30 mu m) was effective in binding ruminal ammonia, which reduced the amount of urea excreted in urine. However, ammonia emissions were only lower for manure from finishing cattle fed CLN with the large particle size. Applying CLN to the pen surface was not effective in reducing ammonia emissions. There were also indications that fed CLN reduced bioavailability of potassium and iron.
