Abstract: Disclosed are compositions and methods for supplementing ruminant feeds. The compositions generally have an enhanced histidine content, e.g., compositions which include at least one ingredient that has an enhanced content of histidine. The histidine-enriched ingredient is commonly derived from a non-animal source. The methods can include feeding ruminants feed compositions that include the histidine-enriched ingredient to improve milk production. In some embodiments, the feed composition includes an E. coli. biomass derived from fermentation of E. coli, wherein the biomass includes (a) an inhibitor of at least one of the endotoxic effects of lipopolysaccharides of E. coli; and (b) histidine, at least one histidine-rich protein or a mixture thereof.