Effect of N Fertilization On Diurnal Phenolic Concentration and Foam Strength in Forage of Hard Red Wheat (Triticum aestivum) Cv. Cutter.

In the southern Great Plains of USA, cattle grazing hard red wheat (Triticum aestivum L.) pastures often develop frothy bloat conditions, which may result in death and economic loss. Bloat is the final result of a series of complex relationships among wheat plant metabolism, cattle genetic make up, environmental conditions, and ruminal microbial activity. Previous research documented condensed tannins (polymerized phenolic compounds) as bloat reducing metabolites. It has been proposed to select wheat cultivars with elevated tannin content to minimize bloat potential. We hypothesized that concentration of phenolic compounds in wheat forage is affected by solar radiation and N fertilization and might affect bloat potential. We measured diurnal (at 7, 10, 13, 16, and 18 hr) phenolic concentration and foam strength (a measure of bloat potential), and seasonal patterns of protein fractions and soluble carbohydrates in forage of “Cutter” wheat fertilized at 33, 67 and 134 kg N ha^-1 during November – March in 2006 and 2007. Phenolic concentrations followed the diurnal patterns of solar radiation and were negatively correlated with foam strength. In 2006, phenolic concentrations were greater in January and February (5.5-5.7 g kg^-1 DM tannic acid equivalent) than other months (2.8-4.2 g kg^-1 DM). In 2007, greater phenolic concentrations occurred in November and December (6.0 - 6.7 g kg^-1 DM) than other months (4.7-5.6 g kg^-1 DM), which was associated with severe drought in winter and spring. The correlations between N fertilization and phenolic concentration and foam strength were inconsistent in both growing seasons. Concentrations of phenolics, insoluble and total proteins, and foam strength declined, whereas soluble carbohydrate concentration increased during the growing season as wheat plants transitioned from vegetative to reproductive growth. Results suggest that phenolics can alter foam strength of wheat forage. Breeding wheat cultivars for high level of phenolics in forage may be one method to prevent bloat in grazing cattle.