Annual and seasonal variation in nutritive quality and ruminal fermentation patterns of diets in steers grazing native rangelands.

The objective of the study was to evaluate the annual and seasonal changes in the nutritive quality and ruminal fermentation patterns of diet consumed by grazing cattle. Diet samples were collected with four esophageal cannulated steers and four ruminally cannulated steers were used to evaluate the ruminal fermentation patterns. Data were analyzed with a repeated measurements design. No year x season interactions were observed for nutritive quality and ruminal fermentation (p > 0.05). However, the crude protein (CP), in vitro digestibility dry matter (IVDMD), Calcium (Ca), metabolizable energy (ME), ammonia nitrogen (NH3N), volatile fatty acids (VFA) and propionate were higher in 2008 and wet season and lowest in 2006 and dry season (p < 0.05). Results confirm that the nutritive quality and ruminal fermentation patterns of diet selected for grazing cattle were affected by year and season and that protein an energy supplementation is necessary to improve productive performance of cattle under these management conditions.