Effect of prolonged high-concentrate feeding on ruminal protozoa concentrations.

Five ruminally cannulated steers, with ad libitum access to feed, were gradually adapted from an all-forage diet to a 75% concentrate diet over a 6-wk period. Three animals were then randomly assigned to an all-concentrate diet (87% whole corn) and the other two were fed a 90% concentrate plus 10% forage diet. These diets were fed for 17 wk and then reversed between groups for 11 additional weeks. Over the last 22 wk, addition of 10% forage to the all-concentrate diet had no effect on the concentration of total protozoa; however, Isotricha and Epidinium percentages increased (P < .05). Although concentrations varied markedly, protozoa persisted throughout the entire period of high-concentrate feeding (both diets) in three of the animals. In contrast, the other two animals were defaunated most of the time, except for the sporadic appearance of Entodinium species for 1- or 2-wk intervals. Average pH in these latter two animals was lower (P < .05) during the entire 28-wk high-concentrate feeding period. Because these two animals had a lower ruminal pH than the other three, even when fed all forage, it seems that the ruminal environment varies between individual animals. Thus, the maintenance of a protozoal population in animals fed high-concentrate diets may be related to physiological conditions such as rate and extent of salivary production, rate of fluid and particulate matter passage within each animal, and so on.