The mode of action of bacterial silage feed additives was proposed by McDonald (1981) who concluded that silages are characterized by having low pH values, usually between 3.7 and 4.2, and containing high concentration of lactic acid. Noton (1982) confirmed that anaerobic bacteria fermentation converts sugary compounds in the material into lactic acid inhibiting normal aerobic bacterial action. If air is kept out of the silage, it is preserved efficiently and stably.As a result of agriculture practices, million tons of agriculture are produced as a secondary or by-products; however, with low nutritive values. Many methods are applied to improve the nutritive value and increase its utilization in ruminant’s nutrition. The biological treatments are the most common with more safe-treated products. In most cases, the biological treatments are paralleled with decreased crude fiber and fiber fractions content with increased crude protein content. Direct-fed microbial and exogenous enzymes to animal are other ways of biological methods for improving nutritive value of feeds. Here in this review, we will try to cover the biological treatments of by-products from different sides view with different types of animals and different animal end-products.