ABSTRACT
Summary Nitrate in forage has been proven to occasionally poison ruminants. Our recent studies, however, have shown that a high concentration of nitrate inhibits butyric acid fermentation to the advantage of the silage quality. This work was undertaken to see the relationship between silage quality and the pattern of nitrate reduction during ensilage. About 140 g of orchardgrass chopped 0.5 cm long was mixed with 0.05% (W/W) of K15NO3 (96 atom % excess) and put in a 250-ml glass bottle, which was immediately sealed to prepare “control silage” or sealed after 24 hours to prepare “air-exposed silage” with a rubber stopper fitted with a fermentation trap for gas absorption. “Glucose-added silage” was prepared in the same way as control silage except that 2% (W/W) glucose was added. The bottles were kept at 30°C and then opened after 1, 2, 4, 7, and 30 days. Nitrate, nitrite, ammonia, and nitric oxide were determined by the Devarda method and 15N by the opticalebission spectrometric method. The pH drop and the lactic acid production were significant only in the control and glucose-added silages after 2 days. Addition of glucose improved silage quality and exposure to air injured quality as evidenced by pH, organic acids, and ammonia. The nitrate-15N decreased rapidly within 2 days to 0.3%, 0.5%, and 17.1% in the air-exposed, control, and glucose-added silages, respectively. Production of small amounts of nitric oxide-15N was observed only in the control and glucose-added silages on the first day and the second day, respectively. Ammonia-15N on the 30th day accounted for 97.7%, 71.5%, and 38.5% of total in the air-exposed, control, and glucose-added silages, respectively. The pattern of nitrate reduction to nitrite, nitric oxide, and ammonia during ensiling is closely related to silage quality. The better the silage quality is, the smaller is the extent of nitrate reduction to ammonia and the larger is the production of nitric oxide.
