1. The nature of the digestion-inhibiting substance in Kikuyu grass (Pennisetum clandestinum, Hochst), containing high levels of nitrate, was investigated using in vitro digestibility techniques.
2. Nitrite, which accumulated during the reduction of nitrate to ammonia, seemed to be the primary factor reducing digestibility. Nitrate and ammonia did not affect digestion in vitro.
3. Nitrite caused a reduction in the cellulolytic, xylanolytic and total microbial population, with a concomitant reduction in cellulase and xylanase activity of the digesta.
4. The mode of action of nitrite on rumen microbial growth was investigated.
5. The possibility that the growth of cellulolytic rumen microbes was depressed by a reduction in concentration of essential branched-chain volatile fatty acids by nitrite was discounted.
6. Although nitrite caused a marked increase in the redox potential, due to its oxidizing properties, the more-positive redox potential did not reduce the digestibility of the grass.
7. The growth of three of the four major cellulolytic bacteria commonly found in the rumen was severely depressed by nitrite, while some rumen bacteria were relatively insensitive to nitrite.
8. Growth inhibition seemed to depend primarily on the extent to which these microbes derive their energy from electron-transport-mediated processes.
9. It was suggested that, due to the sensitivity of some rumen bacteria to nitrite, digestibility and therefore animal performance could be affected long before clinical symptoms of nitrite toxicity become apparent.